520ST reset woes

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Posts: 7
Joined: Sat Jun 26, 2004 2:01 pm
Location: Texas, USA

520ST reset woes

Postby warerat » Mon Jul 19, 2004 5:20 am

Hello all,

I recently salvaged a 520ST that was being thrown away. I got it home, rigged up a mono cable to my VGA monitor and was greeted by a black screen. After probing around in the machine, I noticed the RESET line on the 68000 was pulsing. On a whim I forced RESET high manually and viola! The machine came up! I was greeted by a screen telling me to insert the system disk. Yuck. I pulled out my trusty programmer and burned six EPROMs with TOS 1.04. Got 'em in place and the machine comes up OK (after the RESET trick). I've replaced the 68000 with some spares that I have. Same problem. I replaced the OC 74LS05 driver thinking maybe it was bad. No luck.

So bottom line is that the machine will not come up on it own, nor will it reset properly. If I hold RESET high on the 68000 for a second, the machine comes up. If I push reset after the machine is up, I see a garbled display, sometimes followed by 11 cherry bombs. Could there possibly be something wrong with the 556 timer? Am I looking in the wrong place?

Is anyone familiar with the reset circuitry in the 520ST, or have schematics for it? Any help or insight would be truly appreciated.

Also, the floppy controller IC is pretty hot. Is normal? I got a pair of SF314 floppies, and they seem to read/write OK. Speaking of floppy drives, does anyone have a short floppy cable they're willing to part with?

Sorry for all the questions.



Postby simbo » Mon Jul 19, 2004 12:55 pm


i think the floppy disk drive cable has a few problems at the header

as the disk controller overheats
this is a good place to start

see attached picture and also look in simbo hardware faq section to see the floppy disk drive cable replacement tips

i use a new plug and basicaly cut off the older header from the board
and use a floppy cable from a pc with the twisted normal drive a part cut off with sheering scissors

i see by looking at the schematics for the stfm
{quite similar to a standard st }
{load them with paint shop and change to gray scale then save as bmps}

there is reset lines to most connectors like the keyboard and all the controllers

now you say it pulses
this implies a clocking effect going on ///reset reset reset etc...

look at attachment of reset circuit with the 556

that if a few wires on the floppy header are shorted despite the drives working
there may??? be a problem with the cable or header

i also realise you may have a faulty floppy controller chip
im pretty sure this is where the problem is
so maybe replace that chip next
as a chip can work or work intermitently
{the first sign is like an engine in a car it dosnt like a cold start }
and data lines can do wierd things

some later st computers were fitted with a faulty chip but maybe this is one that has developed a heat problem

BUT more likely is a duff cable

my view is to remove the cable from its header avoiding shorts and check to see if the chip still gets very warm as your going to replace it anyway

if it dosnt proceed to chop off the header with cutters etc as i described in the topic and attached picture as i was doing an ste the other day {not good focus but viewable}

anyway back to the reset circuit it uses a 556 this is a timer
so will provide a pulse for a given time on a reset press and an inital pulse

so you mention that if you hold on the reset it runs as expected this

implies the 556 has developed a fault
but when you hold the reset the 556 gives a negative going pulse inverted by the 7405 for distribution around the circuit

so this implies that there is a fault on the 556 side of the inverter
you can monitor pins 11,13 or 1 and 3

to see if there is still the pulsing at the inverter inputs i think it will be

{althought the inverter should pull this pulse down all very strange}

i think the pulse is coming from the 556
esp if one part of the two part chip package has failed the two output lines one from each are connected to
the inverter
check both are active at the right times
maybe one supplies neg going pulses the other pos going ones
youll have a !reset and a reset lines

but remember that
when you turn the machine on there is an initial pulse to get it going

the next pulse only pulls one set of lines up
as the reset button is only connected to one 555

so this is why there is two 555s
so therefore elfacto reverso the 555 part 556 used to drive the inital pulsesection

has a fault or component fault in its timing setup componets or
has a problem part or a missing or brocken resistor or cap
this would give a similar effect

i think then youll need to replace just a resistor or something similar
or the 556 but always start with the 556

pins 3 and 11 inputs tot he 74ls05 you replaced
connect to the inital pulse only outputs
while 1 and 13 are the reset button

remember this circuit should also give
an inital pulse on the reset only... setup again for the reset period needed to enable the chips

so basicaly all the lines get toggled at powerup and just the reset one there after on a button push

so if you hold the reset button youll get a serial of pulses on pins 1 and 13 of the 74 chip this is normal

but again just one pulse should toggle the 05's outputs high and low

so use a meter or logic probe

i plump for cap 7 is open or gubbed or the 556 is fried

good luck
if you get stuck pm me a scan of the board at that area perhaps i can spot something
but you seem pretty competant and skilled if you replaced the 68000 by desolder it youll find the header a walk in the park

i zipped the pictures up to save board space
You do not have the required permissions to view the files attached to this post.

Posts: 7
Joined: Sat Jun 26, 2004 2:01 pm
Location: Texas, USA

Postby warerat » Wed Jul 21, 2004 3:03 am

Great news! I found and fixed the problem! The machine works great now.

I removed the OC driver and the CPU and was checking the pull up resistances. HALT was pulled up to around 4.7K, but RESET was pulled up to around 20K! The schematic shows only a 1K pull-up. I found the resistor that was directly connected to the RESET line, and lo and behold, it showed around 20K ohms-- really odd for a resistor marked 1K. Upon closer examination, I found a hairline crack right on one of the colored bands so it was easy to overlook. I didn't have any 1K ohm resistors in my scrap box but I did have some 4.7K. Once it was in place, the system came up like a champ. On a side note, the FDC IC now runs a lot cooler, go figure.

I'll run up to the store tomorrow and get some 1K resistors to replace it, although I don't think I'm hurting anything with the 4.7K for the time being.

Once again, many thanks for the links to the schematics and most importantly, your advice!



Postby simbo » Wed Jul 21, 2004 11:19 am

well done :D

see i told you it was a pullup ....

and this is why
a 74ls05 is an inverter with driven collector outputs

so needs a pullup or it fails to invert but will pass a pulse chain as spikes like inverting the pulse chain back as the pulse chain is pwm to a degree and has uneven mark to space becouse of the 556 at boot time

so isnt symetrical square
ive seen this fault also before at it was the rp pack as i mentioned by pm

20k well one for the book when the atari wont auto reset at boot

you can leave the 4.7k insitue this value change wont matter to this type of circuit
1k is actual wrong it should be 10 or as youve used 4.7 1k is a little low 4.7 is better for 5volt lines stuff

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