453 blows HV fuse

Richard Knoppow

I am getting back to a project I've had on the back burner for a long time.
About a year ago I left my 453 running in the shop on a very hot day. When I came back a couple of hours later there was no trace. I discovered the fan had stopped. More investigation found that the HV fuse had blown. I don't remember if the line fuse also blew but don't think so.
I thought the fan had frozen up but found the rotor turned easily and when powered up ran fine. Still does.
Now the problem is that the HV fuse F-937 blows. I checked Q-930, Q-923, Q-913, and Q-914 using the old fashioned method of an ohm meter. All seem to be good. I also made resistance measurements on T-930 and also voltage checks on it to see if there was a short. There does not appear to be.
I am at a loss. If I disconnect the collector of Q-930 the fuse does not blow. If I disconnect the base of Q-930 it does not blow. I also have measured the current though the fuse to see what differences the connections make and to avoid putting many fuses in. I am using a GR metered Variac as an AC source.
The voltage at the base of Q-930 with it disconnected is +11 volts, the handbook says it should be -4.4 volts. I looked at the +75V supply at R-925 and it measured right on the nose. The other side of the 13K resistor should be -4.4 volts. Being disconnected from the base of Q-930 probably affects this.
I've made some other checks, resistor values etc. but can't find anything. Probably I must obtain replacement transistors for all of these guys and try substituting rather than just resistance checks. It would be helpful if someone knows what kinds to get these days.
If anyone has had a similar problem please let me know what you found. Its very frustrating. Also, this is my one and only decent scope so I have none other to trouble shoot with.
Measurements were made using a Triplett 630A and a Hewlett-Packard bench DVM.

Richard Knoppow

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