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

[finom1]

Here is another update from the Old64goat:
"David, I know you had mentioned you did not know how to get out the HV board but do you know if I have to remove the Z-AXIS and the other board on the side of the HV box?

You had asked me if I had a capacity bridge, yes I have a SPRAGUE Telo-Mike TO-5.

However I do not own a transistor tester so all I can do is use my SIMPSON 260 to test the transistor for shorts.

I was getting some sine wave oscillation so could that mean the 2N3055 is good?

If so I do not have to remove it, I need to buy some heat sink compound and a mica washer for the 2N3055.
IF I can get the HV board "box" out I will replace C1198."

Thank you for all your help!!!

[finom1]

More pictures from the Old4goat:

Does anyone know how to pull out the HV Board?

[finom1]

Few more pictures:
Looking for any help removing the HV Board!!!
Thanks again.

[David Hess]

"David, I know you had mentioned you did not know how to get out the HV board but do you know if I have to remove the Z-AXIS and the other board on the side of the HV box?

I took a quick look at my 7603; it uses the same enclosure over the high voltage board.  The enclosure is held on by two side facing screws along the top where the z-axis board is and one down facing screw on the bottom.  In order to gain access to the bottom screw, the power supply has to be pulled out from the back.

You might be able to leave the z-axis board connected and only have to temporarily unmount it to get to the top two screws.

You had asked me if I had a capacity bridge, yes I have a SPRAGUE Telo-Mike TO-5.

That is one cool piece of test gear however I was reading the manual and:

2.1.2 CAUTION.  The TO-5 should NOT be used to measure the capacitance
of small transistor electrolytic capacitors with low doc voltage
ratings, or to check the I-R of transistor ceramics rated 50 volts or less.
Such capacitors require special test equipment to avoid damage during
testing.

However I do not own a transistor tester so all I can do is use my SIMPSON 260 to test the transistor for shorts.

That is usually sufficient because most transistor failures are shorts or opens; I always use the diode test function on my DMMs to test the two transistor junctions separately and then do a base shorted to collector test.  Do not use the Rx10,000 range on your Simpson 260 for testing transistors; the 7.5 volt potential is enough to cause reverse base-emitter breakdown damaging the transistor.

I was getting some sine wave oscillation so could that mean the 2N3055 is good?

Yes, I think so.

If so I do not have to remove it, I need to buy some heat sink compound and a mica washer for the 2N3055.

The mica washer should survive.  The high voltage board is mounted to the inside of the enclosure so the TO-3 transistor goes with it.  Possibly you will not have to remove the 2N3055 but that can be determined after you have the enclosure pulled back.

IF I can get the HV board "box" out I will replace C1198.

Replace C1198 and test the other transistors.

Looking at the schematic again, I now think Q1190 may still be good but test the transistors anyway.  Possibly the only failure is C1198 and that capacitor is known to wear out in this design.

[finom1]

"David, I know you had mentioned you did not know how to get out the HV board but do you know if I have to remove the Z-AXIS and the other board on the side of the HV box?

I took a quick look at my 7603; it uses the same enclosure over the high voltage board.  The enclosure is held on by two side facing screws along the top where the z-axis board is and one down facing screw on the bottom.  In order to gain access to the bottom screw, the power supply has to be pulled out from the back.

You might be able to leave the z-axis board connected and only have to temporarily unmount it to get to the top two screws.

You had asked me if I had a capacity bridge, yes I have a SPRAGUE Telo-Mike TO-5.

That is one cool piece of test gear however I was reading the manual and:

2.1.2 CAUTION.  The TO-5 should NOT be used to measure the capacitance
of small transistor electrolytic capacitors with low doc voltage
ratings, or to check the I-R of transistor ceramics rated 50 volts or less.
Such capacitors require special test equipment to avoid damage during
testing.

However I do not own a transistor tester so all I can do is use my SIMPSON 260 to test the transistor for shorts.

That is usually sufficient because most transistor failures are shorts or opens; I always use the diode test function on my DMMs to test the two transistor junctions separately and then do a base shorted to collector test.  Do not use the Rx10,000 range on your Simpson 260 for testing transistors; the 7.5 volt potential is enough to cause reverse base-emitter breakdown damaging the transistor.

I was getting some sine wave oscillation so could that mean the 2N3055 is good?

Yes, I think so.

If so I do not have to remove it, I need to buy some heat sink compound and a mica washer for the 2N3055.

The mica washer should survive.  The high voltage board is mounted to the inside of the enclosure so the TO-3 transistor goes with it.  Possibly you will not have to remove the 2N3055 but that can be determined after you have the enclosure pulled back.

IF I can get the HV board "box" out I will replace C1198.

Replace C1198 and test the other transistors.

Looking at the schematic again, I now think Q1190 may still be good but test the transistors anyway.  Possibly the only failure is C1198 and that capacitor is known to wear out in this design.

David,
Thank you for all your kindness helping the Old4goat "Bill" out fixing his scope!!!

[finom1]

Another update video from Bill:
Z -AXIS BOARD REMOVAL  -

Thanks everyone for all your help!!!

[finom1]

Can someone please instruct the Old4goat on the next steps to take?

Thank you!!!

[David Hess]

I forgot about the readout board position on your 7633.  You definitely had to remove it.

I cannot see it on your video but on my 7603, there are definitely 2 screws behind the z-axis board which hold the cover on so you have to find a way to get that last screw holding the z-axis board to the high voltage cover removed.  I used a screwdriver with a thin shaft but larger head.  I suppose you could notch that aluminum rail above the screw to gain better access.

I think the screws on the side of the high voltage cover are holding the high voltage board to the inside of the cover.

The 7603 is a little easier to deal with because the readout board is mounted where the 7633 has its storage board which is forward of the high voltage cover.  That was a great drawing that you made for reattaching the cables.

Tektronix packed a lot into the 76xx series oscilloscope chassis although the only major addition to the 7633 is the storage board which goes where the 7603 readout board was.

[finom1]

I forgot about the readout board position on your 7633.  You definitely had to remove it.

I cannot see it on your video but on my 7603, there are definitely 2 screws behind the z-axis board which hold the cover on so you have to find a way to get that last screw holding the z-axis board to the high voltage cover removed.  I used a screwdriver with a thin shaft but larger head.  I suppose you could notch that aluminum rail above the screw to gain better access.

I think the screws on the side of the high voltage cover are holding the high voltage board to the inside of the cover.

The 7603 is a little easier to deal with because the readout board is mounted where the 7633 has its storage board which is forward of the high voltage cover.  That was a great drawing that you made for reattaching the cables.

Tektronix packed a lot into the 76xx series oscilloscope chassis although the only major addition to the 7633 is the storage board which goes where the 7603 readout board was.

Thank you David for helping!!!

[finom1]

This was posted on Old4goat's youtube channel:
"Zeebee19718 hours ago
High voltage compartment dismantling guide: page 4-22 (139 of 320 in pdf document).

HIGH-VOLTAGE COMPARTMENT
The components located in the high-voltage compartment can be reached for maintenance or replace-ment by using the following procedure. Diagram 7 shows the location of components in the high-voltage compartment and color-coding of the wires.
NOTE All solder joints in the high-voltage compartment should have smooth surfaces. Any protrusions may cause high-voltage arcing at high altitudes.
1. Remove the heat radiator/Low-Voltage Regulator assembly as described previously.
2. Disconnect the crt base socket.
3. Disconnect the crt anode plug and discharge it to the chassis. Using an insulated probe or wire, discharge the jack portion of the crt anode connector to chassis ground.
4. Disconnect the multi-pin connectors on the Z-Axis Amplifier board.
5. Remove the screw on the bottom of the high-voltage compartment and the two screws located at the top.
6. Guide the high-voltage compartment away from the instrument chassis. Be careful not to damage any of the components or the pin connectors on the High-Voltage or Z-Axis Amplifier circuit boards. Dis-connect the multi-pin connectors on the High-Voltage board.
7. Using an insulated shorting strap, discharge the exposed connections to chassis ground.
8. Disconnect plug P1196 and the four screws which secure the High-Voltage board to the high-voltage compartment. Now, all of the circuitry in the high-voltage box can be reached for maintenance.

4-22 REV. B, DEC. 1974

Low voltage PSU dismantling guide: page 4-16 (133 of 320 in pdf document).

REGULATOR CIRCUIT BOARD REPLACEMENT.
 1. Remove the four screws which secure the protective cover to the rear of the instrument. Remove the protective cover.
 2. Disconnect the pin connectors.
 3. Remove the four screws which secure the Regulator and Fan-Motor subassembly cover to the rear of the instrument.
 4. Remove the two screws which secure the Fan Motor, and remove the
Fan Motor.
5. Remove the four screws which secure the plate to the heat radiator. Remove the plate.
6. Remove the power transistors from the back of the heat radiator. Note the location of each power transistor.
7. To replace the Regulator board, reverse the order of removal. NOTE After replacing the power transistors, check that the transistor cases are not shorted to the heat radiator before applying power.

4-16 REV. B, DEC. 1974?"

Great detailed information, thank you for sharing!!!

[finom1]

Update from the Pld64goat:
I Am Going IN!!

[guido]

It's interesting to get these out, i've done a few. Start at the back: 1. Remove the heat radiator/Low-Voltage Regulator assembly as described previously. Getting one of the mounting screws out is fun, getting it back in again is even more fun..

Also interesting is if you start swapping the PCB on top of the HV between mainframes (7603 i think it were). There are two versions, where the 15V connector from the powersupply has + and - swapped (not the arrow on the pcb, but the traces on the pcb). And one scope had a "straight" cable and the other one a "crossed" cable to match the PCBs. So i had to swap the pcb and the cable from one scope to the other. If you only swap the pcb it won't work. I know from experience. 

[David Hess]

The more I watch Old64goat's videos and study the 7633, the more I want a 7633 or 7623B.  But I already have a 7603, two 7834s, and other 7000 mainframes so it is difficult to justify.

[Electro Detective]

Mr Old64goat is officially in the 'last man standing' tech arena now, about to face unknown adversaries         

Let's pray he finds a BBQd resistor or two, some dry solder joints, or an easy to replace leaking capacitor, and end this painlessly   

[finom1]

New update from the Old64goat:
It Is Looking More And More Like a Failure -

Please help him if you can!!!

Thank you for helping!!!

[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]

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.

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.

[David Hess]

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.

That is exactly what I mean but the power supply is going to need to be connected to the high voltage board.  The high voltage inverter requires +15 volts unregulated, +5 volts, +15 volts, +50 volts, and +130 volts.

It might be possible to get away without doing this if the high voltage board is extensively tested and refurbished.  The problem is knowing that it is working correctly before reinstalling it.

If I was desperate to avoid a working test of the high voltage inverter while it is removed, then I would change C1198, unambiguously test the transistors in the regulator, test for shorted high voltage capacitors and diodes, test for a short between the high voltage transformer secondaries, and disconnect the input to the high voltage multiplier.  Then I would reinstall the high voltage board and see what happens.  The CRT will not work properly without the PDA but if the -1475 cathode voltage comes up, then something will be visible.

At that point if the CRT works, then the high voltage multiplier may or may not be shorted and it would need to be reconnected to test it.

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.

The standard DMM diode test may not work in some cases for accurate in-circuit measurements.

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.

It is possible although unlikely that the capacitor has excessive ESR without having lost a lot of capacitance.  If it was unambiguously bad, then that could save a lot of time and effort but now we have to consider if there is a more serious problem elsewhere.

The common problems I have read about (1) with Tektronix high voltage inverters include:

1. Leaking or shorted high voltage capacitors or diodes.
2. Leaking or shorted high voltage multiplier.  This is just a special case of #1.
3. Worn out C1198.
4. Bad transistor in the high voltage regulator.
5. Leaking or shorted high voltage secondary on inverter transformer.

I am suspicious in your case that fuse F814 was not blown.  To me that points to a problem with the high voltage regulator because if there was no output, it should drive the inverter very hard blowing the fuse.  On the other hand, absolutely no output at the -1475 test point indicates a short in the high voltage circuits.

(1) I have lots of Tektronix oscilloscopes and none have suffered a high voltage inverter failure and the ones I have bought just to have fun fixing did not have high voltage problems.

[finom1]

Update from Old4goat, here we go:
I Did Not Give up Yet -

You can do it!!!

[finom1]

Dave Hess,
What does Bill have to do next?

This will be such a great success story!!!

With your help he will succeed.

Thank you for sharing!!!

[David Hess]

I had some fun brushing up on my Photoshop skills.  Here is a slightly cleaned up scan of the high voltage board schematic showing what parts I would initially check in circuit for shorts and opens at low voltage.  The board layout I have does not quite match the newer board you have so hopefully you can find everything.

Pay real close attention to C1228 toward the right side of the schematic and check it first.  R1226A and R1226B are the feedback divider which sets the -1475 output voltage.  If C1228 was shorted, then the regulator would hold the output voltage at about 1 volt and the inverter at a very low power level without blowing the fuse which is what you observed.

[finom1]

New update from the Old4goat:
I Am At A BRICK Wall -

Thank yo for any help you can give him!!!