Mac 128k blown fuse on analog board

[td75]

Hi everyone! I'm new to this forum and was hoping someone could help..

I recently acquired an original Mac 128k in good condition but unfortunately does not turn on. I found a bit of corrosion on some of the traces on the analog board which I have cleaned up. I also found the fuse was blown on the analog board and the fuse holders both fell apart when I removed the fuse. The capacitors all look good, no leaking or bulging. I have now replaced the fuse holders and fuse (1.6A 250V slow blow) but unfortunately the fuse blows as soon as I turn it on Is this the correct fuse? This is what I removed so assume is correct..

Any idea what component/s could be faulty that would cause the fuse to blow? Is it worth replacing all the capacitors on these analog boards even if they look good?

The original owner said the Mac worked when they last used it about 10 years ago! He said he thought it may just need a PRAM battery fitted as this was taken out. I remember reading somewhere that these Mac's can still turn on without PRAM battery fitted, but may be wrong..

Any help would be greatly appreciated!

 

[Macdrone]

Caps are most likely bad, age does as much damage as use. Fly back transformer also drawing too much and blowing fuse.

 

[td75]

Thanks for your reply. I will get all the caps replaced and see if this helps.

Is there any way I can test the fly back transformer for faults using a multimeter? If it is faulty any idea if they are repairable or still an available part?

 

[tomlee59]

You can indeed do a basic test with an ohmmeter. See the Classic Mac Repair Guide for the details.

That said, the probability of the flyback being the problem is somewhat low. It's not zero, but the current-limiting circuits in the power supply should act to prevent drawing enough primary current to pop the fuse. Again using the ohmmeter, verify that the bridge rectifier and main filter capacitor aren't shorted. Ditto for the SCR. Do a careful visual inspection of the board (both sides) to see if there are any tell-tale signs of overheating that might be a clue.

 

[James1095]

A fuse that blows instantly almost always means a shorted component on the primary side of the power supply. The most common is the chopper transistor which will be a power transistor or MOSFET on a heatsink near the big filter capacitors on the input side. It could also be a shorted main filter cap or a bad rectifier but the chopper is the first thing I'd check.

A fault on the output side will usually cause the power supply to shut down. Often there will be a soft chirping or ticking sound if you listen carefully, as the supply tries repeatedly to come up but shuts down. Some early switching power supplies, notably those used in the Apple II will whine audibly under this condition.

 

[tomlee59]

Yup, exactly. The "chopper" device in the classic macs is Q11 in the Repair Guide. The SCR often goes south along with Q11, which is why it's mentioned. But Q11 should definitely be checked, as you advise. Q9, too, is frequently a victim.

 

[td75]

Thanks for all the advice on this. I will hopefully get a chance over the weekend to do more tests. Will update as soon as I have tested all the components & hopefully find the culprit!

 

[td75]

Had another look at the board tonight and found two diodes shorting out in position CR24 & CR25 on bridge rectifier. Have replaced all four diodes CR22 - CR25 with part 1N4007.

Have also found CR20 and CR21 rectifiers are faulty. Have read the filter capacitors C24-C26 and C29-C31 are also weak spots. So will get all these replaced.

Is it best to replace all the capacitors on the analog board or just the filter caps? Hopefully this will solve the power issues..

 

[tomlee59]

Sounds like you've made a lot of progress -- congrats!

The only other cap I'd bother with on the analog board is C1. If the others look ok, you can probably leave them be. Once you get the thing to power up, you'll be in a better position to evaluate what other steps (if any) you need to take.

Keep going!

 

[td75]

Great thank you! Will update on any progress..

 

[td75]

Have now also replaced CR20 & CR21 rectifiers. Fuse no longer blows! But Mac doesn't turn on either Speaker is just making a clicking noise now. I have a C1 capacitor (part no ECWF2395JB) on order but I think there must be something else at fault as the current C1 cap does seem okay.

 

[Macdrone]

I am not sure where you are but I have an original analog board with CRT pulled from a dead motherboard unit if you need it. It has been tested with a 512 and a plus motherboard, but it is out of a 128.

 

[td75]

Thank you very much for your offer - I'm based in the UK. It's really difficult to find replacement parts here in the UK, I guess parts are becoming scarce worldwide.

BTW is there an option to get an email when someone reply to a post?

 

[Macdrone]

I think in the preferences for PM's and such you can receive emails. I get them, I probably set it up when I first signed on so I don't know exactly where in the settings.

The board is yours if you want it, but that little crt makes it twice as heavy, I could remove the yoke from it but you could weigh yours to see if trying to get this one is worth it for you. I am not using it and you are more than welcome to it. Last thing I sent over the water over there was expensive but its your call.

 

[td75]

That is very kind thank you. Will send you a PM.

 

[James1095]

Shipping a CRT overseas is going to be risky and expensive, but there is no reason you can't separate the board and the tube. I'm sure there are others here who could use a good tube for their compact Mac.

Clicking likely means the power supply is starting up and shutting down repeatedly. Look for shorted components on the secondary side, particularly the rectifier diodes on the outputs and the horizontal drive transistor in the monitor section.

 

[td75]

Yes probably wouldn't be worth shipping a CRT but the power supply would definitely help I don't even know if the logic board and CRT are working till I can get this to turn on..

I won't give up yet! I will keep looking and will probably end up replacing every component on this board!

Could a faulty CRT or logic board prevent power supply turning on?

 

[James1095]

A fault in the logic board could. It's not common, but a shorted IC could cause the supply to shut down and tick like that. It would be extremely unlikely for a CRT to fail in such a way as to cause this. As long as the vacuum is intact, the tube is probably fine.

 

[tomlee59]

I won't give up yet! I will keep looking and will probably end up replacing every component on this board!

Don't do that -- use test equipment and logic to narrow down the suspects. Judicious use of an ohmmeter will get you very far, so you don't need very sophisticated equipment for the majority of problems.

Did you check the SCR, for example? It frequently is destroyed by other faults with the power supply. If you've fixed the original problem, but not replaced a shorted SCR, you'll get symptoms very similar to what you seem to be reporting.

Also, make sure that the voltage adjustment pot hasn't been turned to a high voltage setting. That, too, will trigger a periodic shutdown behavior. I recommend setting the pot to the minimum voltage setting to start. Viewed from the foil side of the analog board, that corresponds to a maximally clockwise position (the opposite of what one might expect).

Check for short circuits across any of the supplies.

Could a faulty CRT or logic board prevent power supply turning on?

The CRT is highly unlikely to be a source of this problem. However, do check the logic board. Once again, a simple ohmmeter check will be revealing.