Twentieth Anniversary Mac PSU replacement

[GRudolf94]

Well, one of the wires should have a voltage at all times. I thought it was yellow. In any case, it is probably one of the wires that are the only one of its color. If not, then you have more troubleshooting to do. The normal order would be to go from the diode bridge forwards, but it could be that the PWM (the TOP226) failed as discussed previously, as it's one of the most sensitive parts in there, and there was an overvolt condition.

 

[chrisrueckert]

MOVs do have a lifespan so thats not surprising. Also a surge could have done this as well.

the TOP226Y and 1ohm fusible resistor are a known weak spot in these PSUs, knocking out standby voltage. When the TOP controller fails, it opens the 1ohm resistor.

Could you point me to the location on the board where I could find the TOP226Y? I found a few similar ones but not this particular part:

 

[GRudolf94]

TOP200 is the PWM there, not a 226 then.

 

[chrisrueckert]

Ok, so I switched the TOP200. No success.

Coming back to the task to measure the voltage of the yellow wires: Which one of the following table do we want to measure?

Pin​	Connector	Label
1​	J2 Pin 7	AMP OE
2​	—	—
3​	J2 Pin 14	AMP RHS
4​	—	—
5​	J2 Pin 10	SPKR RHS
6​	J2 Pin 9	SPKR RTRN
7​	J2 Pin 8	SPKR LHS
8​	J2 Pin 12	AMP RTRN
9​	J2 Pin 13	AMP LHS
10​	J2 Pin 4	PFW
11​	J2 Pin 6	-12V
12​	J2 Pin 2	5V
13​	J16 Pin 2	5V
14​	J16 Pin 6	GND
15​	J2 Pin 3	GND
16​	—	—
17​	J2 Pin 1	5V
18​	J16 Pin 3	5V
19​	J16 Pin 7	GND
20​	GND	GND
21​	J16 Pin 8	GND
22​	J16 Pin 1	12V
23​	J2 Pin 5	12V

 

[techknight]

Ok, so I switched the TOP200. No success.

Coming back to the task to measure the voltage of the yellow wires: Which one of the following table do we want to measure?

Pin​	Connector	Label
1​	J2 Pin 7	AMP OE
2​	—	—
3​	J2 Pin 14	AMP RHS
4​	—	—
5​	J2 Pin 10	SPKR RHS
6​	J2 Pin 9	SPKR RTRN
7​	J2 Pin 8	SPKR LHS
8​	J2 Pin 12	AMP RTRN
9​	J2 Pin 13	AMP LHS
10​	J2 Pin 4	PFW
11​	J2 Pin 6	-12V
12​	J2 Pin 2	5V
13​	J16 Pin 2	5V
14​	J16 Pin 6	GND
15​	J2 Pin 3	GND
16​	—	—
17​	J2 Pin 1	5V
18​	J16 Pin 3	5V
19​	J16 Pin 7	GND
20​	GND	GND
21​	J16 Pin 8	GND
22​	J16 Pin 1	12V
23​	J2 Pin 5	12V

Did you actually test the 1 ohm resistor associated with it to see if it was open? because thats how you determine if the TOP Failed or not...

 

[chrisrueckert]

It is alive! Just realized that I forgot to connect one plug in the PSU. Thank you all provided here in this thread.
In summary, the overvoltage caused the MOV and TOP200 to fail. I sourced these parts from a DPS-150GB B PSU for a Performa 64/6500. Once these were replaced, the TAM works flawlessly.

 

[GRudolf94]

Excellent. Sourcing new ones is cheap, in case you need to restore the 6500 PSU

 

[Byrd]

It's good to know the 6500 PSU is pretty much the same as what is housed in the TAM's sub box - are there any massive differences in the design?

 

[chrisrueckert]

They are built on the same basis, however, they are different in details. You'll finde a nice write-up over at Tinkerdifferent.

 

[Byrd]

@chrisrueckert thanks for the link - I'm going to have to delve inside mine one day (fan seems suspect) - did you find any screws at the top under the rubber - or did it all come apart prying the base off?

 

[chrisrueckert]

Some of the screws are under the rubber. You'll probably feel them then you push the rubber a bit. Their position is the same as with the P6500 PSU. Note the additional two screws holding the transformer in the TAM PSU but you won't need to loosen them to remove the case.

 

[daanvdl]

I replaced the two MOVs with spares from another P6500 PSU. Connected to mains did not blow more parts, luckily. So far so good. However, the TAM doesn't power up. What's the best way to test if the PSU delivers power to the TAM?

If there is 5V standby but the power supply doesn't turn on, it may be due to the LM393/2N2222 combination on the PSU, which can get damaged. These components are often part of a control circuit that enables the main output, and they can be blown in a chain reaction if the original TOP200 (or similar) fails shorted.

In my case, the fault went deeper: I discovered carbon tracking (carbonization) between pins of the TOP200 due to a previous overvoltage or burn-in event. This caused a leakage path that only manifested under high voltage, repeatedly blowing the TOP200, LM393, and 2N2222 even after replacement.

Check for continuity or low resistance between pin 1 and pin 4 of the LM393 — this usually indicates internal damage. Also test the 2N2222 for shorts between collector, base, and emitter.

If you're debugging a similar failure, it’s worth cleaning the PCB around the TOPSwitch thoroughly and inspecting for carbonized areas between pins. I had to scrape between the pins until I measured infinite resistance before the circuit finally stabilized.