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Mac Classic Analogue Board Repair - Low 12Vdc (and other) Supply Rails

TheMightyMadman

Active member
Hi all,

I've recently got my hands on three compact Macs - a Classic and two Classic IIs.
I've got the Classic and one of the Classic IIs working well, which is great.
However, the other Classic II is putting up a fight.
Not only is the digital board not working (recapped but needs a new CPU due to corrosion), the analogue board is also faulty.

It was originally completely dead, having suffered from extremely bad capacitor leakage.
Since then, I have:
  • Replaced all of the electrolytic capacitors (except for the mains filter capacitor).
  • Replaced the two RIFA-type mains filter capacitors.
  • Checked power switch, fuse, and IEC connector joints - all OK.
  • Replaced several parts that had been affected by leaked electrolyte:
    • DP6 (MBR1045 Shottky diode), which was badly corroded.
    • DP3 and DP4 (1N4148s).
    • TDA4605 (PSU driver IC).
    • CNY17G (optocoupler).
    • QP2 IRFBC40 (output transistor).
  • Desoldered and cleaned underneath most major parts, including mains transformer, flyback transformer, connectors, etc.
The board is now outputting voltages, however these start off very low when cold (approx 10.5Vdc and 3.7Vdc), then increase as the board warms up (up to approx 11.2Vdc and 4.2Vdc). At this point, high voltage starts to kick in, and the CRT only displays a horizontal line - I turn it off every time when this happens so as not to burn the CRT, so I don't know how high the voltages end up.

I replaced DF1 (1N4001) and DF2 (1N4148) in case there was also an issue with the vertical deflection circuit, but I suspect that this is due to the weird low-voltage supply rails; RF7 (3.3 Ohm fusible resistor) tests OK; TEA2037A has been socketed and cleaned but I don't currently have any spares.

Even though this will probably end up as a parts machine, I'd like to get it working.

Has anyone had similar symptoms, and if so did you manage to fix it? Thanks!

Many thanks,

Adam
 

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Nixontheknight

Well-known member
Hi all,

I've recently got my hands on three compact Macs - a Classic and two Classic IIs.
I've got the Classic and one of the Classic IIs working well, which is great.
However, the other Classic II is putting up a fight.
Not only is the digital board not working (recapped but needs a new CPU due to corrosion), the analogue board is also faulty.

It was originally completely dead, having suffered from extremely bad capacitor leakage.
Since then, I have:
  • Replaced all of the electrolytic capacitors (except for the mains filter capacitor).
  • Replaced the two RIFA-type mains filter capacitors.
  • Checked power switch, fuse, and IEC connector joints - all OK.
  • Replaced several parts that had been affected by leaked electrolyte:
    • DP6 (MBR1045 Shottky diode), which was badly corroded.
    • DP3 and DP4 (1N4148s).
    • TDA4605 (PSU driver IC).
    • CNY17G (optocoupler).
    • QP2 IRFBC40 (output transistor).
  • Desoldered and cleaned underneath most major parts, including mains transformer, flyback transformer, connectors, etc.
The board is now outputting voltages, however these start off very low when cold (approx 10.5Vdc and 3.7Vdc), then increase as the board warms up (up to approx 11.2Vdc and 4.2Vdc). At this point, high voltage starts to kick in, and the CRT only displays a horizontal line - I turn it off every time when this happens so as not to burn the CRT, so I don't know how high the voltages end up.

I replaced DF1 (1N4001) and DF2 (1N4148) in case there was also an issue with the vertical deflection circuit, but I suspect that this is due to the weird low-voltage supply rails; RF7 (3.3 Ohm fusible resistor) tests OK; TEA2037A has been socketed and cleaned but I don't currently have any spares.

Even though this will probably end up as a parts machine, I'd like to get it working.

Has anyone had similar symptoms, and if so did you manage to fix it? Thanks!

Many thanks,

Adam
If the voltage is still reading as low, there is a potentiometer on the analog board to adjust the voltages until they are within spec, this will affect all the voltages, so make sure you don't overvolt any lines you didn't say were low. And as long as the CRT is kicking in, that does mean that the high voltage is at least high enough for the crt to work
 

TheMightyMadman

Active member
If the voltage is still reading as low, there is a potentiometer on the analog board to adjust the voltages until they are within spec, this will affect all the voltages, so make sure you don't overvolt any lines you didn't say were low. And as long as the CRT is kicking in, that does mean that the high voltage is at least high enough for the crt to work
Thank you 😊 I meant to mention in my original post that the B+ adjustment potentiometer PP1 doesn’t seem to be affecting the voltages at all. I’ve cleaned the pot and tested it out of circuit and it seems OK.
 

alexGS

Active member
Thank you 😊 I meant to mention in my original post that the B+ adjustment potentiometer PP1 doesn’t seem to be affecting the voltages at all. I’ve cleaned the pot and tested it out of circuit and it seems OK.
How did you go with this one?
I have yet another Mac Classic analog board with problems.

Like you, I had no vertical deflection at all, but I actually solved that by changing RL2 - a low-ohms tiny resistor under the flyback transformer - it had gone open-circuit.

Now I have the usual wobbly-screen problem, though the machine now boots since I cleaned the logic board - and swapped the flyback transformer (!) - otherwise it was failing with ticking sounds. Have done everything else you’ve done… interested to hear what you did next please :)

Cheers
 
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