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Tektronix 7633 Scope, Can anyone please help Old64goat!!!

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finom1:
Any ideas or suggestions are welcome.

Can everyone share this video with people that you know that have Tektronix service experience or anyone with the capability to resolve this issue Bill is having?

Thanks everyone for trying to help!!!

David Hess:
As I recall, a pair of those heavy wires from the rectifier board go to the cooling fan.  I ended up adding an inline Molex connector to the wires going to the cooling fan in my 7603 when I removed the fan to lubricate its bearings.  My 7603 is one of the weird ones which has an engineering change notice fan which is not documented in any of the service manuals.  The fan in my 7603 matches the one in your 7633 probably meaning that both were late production units.  That may be related to the status of your high voltage board; see below.

I have marked in green what I am sure are L1198 and C1198 in the photograph below.  I am actually somewhat relieved that it is an aluminum electrolytic capacitor because I have been trying to verify the exact type without results.

However the board in the photo does *not* match the board layout shown in my scan of the 7633 service manual.  Besides other minor differences, L1198 and C1198 are swapped in position.  The 7633 was produced for a long time so it would not surprise me if some of the boards had major revisions.  Tektronix either released updated service manuals or engineering change notices describing modifications made based on serial number.  A later service manual will at least show that the board was changed in the parts list.

As far as how to proceed, I really think the whole board should be removed whether C1198 can be changed in place or not.  All of the aluminum electrolytic capacitors should be changed, the transistors in the high voltage regulator should be tested, and I would also test the high voltage diodes and maybe capacitors although the high voltage parts cannot be tested well without a high voltage low current supply; at least they can be checked for low voltage shorts.

I would try to setup the oscilloscope so the board can be operated with only the CRT heater connected or a suitable load resistor used in place of the CRT heater.  (1) Then the high voltage output can be verified before reinstalling the board.

I know the above would be a lot of work but I think it would be worth it and better than parting the oscilloscope out but of course it is up to you Old64goat.  I see that you are in the US but I have no idea of which area; maybe an EEVBlog (or TekScopes@yahoogroups.com or TekScopes2@groups.io) member who can help you out is close.

(1) The CRT heater's resistance increases significantly as it warms up (4 times?  For incandescent bulbs it is like 10 times.) and I have no idea what value it is at operating temperature so it may be better to use the actual CRT heater for testing.

finom1:

--- Quote from: David Hess on May 06, 2017, 09:05:11 pm ---As I recall, a pair of those heavy wires from the rectifier board go to the cooling fan.  I ended up adding an inline Molex connector to the wires going to the cooling fan in my 7603 when I removed the fan to lubricate its bearings.  My 7603 is one of the weird ones which has an engineering change notice fan which is not documented in any of the service manuals.  The fan in my 7603 matches the one in your 7633 probably meaning that both were late production units.  That may be related to the status of your high voltage board; see below.

I have marked in green what I am sure are L1198 and C1198 in the photograph below.  I am actually somewhat relieved that it is an aluminum electrolytic capacitor because I have been trying to verify the exact type without results.

However the board in the photo does *not* match the board layout shown in my scan of the 7633 service manual.  Besides other minor differences, L1198 and C1198 are swapped in position.  The 7633 was produced for a long time so it would not surprise me if some of the boards had major revisions.  Tektronix either released updated service manuals or engineering change notices describing modifications made based on serial number.  A later service manual will at least show that the board was changed in the parts list.

As far as how to proceed, I really think the whole board should be removed whether C1198 can be changed in place or not.  All of the aluminum electrolytic capacitors should be changed, the transistors in the high voltage regulator should be tested, and I would also test the high voltage diodes and maybe capacitors although the high voltage parts cannot be tested well without a high voltage low current supply; at least they can be checked for low voltage shorts.

I would try to setup the oscilloscope so the board can be operated with only the CRT heater connected or a suitable load resistor used in place of the CRT heater.  (1) Then the high voltage output can be verified before reinstalling the board.

I know the above would be a lot of work but I think it would be worth it and better than parting the oscilloscope out but of course it is up to you Old64goat.  I see that you are in the US but I have no idea of which area; maybe an EEVBlog (or TekScopes@yahoogroups.com or TekScopes2@groups.io) member who can help you out is close.

(1) The CRT heater's resistance increases significantly as it warms up (4 times?  For incandescent bulbs it is like 10 times.) and I have no idea what value it is at operating temperature so it may be better to use the actual CRT heater for testing.

--- End quote ---
David, Bless you for all this detailed information. Great picture, worth a thousand words.

Thank you for sharing your wisdom with Bill!!!

finom1:
New message from The Old64goat:

I have a question for David Hess:
===================================================================
Here is a quote from David.

"I would try to setup the oscilloscope so the board can be operated with only the CRT heater connected or a suitable load resistor used in place of the CRT heater.  (1) Then the high voltage output can be verified before reinstalling the board"

===================================================================
Do you mean run this HV board on the bench?
I had thought of that but there are so many plugs disconnected from that scope I can't see how I could do it.
I would have to ground the case, I would have to feed the +15 volt unregulated voltage to and and who knows where to insert them.
Other than the +15 volts do I need any other voltages?
If so where do I insert them?
I can use some clip leads from the low voltage rectifier board once I find what they are.
BIG complex job to say the least.
I am not having much luck testing the transistors, too hard to see the pins and of course I am testing them in circuit with my DMM on "diode" test.
David, once again THANKS for all your help

finom1:
Another update from The Old4goat:

OKAY here is what I found now that I can see the board clearly:
C1198 which should be 47uf reads 44.1uf on my Chinese digital capacity handheld meter.
There is no leakage spilling out of that cap, in fact everything on that board is very clean except around the red HV lead and the components near it, there is a very light grey deposit like you find near any HV anode lead.
Under the board there is a yellow  .1uf @600 volts, I tested it in circuit like I did with C1198 and it reads very close to .1uf.

Thank you for all your help.

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