Mac IIsi Power Supply Problems

[Big Bird]

I pulled three IIsis out of storage that worked when I put them away over a year ago, and now two of them have dead power supplies. I was able to narrow the problem down to just the power supplies by swapping components into and from the working unit. Last night, I tore into the power supplies. Swapping the AC input connector from the working power supply into either of the non-working ones fails to fix the problem. The fuses check good. All of the components on the board level that I can figure out to test have continuity.

When power is applied to the machine with either of the two non-working power supplies, the green light connected to the logic board flashes momentarily and the fan grunts. The relay in the power supply itself momentarily connects, then disconnects.

Any ideas on what could be causing the relay to kick back out and preventing the power supply from initiating a complete startup? I'm sure there's someone here with more technical/electrical knowledge than myself that could be of help. Thanks!

 

[Scott Baret]

IIsis have lots of strange issues, from the keyboard woes discussed in Charles Rubin's "What do I do Now?" books to strange power problems.

I've had something similar with a IIsi where the speaker doesn't work and/or the green light doesn't come on. Everything else works fine though.

I also had a somewhat similar problem with a IIcx a while back. It abruptly turned on after being plugged in for a half hour or so. It refused all of my keyboard and power button requests to turn itself on before that.

My guess is a capacitor problem somewhere, as they seem to be the culprit behind most of the oddities on older Macs as they age. I don't know how much research has been done with IIsis and their capacitors but if someone wants to investigate further that would be great.

 

[Unknown_K]

Leave it plugged in a while then try turning it on from the reset switch in the back.

 

[MacMan]

As mentioned, this is a problem that could be caused by leaking capacitors. Have a look on the motherboard for leakage, (it looks like someone has spilled cola on it), as this could be shorting out components. If any is found then it can be cleaned off with isopropanol or another cleaning alcohol.

Another method is to stick the motherboard in the dishwasher but I find that gentle cleaning with alcohol and a q-tip works fine.

This is certainly a cause of problems in IIcx Macs, have a look at one of my RetroChallenge blogs about this issue here.

 

[equill]

The principal problem with IIcx/IIci/Q700 Mac PSUs has to do not with capacitors but a diode in the PSU. It changes value with age, and eventually will not properly bias the sawtooth oscillator in the PSU. That causes loss of the essential TRKL voltage on which soft-power switch-on depends.

There is a published remedy (in brief, putting another diode in series with the original to increase the biasing voltage drop over the pair of diodes). It's not a job for the faint of heart, especially if the PSU's filter capacitors are not completely and deliberately discharged before the work is begun. The problem is commoner by far in Astec rather than Delta PSUs.

de

 

[Big Bird]

I'm familiar with the IIcx/IIci/Quadra 700 issues. I have at least one machine that exhibits such problems myself and, though I haven't taken the time to work on it, I've found that the power strip workaround is acceptable.

It's definitely not a motherboard problem. I've already isolated it specifically to the power supply itself.

Is the problem with the IIsi the same as that exhibited in the IIcx/IIci/Quadra 700 Macs? It's not the same power supply, and the IIsi is a unique form factor, but I suppose the symptoms are similar. Does anyone have a schematic or something for the IIsi PSU?

 

[equill]

As you could undoubtedly surmise for yourself, a different PSU case implies nothing about the contents. The IIsi is certainly of its breed, and derived from the IIci, but does it have an auto-restart switch? Absence of that could make a difference. Not even a site such as this has much info. about PSU design. Showbaffum wrote that he didn't know of any schematics, but you may be able to do better than that if you can catch tomlee59's eye.

de

 

[equill]

I have just stumbled again onto a page that I first found some years ago. It has schematics for the PSUs of several of the Mac II family. Although none of them is the IIsi, there may be some help there for you.

The page also carries an article by our own tomlee59, which, if we ask him very nicely, he may some day accompany with similar dissertations about other early Macs.

de

 

[Blessed Cheesemaker]

I have just stumbled again onto a page that I first found some years ago. It has schematics for the PSUs of several of the Mac II family. Although none of them is the IIsi, there may be some help there for you.
The page also carries an article by our own tomlee59, which, if we ask him very nicely, he may some day accompany with similar dissertations about other early Macs.

de

Interestingly, on his page he also has a link to Bomarc Services. If you scroll down the page at this link

http://www.bomarc.org/catalogsmenu.php

you will find schematics for more Mac logic boards and PSU's. Hmmm...1.50 a sheet, minimum order is $10, I may buy a bunch...I don't see any Quadra 700 on there, though (plenty of IIsi stuff)...