Fighting with 630-010B Plus International Analog Board

[solidpro]

Hi Everyone

I've been gradually self-teaching PCB repair and getting better each day. But I'm struggling with my Mac Plus analog board (630-010B)

I have replaced all the capacitors, except C1 (sometimes labelled C2, which is the unusual 3.9uf bipolar cap, which I have taken from another known-working-good board) and gone from a non working flyback to a working flyback and it works with a 128k logic board. However, using a Mac plus board I get Horizontal zebra (sisami) stripes - something which nobody online has ever seemed to reference on a plus.

Anyway, to cut a long story short, all the voltages are stable, but low, and outside the adjustments on the analog board itself (which seem to adjust only about .5v each way). This low voltage is what I suspect to be the cause of the zebra stripes on a Plus board.

The +12v rail is measuring 10v (on the J2 CRT connector + J4 logic connector + FDD connector) and the +5v is measuring about 3.2v

At this point i'm stuck with my own limitations. Any ideas what might cause this steady low voltage across the board?

Any ideas gratefully received! 

Thanks

 

[bibilit]

Strange thing, The Analog Board can be swapped from a 128 to a Plus without issue, so if it's working with the 128 board, should also work with the Plus board, unless you Plus board is faulty.

 

[solidpro]

Strange thing, The Analog Board can be swapped from a 128 to a Plus without issue, so if it's working with the 128 board, should also work with the Plus board, unless you Plus board is faulty.

I have 4 x Plus boards which test/boot absolutely fine with a different known good working Mac Plus... I also tested the CRT with another Plus, and it's a brand new flyback....! We're down to the analog board.

When I say the 128k works, I'm not being entirely full with the facts - I get a 040A80 sad mac, but I believe this to be a RAM issue with the 128k board. The main thing is that it's booting and working rather than horizontal. I just assumed 128k boards have a much higher tolerance to low voltage than a Plus...

 

[bibilit]

A usual suspect is the Optoisolator (U3 IIRC) 

As is in the regulation side and prone to failure, i will try a swap (voltages are too high or too low when they go bad)

 

[solidpro]

A usual suspect is the Optoisolator (U3 IIRC) 

As is in the regulation side and prone to failure, i will try a swap (voltages are too high or too low when they go bad)

I switched U3 with one from a spare board and it's exactly the same... It could be two bad ones but that's less likely than when I started....

 

[bibilit]

Leaky diode ?

 

[solidpro]

Still only 10.4v on the +12v rail.

So I'm reaching my wits end now! I've replaced CR20 and 21 with new. I've replaced R39 with new. I've replaced every single cap and then some.

I can't understand why the voltage adjustment R59 doesn't do anything. I traced the tracks to R39 thinking it was to do with this but replaced it changes nothing... 

Does anyone know what allows R59 (the adjustment for voltage) to do it's job? Or it is something else.

 

[Joe]

Are you measuring the 12V rail with or without load? Maybe something is pulling too much current of this rail, causing the voltage drop. I usually disconnect logic board, FBT and the vertical drive circuit, which all feed of the 12V rail, when troubleshooting this scenario.

Also, if you have access to a benchtop power supply, you can inject 12V directly on the analog board (with the Mac disconnected from the mains) and check current draw and troubleshoot the feedback circuit (see below). Like bibilit mentioned, the opto-coupler is usually a prime suspect, and on rare occassions I have also had to change the LM324 op amp.

Here is the feedback circuit that controls the switching transistor:



And the explanation to the above:

The feedback control circuit monitors the +12V supply voltage through a voltage divider comprising R35, R56 and R38. 
This divided-down voltage is compared with a 6.2V zener diode-derived reference voltage (from R34 and CR19). 
The system tries to maintain equality of the voltages presented to the input terminals of the op-amp by driving the 
switching converter more or less hard as necessary. 

An optoisolator (U3) couples the control circuitry, which is all referenced to digital ground, to the core switching 
converter, which is referenced to primary ground. Resistor R56 (located just above the speaker) allows fine adjustment 
of the output voltage. Stability of the feedback system is assured by C21, R37, C27 and C28.

If the +12V supply voltage is too low, the LED inside the optoisolator is driven with less than normal current. This, 
in turn, reduces the current through the optoisolator’s transistor, which ultimately commands the switching core to 
increase its output. The opposite happens if the supply voltage is too high.

 

[desertrout]

@solidpro Did you manage to resolve this issue? It sounds similar to mine...

 

[desertrout]

nm - it was the 4N35 optoisolator at U3....