Greetings,
I've been a lurker here for a bit, but stepleton, on another website, suggested I should be asking my questions on a more Lisa-centric site, so here I am.
I've got a non-working Apple Lisa 1.2A power supply.
When the Lisa's power button is pushed, the power supply lights up the switch, makes a few noises like it wants to work and then shuts down. I stopped pushing the button and took it apart. I've found a few things that I was hoping someone here could take a look at and let me know if they look correct. There may be photos of the underside of 1.2A boards somewhere but I am having no luck finding them.
The first thing is this bodge wire from the C12 (?) capacitor to the R14 resistor. Does that look right? Also, the R14 resistor seems to be 'soldered' to the R15 resistor as well but it looks like that is by design since the vias appear to be touching.
1,2a2.jpg1.2a1.jpg
The second thing is some board burn around the R24 resistor. I measured with a multimeter and it shows 149 ohms, which I think is still in spec. I also reflowed the solder to the vias on both since they seemed to be cracked. Anything unusual about that much burn on such an old power supply or does that burn point to a specific problem elsewhere?
Lisa 1.2a mu.jpgLisa1.2a 2.jpg
I've ordered new caps and safety caps and abandoned testing until those arrive and are installed just to rule out the maintenance issues I would be doing anyway.
Does anything jump out here as especially wrong or symptomatic of other issues?
One last question, does the Apple 1.2A power supply work with all Lisa variants? I read somewhere that it might struggle powering a 2/10 with a Widget?
Thanks!
Welcome! PSUs are a dark art to me, and I won't be able to offer much technical advice on this. But I do happen to have a working 1.2A PSU and pictures might be helpful for comparison purposes. Here you go.
You are correct that the 1.2A PSU was developed for the Lisa 1 and Lisa 2/5. The Lisa 2/10 came with a 1.8A PSU to provide the incremental power needed by the Widget. It is not recommended to use a 1.2A PSU, which are known to be somewhat fragile by comparison, in a Lisa 2/10. The later 1.8A PSU (made by DataPower) can be used in all Lisas.
Quote from: mst3k on Yesterday at 01:29:49 PMthis bodge wire from the C12 (?) capacitor to the R14 resistor. Does that look right?
Also, the R14 resistor seems to be 'soldered' to the R15 resistor as well but it looks like that is by design
Yes, that is standard and by design. See the schematic: https://lisalist2.com/index.php/topic,292.0.html (https://lisalist2.com/index.php/topic,292.0.html)
Quoteboard burn around the R24 resistor
Anything unusual about that much burn on such an old power supply or does that burn point to a specific problem elsewhere?
In my experience, those burn marks are common for a well used 1.2A PSU. Increased load such as internal spinning hard disks and additional circuit boards does make it worse.
While burn marks often/usually point to a failure/problem, those specific burn marks don't.
R24 does sometimes become brittle and break, and I suppose it is possible for it to be intermittent, allowing the PSU to work when cold but not when it warms up. I suggest replacing it with a higher power variant say 1 Watt, spaced above the board a bit for better air circulation like the other power resistors.
While you're in there, adding a small low performance (quiet) fan will increase the longevity substantially.
Perhaps Patrick et al will have other suggestions...
Quotedoes the Apple 1.2A power supply work with all Lisa variants?
It does work with all variants, but longevity is an issue when heavily loaded. If you have a working Widget, you probably want to preserve it rather than use it, so substitute a modern solid state emulator (IDEfile, ESProFile, etc.) which will ease the PSU load too.
The "powers up but then shuts down" may indicate the voltage output needs adjustment. Measure the +5 rail on the I/O Board (you need to defeat the safety interlock at the top of the PSU) to see if it is within spec (4.75 to 5.25V).
Quote from: ried on Yesterday at 01:55:58 PMYou are correct that the 1.2A PSU was developed for the Lisa 1 and Lisa 2/5. The Lisa 2/10 came with a 1.8A PSU to provide the incremental power needed by the Widget. It is not recommended to use a 1.2A PSU, which are known to be somewhat fragile by comparison, in a Lisa 2/10. The later 1.8A PSU (made by DataPower) can be used in all Lisas.
Thanks for the info and the photos. The back of your board looks very clean unlike my scarred one. I guess it could be explained with something as simple as low use versus high use, but I like yours better.
Quote from: sigma7 on Yesterday at 03:05:02 PMThe "powers up but then shuts down" may indicate the voltage output needs adjustment. Measure the +5 rail on the I/O Board (you need to defeat the safety interlock at the top of the PSU) to see if it is within spec (4.75 to 5.25V).
Is that a voltage potentiometer at R29? I noticed when I took the PSU apart that the factory putty/seal/glue was broken there and the settings had obviously been fiddled with. Is there enough voltage variation from that pot to stop the machine from booting?
I will check the 5v rail.
Thanks to you both for the help.
Quote from: mst3k on Yesterday at 05:35:11 PMIs that a voltage potentiometer at R29?
Is there enough voltage variation from that pot to stop the machine from booting?
... the factory putty/seal/glue was broken there and the settings had obviously been fiddled with.
Yes and yes.
Although the PSU has "remote sense", it feeds back +5 from the motherboard's edge connector, so does not compensate for voltage drop across the motherboard and through the additional card edge connectors. In addition, it does not compensate for voltage drop through the ground wires in the internal wiring harness. As a result, it is not uncommon for the voltage to have been adjusted due to voltage droop due to add-ons, or if components have been replaced during a repair.
If set too high the overvoltage protection kicks in and shuts it down.
I'm not sure if you can set it low enough that the electronics misbehave, but I expect so.
IIRC, beware that it is CCW to increase.