Mac Classic problems

[Cinan]

Hi all,

I have a Mac Classic with a checkerboard pattern. I have recapped the analog and the logic board. I have proved the analog board works with a known working logic board (Board B) and I have proved the Logic board (Board A) also doesn't wok with the known working Analog board.

The symptoms I am getting are the checkerboard (with two different ROM chips being tried). The power rails are as follows measured at the 19 pin connector on the back of the logic board :

-12v is good at -12v
+5v is good at +5v
+12v is bad only measuring 1.8v

So my question is what component controls the +12v once it enters from the analog board ? The Analog board must be passing the right voltage as Board B (working logic board) works ok with the Analog board and this board (B) is showing +12v at the pin 8 using the same analog board.

Does VR1 have something to do with this next to C8 ?

Logic Board is 630-0390-C

 

[bibilit]

Hi, VRI is a 79L05 so a - 5 Volt voltage regulator, not related to your + 12V

In the other hand, C8 (47µf/16v) is the cap directly connected to the +12v rail.

 

[Cinan]

thanks but what would pull it down to 1.8v if C8 is new ?

 

[Cinan]

I suppose the obvious questions is the checkerboard likely a symptom of this or not related ?

 

[Cinan]

I can't actually any usage of the +12v on the schematic other than the output of the J6. So would this cause an issue with no booting to the beep and happy screen. If all chips are using +5v and that is reading ok can I assume the problem is somewhere with a trace ? I have checked about 40 traces already mainly focused on the connections between ROM, CPU, VIA, BBU, SWIM and RAM.

 

[Cinan]

I fixed this. Was a brown trace from Pin 1 of J12 that went past C8. It wasn't including C8 therefore impacted the 12v rail.

 

[Cinan]

Well I thought I had it fixed. On Tuesday was all good, nice steady scene, nice clear sound. Turned it on this morning and there is "interference" ripples on the screen. So I tried the logicboard in a known working Mac Classic and it is fine so this new problem is on the Analog Board. The board has been recapped (recommended caps) + today I have desoldered a lot of components and recapped a few more caps. I also replaced PP1 with another PP1 of another classic but no change. I have turned all the screen adjustment pots to make sure they weren't "stuck". I have done

all the suggestions in this forum for that symptom without any luck, any suggestions what to try next ? Is this likely a voltage issue (voltages are strong and stable) or a Cap issue or a Screen driver issue ?

Video of what is happening attached.

View attachment IMG_2325.MOV

 

[pfuentes69]

(Deleted my own reply as it was irrelevant)

 

[joshc]

What voltages do you get now? I assume they are fluctuating/outside of normal tolerance given the rippling/wobbling on the screen.

I've repaired quite a few Classic analog boards and tended to find they wouldn't be completely stable until the optocoupler (OP1) was replaced. You may also need to adjust the voltage potentiometer (PP1).

I would also check for bad/dry solder joints.

Can you upload some photos of your analog board, the top and bottom?

 

[dochilli]

There are a lot of parts on classic AB that can be defect: DP3 and DP4 (1N4148), TDA4605, CNY17G, IRFBC40.

 

[Cinan]

So I have done some more investigations, all electrolytic caps are now replaced on the analog board, DP3 and DP4, CNY17G, I have swapped TDA4605 and TEA2037A with another classic all without any changes.

I have noticed that the MOSFET IRF740 in the large heatsink gets warm on this machine but not as warm on a working classic ?

My next step will be to test/replace IRFBC40.

Question : would IRF740 getting warm have an impact here ?

 

[Cinan]

I have narrowed it down to the brightness controls now (I think). See video, when brightness is increased so does the interference. I have checked most components in this areas and all look good, I have moved the Pot around checked continuity between components. I just need to check the two transistors in this circuit.

Voltages are 5.02v and 12.11. I have adjusted the voltages through pp1 without any changes.

Does anyone have any experience in this area ?


View attachment IMG_2371.MOV

 

[Cinan]

So I solved this problem. Was the flyback. I started testing in the contracts and brightness circuits given the visual changes when brightness was adjusted. All looked ok from a component perspectives I tested the output of the Contrast Pot on my scope and it showed a lot of noise, then moving to the input of the Pot showed the same so pointed to a dodgy voltage output of pin1 of the Flyback. I then swapped the Flyback with a known working one and all was ok.

The Flyback looks perfect, is there anyway of fixing a flyback that works ok other than the "interference" out of pin1 ?

Does anyone have a spare 157-0129 Flyback ? Really annoying how Apple change the PCB layout so you can't use different size Flyback like the earlier Mac's !!