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BTW, if you go the Quadra route to run A/UX, don't get a 631 or other model that has an IDE drive instead of SCSI. A/UX doesn't support IDE drives out of the box. Not sure if anyone ever made a mod to A/UX to support one.

I have fond memories of the IIsi I had. I remember the 20 MB drive I bought for it cost about $900. I had A/UX running on it along side Mac OS. It was definitely a nice size machine for that era.

The pin outs of the 8530 chip would not change, unless they substituted another variant. If you don't have it, this appears to be the ROM 03 schematic https://downloads.reactivemicro.com/Apple%20II%20Items/Hardware/IIgs/Schematic/, which is formatted differently to the ROM 01 schematic I have.

The SCC diagnostic test performs a exhaustive test of all the values that can be written to the SCC register 2 (interrupt vector, via channel A only), registers 12 and 13 (the baud rate generator counter registers) and register 15 (the external status/interrupt control register, bits 7, 6, 5, 4...

It is the Y2 crystal. It is the baud rate clock (SYNC.L, RTXCA.L, RTXCB.L), and the PCLK signal (CREF.H from the Mega 2 chip) provides the timing signal for the remainder of the 8530 operation, which includes the read/write of the 8530 internal registers. Thus if PCLK is good but the baud rate...

Yes, I suspect the crystal could be the cause of that error code. Do you have an oscilloscope handy to see if it is working? Be sure to use a 10x attenuation on the probe. Commonly a crystal might have capacitors to ground on each leg, but the schematic doesn't show any.

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...

Check that R21, C16, D13, D11 and C14 are not shorted.

Oh, forgot I do actually have the schematic.

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...

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.

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.

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

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.

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...

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.

Can you tack two wires to the leads of C5 so that you can measure it while in operation?

Don't assume that because a trace is tiny it can't carry a higher voltage. It is the current the trace carries that is the important factor in trace area (and thus width). Did you mean to say that C5+ pin is connected to the LM324M pins 3 and 10? What supply voltages are supplied to the op amp...

The metallic paint is there to reduce radio frequency interference generated by the IIsi, but with all the noise in the current environment from switching power supplies, etc, you may not notice any problems unless you are an amateur radio operator or you have one nearby. Looks like if it comes...

You might try a quick reheat of the pins for Y2 and the pins 12+13 on the 8530 just to see if it is a cold solder joint.