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453 blows HV fuse

bc
 

Sounds like you have something to investigate. Be careful not to
electrocute yourself when finding the source of the arc, it's not
supposed to arc! Try sticking plastic, move things apart, etc. to see
if you can get the arcing to stop.

The voltage on the base is fine when the HV isn't actually running -
it's both a symptom and a result but not a cause. I would suspect the
voltage is around -4.4V (or much lower than 11V) when you have the line
voltage turned down. At that point it's actually running and you're
getting HV. When the HV is not operating, the only way to get things
going is to turn that transistor on, and at -4.4V it may not be enough
to get things going.

The presence/operation of HV is what causes the voltage to go down to
-4.4V, if that would help explain what's going on here.

On Sat, 2018-04-28 at 14:24 -0700, Richard Knoppow wrote:
    Still have to follow up and make more measurements. Its the 
fuse on the 20V supply to the HV assembly. Tek calls it the HV 
fuse even though its in a low voltage line. Probably less than an 
amp flows. I can see the arc. I thought at first it was along the 
1meg resistor that feeds the HV to the CRT but its on a lead 
underneath that, probably on one of the caps on either side of 
the 1Meg resistor. Its a sort of corrona to the air. Very odd. 
Its this arc that's drawing all the current. Making measurements 
here is difficult both because of the narrow range where the arc 
is not happening and because of the somewhat cramped space.
     The base voltage on Q930 is wrong. Supposed to be -4.4 volts 
but I get about +11 volts. This is with the fuse out so voltages 
are probably pretty upset. It would turn the transistor on hard. 
Maybe an illusion. Anyway, I will make some more measurements and 
make sure the other supplies are working. I know the +75V supply 
is OK but first thing should always be to check the PS.
    Don't think its an insulation failure.
    I need to study the handbook and schematic further to see 
what I want to measure. Very strange problem although I suspect 
someone else has encountered it before.

Richard Knoppow
 

Still have to follow up and make more measurements. Its the fuse on the 20V supply to the HV assembly. Tek calls it the HV fuse even though its in a low voltage line. Probably less than an amp flows. I can see the arc. I thought at first it was along the 1meg resistor that feeds the HV to the CRT but its on a lead underneath that, probably on one of the caps on either side of the 1Meg resistor. Its a sort of corrona to the air. Very odd. Its this arc that's drawing all the current. Making measurements here is difficult both because of the narrow range where the arc is not happening and because of the somewhat cramped space.
The base voltage on Q930 is wrong. Supposed to be -4.4 volts but I get about +11 volts. This is with the fuse out so voltages are probably pretty upset. It would turn the transistor on hard. Maybe an illusion. Anyway, I will make some more measurements and make sure the other supplies are working. I know the +75V supply is OK but first thing should always be to check the PS.
Don't think its an insulation failure.
I need to study the handbook and schematic further to see what I want to measure. Very strange problem although I suspect someone else has encountered it before.

On 4/28/2018 9:32 AM, b c via Groups.Io wrote:
Okay yeah looks like the data collected do line up with each other and
your CRT HV seems to be working at least to a point.
This is F937 (2A) that you're seeing burn? I would call this the "HV"
fuse, though there are many other voltages that are quite high. How
many amperes is flowing through the fuse you're working with when the
CRT starts glowing at that circa 90V line voltage you found?
Perhaps it's just insulation failure, these scopes are quite old now.
This could cause higher current drain and burn the fuse. Are you
saying the high tension second anode lead is leaking charge? Can you
stick some plastic in its path to prevent the arcing discharge?
--
Richard Knoppow
dickburk@...
WB6KBL

Roger Evans
 

Isn't it normal for the trace to be distorted with the beam finder? The beam finder applies a fairly hard clipping to X and Y amplifiers to keep the spot on screen.

When you say there is something arcing in the HV box can you see the arc? The high voltage capacitors (C906, C976, C952, C954, C953, C961) are all likely points of failure in a scope of this age. I had to replace two of the 2.5kV rated capacitors in a 454 but they only showed any sign of leakage with a 1kV insulation tester so low voltage testing will not reliably find high voltage faults.

Roger

bc
 

Okay yeah looks like the data collected do line up with each other and
your CRT HV seems to be working at least to a point.

This is F937 (2A) that you're seeing burn? I would call this the "HV"
fuse, though there are many other voltages that are quite high. How
many amperes is flowing through the fuse you're working with when the
CRT starts glowing at that circa 90V line voltage you found?

Perhaps it's just insulation failure, these scopes are quite old now.
This could cause higher current drain and burn the fuse. Are you
saying the high tension second anode lead is leaking charge? Can you
stick some plastic in its path to prevent the arcing discharge?

On Fri, 2018-04-27 at 14:01 -0700, Richard Knoppow wrote:
    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.

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
dickburk@...
WB6KBL

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.

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
dickburk@...
WB6KBL