Re: 453 blows HV fuse

bc
 

I had my 2465 burn HV fuses. However it ended up being that I couldn't
find any failed transistors, coil was good via a ring tester, so it had
to be the CRT - which I happened to have cracked but didn't know it at
the time. At least for me, the event that stopped blowing fuses is
when I disconnected the CRT socket.

Is D940 still good?

Do you have a ring tester to check the transformer?

Try running with the back CRT socket disconnected or perhaps one of the
second anode rectifier tubes pulled (V952/V962), does it still blow?
You may want to use a light bulb in the meantime to diagnose this
problem, fuses get expensive fast.

On Thu, 2018-04-26 at 16:42 -0700, Richard Knoppow wrote:
     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.

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