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Tektronix 7633 Scope, Can anyone please help Old64goat!!!
finom1:
From youtube:
Zeebee19712 hours ago (edited)
Diodes CR1202, CR1203 type VG-12X http://pdf.datasheet.live/d016da9f/microsemi.com/VG-12X.pdf
"Improved" Tektronix voltage doubler schematic: https://ibb.co/hhSCY5
You can use other transformer secondary lead to connect ac test voltage to the doubler but it is risky without current limiter (very thin secondary coil wire). You can also use signal generator and connect it to the primary side, but you don't have a HV probe (you can control filament voltage only).?
David Hess:
--- Quote from: finom1 on May 11, 2017, 06:06:02 pm ---Diodes CR1202, CR1203 type VG-12X
--- End quote ---
The high voltage diode selection is not all that critical. They need to be high voltage, voltage derated, and fast recovery.
--- Quote ---You can use other transformer secondary lead to connect ac test voltage to the doubler but it is risky without current limiter (very thin secondary coil wire). You can also use signal generator and connect it to the primary side, but you don't have a HV probe (you can control filament voltage only).?
--- End quote ---
I like this idea but it may be perfect for Old4goat in a different form.
Disconnect Q1195 and drive the collector winding with a standard function generator. Now the input voltage can be carefully controlled and monitored and the output voltages can be measured. No load will be needed on the CRT winding and no other connections to the board will be needed or are even desired; disconnect everything.
If there is a low voltage problem on the secondary side which seems likely, then it will be possible to track down without using a high voltage probe.
Termination of the function generator's output is not required because of the low frequencies of up to about 40kHz however function generators usually have a 50 ohm series termination limiting them to about 300 milliamps into a short. This should be enough but if it is not, then a small single ended push-pull amplifier will help drive the inverter transformer to higher outputs.
finom1:
New update from The Old64goat:
Any help would be greatly appreciated!!!
oldway:
I have not intervened until now because I have repaired many analogue oscilloscopes of different brands, including Tektronix, but never a 7633.
For me, there are only two possible faults: faulty transformer or faulty voltage doubler.
You worry about things you do not have to worry about: desolder the wire of the voltage doubler that goes to the transformer, whatever the insulation there is on it. You don't have to worry about this insulation !
Power on the scope with the HV board without the voltage doubler and check if you have the - 1475V on the cathode of the tube.
There is no problem to let this auto-oscillator working without load because it is not a flyback and it has a regulation circuit of the -1475V voltage.
If you have the correct -1475V voltage, it is the voltage doubler that is defective, if not, it is the transformer that is dead.
Since there are many connectors on this HV board and that there is a risk of error in reassembling the scope, David Hess's proposal is also a good option.
Again, desolder the wire of the voltage doubler that goes to the transformer.
It seems to me that 40Khz is a bit high, I believe that these auto - oscillators work rather at a frequency close to 25 Khz.
Connect the probe (x10 or x100) of your oscilloscope between the ground and the terminal where you have desoldered the voltage doubler wire and feed the transformer primary as you do with 25Khz, but starting from 0V ... (filament of CRT NOT connected !)
Slowly increase the output voltage of your function generator and check the voltage at the secondary with your oscilloscope.
If you have a very low voltage or nothing, the transformer is faulty.
Nb: some scopes as Philips PM3264 have well known issues of corona discharges, but their HV voltage is 17KV's, not 7KV's as this Tektronix 7633.
David Hess:
--- Quote from: oldway on May 12, 2017, 06:48:49 am ---For me, there are only two possible faults: faulty transformer or faulty voltage doubler.
--- End quote ---
These two faults would normally be my first suspicion as well except for the following:
1. There is little or no history of transformer faults in the 76xx series storage oscilloscopes. (1)
2. There is little or no history of high voltage multiplier faults in the 76xx series storage oscilloscopes. (1)
3. If there was a transformer short or a high current short on the secondary side, then the high voltage regulator circuit should drive Q1195 hard enough to blow fuse F814.
4. Nothing was measured on the secondary. Even with a short, I would have expected something on the secondary side if the primary side was oscillating at all which it apparently was but at a very low level.
Now maybe there are two faults with a shorted transformer or output and damage to the high voltage regulator circuit. One could have caused the other.
One odd thing I noticed about the 7633 high voltage inverter is that unlike the 465 series high voltage inverter which is practically identical, the 7633 implementation has no primary side over-voltage protection. I wonder if they did not think it was needed or left it out to save space. I mention this because if it ran away, then the transformer or secondary side would likely have been damaged.
--- Quote ---It seems to me that 40Khz is a bit high, I believe that these auto - oscillators work rather at a frequency close to 25 Khz.
--- End quote ---
I could measure the frequency of the high voltage inverter on my 7603 but it uses a different configuration with balanced drive using two 2N3055s. The 465 series which uses practically the same inverter configuration as the 7623A and 7633 operates at roughly 50kHz.
(1) Maybe there is no such history because the high voltage inverter and multiplier are so difficult to access and diagnose that people give up on it.
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