Issue with PSU in Philips VG-8235/20

بواسطة retro69

Rookie (32)

صورة retro69

28-03-2022, 12:20

Hello,
Just last week end my Philips VG-8235/20 stopped working. I have switched it off, removed the cartridge, re-inserted the cartridge and switched it on again. The computer did not boot up and the power LED remaind dark as well.
In the meantime I have checked the PSU output.
The +12 V works fine, but on -12 V there is basically nothing and on +5 V only about +2.6 V are achieved and not stable either.
I am now not sure what the specific problem is. Looking at the circuit diagram I see that both on -12 V and and +5 V there are capacitors so they might be faulty. In contrast, on +12 V there are no capacitors and that might be the reason why this still works.
On the other hand it appears to me that in the end both -12 V and +5 V are fed by the voltage regulator STK7561A.
Does anyone have an idea how to narrow down the problem ?
Thank you very much.

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بواسطة Jipe

Paragon (1572)

صورة Jipe

28-03-2022, 13:55

بواسطة meits

Scribe (6528)

صورة meits

28-03-2022, 19:52

Check the -12V regulator L7912 (U2). Getting that thing back to -12V will probably get your 5V back as well.

بواسطة retro69

Rookie (32)

صورة retro69

28-03-2022, 22:10

Reworking the solder joints did not have any effect. I have just ordered a new voltage regulator.

بواسطة retro69

Rookie (32)

صورة retro69

16-04-2022, 20:38

It was the STK7561A. After replacing the part the computer started again and works fine. However, I have noticed that the -12 V rail shows just -10 V without load. I have read afterwards that the Voltage may be outside of the spec without load, so I guess I should measure again. However, if the -12 V regulator is not working properly either I may want to replace it as well. It appears that the 7912 type A is not available any more. I could take a 7912 type CV. Does anybody know if this fits as well ?
Thank you for your help, I am really happy that my beloved VG-8235 runs again.