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IIGS Power Supply Burned Trace

JRotar

Member
Hey all,

I'm working on recapping a IIGS power supply - Dynacomp - that had pretty much no voltage output. I replaced all the caps on the low voltage side, but at this point though I did not replace the filter caps since they weren't Rifas and looked ok, and I did not have the 100uF main caps. I have since replaced all of that but haven't tested yet because I want to ask about a strange burned trace.

Sidenote I had a dead 100kohm resister I replaced too.

So when I tested it I actually got some life out of it including a whine sound (yeah, I tested it on the machine against my better judgement but I didn't have anything to put a load on it) and then it died along with a strange smell. I thought it might be those filter caps starting to give out. The smell was hard to tell but it could have been the transformer as it was in that area but also near the filter caps too.

While I was replacing the filter caps, I noticed the burnt trace. Which is near the transformer. See pic - this was after I cleaned the burned mask off. It has continuity still.

Any idea what could have caused that? I'm a little nervous to plug it in and test it again with the new filter caps after seeing it.

Thanks!
 

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cruff

Well-known member
Which side of the transformer is it on? Possibilities are shorted diodes or capacitor on the secondary side. Shorted transformer windings are also a possibility, but probably a much lower probability. Check for other shorts accidentally introduced while changing the capacitors.
 

JRotar

Member
It's on the way to the transformer actually. It looks like mains voltage going to the transformer. My finger in this pic shows where it comes in and then after that is where it burned before going into the resistor to the left (90ohm-ish, tested fine, burned trace stops before hitting resistor) after the two diodes and then into the transformer. I believe I checked all the resistors and the diodes that make up the bridge rectifier circuit but good idea to try further.

When this burning happened, I didn't touch anything on the high voltage side yet so I feel like it's unlikely the low voltage side recap would cause this. Attached pic is post high voltage re-cap and I haven't tested since I found this burnt trace.

The wire is white so that implies it's the neutral AC I would think....maybe that does imply an AC short. Perhaps it was a filter cap but I feel like they would have blown up before burning this trace
 

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cruff

Well-known member
The caps are installed with the proper orientation? Excessive current drawn on the secondary side could cause increased current draw on the primary side. Also can you check the resistance of the transformer windings?

Oh, it turns out I have a Dynacomp supply in my IIgs, I can check some stuff possibly. What is the part number of your supply?
 

JRotar

Member
The caps are installed with the proper orientation? Excessive current drawn on the secondary side could cause increased current draw on the primary side. Also can you check the resistance of the transformer windings?

Oh, it turns out I have a Dynacomp supply in my IIgs, I can check some stuff possibly. What is the part number of your supply?
I cross-referenced with Mac84's recapping guide and they are installed correctly.

I've never really dealt with transformers before since most of the time they just work :ROFLMAO:. What's the proper way to test resistance on it?

My Dynacomp is LR62786 or Apple 699-0126

If your IIgs is moderately equipped with expansion cards, look into a replacement like the Universal PSU kit. https://wiki.reactivemicro.com/Universal_PSU_Kit

The stock units are under powered and 30+ years old at this point.
I was thinking about that but part of the fun is trying to figure some stuff out but if it is too much of a mystery I'll probably do that. But I don't want to give up! haha
 

cruff

Well-known member
I has the same Apple model number, but the Dynacomp label says LR38879. Let me look at mine and see what I can figure out.
 

cruff

Well-known member
Also I'm pretty sure that what you were pointing at is the feedback transformer from the low voltage side to the high voltage side
 

cruff

Well-known member
For a start, check that all of the diodes on the low voltage side (to the left of your picture #2) are not shorted. Start with the big diode attached to the heat sink in the middle of the board next to the transformer. I count 12 others.
 

JRotar

Member
For a start, check that all of the diodes on the low voltage side (to the left of your picture #2) are not shorted. Start with the big diode attached to the heat sink in the middle of the board next to the transformer. I count 12 others.
I tested every diode with the multimeter in circuit and anything that looked weird I removed and they tested fine (about 4-5 didn't test well in-circuit but tested fine out of circuit)...I know in-circuit isn't the best but it usually seems that anything wildly out of spec will show up. I removed all the voltage regulators and they tested fine as well.

I also removed all the triax transistors and tested them on my Peak LCR and then tested them with the multimeter (base to emitter, etc) and they all passed.

I do want to ask a favor and that in the bridge rectifier circuit, early testing I did in-circuit but removed one or two of the triax transistors (four total)...I didn't notice until I removed them one-by-one that there are two types. I'm wondering if you can tell me the part numbers of each one to make sure I put them back properly. They are either 2907A or 2222A according to my model. Attached pic shows that bridge rectifier circuit.

I triple checked the caps orientation and values and everything is correct.

I also did blow the fuse so I had to order a new one and plus some clips (so I can replace it repeatedly haha) so I haven't been able to test. That might be another clue that something caused it to blow, although the burning trace might have. Maybe there's something else that's shorted that I haven't found yet - or I did stupidly put the transistors in the wrong order back.
 

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cruff

Well-known member
Here you go. Those four transistors are not a bridge rectifier circuit, they appear to be part of the feedback control circuit. I haven't had time to reverse engineer the schematic yet.

Q-locations.jpg
 

JRotar

Member
Here you go. Those four transistors are not a bridge rectifier circuit, they appear to be part of the feedback control circuit. I haven't had time to reverse engineer the schematic yet.

View attachment 37896
OMG Thank you!!! I actually DID reverse 2 of them D'oh! Once I get the fuse holder and fuses tomorrow I can put those back and try again.

Do you have a link to those schematics? All the links I've come across are dead and the one FTP server I found with a ton of them don't have the Dynacomp
 

cruff

Well-known member
I don't have a schematic yet, unfortunately, I haven't had time to reverse engineer the board. But your overheated trace is in the low voltage side of the feedback circuitry, so concentrate your efforts on that side of the board. Next would be to check the resistors on that side have values in tolerance.
 

JRotar

Member
I don't have a schematic yet, unfortunately, I haven't had time to reverse engineer the board. But your overheated trace is in the low voltage side of the feedback circuitry, so concentrate your efforts on that side of the board. Next would be to check the resistors on that side have values in tolerance.
Oh gotcha! Now that I re-read it that makes more sense that you're doing it.

That's very helpful - I removed most of the diodes on that side to test and thanks to my manhandling I can tell which ones haven't been tested out of circuit - so I'll probably do that as well as look up every resistor. Thanks!
 

cruff

Well-known member
Oh, forgot I do actually have the schematic.
 

Attachments

  • DynaComp Schematic 2.0.pdf
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  • DynaComp PSU PCB 2.0 C.pdf
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  • DynaComp PSU PCB 2.0 BW.pdf
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  • DynaComp Parts list v2.0.pdf
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cruff

Well-known member
You might check that zener D24 generates the correct voltage, if it was incorrect it could trigger SCR1 which would likely draw excess current through that trace that overheated. Double check D5 and D6 are not shorted. If you have an IR camera available you could also quickly check for possible bad components that are overheating.
 
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