SM 124 black screen BUT power LED ok and sound

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Chandler
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SM 124 black screen BUT power LED ok and sound

Postby Chandler » Fri Dec 14, 2018 9:06 pm

Been trying to repair this SM 124, not seen this problem before

When powered up the screen is black but the green power LED is bright and when keys are hit on the ST you can hear the beep through the sm124 speaker and nice and loud too

So far i have changed the following:

LOPT
BU806 Transistor
ALL caps
AN 5790
TDA 1170D

It's definately something on the main PCB as i have another working PCB and when connected up to the CRT it all works ok so i know the CRT is ok

Can anyone throw any light onto what i could look at next? :)

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frank.lukas
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Re: SM 124 black screen BUT power LED ok and sound

Postby frank.lukas » Sat Dec 15, 2018 7:05 am

may be the line transformer is faulty ...

when you have a sheet of paper it must stick on the screen when the sm124 is power on.
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Re: SM 124 black screen BUT power LED ok and sound

Postby Chandler » Sat Dec 15, 2018 11:19 am

Hi Frank

Line transformer? do you mean the LOPT / Flyback (Line Output Transformer) ?

I have changed it to a known working new one

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Re: SM 124 black screen BUT power LED ok and sound

Postby frank.lukas » Sat Dec 15, 2018 11:31 am

Chandler wrote:Hi Frank

Line transformer? do you mean the LOPT / Flyback (Line Output Transformer) ?


Yes
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Re: SM 124 black screen BUT power LED ok and sound

Postby Chandler » Sat Dec 15, 2018 12:12 pm

Yes i changed the Line Transformer

It's something on the main PCB as i changed the main PCB to a known working one and everything then works ok

So i'm guessing testing diodes and individual part on the PCB now?

I believe the PSU part is ok otherwise i wouldn't have the bright green power LED or sound

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Re: SM 124 black screen BUT power LED ok and sound

Postby Chandler » Sat Dec 15, 2018 1:12 pm

I should also mention there is NO CRT glow from the end of the tube with the faulty PCB

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Re: SM 124 black screen BUT power LED ok and sound

Postby siriushardware » Wed Dec 19, 2018 10:56 pm

Could you be a bit clearer - are you saying you have another PCB and when you put that in the monitor it does work and the tube heater does glow?

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Re: SM 124 black screen BUT power LED ok and sound

Postby Chandler » Mon Dec 24, 2018 3:10 pm

I do have another PCB and yes with that fitted the monitor works 100%, the reason i don't use that particular PCB is it has a completely different issue of making a constant faint buzzing sound through the speaker

Also to note; the tube neck doesn't glow on the SM 124 as i checked when it was working (with good PCB)

So back to the faulty PCB, i think it is something to do with powering the FBT / LOPT (B+)

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frank.lukas
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Re: SM 124 black screen BUT power LED ok and sound

Postby frank.lukas » Mon Dec 24, 2018 4:10 pm

Service Manual -> https://www.atariworld.org/files/docs/A ... Manual.pdf

Check the trouble chart and the voltage chart ...
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Re: SM 124 black screen BUT power LED ok and sound

Postby Chandler » Tue Jan 15, 2019 8:11 pm

Totally lost with this one

To be honest the service manual for these is pretty useless, done all the tests it suggests and nothing

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Re: SM 124 black screen BUT power LED ok and sound

Postby siriushardware » Wed Jan 16, 2019 11:03 pm

Maybe one small step at a time?

You say the tube neck doesn't glow on a working unit, I'll take your word for that, they are sometimes very very dim. In most cases if you put all the lights out and look, you can see at least a dim glow which will give you some confidence that the tube heater is being powered. Unusually, the tube heater supply in this is not derived from an output on the Line Output Transformer, instead the tube heater seems to be powered straight from the 12VDC output of the main regulator.

You have the power LED on, and you have sound so that suggests you have 12V coming out of your power supply, but you should still check that anyway just to make sure the voltage is correct, because a fault in the 12V regulator could potentially pass the unregulated input voltage straight through onto the 12v rail. You should be able to find +12VDC and 0V on the inner two and outer two pins of connector P901.

If you have that, check the output of the +5V regulator as well, IC301.

Check the voltage from both ends of R701 (120R) down to 0V - should be +12V on one end and not much less than +12V on the other end. This resistor supplies the line output oscillator, without which there will be no drive to the Line Output stage.

Check at the junction of D707 / C716. Again, there should be +12V there, that's the supply point for the line output transistor and transformer.

Do you have an oscilloscope?

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Re: SM 124 black screen BUT power LED ok and sound

Postby Chandler » Thu Jan 17, 2019 10:21 am

Thanks for your reply

I have no scope

I should mention too that there are 2 types of SM 124, both look the same outside but are totally different inside

The early type has the power supply on the left side of the monitor with a large transformer

The later type has the power supply built into the main PCB

The type i have is the later type with power supply built into the main PCB

Usually the manual relates to the early type SM 124 which isn't much use!

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Re: SM 124 black screen BUT power LED ok and sound

Postby siriushardware » Thu Jan 17, 2019 8:06 pm

Ok, that is a -bit- unhelpful, no point in our trying to talk you through it if that is not the circuit of your monitor. I had a little phase a while ago where I was trying to collect all of the diagrams available for the Atari stuff that I personally have, so I'll have a hunt through to see what I have.

I have an SM124 here which I have owned from new, but (touch wood) I have never had a problem with it so I don't even know which version mine is inside.

Edit: Just had a look through the manual which was linked to earlier, and there are actually three circuit diagrams in that document. The first one you come to is the three-board version with the power supply separate and that's the one I was making suggestions from, but there are two other schematics right at the end. Would you be able to identify which one of those is the one for your monitor?

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Re: SM 124 black screen BUT power LED ok and sound

Postby czietz » Thu Jan 17, 2019 8:13 pm

Service manuals for both SM124 variants are available: https://www.dev-docs.org/docs/htm/searc ... 24+service. Chandler probably has the MT-21. Obviously, the reference waveforms shown on the schematic are only helpful if one has a scope.

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Re: SM 124 black screen BUT power LED ok and sound

Postby siriushardware » Thu Jan 17, 2019 8:41 pm

True, but there are some basic things which can be checked without one. Of the three, the MT-21 seems, naturally, to be the most complicated.

However, we can say that the input SMPSU seems to be working as the power LED is on, and the audio amplifier is working - both run from 12V.

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Re: SM 124 black screen BUT power LED ok and sound

Postby czietz » Thu Jan 17, 2019 9:08 pm

The third schematic in the document that Frank posted above is not an SM124 but an SM125. Compare it with the SM125 service manual: https://www.dev-docs.org/docs/htm/searc ... +schematic. Afaik, MT-9 and MT-21 are the only two variants of the SM124.

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Re: SM 124 black screen BUT power LED ok and sound

Postby siriushardware » Thu Jan 17, 2019 9:40 pm

Thanks for the pointers - it turns out I did have both versions of the SM124 manual in my offline stash. We'll wait for Chandler to catch up again and grab himself a copy of the MT21 manual if he doesn't already have it.

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Re: SM 124 black screen BUT power LED ok and sound

Postby Chandler » Thu Jan 17, 2019 10:37 pm

Yes i have the MT-21 type, just downloaded the correct manual

Page 7 and 8 are quite useful and shows that the power supply should give out 12V and 27V

I'm sure the 12V is ok due to LED and sound, but not checked the 27V ..

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Re: SM 124 black screen BUT power LED ok and sound

Postby siriushardware » Fri Jan 18, 2019 12:55 am

Yes, check for +27V on C714 +Ve near the line output stage as well as at the power supply.

Also check the voltage with respect to 0V on both sides of R707 which goes to pin 5 of IC701, and on both sides of R605 which goes to pin 1 of IC601. The voltage on both ends of both resistors should be about 12V. Both of those (low value) resistors are at least partly intended to function as fuses and burn out if the ICs they are supplying draw too much current.

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Re: SM 124 black screen BUT power LED ok and sound

Postby Chandler » Tue Jun 18, 2019 6:49 pm

checked for 27V at C714 and it was 33V, so i adjusted the B+ pot back to 27V

Still no joy, but at least i know the power supply is ok i guess

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Re: SM 124 black screen BUT power LED ok and sound

Postby Chandler » Wed Jun 19, 2019 8:50 pm

Actually thinking about it, i tested the other PCB i have and the B+ was 26.4V so i adjusted it to 27V

Back to this non working PCB with 33V B+ this doesn't seem right as adjusting it back to 27V the adjustment pot was at the minimal setting and can't go back any further

Could something be causing this PSU to output a higher voltage on this B+ line? and maybe that has caused something to pop

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Re: SM 124 black screen BUT power LED ok and sound

Postby siriushardware » Wed Jun 26, 2019 7:15 pm

Sorry Chandler, it took you a long time to come back on this so I stopped looking.

When you adjusted the +27V supply, did you notice what happened to the +12V supply? The B+ adjustment very likely adjusts BOTH rails but the important one to have exactly right in this case is the +12V supply, so reset that to the correct voltage (12V) regardless of where that then takes the +27V rail.

I have a feeling the +27V is higher than usual because it is not loaded, that is, there is nothing running on it.

After adjusting the 12V rail back to 12V, please measure the voltage with respect to 0V on each end of 1 Watt 1R resistor R608 which is associated with IC 601, and measure the voltage on each end of 18R resistor R708 which is associated with IC 701. In all cases you should see about 12VDC.

Alternatively, power OFF and use a low ohms range on your meter to check the values of those two resistors in-circuit. R608 should measure 1 ohm or perhaps a little less, and R708 should measure 18 ohms or less. If either of them measure significantly more than their marked value (in the hundreds of ohms or higher) then that resistor is probably open-circuit.

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Re: SM 124 black screen BUT power LED ok and sound

Postby Chandler » Fri Jun 28, 2019 1:01 pm

Thanks for your reply

I have removed and tested R608 which measures 30KOhm and R708 11KOhm

So they are both correct as these are the values given in the service manual

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Re: SM 124 black screen BUT power LED ok and sound

Postby siriushardware » Mon Jul 01, 2019 5:26 pm

Sorry, in that case I obviously misread the part numbers of the resistors I wanted you to check. (The markings are very blobby in both cases). I've now cross checked the values with those in the component list - I should have done that in the first place. Here are the resistors (marked on a clip from the diagram) that I want you to check.

They are:
R605 (I Ohm, 1 watt)
R707 (18 ohm).

Note that because these are such low values, you don't need to take them out of circuit to check them, just leave them in place and measure their resistance in-circuit, with the power off as always when measuring resistance of course.

Even if they are OK when measured that way, please then power the monitor up and measure the voltage (with respect to 0V) on each end of both of those resistors.
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Re: SM 124 black screen BUT power LED ok and sound

Postby Chandler » Tue Jul 02, 2019 7:17 pm

OK so i have removed and tested both R605 and R707 and they are both good and measure the correct resistance


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