Tektronix 465M HV problem

[Steve Turner]

Greetings.

I recently acquired a nonworking Tek 465M to serve as a source of
spare parts for my main scope (also a 465M). After receiving the dead
scope, I thought it would be fun and educational and maybe even
worthwhile to try to fix it, so I would have a spare scope instead of
just spare parts. Fortunately, I have a full set of schematics, which
partially make up for my complete lack of formal electronic
education...

The scope has no trace or HV. After removing the horizontal and
vertical modules and the CRT, I discovered a blown 0.25A fuse on the
mainboard. This fuse supplies the HV oscillator with +32v (unreg),
and I verified that, yep, it blows instantly on powerup. All other
supply voltages are right on the money.

I have checked the transistors, diodes, and electrolytics in the HV
oscillator and nearby HV regulator circuitry, and there are no obvious
shorted (or open) junctions. I did discover that applying the nominal
+3.9v to the base of Q548, which is part of the HV regulator, prevents
the fuse from blowing and allows the HV oscillator to be powered.
However, it still refuses to oscillate, meaning no drive for the
transformer and HV module.

At this point, I'm kinda stuck. I think I've done my Google homework
and found lots of interesting things, such as the fact that there are
a number of Tek experts on this forum, but I was unable to find the
solution to my problem. I'd be most appreciative of any pointers.

I'll try to post the relevant schematic to
alt.binaries.schematics.electronic.

Thanks.

Steve Turner

 

[Jamie]

by you applying Biasing on the regulator you may have done nothing more
than shut down the Oscillator..
try to vary that voltage and see if its it gradually coming on and
your getting excessive current build up as it attempts to drive the
output module? if this is the case then you most likely have a shorted
high voltage Transformer or High Voltage tripler shorted or in many
cases the high voltage unit which has both the coil and tripler build in..


Steve Turner wrote:

Greetings.

I recently acquired a nonworking Tek 465M to serve as a source of
spare parts for my main scope (also a 465M). After receiving the dead
scope, I thought it would be fun and educational and maybe even
worthwhile to try to fix it, so I would have a spare scope instead of
just spare parts. Fortunately, I have a full set of schematics, which
partially make up for my complete lack of formal electronic
education...

The scope has no trace or HV. After removing the horizontal and
vertical modules and the CRT, I discovered a blown 0.25A fuse on the
mainboard. This fuse supplies the HV oscillator with +32v (unreg),
and I verified that, yep, it blows instantly on powerup. All other
supply voltages are right on the money.

I have checked the transistors, diodes, and electrolytics in the HV
oscillator and nearby HV regulator circuitry, and there are no obvious
shorted (or open) junctions. I did discover that applying the nominal
+3.9v to the base of Q548, which is part of the HV regulator, prevents
the fuse from blowing and allows the HV oscillator to be powered.
However, it still refuses to oscillate, meaning no drive for the
transformer and HV module.

At this point, I'm kinda stuck. I think I've done my Google homework
and found lots of interesting things, such as the fact that there are
a number of Tek experts on this forum, but I was unable to find the
solution to my problem. I'd be most appreciative of any pointers.

I'll try to post the relevant schematic to
alt.binaries.schematics.electronic.

Thanks.

Steve Turner

 

[Steve Turner]

Jamie <jamie_5_not_valid_after_5_Please@charter.net> wrote:

by you applying Biasing on the regulator you may have done nothing more
than shut down the Oscillator..
try to vary that voltage and see if its it gradually coming on and
your getting excessive current build up as it attempts to drive the
output module? if this is the case then you most likely have a shorted
high voltage Transformer or High Voltage tripler shorted or in many
cases the high voltage unit which has both the coil and tripler build in..

Thanks for your reply.

I did as you suggested, and found that lower bias voltage on Q548
results in a larger current through the fuse. Current is around 50 mA
at a 3.5v bias level. At this point I probed around a bit with the
scope and ... lo and behold ... the HV oscillator was oscillating
after all -!! I don't know why the difference from my original
observation. The waveforms look a bit funny though. I'm out of time
tonight but will play with this some more tomorrow.

Thanks again for your feedback.

Steve Turner

 

[Jim Yanik]

Steve Turner <srturner1@spamnet.att.net> wrote in
news:db8830lnkh3bvhodq7ruhp1is22ib1t419@4ax.com:

Jamie <jamie_5_not_valid_after_5_Please@charter.net> wrote:

by you applying Biasing on the regulator you may have done nothing
more
than shut down the Oscillator..
try to vary that voltage and see if its it gradually coming on and
your getting excessive current build up as it attempts to drive the
output module? if this is the case then you most likely have a
shorted high voltage Transformer or High Voltage tripler shorted or
in many
cases the high voltage unit which has both the coil and tripler build
in..

Thanks for your reply.

I did as you suggested, and found that lower bias voltage on Q548
results in a larger current through the fuse. Current is around 50 mA
at a 3.5v bias level. At this point I probed around a bit with the
scope and ... lo and behold ... the HV oscillator was oscillating
after all -!! I don't know why the difference from my original
observation. The waveforms look a bit funny though. I'm out of time
tonight but will play with this some more tomorrow.

Thanks again for your feedback.

Steve Turner

Did you read the circuit description in the service manual?

--
Jim Yanik
jyanik-at-kua.net

 

[Steve Turner]

Jim Yanik <jyanik@abuse.gov> wrote:

I did as you suggested, and found that lower bias voltage on Q548
results in a larger current through the fuse. Current is around 50 mA
at a 3.5v bias level. At this point I probed around a bit with the
scope and ... lo and behold ... the HV oscillator was oscillating
after all -!! I don't know why the difference from my original
observation. The waveforms look a bit funny though. I'm out of time
tonight but will play with this some more tomorrow.

Did you read the circuit description in the service manual?

Yes, several times, until I sort of understood it in a general way.
After Jamie's feedback I played with the "artificial" bias on the HV
regulator transistor and found that this level 1) dramatically affects
the current through the HV oscillator, and 2) prohibits oscillation
entirely if above a certain level. Maybe this is obvious to you but
'taint to me. At least I can get some scope waveforms now, although
they are not good waveforms and bear little resemblance to the ones in
the service manual.

Even with the HV oscillator oscillating, the other end of the
transformer doesn't show +95v like it's supposed to. Maybe the
transformer is "bad"?

As much as anything, I was wondering if anyone else had had a similar
experience with this model scope. If so, I wasn't able to find it
with Google.

I appreciate all comments. Although I'm familiar with most electronic
fundamentals in a textbook sense (having waded through Horowitz & Hill
on my own), I have zero practical experience.

Steve Turner

 

[Steve Turner]

Jim Yanik <jyanik@abuse.gov> wrote:

Did you read the circuit description in the service manual?

Forgot to mention one thing ... I'd be happy to transcribe the circuit
description here if anyone wants. (Maybe not as quickly as I'd like
though, since I have in-laws arriving for the weekend...)

Steve Turner

 

Steve,

Hi!

BTW, I also got a non-working Tek 465M 2 days ago from eBay and managed to
get it working 2 hours ago thanks to the schematic you've posted earlier .

I have similar problem like the one your describing but without the blown
fuse for the +32V unregulated line. All the +5, -5 and +32V are good. I did
check the +95V testpoint and got ~ +13V. I also checked all the HV related
components in-circuit and the +95V line for any shorts, all appeared to be
OK. I decided to take the Q552 off circuit and noticed that the B-E and B-C
forward voltage was a bit off the 0.5-0.6V range. Bought the replacement
part (2SC2527) this afternoon, soldered the transistor after dinner and
viola! it's working.

Yours might have a different component/s thats faulty. Try to check the
components off circuit.

Cheers!

Jo

Note: my email is valid once the extra 't' is removed.

Even with the HV oscillator oscillating, the other end of the
transformer doesn't show +95v like it's supposed to. Maybe the
transformer is "bad"?

 

[TekMan]

Steve Turner <srturner1@spamnet.att.net> wrote in message news:<hsra305mllcep261b86l6ggd3t0kpodrdg@4ax.com>...
:
.....snipp...

'taint to me. At least I can get some scope waveforms now, although
they are not good waveforms and bear little resemblance to the ones in
the service manual.

Even with the HV oscillator oscillating, the other end of the
transformer doesn't show +95v like it's supposed to. Maybe the
transformer is "bad"?

Either transformer bad or too much load. Latter
might be in the +95V path Or too muc hload due to partly damaged HV multiplier.

As much as anything, I was wondering if anyone else had had a similar
experience with this model scope. If so, I wasn't able to find it
with Google.

yepp, I had once a bad multiplier, causing same sympthoms.

I appreciate all comments. Although I'm familiar with most electronic
fundamentals in a textbook sense (having waded through Horowitz & Hill
on my own), I have zero practical experience.

Steve Turner

hth,
Andreas

 

I mean

Note: my email is valid once the extra 'l' is removed.

joemango@tellus.net wrote:

Steve,

Hi!

BTW, I also got a non-working Tek 465M 2 days ago from eBay and managed to
get it working 2 hours ago thanks to the schematic you've posted earlier .

I have similar problem like the one your describing but without the blown
fuse for the +32V unregulated line. All the +5, -5 and +32V are good. I did
check the +95V testpoint and got ~ +13V. I also checked all the HV related
components in-circuit and the +95V line for any shorts, all appeared to be
OK. I decided to take the Q552 off circuit and noticed that the B-E and B-C
forward voltage was a bit off the 0.5-0.6V range. Bought the replacement
part (2SC2527) this afternoon, soldered the transistor after dinner and
viola! it's working.

Yours might have a different component/s thats faulty. Try to check the
components off circuit.

Cheers!

Jo

Note: my email is valid once the extra 't' is removed.



Even with the HV oscillator oscillating, the other end of the
transformer doesn't show +95v like it's supposed to. Maybe the
transformer is "bad"?

 

[Steve Turner]

joemango@tellus.net wrote:

BTW, I also got a non-working Tek 465M 2 days ago from eBay and managed to
get it working 2 hours ago thanks to the schematic you've posted earlier .

Excellent!

I have similar problem like the one your describing but without the blown
fuse for the +32V unregulated line. All the +5, -5 and +32V are good. I did
check the +95V testpoint and got ~ +13V. I also checked all the HV related
components in-circuit and the +95V line for any shorts, all appeared to be
OK. I decided to take the Q552 off circuit and noticed that the B-E and B-C
forward voltage was a bit off the 0.5-0.6V range. Bought the replacement
part (2SC2527) this afternoon, soldered the transistor after dinner and
viola! it's working.

That is very interesting information. Your symptoms sound similar
except for the +32v fuse blowing. I may try this solution, if I can
figure out how to get underneath the mainboard without breaking
anything. Doesn't look like a trivial process.

Yours might have a different component/s thats faulty. Try to check the
components off circuit.

I guess the problem is how to do this when the mainboard is still
screwed down in the chassis. I have isolated a couple of
electrolytics (w/axial leads) by snipping one of the leads on the top
surface of the board. This can later be soldered back into place.
I'm not sure I like this approach. It's easier than dismounting the
component from the board, but I dislike connections held together by
nothing but solder...

Thanks much for the feedback.

Steve Turner

 

[Steve Turner]

and7@bigfoot.com (TekMan) wrote:

Steve Turner <srturner1@spamnet.att.net> wrote in message news:<hsra305mllcep261b86l6ggd3t0kpodrdg@4ax.com>...

Even with the HV oscillator oscillating, the other end of the
transformer doesn't show +95v like it's supposed to. Maybe the
transformer is "bad"?

Either transformer bad or too much load. Latter
might be in the +95V path Or too muc hload due to partly damaged HV multiplier.

That's what I'm afraid of. However, although the HV oscillator is
oscillating in some fashion, the waveforms are quite a bit different
than the ones shown in the service manual. So I'm not sure if the low
voltage means that the transformer or HV multiplier is faulty, or if
the transformer simply isn't being driven properly. I need to do some
more looking at this, but have house guests at the moment...

yepp, I had once a bad multiplier, causing same sympthoms.

Do you recall if you were able to find a replacement?

Thanks much for the comments.

Steve Turner

 

[Jo]

Steve Turner wrote:

joemango@tellus.net wrote:

BTW, I also got a non-working Tek 465M 2 days ago from eBay and managed to
get it working 2 hours ago thanks to the schematic you've posted earlier .

Excellent!



I have similar problem like the one your describing but without the blown
fuse for the +32V unregulated line. All the +5, -5 and +32V are good. I did
check the +95V testpoint and got ~ +13V. I also checked all the HV related
components in-circuit and the +95V line for any shorts, all appeared to be
OK. I decided to take the Q552 off circuit and noticed that the B-E and B-C
forward voltage was a bit off the 0.5-0.6V range. Bought the replacement
part (2SC2527) this afternoon, soldered the transistor after dinner and
viola! it's working.

That is very interesting information. Your symptoms sound similar
except for the +32v fuse blowing. I may try this solution, if I can
figure out how to get underneath the mainboard without breaking
anything. Doesn't look like a trivial process.

A shorted Q552 obviously will cause a blown up +32V unreg fuse. Besides a
transistor is more likely to fail than a transformer. Accessing the main board is
just the matter of locating the hidden screws. Two on the rear below the CRT neck
and two on the middle near the four screws that holds the CRT itself. The rest of
the five or so are visible and of the same typeas the other four.

Good luck!

Jo

 

[Steve Turner]

Jo <joemango@tellus.net> wrote:

A shorted Q552 obviously will cause a blown up +32V unreg fuse. Besides a
transistor is more likely to fail than a transformer. Accessing the main board is
just the matter of locating the hidden screws. Two on the rear below the CRT neck
and two on the middle near the four screws that holds the CRT itself. The rest of
the five or so are visible and of the same typeas the other four.

It looked to me at the outset that Q552 was the most likely culprit
for the symptoms I was seeing, so it was one of the first parts that I
"tested" (to my limited ability). It does not appear to be shorted
outright. However, I'm willing to believe that my testing is not
revealing the fault, so I think I'll go ahead and replace it with the
2SC2527 replacement device that you specified earlier. I don't have
much hope of locating a replacement transformer or HV module, so it
will probably be that or nothing anyway.

Thanks also for the tips on mainboard removal.

Steve Turner