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| Mac 512k analog board problem |
Posted by: VintageTech on 2022-05-31 01:14:44
I recently got a not working macintosh 512k. I replaced all electrolytic caps and the mac was running again.
My happiness didn't last very long though. The housing got very hot at the top of the Q2 (International 230v) during operation. I therefore assumed a defective Q2 BU 406 and a defective T1 flyback. I then replaced the Q2 and the T1. Unfortunately, the problem with the overheating Q2 has not gone away.
My question is now, could it be a bad T2 or the R11 Resistor reason for overheating?
Could be a problem in the neckboard of the crt?
Would it be possible to check the values in circuit with a multitester and find the bad part? Are there any important reference values to check? Why is the bu 406 overheating?
I already measured the values of the bu 406 to ground.
I have got:
Base -82.0 mv
Collector 20.49v
Emitter -22.0mv |
Posted by: Cinan on 2022-06-07 19:59:27
Do you know the replacement Flyback is a working one ? You can test with a Oscilloscope if need be.
Is the machine actually working and displaying anything ?
Is there a noise inferring the crowbar circuit is being initiated ?
I had a similar problem - https://68kmla.org/bb/index.php?threads/240v-power-circuit.38381/#post-423160 |
Posted by: VintageTech on 2022-06-08 00:28:13
The CRT is working. The picture is good. I replaced the bu 406 with a cheap china version (have a little foldover on the right side. Should have keept the original Fairchild on the board).
The Flayback is working working. It came from a working SE 30 analog board.
There is no special noise.
It would be a shame, if it would not be possible to repair the board. I my view could it be only a minor thing.
I am not an expert in transistors and power amplification circuits. Would it be sensible to put a resistor with a higher value in the place of the R 8 (100Ohms) Resistor? |
Posted by: VintageTech on 2022-06-10 00:57:10
I lowered the resistance of the R 11 (R8 sketch above) by 20 Ohms). BU 406 still getting hot. |
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