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Unidentified Apple II / clone

There is something that looks like a 79xx, but there are no specific markings on it.

Is there a simple way to test/identify this thing?

Thank you.
 
The center pin of the thing will be grounded. One side will be connected to the -5 output and the other will most likely be facing a diode and a capacitor coming off the secondary of the switcher transformer. If that seems to line up, then it's almost certainly a 7905 regulator.
 
Well it's not a 7905. What I thought might be the regulator was in fact just an SCR used to short the +5 and +12 VDC out to 0 as a safety in case voltage regulation fails. Did a lot more digging, including pulling up Apple's patent for the original PSU and lo! It matches this PCB exactly. :)

http://www.wikipatents.com/US-Patent-4130862/dc-power-supply

(#88 is the component in question.)


I learned about the SCR and its purpose from a great post by rocketscientists over on applefritter.com:
http://www.applefritter.com/node/24037

Going to be re-reading this quite a few more times to see what I can learn.
 
I found a schematic for the Apple III Power Supply. It looks to be pretty close to the Apple II unit. Note that the -5 is derived (through a regulator) from the -12. Note that the -12 is just a simple half-wave rectifier with a pi-section filter. So, assuming that your supply is similar, you check the diode, the secondary on the transformer and the series inductor (I assume you've checked the caps). And also note that there's a 7905 in there that should be checked.

Don't know if this will help, but maybe.
 
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My wife replaced two major caps and touched up a number of points in case of cold solder fractures (wave soldering is a mess). She is going to replace all the electrolytic caps since they are pretty cheap; even though this is getting more expensive than adapting a newer supply, we are sticking to reasonable preservation guidelines. Cross your fingers for us!

I don't think the Apple II clone PSU has a 7905, or I am blind. But what the heck is that T3 transformer with the concentric rings in the III schematic? And have they eliminated the opto-isolator in the new version?

Cheers!
 
Looks like they did indeed replace the opto with a toroidal transformer (the idea is to keep the high- and low-voltage sides separated).

Are you sure you're on the right track? If you started with a good 5V Vcc then it sounds like a fairly trivial problem in the -V side, probably a 7905 as Chuck suggests. Of course it's possible that after being awakened from a long hiatus some other caps are gradually failing, but if replacing some caps has lowered the 5V by 50% I'd double-check your work. No chance you accidentally switched from 110V to 220V (if it has a switch)?

Are you using any kind of dummy load?

Did the II+ supply schematic that Rocketscientist refers to actually get posted somewhere? That patent schematic is pretty incomplete and not really very useful.
 
I could see a simple zener diode shunt regulator for the -5; it's only there to provide substrate bias for the 4116 DRAM. You could also have a 79L05 regulator which comes in a TO-92 package and so looks like a transistor.

But the reason for the lack of -12 has to be pretty simple.
 
Well, I just opened two clone supplies and although they're quite different there's no sign of any regulators on the output at all; looks like it's all done in the feedback loop.

I didn't remove the boards so I can't trace anything; one of them (a THB-43H) has an LM324 straddling the low-high voltage divide and an SCR and a dual diode both in TO-220 packages and some more diodes, toroidal chokes, caps and resistors; looks like it uses a small transformer for the feedback.
The other one (a PSA-2B) uses an optoisolator for feedback and has a trimpot and a TO-92 marked UA43 1AWC; only the crowbar SCR, some heatsinked (and not-) diodes, ferrite-core chokes and the usual electrolytics.
That's all, folks.
 
Interesting. I'm just interjecting to respond to Mike's questions:

I'm not sure I'm on the right track. But numerous electrolytic caps had left puddles on the PCB, enough that my wife just said screw it, better to replace the bunch since we can't verify their operation. She had already checked all the TO-92 packages (3904 and 1802) for a match as well.

I had exactly the same thought regarding an accidental 110/220 switch and checked this immediately after we got the reduced voltage results. But I haven't been testing under load either.

Cheers!
 
Chuck(G) said:
Mike, the uA431A is a voltage regulator IC. Datasheet here. But that -5 has to come about somehow--so I'd look for a zener diode somewhere on the output.
Click to expand...
Don't know how I missed that in my search, but I figured it probably was a regulator like the LM324 in the other one; I didn't mean that there are no regulators, just no high-current ones in-line, and presumably at least one of those diodes is indeed a Zener.

Kind of sounds like a simple problem in the V- output section might have turned into something more complicated...
 
Heads up, I included pictures of the associated Apple II manual schematic and a pano of the PCB in the original post. Note a number of caps are currently removed.
 
Kludgy's wife here. Kludgy is too sad to post. We replaced all the capacitors and the power supply is still dead.

The next step is to troubleshoot the power supply live. That should be fun.
 
Dear Mrs Kludgy,

Tell Kludgy not to be sad. The harder the diagnosis and fix, the sweeter the success when you finally crack it!

After all, that's why where are in this hobby. (Isn't it??). It's a masochist's dream! :D

Tez
 
Kludgy said:
Kludgy's wife here. Kludgy is too sad to post. We replaced all the capacitors and the power supply is still dead.

The next step is to troubleshoot the power supply live. That should be fun.
Click to expand...

Strangely enough, that's where I would have started, assuming i didn't get any magic smoke when I plugged it in. Just stay away from the side of the PSU with line current on iit and you should be fine.

Before you do this, however, have you checked for cracked traces? On a lot of these old PCBs, they're not uncommon.
 
I'm not sure if this has being mentioned, but the orginal motherboard was fitted with 2716 eproms, but later versions of clones were fitted with 2732 eproms, which is why there are only 3 instead of 6 eproms.
 
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