[SOLVED] Atari 520 STFM short circuit in main board +5V

Moderators: Mug UK, Zorro 2, Greenious, Moderator Team

Enzo1960
Atarian
Atarian
Posts: 4
Joined: Sat Mar 18, 2017 9:38 am

[SOLVED] Atari 520 STFM short circuit in main board +5V

Postby Enzo1960 » Sat Mar 18, 2017 10:12 am

Hello!
I recently bought an used Atari 520STFM, but it reveals not working.
At video output it shows various vertical bars/garbage.
I opened it, checked capacitors value & ESR, all regular.
But when I check voltage on RAMs they shows a negative value!
I measured voltage in J10 connector and they are regular (+5v, +12v)
So I disconnected power supply unit and measured resistances.
It shows a short circuit in all +5V -> GND circuit!
So I tried to isolate the problem: all RAMs chip, TTL chip, ROM chip shows 0 ohm resistance between Vcc and GND. I removed one side inductor L1 to isolate ram power supply and they don't show shorts anymore.
After some tries, I discovered that short disappear if I remove JP4 wire.
Unluckly service manual diagrams and schematics don't show cabling between power lines.
Any help would be appreciated, thanks in advance!
Last edited by Enzo1960 on Tue Apr 04, 2017 3:27 pm, edited 1 time in total.

rj1
Atari freak
Atari freak
Posts: 56
Joined: Mon Sep 28, 2015 8:01 pm

Re: Atari 520 STFM short circuit in main board +5V

Postby rj1 » Sat Mar 18, 2017 11:49 am

Might want to check decoupling capacitors.
I once had a short on a decupling 100nF ceramic capacitor next to one of RAM chips, on Mega 1.
Ceramic caps short sometimes when there is a physical force on the PCB - they might break inside.

Enzo1960
Atarian
Atarian
Posts: 4
Joined: Sat Mar 18, 2017 9:38 am

Re: Atari 520 STFM short circuit in main board +5V

Postby Enzo1960 » Mon Mar 20, 2017 6:56 pm

Thank you, although, because all in parallel, I've to disconnect almost one pin from every cap.
But, in my opinion, perhaps I'm failed diagnosis.
It is possible that Atari has a strong absorption pn +5v, and modern tester shows it as a short circuit. I'm thinking to build a diagnose card, so I can run tests...

rj1
Atari freak
Atari freak
Posts: 56
Joined: Mon Sep 28, 2015 8:01 pm

Re: Atari 520 STFM short circuit in main board +5V

Postby rj1 » Tue Mar 21, 2017 8:04 am

No good circuit will show you a 0ohm resistance on supply. Diagnose cart won't help if you have a short.
If you have a high resolution multimeter and sharp probes you could narrow it down to a small area.
That is what I did and it worked pretty well, there was a visible difference in resistance on some mili ohms digits or something like that.

mikro
Atari God
Atari God
Posts: 1308
Joined: Sat Sep 10, 2005 11:11 am
Location: Brisbane, Queensland, Australia
Contact:

Re: Atari 520 STFM short circuit in main board +5V

Postby mikro » Tue Mar 21, 2017 10:29 pm

I'm selling my 520 STFM from $1 here: http://www.ebay.com.au/itm/252818958708 ... if you are willing to pay postage fees from Australia, you can have it for free.

Enzo1960
Atarian
Atarian
Posts: 4
Joined: Sat Mar 18, 2017 9:38 am

Re: Atari 520 STFM short circuit in main board +5V

Postby Enzo1960 » Thu Mar 23, 2017 11:10 am

My digital tester shows short circuit, but I putted it in "continuity circuit" test mode. If I measure voltage with power supply connected it shows +5v, +12v. Otherwise if a similar short will exist on +5v line, surely some component will be hot, and no video appears.At video appears some sort of garbage, always the same. In next days I'll build diag cart and let you know was happened, thanks!

Enzo1960
Atarian
Atarian
Posts: 4
Joined: Sat Mar 18, 2017 9:38 am

Re: Atari 520 STFM short circuit in main board +5V

Postby Enzo1960 » Fri Mar 31, 2017 4:40 pm

Today I built my Atari Diagnostic cartridge,so I hope to solve mystery:
Result: RS232 output data, and, with a VT100 terminal I can read this line, repeated in loop:

I1 RAM/data line error 0000000000000000 0000000000100000 000008

I searched in service manual how this output can help to identify defective RAM, but no answer.
So, in my opinion, it could indicate Ram chip n.11, lower row, counting from left to right?
I try to desolder it meanwhile..
EDIT
I desoldered that chip (it is named U18 on board), put a socket and substitute with other one.
Problem remain, and output from serial terminal, via diagnostic cartridge, remain the same.
So, I tried to remove chip from socket, leaving it empty, and run test again. Output now is:

I1 RAM/data line error 0000000000000000 0000000000100100 000008

So, seems that U18 is represented by 3rd from right bit in second 16bit word.
Thus, 6th from right bit in 16bit word, what chip indicate?

EDIT2
I substitute U43 (6th from left, on board) but same result.

LAST NEWS
At least, I individuate failed RAM.It is U28, 3rd from right in main board.
I discovery mistery how diagnostic card indicate failed ram: if interested ask me and I will open new post explaining logic behind these "magic numbers".
But, repair is not finished. After substitution of failed ram, still 520ST doesn't work.
This time, diagnostic card outputs on RS232:

I2 RAM disturbance 0000000000000000 000000000010010 000008

This is a failure in these other 2 ram chips? (U17, U25 if you've discovered diagnostic cart logic too)
Despite my desolder gun runs out so I've to wait until I repair it to check these 2 drams....


S O L V E D
I substituted U17, U25 and Atari goes on!
Soon I create, with your permission, a little guide about how to find bad rams following numeric error codes of the cartridge.


Social Media

     

Return to “Guides”

Who is online

Users browsing this forum: No registered users and 1 guest