Classic II Analog Board Repairs

[liamoc]

Hi all,

I have a Classic II with a working logic board but the analog board was not in good shape. Some capacitors were leaking slightly and the fuse was blown. I replaced the fuse and I replaced all the capacitors and cleaned the board, but as soon as I tried to turn the machine on I saw a flash and the new fuse was also blown. Does anyone have any idea what component of the board could be at fault here? Is this something I could repair?

 

[PacificState]

Yeah, the fault may have existed beforehand. Few things:

- Are the replacement capacitors of the correct value and the right way round?

- Not sure if it's like the SE/30, but can you check the power supply separately?

Otherwise, you could check the board visually for clues.

 

[dochilli]

A lot of things can be wrong with Classic analog boards (the two Diodes DP3 and DP4 (1N4148) are almost bad everytime, TDA4605, CNY17G, IRFBC40).

 

[liamoc]

Would the diodes being bad cause the fuse to blow though? I'm very unfamiliar with how analog boards work.

Edit: I just tested the diodes and I think they might be faulty. I get ~3 in one direction and ~0.5V in the other direction in diode test mode. I don't really know what that means but neither side is 0L which seems bad?

Edit: Also, is it safe to replace the CNY17G-3 with a CNY17G-2? The main difference seems to be the CTR Min, which is 63% rather than 100%.

 

[bibilit]

Have a look at the IRFBC40, often the problem here, if legs are shorted, the fuse will blow.

another issue is the bridge rectifier, but pretty rare, only seen one bad.

your best solution is to remove both and have them tested out of the board, as both are connected and will lead to wrong readings otherwise.

 

[liamoc]

OK, I tested the MOSFET and indeed all legs seem shorted. If I replace the MOSFET, should I replace other components too? (this thread seems relevant: https://68kmla.org/bb/index.php?thr...g-board-blowing-fuse-and-mosfet-irfbc40.6787/)

 

[dochilli]

I would change the diodes (DP3, DP4) and TDA4605, CNY17G, IRFBC40. I used CNY17G-3, but do not know, if -2 will work.

 

[joshc]

It is pretty common for multiple parts to need replacing on these boards, they are probably one of the most unreliable analog boards Apple ever made.

 

[bibilit]

Most of the time, only the Mosfet will be responsible for a blown fuse, but yes all the above parts are prone to failure.

DP3 and DP4 can be replaced at the same time, dirty cheap.

 

[liamoc]

Hi all,

I replaced all the mentioned components, and now the machine turns on and the CRT turns on! But the display is very garbled, it sort of looks like it's "rolling" and wavy. Seems quite bright too. Any ideas what is wrong?

 

[bibilit]

Too bright probably is a matter of voltage too high, what figure are you getting in the 5 volt rail ?

 

[AndiS]

Edit: Also, is it safe to replace the CNY17G-3 with a CNY17G-2? The main difference seems to be the CTR Min, which is 63% rather than 100%.

I replaced it with a CNY17-3 (without the "G") in my Mac Classic and it works without problems. The Analog Boards should be the same, so I can recommend this.

 

[liamoc]

Too bright probably is a matter of voltage too high, what figure are you getting in the 5 volt rail ?

How do I find this? I have a multimeter etc. but I don't know where to put the probes (I'm also a bit hesitant to put my hands near a CRT when it's on...)

 

[AndiS]

you can measure the voltages from the external foppy port. I dont remember the exact pin but, but this can be found online.

 

[bibilit]

on the hard drive Molex connector or yes, external floppy port.