IBM PS2 Mod 70 - 8570-E61 power problems

[Carl Gerdes]

Had this PC since I retired from IBM in 1991. It has hung around in my garage all that time, so thought I would see if it would power-up. Of course it did not - no surprises here. No fans, no lights - nothing spinning. Thinking I would look carefully at the power supply I cleaned everything up and removed the PS. With the idea of looking for suspicious capacitors, or what ever. BUT WAIT, Very interesting -- the PS comes to life while unplugged from the mother board. Checked voltages along the conductor and have: -12v, 12v and 5v on various pins. However, plugging in the mother board and, again, no lights or fan. Dead!!

Would like suggesting on what to look at next. Much thanks... carl gerdes

[lutiana]

I would suggest a careful examination of the motherboard for any leaking or bloated capacitors. Also check the voltage lines for a dead short to ground.

[bear]

You're almost certain to have a shorted tantalum on the planar.

[Carl Gerdes]

Thanks to both for your reply. First, my MB has no leaking or bloated capacitors as I suspected. All caps are little yellow caps seen in the attached pix. Also the terminal to the PS can be seen in the pix and that is where I checked the voltage. Bear, I'm not sure I know what a tantalum is. Is that what I call the little yellow caps? If so there are a large number of them on the MB. Any hints on how to find the shorted one? Thanks in advance for you response.

Attachment 64852.

[WBST]

"Our" (meaning the MCA Mafia/Militia's) primary information sources are here:
http://ps-2.kev009.com/ohlandl/ps-2.html which is a subset of: http://ps-2.kev009.com/ohlandl/
and here:
https://www.ardent-tool.com/
with the Model 70 power details here:
http://ps-2.kev009.com/ohlandl/8570/...570_Power.html

The three IBM Model 70 planars plus Reply upgrade can be found here:
https://www.ardent-tool.com/8550_8570/index.html

[bear]

Is that what I call the little yellow caps?

Yes. It refers to their chemistry/physical construction. It's common for one or more to short with age, and since they're overwhelmingly used for decoupling (fitted between +V and ground) the most obvious sign that one has failed in this way is a) no power (with a power supply featuring overload protection), or b) fire (what shorted tantalums do when powered with more than a tiny amount of current for more than a tiny amount of time).

You're looking for a short in a voltage rail on the motherboard. Measure the resistance between ground and any point that is supposed to be connected to any of the power rails. If you find one at 0 ohms (or within the tolerance of your measuring device), then you've found a shorted power rail. Isolating which device on the rail has the short is harder. I don't know of a better method that produces consistent results besides to remove devices one at a time until the short disappears.

[Carl Gerdes]

You're looking for a short in a voltage rail on the motherboard.

I did as you suggested and found no shorts on the voltage rail. However, the -12 pin was showing a resistance of <200 ohms and I decided to remove its tanalum. Turns out it was easy to find as it is close to the voltage rail. Anyway, it's gone, and we have power up. It booted to Basic with no hard drive or floppy in the machine. It does not recognize either drive, so that my next challenge. Thanks for you help.

[Carl Gerdes]

You're looking for a short in a voltage rail on the motherboard.

Thanks for the advice. I did measure the resistance of each voltage on the power rails. They were all 100k ohms or more, except for the -12 pin. I decided to trace its connection to what I thought might be its capacitor. Turns out it was C2 which was close by and traceable. I tried to unsolder it, but gave up and cut it out. We now have power and system boots up to Basic screen. Do I need to be concerned with trying to replace the capacitor if I can determine size?


Error codes presented at boot-up time indicate that the date/time needs to be set. I did replace the battery, which guess resets the date/time. Seems I need a reference disk to set the date/time, but can not get the floppy to read any old disk I have. Any suggestions on where to get help would be appreciated.

Thanks for you help.