Fixing QC1920

[Jbliss]

Hi Im trying to fix a old oscilloscope the digitech QC1920. the + and -12volt rails were both down I replaced the regulators and there working however there is a separate PCB with a hidden fuse which keeps blowing. I have tried putting a larger fuse for trouble shooting when I did the the 12v rail dropped to 1 v i believe this seperate PCB is loading down the 12v rail but I can't work out whats wrong. The fuse is directly connected to the + and - 12 volts.

Can some please help
this is the only scope i have and would like to try and get it working again.

[Jbliss]

here are the shematics

[Jbliss]

here

[vk6zgo]

I have a very similar looking CRO,which died on me on one occasion..

I found that V709 or V710 was faulty (can't remember which,but the former seems the most likely).

It was some obscure part number,but I replaced it with a transistor of the same package salvaged from a switchmode supply in an old piece of equipment.
Would have liked to have replaced both with the salvaged pair,but one of them ended up with a short leg during extraction.

It didn't matter---the beastie fired up perfectly!

[singapol]

Hi,
      I don't see a hidden fuse you refer to...unless it's the fuse F701 before 7812. Anyway both 7812 and 7912 are part of switching power supply circuit developing 140V and 220V to feed vertical and horizontal drive circuit for the CRT ( cathode ray tube).

Most likely drive transistor V709/710 may have shorted. I hope the transformer is still good...
Schotkey diodes V711 - V714 may have shorted. ( They will measure low resistance but they are good,
only if they measure dead short from both polarities can you be sure they are defective).So are the high voltage electrolytic filter caps C715/716 respectively. Also check R703 47 ohm and R704 430 ohm.

Hopefully you can find the fault. Good luck.

Edit: Most likely vk6zgo has confirmed my suspicion...but check the rest of the components anyway.

[Jbliss]

Hey Guys Thanks

I found the shorted Transistor and have replaced it with a new one i could not find the exact one. But It Works !!!! however the new transistor makes really quiet noise Is this a problem