MegaSTe hardware problems -help locate it please!

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Re: MegaSTe hardware problems -help locate it please!

Postby Fujiyama » Tue Jul 04, 2017 9:59 am

troed wrote:
Fujiyama wrote:Cool!
May I ask if the MOSFET transistor listed there was also included, or just the caps?

Did you notice any difference before/after?


Only the caps. Also, I recapped for someone else so I have no knowledge on workings before/after.


Perhaps not a noticeable difference, but the "ticking timebomb" issue is gone for several more years :lol:


You mentioned receiving replacement capacitors from Exxos with the values listed here and almost all of them differ from their original capacity values:
400V 120uF - Replacement 150uF 400V
400V 10uF - Replacement 10uF 400V
16V 1,000uF x 2 - Replacement 16V 1500uF
10V 2,200uF - Replacement 16V 4700uF
16V 4700uF - Replacement 16V 4700uF
10V 680uF x2 - Replacement 16V 1500uF
10V 100uF - Replacement 10V 100uF
22uF 25V - Replacement 10uF 50V (not a typo!)
3.3uF 50V - Replacement 10uF 50V
4.7uF 50V - Replacement 10uF 50V
25V 100uF - Replacement 220uF 50V (needs to be mounted higher due to resistor in way)
25V 220uF - Replacement 220uF 50V

Do you know why and if it's best I order the exact same values as before (or as close as possible) or go with Exxos' choice? I'm ordering those capacitors along with other components from an electronics supplier, so I won't be getting Exxos' recapping kit.
(I don't want to modify it without knowing what I'm doing and something that can cause problems/electrical risks later).
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Re: MegaSTe hardware problems -help locate it please!

Postby troed » Tue Jul 04, 2017 11:33 am

Fujiyama wrote:Do you know why and if it's best I order the exact same values as before (or as close as possible) or go with Exxos' choice?


I trust exxos' reasons for having changed the values (I think he discusses why in the long description about the recapping).

/Troed

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Re: MegaSTe hardware problems -help locate it please!

Postby Fujiyama » Wed Jul 05, 2017 7:27 pm

I suppose the power supply is re-assembled and the computer delivered back to its owner, but do you happen to remember anything about the 400V replacements? There aren't any Panasonic FR-series caps available with those values (150uF/400V and 10uF/400V), so I'm curious as to which caps Exxos included in the package...
As I understand it, he supplies Panasonic FR-series caps for the rest, having read between the lines in his comments about recapping Atari PSU units on his website.
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Re: MegaSTe hardware problems -help locate it please!

Postby Fujiyama » Wed Jul 05, 2017 7:57 pm

siriushardware wrote:When you get your regulator, we'll suggest a way to test it for basic functionality before you put it in the machine. Do you have a bench power supply which can provide either a variable DC voltage or one or more DC voltages? Do you have an experimenter's 'breadboard'?

To be honest, I would say that the 78L82 is not the sort of exotic high value semi which people are likely to bother to fake.


I hope you're right.
It just arrived so I'm ready to pop it in.

Were you thinking it's a good idea to test it outside of the computer just in case it's a fake/non-working etc. component I've received?
I don't have a bench PSU, but couldn't I just connect tap power from the Mega STe to test it?
According to the datasheet this would be simple, attaching it as follows:
78L82 pinout.png


The datasheet is a little unclear about its input voltage, but I believe it says (in the "Output voltage" section in the 78L82 datasheet) to use between 11 and 23V, so I'll attach the "input" pin to +12V, then measure the output voltage between GND and the "output" pin before soldering it back into the Mega STe.
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Re: MegaSTe hardware problems -help locate it please!

Postby siriushardware » Wed Jul 05, 2017 8:31 pm

Yes, that's pretty much it. Connect +12V from a 12V power supply to INPUT, connect 0V of the 12V power supply to GND.

Power on, and then check the output voltage between OUTPUT and GND with a meter.

It should be round about, not necessarily exactly 8.2V.

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Re: MegaSTe hardware problems -help locate it please!

Postby Fujiyama » Wed Jul 05, 2017 9:01 pm

Fujiyama wrote:I suppose the power supply is re-assembled and the computer delivered back to its owner, but do you happen to remember anything about the 400V replacements? There aren't any Panasonic FR-series caps available with those values (150uF/400V and 10uF/400V), so I'm curious as to which caps Exxos included in the package...
As I understand it, he supplies Panasonic FR-series caps for the rest, having read between the lines in his comments about recapping Atari PSU units on his website.


I usually don't reply to my own postings :P
.... but if I'm not mistaken about what I read in another forum, low-ESR type capacitors aren't needed in this part of the PSU section because they're for the input stage (100-110V/220-240V depending on where you live) as opposed to the high frequency section of the PSU which I believe really needs low-ESR caps (to prevent voltage loss on the outputs and internal heat buildup due to higher internal capacitor resistance (high ESR).
So I'm concluding that just about any quality brand capacitor with 105 degrees C and long life (10 000 or 12 000 hours) ratings will qualify. Exxos' choice of replacing the 120uF/400V cap with 150uF/400V is probably for slightly improving the input voltage fluctuation, but I'm not sure. Is this a safe thing to do, or should I just stick to the original 120uF value if I can find it?

Are my assumptions/understanding above correct?
Attached is the Mega STe power supply schematic. The two 400V capacitors are marked in red.

Atari_PSU_400V_caps.png
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Re: MegaSTe hardware problems -help locate it please!

Postby Fujiyama » Wed Jul 05, 2017 9:06 pm

siriushardware wrote:Yes, that's pretty much it. Connect +12V from a 12V power supply to INPUT, connect 0V of the 12V power supply to GND.

Power on, and then check the output voltage between OUTPUT and GND with a meter.

It should be round about, not necessarily exactly 8.2V.


Thanks. I'll try to check it tonight and will report back here.
The 78L82 already in the computer gave out about 8.4V which I thought was a lot, but according to the datasheet it can fluctuate between 7.8V and 8.6V so it should be OK I guess.
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Re: MegaSTe hardware problems -help locate it please!

Postby siriushardware » Wed Jul 05, 2017 9:10 pm

By the way, while you are waiting for your replacement 7406 there's something you can try. Take IC301 out and put it into your experimenter's breadboard, set your multimeter to ohms and make the following measurements.

With the black meter lead on pin 7, measure the resistance from:-
-Pin 7 to pins 1, 3, 5, 9, 11, and 13 - all six resistance measurements should be the same.
-Pin 7 to pins 2, 4, 6, 8, 10, and 12 - all six resistance measurements should be the same.

With the red meter lead on pin 14, measure the resistance from:
-Pin 14 to pins 1, 3, 5, 9, 11, and 13 - all six resistance measurements should be the same.
-Pin 14 to pins 2, 4, 6, 8, 10, and 12 - all six resistance measurements should be the same.

The logic(?) behind this is that the 7406 is basically a box containing six identical elements, so if, when comparing all the elements, you see one which seems quite different, then the chances are that element is damaged and the others are OK.

If you measure the input and output resistances of all the gates and all the inputs seem identical and all the outputs seem identical, that is not absolute proof that the device is OK as there could still be an 'internal' fault within one of the gates which does not affect the resistance measured on the device pins.

But if you do the measurement and you find that (say) five inputs are in the high megaohms and and one has an input resistance of just a few K or ohms, then you can be pretty sure that gate is damaged.

You can apply this same crude testing method to any logic device (or indeed any semiconductor device) which contains two or more identical elements. Any measureable difference between identical parts in an IC is a fair indication that part of the IC is defective.

Ref: Capacitors in your PSU, those high voltage ones immediately after the bridge rectifier run at low frequency and don't need to be low-ESR types, but most of the rest, especially on the output / low voltage side of the power supply, do. C8 does need to be a low ESR / high frequency type because its job is to smooth the high frequency ripple on the supply being fed back from the transformer via D6.

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Re: MegaSTe hardware problems -help locate it please!

Postby Fujiyama » Thu Jul 06, 2017 9:23 am

The new 78L82 appears to be working correctly. With the Mega STe +12V input it outputs +8.38V, so I'll solder it back into the main board and at least get the audio working again.

Good news about the 7406: I found that Farnell actually has an SN7406N as well as the SN74LS06N which I found earlier, so I'll be ordering a couple of both.If I get the time I'll perform the breadboard tests above. You're just talking about measuring internal resistance, right? No need to add power to the + and GND pins?

I'm learning a lot about recapping when reading this and other threads, which is very useful.
One assumption I've had is the when you recap you need to change all of them to the best possible low ESR types you can afford or justify. Not so according to another thread on the subject. Actually I've been told that the Panasonic FR-series caps I've chosen for the PSU might be "too good" for the (almost 30 year old) technology and that they might cause stability problems with oscillation because of the feedback loop compensation which would be incorrect. I'm not sure what all that means but I'm guessing that the original circuit was designed with the component flaws in mind (i.e. no ultra-low ESR caps available back in the late 80's/early 90s), so when the caps are replaced with ultra-low ESR caps their values will be different and thus cause various issues with the PSU.
On the other hand, Exxos (who seems knowledgeable in all of this) says he uses mostly Panasonic FR-series caps when recapping Atari power supplies:
NOTE2: I keep getting asked about part numbers I use for these kits. Unfortunately I do not list them anymore simply because by the time I have sorted part numbers out and updated my site, the part numbers are no longer valid or discontinued. With several PSU's now on this part all needed various capacitors, it would be a huge undertaking to keep updating this page (time I just do not have unfortunately). Generally I go with the Panasonic FR series as they are really good ESR values. Please don't fit "any old capacitor sold around the Internet" as the cheaper capacitors are not suitable for switchmdoe designs and can easily overheat or explode! I am trying to update my store with new re-cap kits so please keep checking there.


So that's what I've been adding to my order as well (except for the two 400V caps). It's not complete and sent in though, so I can always change them to other caps if really necessary. It's hard to tell exactly which caps Exxos is selling in his "MSTE PSU REPAIR KIT PHIHONG PSM-5341 240V type £25 1 In Stock [0064]" but judging from the photo there's one red cap there which I can barely make as 150uF/400V. It seems to resemble one of Wurth's two available which is probably fine except they only have a 2000 hour lifetime. I prefer having the same lifetime for all the caps so when it's time to recap again there's no guessing which one is the oldest (or unnecessary changing of all the other expensive low ESR caps just because the low-life Wurth cap has expired). I didn't succeed in finding anything suitable with 150uF, but optinf for the original 120uF value I ended up with a Rubycon BXW-series 120uF/400V. It's got a 12000 hour lifetime at 105C (actually better than the 10000 hours for the Panasonic low-ESR caps) but is a little high at 30mm. Still it should fit inside the PSU casing.
For the 10uF/400V I opted for Panasonic's EE-series 10uF/450V rated at 10000 hours at 105C (just like the other Panasonic caps (the low ESR ones) I'll be using in the PSU).

I did start posting about this in a separate Atari recapping thread, but responses there has been minimal and the discussion flow here is already going, which is fine by me :D
I think this info might be useful for others thinking of recapping the same PSU even though there is some info already available.
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Re: MegaSTe hardware problems -help locate it please!

Postby siriushardware » Thu Jul 06, 2017 4:33 pm

Fujiyama wrote:
The new 78L82 appears to be working correctly. With the Mega STe +12V input it outputs +8.38V, so I'll solder it back into the main board and at least get the audio working again.

I found that Farnell actually has an SN7406N as well as the SN74LS06N which I found earlier, so I'll be ordering a couple of both.If I get the time I'll perform the breadboard tests above. You're just talking about measuring internal resistance, right? No need to add power to the + and GND pins?

(Caps)...
....I'm not sure what all that means but I'm guessing that the original circuit was designed with the component flaws in mind (i.e. no ultra-low ESR caps available back in the late 80's/early 90s), so when the caps are replaced with ultra-low ESR caps their values will be different and thus cause various issues with the PSU.


Glad your 78L82 turned out to be 'real' and is apparently working OK. You may find that 8.38V goes down a little bit once there is a load running on the output from the device. Once you have it in, please also check again for the minus supply (I think minus five volts) on pins 11 of the LF347 ICs and pins 3 of the DAC ICs, Your audio-out section still won't work if you don't have that minus supply, or at least that's how I read the situation.

Yes, when testing the 7406 out of circuit you're just looking for the resistances of all the inputs to be the same, and the resistances of all the outputs to be the same. As I'm sure you know, resistance measurements should never be taken with the power on. It doesn't really matter now that you have managed to find 7406s from a reputable source.

There is some sense in what you are saying when you worry that modern capacitors might be too 'good'. On the other hand, Exxos has demonstrated over and over again that he is a hands-on practical, seriously competent technician / designer, so if he says that a certain type or set of values should be used you can be sure that he will have tried them himself. If you're still worried, my suggestion (which actually applies whenever you re-cap anything) is that when you are removing the original capacitors you mark each one with the position it originally came from - so if you replace C8, write 'C8' on the body of the original capacitor just after you remove it. Do this with each capacitor one at a time as you take them out so you can never get them mixed up. If you get into a situation where the cumulative effect of recapping a power supply is that it stops working, you will then have the option of putting the original capacitors back in exactly where they originally came from.

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Re: MegaSTe hardware problems -help locate it please!

Postby Fujiyama » Fri Jul 07, 2017 1:26 pm

siriushardware wrote:If pin 11 on both LF347s (not LF357, as you say) is really positive, that's a definite problem.

Those pins (and pin 3 of the two DAC ICs, U603 and U604) are all supposed to be at minus 5 volts. You have the -5V coming out of the PSU, so where it it going missing? Can you check the voltage on both ends of R604?

Locate R604 on the diagram, (Audio section, upper left). According to me, the feed into the left hand side of R604 is minus five volts. Or is that just my terrible diagram again?


Don't worry about the diagram reading -I think we have equally bad ones and have to make estimated guesses now and then to decipher what it actually says :)

Well, the 87L82 regulator is back in (from now on I'm going to be VERY careful whenever I remove/insert the PSU as it's all too easy to knock over that regulator as it's a tight fit with the fan and edge of the PSU mounting frame).
Audio is back and I have some further good news about voltage readings. I don't know if I got it wrong last time around (most likely), or if the regulator caused the problems, but this is what I just measured:

U608 (LMC1992) pin 28 shows +8.3V (PS: I believe I misquoted it as U508 earlier, but U608 is correct)
U605 (LF347) pin 4 shows +8.3V
U607 (LF347) pin 4 shows +4.8V
U605 (LF347) pin 11 shows -4.76V (correct polarity this time, or me reading the meter correctly!)
U607 LF347) pin 11 shows -4.76V (correct polarity this time, or me reading the meter correctly!)

I also checked both ends of R604 (I didn't check which end goes to which component though), and got these numbers:
-5.033V (R604 side which goes to the PSU -5V line)
-4.759V

I couldn't visually or electrically check verify where the other end (-4.759V) of the resistor was attached as I had put the PCB back in the MSTe bottom-casing, but according to the schematic it should go to the + end of C611 (100uF electrolytic) and one end of the C612 (0.1uF). The other ends of these two capacitors go to GND.

Still "No stereo sound available" when accessing the audio CPX control panel, so I rechecked if U705 (74LS244) pin 11 went to GND via the large DIP switch block (DIP#8) which it did. And yes -the DIP switch does enable/disable it so that's working as it should.
The problem must lie somewhere else.

I haven't gotten round to checking the 7406 chips as you suggested yet, but other than that it appears I can't do much more now before receiving replacement parts.
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Re: MegaSTe hardware problems -help locate it please!

Postby siriushardware » Fri Jul 07, 2017 5:07 pm

Fujiyama wrote:
Well, the 87L82 regulator is back in (from now on I'm going to be VERY careful whenever I remove/insert the PSU as it's all too easy to knock over that regulator as it's a tight fit with the fan and edge of the PSU mounting frame).
Audio is back and I have some further good news about voltage readings....

U608 (LMC1992) pin 28 shows +8.3V (PS: I believe I misquoted it as U508 earlier, but U608 is correct)
U605 (LF347) pin 4 shows +8.3V
U607 (LF347) pin 4 shows +4.8V
U605 (LF347) pin 11 shows -4.76V (correct polarity this time, or me reading the meter correctly!)
U607 LF347) pin 11 shows -4.76V (correct polarity this time, or me reading the meter correctly!)

I checked both ends of R604 (I didn't check which end goes to which component though), and got these numbers:
-5.033V (R604 side which goes to the PSU -5V line) -4.759V...



Yes that's OK - the only reason I targeted R604 so specifically is, that was your number one suspect if the voltage on the pins 11 / pins 3 was really positive rather than negative. R604 would most likely have been open-circuit in that case, but your most recent results override all that.

I've never seen a MSTe so in some ways I'm swordfencing blindfolded here. If the 78L82 really is in such peril every time you move the PSU in or out I might have suggested you mount it on the underside of the PCB out of harm's way, assuming there is enough space to do so. But you should hopefully get to the point where the PSU won't need to be moved again for quite some time, so just try not to knock it off again in the meantime.

Fujiyama wrote:Still "No stereo sound available" when accessing the audio CPX control panel, so I rechecked if U705 (74LS244) pin 11 went to GND via the large DIP switch block (DIP#8) which it did. And yes -the DIP switch does enable/disable it so that's working as it should.
The problem must lie somewhere else.


I'm assuming this control panel is a MSTe special as I've never heard of it - can you (or anyone who knows this aspect of the machine) explain how the control panel 'knows' or interrogates the status of the stereo audio hardware? I can only assume it sends some sort of digital query to some part of the sound hardware and expects to get a reply, and doesn't.

Fujiyama wrote:It appears I can't do much more now before receiving replacement parts.


Agreed. Keep us updated, It's been interesting so far.

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Re: MegaSTe hardware problems -help locate it please!

Postby Fujiyama » Fri Jul 07, 2017 8:15 pm

I don't have a dedicated workspace for troubleshooting this so I have the motherboard stay in its "base" (lower part of the computer casing) which allows me to safely secure the PSU whenever I apply power, and adding support to the PCB so it won't be under any strain). But I will pay extra attention to whenever I need to move the PSU around.
Regarding the recapping issue I've posted a schematic with the replacement/new values as well as a complete BOM (which might be of interest for anyone else needing to perform the operation), posted here, in that other recapping thread.

Regarding my comment/question about those low ESR Panasonic FR-series capacitors being "too good".... I got that cleared up in a different thread where this was regarding the 400V capacitors before the high frequency part of the PSU. So I've added a note about that in the Atari recapping thread I just mentioned. Your suggestion about marking each PSU capacitor one by one when I remove them is an excellent idea! I'll be sure to do that in case I mess up.

The only questions left surrounding this power supply are:
a) is changing the values of some capacitors as Exxos has done OK and electrically safe?
(as opposed to replacing the exact same values, but with low ESR caps -except for the two 400V ones that is)

b) is keeping the same value as before for C6 (120uF/400V) OK (now that many other capacitor values have been changed and thus might interact with each other in a different way from before), or is it essential that I also replace C6 with the Exxos' new value (150uF/400V)?


A final question about recapping the Mega STe (and STe as well actually): there are several electrolytics on the motherboard of these 25-30 year old computers and I'm sure they could need recapping as well. Given what I've now learnt about "too good" capacitors, should these be replaced only with "normal" (but high quality/renowned brand) capacitors (and NOT low-ESR types)? And does that include the big one (C227: 470uF electrolytic) near the power supply connector which I assume are there to further stabilise the +5V power line? See image below (centre senction of page 2 (of 11) from the Mega STe schematic) where I also shouldn't use a low ESR type?.
C227_mste.png



The resistance measure tip without powering on is indeed useful. I wasn't entirely sure what the test was about so reminders like that are always welcome! And yes, having sourced the 7406 I should be fine in any case.
On a slightly different note, I took a look at the PC900 opto isolator (U302) inside the computer and it's actually labelled something entirely different: "HIL3 048J" with an "H" logo that I don't recognize from any other semiconductor. The exact same one is used in my STe as well, so I was hoping it could be easier to source that part, but so far nothing pops up when doing any online searching. Perhaps a proprietary part ordered by Atari at the time, but compatible with the PC-900 (it says "PC-900" in the schematics).

I don't know how that "Sound" CPX works. I don't think it's Mega STe-specific because as far as I can remember it's also used with my STe. Hopefully replacing that 7406 will fix a number of things....

Glad you've found the thread interesting. I'm very grateful for your and others' help I've received here and aim for a fully working computer in the not too distant future (the next weeks will be quite busy, but I'll be sure to post here once I get to try out the new parts). Also I'm learning a lot!
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Re: MegaSTe hardware problems -help locate it please!

Postby Fujiyama » Fri Jul 07, 2017 9:36 pm

Greenious wrote:I found a pic of what you should be seeing on the scope:

dc ripple.jpg

The dotted lines is the power coming out from the rectifier, the filled line what you see after your caps are done smoothing the voltage.


Thanks. I'm a 'scope newbie so that's useful info. I've also ordered a proper probe (1x, 10x) as the oscilloscope only came with a pair of alligator clip wires attached to the BNC connector. Probably to save a few cents :wink:
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Re: MegaSTe hardware problems -help locate it please!

Postby Greenious » Tue Jul 11, 2017 11:37 pm

Fujiyama wrote:
Greenious wrote:I found a pic of what you should be seeing on the scope:

dc ripple.jpg

The dotted lines is the power coming out from the rectifier, the filled line what you see after your caps are done smoothing the voltage.


Thanks. I'm a 'scope newbie so that's useful info. I've also ordered a proper probe (1x, 10x) as the oscilloscope only came with a pair of alligator clip wires attached to the BNC connector. Probably to save a few cents :wink:


Np.

As for replacing the caps, the only thing that really does not matter, is the voltage rating, as long as it is the same or more.

The rest, you should generally try to be same, or very close. Overkill is not what we want, since it brings with it a host of other potential problems.

Overdoing it with the caps is like feeding a circuit that needs 1 amp with a 500 amp PSU, as long as everything works, there is no problem, but if something fails, you potentially fry the entire thing with 500 amps instead of 1 amp, causing more damage than necessary. (obviously I am exaggerating, but I think you get what I mean) And the existing ones did work for 25-30 years, so why question the original engineers? And given the lifecycle of decent electrolytic caps, "going overboard" likely won't buy you that much more time anyway, so it's just a waste of money...

Anyway, I'm going to refrain from commenting this subject anymore. Apparantly there is some that takes offence to differing opinions... (Which is kinda ridiculus considering that they supposedly are adults... :roll: )
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Re: MegaSTe hardware problems -help locate it please!

Postby Greenious » Wed Jul 12, 2017 12:02 am

Fujiyama wrote:Did you notice any difference before/after?


Actually, replacing the caps should improve both picture quality and sound after 20+ years.

Even though it might not be absolutely necessary to replace the caps for some time more, the degradation by now is likely big enough to be clearly visible and audible, if you actually look/listen and compare to a machine that's had a recap. Especially during things like floppy access (varying loads) I wouldn't be surprised if you can see anomalies on the screen on a machine that haven't been recapped.

Otoh, that degradation can be seen on machines not even 5 years old (caps degrade over their lifetime, not overnight), and when recapping is worthwhile is a judgement call.
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Re: MegaSTe hardware problems -help locate it please!

Postby Fujiyama » Thu Jul 13, 2017 1:24 pm

I for one appreciate your opinion, differing or not, so keep doing what you do :D
I will definitely recap but with all the differing info floating around the Internet it gets confusing.
It makes sense to keep to the same values like you say as that's what the designers chose for a purpose, so I'll go for that. But with low ESR caps as mentioned earlier or just normal quality brand ones? And for the motherboard caps?
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Re: MegaSTe hardware problems -help locate it please!

Postby siriushardware » Thu Jul 13, 2017 6:07 pm

Greenious wrote:The existing ones did work for 25-30 years, so why question the original engineers?


I actually agree with this view for the simple reason that, although I have been a hardware technician for the whole of my working life and have a fair understanding of how things work, my understanding is not so great that I would just be able to design a switch-mode power supply and make it work. Therefore, if I see a design and a set of component values which has worked for decades, it would not occur to me to do anything other than keep it working the way it is. I simply do not know enough about switch-mode PSU design to be able to overrule the choices of the original designer.

However, as seen from countless examples during my working life, designers don't always get it right and it's common if you work in the repair trade to come across certain faults which keep happening over and over again on the same models, perhaps because a particular component was underrated, or because an electrolytic capacitor was fitted too close to a heatsink and tends to be baked dry over time. In the latter case common sense tells you to fit a more temperature resistant component and to fit it lying on its side away from the heatsink which cooked the original component. But those are the only sorts of design modifications I would normally consider making.

There are other people - Exxos for example - who clearly do have the level of understanding which gives them the confidence to able to overrule the original design choices or even, as in Exxos's case, to design an entirely new replacement ST power supply from scratch.

Neither approach is wrong - Exxos and others like him do what they do because they can, and the rest of us just have to have faith in the original designs and keep them working the way they were built.

Fujiyama, your question about low-ESR capacitors - I have repaired all kinds of switched-mode PSUs many of which were dead purely due to aged capacitors. I always fit 105 degree low-ESR types in the positions where the capacitors are handling high frequencies, on the assumption that the designers chose the lowest ESR capacitors available to them at the time.

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Re: MegaSTe hardware problems -help locate it please!

Postby Fujiyama » Sat Jul 22, 2017 10:14 am

What you say makes perfect sense.
Having electronic musical instruments in addition to the Atari ST computers from several decades back (along with the ability to solder/desolder) I really can't skip the decapping thing.

You say that you've used low-ESR caps when recapping SMPS units, but I've been warned against doing this in other electronics related forums as it can destabilize the PSU and case oscillation in the feedback loop, so I've been advised to pick similar ESR valued caps from the time period the PSU was designed. Of course there's no way to measure the ESR values of the PSU caps when it was new, and with the obscure branded caps there's no way to look up their specs either, so for me that would be pure guesswork.

It does appear that a fair number of people here have recapped their Atari power supplies, and the general concensus here at least, though differing from the one above in many dedicated electronics related forums, is to use low-ESR caps, and I assume that if something was to go wrong (I have no idea what oscillation around the feedback loop will result in) it would be noticeable and people would post about it....
Then there's the mystery around why Exxos has chosen different values than the original caps -if these are precicely calucated values for improvement, if they're precicely calculated values based on compensations in the circuit because of the much lower (I assume) ESR caps than originally used, or if it's really just random guesswork in hopes for improvement. I mean no offense to Exxos or anyone else by saying this, but I just don't know enough about the matter to decide which way to go, so perhaps I should just leave the recapping until later so I can go ahead and order the other malfunctioning parts in the Mega STe.
There's also the matter of choosing the right type of caps for the motherboard, which I intend to do along with recapping the PSU.
Apart from the low-ESR Panasonic FR-series capacitors' 105C temperature rating and 10 000 hour rating I have found "normal/general purpose" capacitors (also from Panasonic -their EB-series) which have the same hour/temperature rating, according to this Panasonic capacitor series description.

In the midst of summer holidays and other things to keep the days busy I think I just need to get those ICs and other parts ordered, and work out a "recapping" order later, when all the details have been sorted.
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Re: MegaSTe hardware problems -help locate it please!

Postby siriushardware » Mon Jul 24, 2017 12:23 am

I would really prefer Exxos to speak for himself, but from observation of many past posts from him the way he works is, he notices problems with the systems as designed - for example, a PSU recapped with the original values may still not perform perfectly, may have excessive ripple on the output or not have perfect regulation, and then he sets about trying different things to see what will improve the situation. His preferred capacitor values are probably the result of one such investigation.

Sometimes the performance of the original design was limited by the quality of the components then available, as you have observed yourself, and sometimes designers have used the minimum capacitor values they can get away with because that keeps the cost to a minimum. For example in switch-mode PSUs I have often seen two or three large value (say 4700uF) capacitors in parallel to provide the output smoothing, with holes provided in the PCB for two or three more which could have been fitted but were not, possibly for no other reason than to save the cost per unit.

I have a Yamaha SY22 synth (from the late eighties) in which some of the capacitors around the headphone amplifier IC leaked and the electrolyte dissolved a track, disabling one channel of the headphone output. So I agree there is almost nothing which is immune to potential capacitor problems. I prefer it when they just dry out, as capacitors placed near to hot objects like transformers and heatsinks typically do: At least then they don't really do any damage.

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Re: MegaSTe hardware problems -help locate it please!

Postby Fujiyama » Thu Aug 24, 2017 8:41 pm

Holidays are over and the parts received including the SN7406N which I inserted in the U301 socket.
Unfortunately my problems still aren't gone.

Actually I'm having a hard time pinpointing what area to look for the problem because there are different problems all the time. From a power-on or a cold reset I can get 4 bombs, no keyboard response, or everything looking just fine and going to the TOS desktop.
Aren't these random type of things what people with bad PSUs experience? Maybe I should just recap the PSU right away before looking at the rest of the MSTE. I ordered (and have received) the values Exxos suggested and chose Panasonic FR series capacitors (low-ESR) for everything but the two 400V capacitors which I couldn't find in their series and are "regular" ones instead.

I need to re-read my notes and this whole thread again to refresh all the details as it's been a while, but I do believe I discussed MIDI problems a while back.
The strange thing now is that I haven't been able to start Notator even once. It starts but freezes while the startup screen is showing.
I managed to get another MIDI program running though (SMF-play, which is a MIDI-file player) and it successfully played a MIDI file on a synth attached to the MIDI OUT port. But that worked before as well, as far as I remember. It was the IN I was having problems with, and one strange thing is that I've had the mouse malfunction (no mouse pointer or movement from the mouse) with both MIDI IN and OUT connected to their respective connectors on a MIDI synth. I should probably desolder that optop-isolator, put in a socket and get a replacement of some sort (was there any conclusion as to what currently available opto-isolator would work in its place?)

What else could be affecting MIDI as well as other key elements when booting?


Oh, I've removed the MonSTer board (just in case) and have been booting from the trusty old internal SCSI HDD instead, but there's no difference so I don't think the MonSTer is the cause of all this. I've also re-attached the floppy drive as I've heard removing it can cause multiple bombs while booting. So that should eliminate potential causes of my issues.
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Re: MegaSTe hardware problems -help locate it please!

Postby siriushardware » Sun Aug 27, 2017 7:35 pm

Since you've replaced the 7406 (which was one possible avenue of enquiry) and you still don't get consistent behaviour maybe now is the time to either substitute the power supply (perhaps with an appropriately wired ATX PSU) or recap your existing PSU. If you do use an ATX unit be extra careful not to make any wiring mistakes, as they can supply so much current especially from the 5V supply that any miswiring could cause a hell of a lot of damage even in the short time it would take you to realise there was a problem and switch it off.

Now that you have changed the 7406 which includes the gates involved in MIDI thru, then, if you can, try that MIDI thru test I suggested a few posts back in this thread. If that works, it will prove the PC 900 opto isolator is working (or not) regardless of any problems further in at the ACIA or processor system level, and then you can stop worrying about trying to source a replacement. (Or if the test doesn't work, you can start worrying about trying to source a replacement).

With regard to Notator, does your machine actually have enough RAM to run Notator? (Sorry, but I don't recall how much RAM you said it had). I could be wrong but I seem to remember that some Steinberg software (Cubase 2?) can't run in a mere 1MB of RAM. Maybe Notator needs more than the stock amount of RAM as well? But to be honest, until the machine can be made to start reliably every time, I would not worry about which software it does run and which it does not. Solve the hardware problem and you will almost certainly solve the software problems as well.

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Re: MegaSTe hardware problems -help locate it please!

Postby Fujiyama » Sun Aug 27, 2017 8:32 pm

Nice to hear from you again Siriushardware :D
You have many good comments and suggestions that I've learnt a lot from, and your posting above is no exception. Yes, i'll give the MIDI thru thing a go, but given that my MSTE seems so unstable (sometimes it powers up fine, sometimes it gives 4 bombs, sometimes other problems) I think I should recap soon -the sooner the better.
Luck has it that my every so successful desoldering pump stopped working the other day, but I'm hoping for a replacement. This cheap tool (roughly 10 Euros/GBP/US$) looks like a soldering iron (40W) but has a hollow tip with a manual solder sucker built-in and makes desoldering soooo much easier and better than anything else I've tried. I hope to get a new one very soon as I don't want to desolder anything the old way (soldering iron + solder pump) any more as it's too easy to damage PCBs.

My MSTE as well as the STE both have 4 MB of RAM. I have no idea why I couldn't get Notator to load properly. Eventually (after posting) I finally got it working, but I couldn't get it to receive any MIDI data (though MIDI OUT worked OK). I also tried exchanging the MIDI IN/OUT cables, but they were both OK as well. I should probably go try out the MIDI stuff you suggested while I wait to get the PSU recapped.

Oh, I don't plan to get an ATX PSU for the MSTE, but will likely sell it as a complete Atari package when/if I get it fixed. The STE will be the one with the ATX PSU, but I haven't bought anything yet as I don't know how many watts I would need for all the add-on hardware it'll have. The STE fortunately works fine but there's a lot of hardware projects I need to figure out before I end up with the end product the way I plan it. Too much to do, too little time .... :lol:
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Re: MegaSTe hardware problems -help locate it please!

Postby siriushardware » Mon Aug 28, 2017 9:14 am

I share your preference for 'proper' desoldering tools - I use an electrically assisted one (one with an electric pump which applies constant suction through the hollow tip) myself, although they are usually a bit more expensive than your handy 10 Euro tool. Since I work in electronics repair there is always something decent lying around that I can 'borrow' during my lunch break, If I need to do any delicate work like that on my own stuff.

On a different track, does your machine have any ICs fitted in square PLCC sockets? The sockets are notorious for going unreliable. I had to replace the CPU socket on my 1040STe for that reason.

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Re: MegaSTe hardware problems -help locate it please!

Postby Fujiyama » Mon Aug 28, 2017 10:28 am

They're sending me a replacement desoldering iron, so I'm hoping to recap the PSU later this week.

Yes! At least the CPU is socketed, and as I have a Mega STe version MonSTer board fitted (and awaiting Alanh to ready another batch of STe versions) I've been swapping it over between the two machines. That and removing it to access those ICs underneath, on the MSTE motherboard.
Now that you mention the sockets I should probably do the obvious thing and look for any bent pins. I don't have the MSTE in front of me right now, but I think there might be some other socketed square chips as well. Good catch!
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