Mac Classic II dim CRT + jail bars

[howardc64]

A little repair history

- Changed out all the leaky caps on logic board 10 years ago. Replaced the battery. Everything worked great except doesn't keep date across power cycle (didn't before the repairs). Never found the problem.

- Messed with it 10 months ago. A friend has better soldering iron + flux, retouched all the caps I replaced. Worked great afterwards, doesn't keep date just like before even though battery still reads 3+ volts.

- Just power it up again, no bong sound and vertical jail bars. CRT screen is much dimmer than the past with dark wavy band slowly running top to bottom repeatedly. Cleaned parts of the logic board (around where recapped) with alcohol, no dice.

Analog board issues? This board's never been touched. I see no obvious signs of failed electrolytic capacitors. But of course failure can be hidden away on the bottom (Just fixed a garage door that had failed cap in AC/DC convertor, leak was at bottom of the cap hidden away from plain view)

 

[CharlieFrown]

I have the same issue with my Classic II and I doubt it has anything to do with AB. Analog board re-cap solved "vibration" on screen and it worked fine. I could even play some games,  but after I replaced already weak caps on motherboard it started to give me no bong, random vertical bars.  Cleaning the board with IPA, acetone didnt help. 

Is there any way you could diagnose the problem when there are no visible damages to the logic board. Oscilloscope, infrared camera?

 

[howardc64]

Ok, interesting update...

Powered it up again tonight to take some voltage readings off of the floppy connector (from page 8-9 of this link https://68kmla.org/files/classicmac2.pdf) .Same as yesterday, no bong sound and dark CRT with jail bars.

Then while probing a pin (pin 6 I think) on the floppy connector, I heard bong and the system kept on triggering the reset sequence so it was bong over and over.

Eventually system did power on without reset but HDD didn't spin up (looking for disk icon as well as no HDD sound) and the CRT had this arcing sound every 1 second and the screen image got a little jolt every time.

Pin 7-8 would fluctuate +- 0.3Vs during measurement.

Eventually, I powered it down and powered it back up and bingo... everything was normal! Booted up, HDD spun, got into my desktop, CRT looks bright and image is stable. Floppy connector measures on pin 5,6,7,8 are -11.98, +5.16 +11.25 +11.25 respectively

Last 2 voltages should probably be closer to +12V? Seems like I might have a power supply problem?

I'll give it another try in a day or so and see if any changes.

 

[CharlieFrown]

it makes sense, new caps with full value need more power than before.

My power supply wasnt recapped either.

 

[howardc64]

Another update...

Left it unplugged for 2-3 hours and plugged back in and restarted. Same pattern

- no bong, dim CRT, jail bars. Checked the voltage at floppy pin 5,6,7,8. All read around 0V

- after a min or so, repetitive restart+bong

- after another 1/2 min or so, starts booting and no HDD

- eventually boots and all is well

Clearly, something in the power supply have problems. Will be checking J4 connector, diodes+caps in the power supply circuitry per repair guide. Hopefully my Mac Classic II's power supply circuit is same/close as the repair guide's Mac Plus.

 

[howardc64]

it makes sense, new caps with full value need more power than before.

My power supply wasnt recapped either.

Check your ext floppy connector pin 5,6,7,8 and see if you read -12V,+5V,+12V,+12V. This probably gives pretty good indicator if logic board has all correct voltages. Page 8-9 here https://68kmla.org/files/classicmac2.pdf

 

[howardc64]

Took out Analog board and look what I found! A bunch of leaky capacitors near the 12/5V power output and crowbar circuit (close to figure 15, page 22 of this doc https://68kmla.org/files/classicmac2.pdf)Hopefully replacing these will fix it.

Pretty amazing the Mac still manage to boot up properly after warming up for awhile

Pics here https://drive.google.com/open?id=0Bwns7f1Ly3k1Wlo3WlhvZGZCWmc

 

[bibilit]

Yes all Classic i have seen lately have the same leaking caps in this area.

 

[howardc64]

Yes all Classic i have seen lately have the same leaking caps in this area.

Replacing that block of capacitors fix the problem? I just ordered all the parts.

And are the 2 nearby diode usually good or bad?

 

[howardc64]

 A bit more googling shows this group of capacitors on the power output and crowbar circuit has very high failure rate. The reason is they are close to a heat source (transformer). Mac Classic II has a fan but these capacitors are densely packed with little air flow. The capacitor furtherest from the fan appears to have the worst leak.

I am planning to do the following on the repair and was wondering if anyone tried it

- Mount the capacitors in a way to allow more air flow to reach all of them

    - mount it higher away from the board to allow air flow through the bottom

    - stagger mount them height wise so every cap gets air flow

    - bend the long leads so they are away from each other

Not a ton of room but I figure its better than packing them tightly and seated tightly against the board. Thoughts?

 

[CharlieFrown]

do you guys think that you usually  need  a new optocoupler (isolator) after Classic II recap? 

 

[bibilit]

I will change it for the sake of mind, seems to be also a weak part on those boards.

 

[howardc64]

Just following up on my successful repair of the analog board.

Replaced all the brown color high temperature low ESR caps near the transformer. The screen is nice and stable afterwards but system didn't boot. Eventually got it to work so likely was just the analog board's connector to the logic board.

Only 1 of the capacitor was bad. It was the fat 2200uF 16V one. After heating it up, my capacitor tester doesn't even recognize it as a capacitor.

Still can't keep time across power downs (battery reads 3.6V) so tracing that next on the logic board.