tektronix 932a scope malfunction

[radiolag]

My 932a scope shows a blinking power light when I turn it on. No display at this time. I checked two internal fuses but both OK. Any suggestions?

 

[ModemHead]

The **broken link removed** says that the power lamp will blink if the line voltage varies more than about 10% from nominal. So check the 120/240 voltage selector switch setting. Check your line (mains) voltage with a multimeter. If those are OK, then it's likely something has failed in the power supply. Perhaps one of the electrolytic filter caps, which would be a common failure in equipment of this vintage.

 

[radiolag]

Thanks, I checked again and found a 1/16 amp fuse in the 125 VDC line to the HV supply blown. Unfortunately replacing it with a 1/4 amp fuse brought smoke and heat from a 10 ohm resistor in series with the HV supply. If the HV supply is gone I don't think I have many options with a scope of this age.

 

[ModemHead]

Just for grins, check transistor Q458 for a collector-emitter short. It's the chopper transistor for the HV supply transformer, and this would probably cause the fuse to blow. And maybe Q454 also.

 

[radiolag]

Thanks for the help. Q458 checked good (junctions and C/E short checks). Also Q454. Also pulled C458 out of the circuit but no change. I suspect the HV transformer, but even if it is that I don't suppose new ones are available. Also, it's not easy trouble shooting without a schematic! Again, thanks

 

[ModemHead]

Don't give up, that current has to be going somewhere! Is C458 one of those dipped tantalum caps that are famous for shorting out? Check that too. Otherwise, keep disconnecting things from that 100V unreg supply until the short disappears.

Forgive me if I misunderstand, but if you don't have a schematic then there is one in the service manual I linked to in post #2. It's pretty readable for a free copy.