Questions on the Lisa 1.8 power supply

[Juror22]

I have a Lisa that has yet to start up, but I have a little time to work on it now.  I got a power supply with the Lisa that was assumed non-working (since I don't know whether or not the Lisa has other issues.  I have had it apart and replaced the caps (the filter ones were already replaced, before I got it) but I cannot get it to start up.  The standby +5 current is there, and I think that doing the following (from the badcaps forum) should allow it to start, but I'm not 100% sure that I interpreted the process correctly.

The ON/OFF control signal should be >VR (2.52V, which feeds PIN5 of the ICZ3B-LM393 to keep the output High, when you apply 3VDC to pin6 (ON/OFF) of ICZ3B, the output will swing low and turn on the power supply.

(Yes I have the same power supply, being referenced)  I believe this means that the power supply will switch on if I:

1) plug in the power supply

2) fake out the back panel switch SW1

3) apply +3VDC to the 6th pin of Z3 on the DataPower 'daughter card' (momentarily?)

At that point, a properly running power supply should start supplying power on the +12V , etc. edge connectors.

My supply doesn't do anything at that point and I need a functioning power supply, before I can move onto testing the rest of the computer.

A couple of outright questions.

1) is the process above correct, assuming the 1.8A Power supply from DataPower (at this point I have 3) that are the same and correspond to the pdf schematics available on the BC forum?

2) is there a better way to 'trick' the power supply into starting up, using a pin on the edge connector (similar to the IICI /IICX trick?) outside of the computer?

3) is checking for the voltages on the edge card sufficient, or should there be an additional load attached? (a HD or two, maybe a fan?)

4) are there any known schematics for a substitute for these power supplies? (there was a posting in MacRumors a LONG time ago by someone that claimed to have sorted out an ATX conversion)

I have a very basic understanding of power supplies (including safety precautions and what NOT to touch), but don't have experience with an oscilloscope, so advanced diagnostics are currently beyond my abilities, but I'm not convinced that they are needed, yet.

 

[uniserver]

i think those round looking motorola power transistors burn out in that psu… look up that model number on it and see if they are still available.

those same things go bad with ham radio power supplies all the time as well.  All though with the ham radio PSU's they are always ( even the very worst design ) better cooled then any of the lisa psu's 1.2a - 1.8a.

 

[Juror22]

You mean these guys?

 ...and do the lower numbers have any significance (do I have to find one that matches 8426, or just a 2N6308?)

 

[uniserver]

middle #

 

[maceffects]

Yep, pretty common on the 1.8amp PS. Uniserver is right, that's most likely your issue.

 

[snuci]

I'm sure you've got this covered but just in case, there is also a front cover switch that acts the same way as the back switch in killing the power supply.  You have to "fake out" both switches.

 

[Juror22]

Thanks Uni/Maceffects, I've ordered a couple of them.

Snuci, I'm not sure where the other switch is - I see sw1 ( the one with the little lever that is deactivated by the rear cover), but I don't see another one that you mentioned, could you help me to zero in on that?

 

[snuci]

The switch is highlighted in red.  You can also jam a q-tip or cotton swab in there (that's what I use)



(Image was originally from Digibarn but I cropped it to zero in on the area and circled the switch)

 

[Juror22]

I did not see that one!

Thanks Snuci

 

[ScutBoy]

Also check that the plastic post that should engage those switches isn't broken off on the back panel or front panel. If the machine gets jostled around in shipping, etc. they very often break.

Don't ask me how I know this... At least it's fairly easy to fix.