Re: 453 blows HV fuse

Richard Knoppow

Some progress. First to answer your questions; D940 appears to be good.
I do not have either a ring tester or a bulb tester. What I use is a metered Variac and put a Triplett 630A across the fuse holder to measure the current. Its set on the 12A range and I can see clearly when the current begins to rise with line voltage. I tried disconnecting the CRT at its socket and also disconnected the HV lead to the helix at the HV compartment. Made no difference but thank you for the suggestion.
So far, here is what I've found. When the line voltage begins to get to normal something in the HV compartment begins to arc. I found its a lead from the HV transformer to the filter for the "helix". Since there was HV I decided to see if I was getting any sort of trace. At around 90V line voltage (I have to note this again) I could get a trace with the beam finder. The trace was distorted and that makes me think I've been chasing my tail on the HV. I now think the problem is in one of the lower voltage supplies. I have errands to run today so will get to this tomorrow. It would be great if I had a second decent scope but I don't so will have to trace this down via a voltmeter. In any case I think the presence of a trace of sorts indicates the HV is OK and the CRT is OK. I now need to study the other power supplies to find the problem. I can run the scope at full line voltage with the HV fuse removed. The distorted trace should give me some clue as to what has failed. My guess is a filter cap somewhere since the thing overheated.
Any suggestions would be welcome. In any case I now have some confidence that I can fix the thing.

On 4/27/2018 1:57 AM, b c via Groups.Io wrote:
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.
Richard Knoppow

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