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Tektronix 7633 Scope, Can anyone please help Old64goat!!!
David Hess:
--- Quote from: finom1 on May 07, 2017, 01:20:11 am ---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.
--- End quote ---
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.
--- Quote ---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.
--- End quote ---
The standard DMM diode test may not work in some cases for accurate in-circuit measurements.
--- Quote from: finom1 on May 07, 2017, 01:21:49 am ---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.
--- End quote ---
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!!!
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