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Old 16th Oct 2017, 11:50 am   #121
OldTechFan96
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

The transformer is outputting the AC voltages is should. The resistance between one side of the transformer to the other(yellow to pink 2) is 1.359K ohms. I'll discount it as a possible source of trouble and focus on the -20V section of the power supply.
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Old 16th Oct 2017, 5:05 pm   #122
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

Hello,
HTML Code:
If there is a short to ground:
it can be such capacitor or (if has it) a tantalum...
Regards, Karl
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Old 19th Oct 2017, 4:05 pm   #123
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

I have replaced TR683, TR684 and TR681 with BC212s as two of them tested bad. The voltage on the negative end of C684 is too high at -27V. I heard a fizz like sound and shut off the scope. There was a burning smell which I'm sure came from the transformer.

I'd like to do more voltage testing but I'm concerned about damaging other parts of the scope or burning out the transformer. It's a shame as I've just bought a sweep generator.
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Old 20th Oct 2017, 12:23 pm   #124
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

I noticed that D606 was bad and replaced it. Not long after I switched on the scope I saw a small reddish glow coming from the transformer. I wonder if a bad component is causing this or the transformer is at fault?

Is it worth it trying to fix this scope?
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Old 30th Oct 2017, 3:23 pm   #125
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

I have been having another look at this scope and found that the transformer won't fizz with the 600VAC secondary removed. I think that section of transformer has developed this fault after the power supply was sorted, hence the trace disappearing. I've went through the power supply section and tested many components; all of them tested good.

My diagnosis at the moment is a faulty mains transformer.
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Old 30th Oct 2017, 4:16 pm   #126
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

If the transformer had a fault, eg shorted turns or insulation failure then disconnecting the supply to the 1.6kV and 5kV circuitry would not cure the fault condition. Somebody correct me if I am wrong but I don't think the presence of the HV power supplies, rectifiers etc has a significant effect on the voltages at the transformer windings. Adding the load does of course draw more current, but if the load current is excessive causing the transformer to overheat then the problem is in the load circuitry and not the transformer itself.

Can you confirm that what you have done here is to detach only the yellow wire from the transformer?
If you reattach the yellow wire then you can isolate the 5kV circuitry by lifting one end of C607, and in a second test you can reattach C607 and lift one end of C601 and one end of R601 to isolate the 1.6kV circuitry. Hopefully this could distinguish between faults on each of the two parts of the high voltage circuitry. Clearly you need to take extreme care with the high voltage components and wait some minutes for everything to discharge (I am sure this has been said many times already).

Also I have just noticed that the -20V regulator relies on getting -200V from D624 in order to supply the collector voltage on TR681 via the 1M collector load resistor. So if the -1.6kV supply is not working there is no -200V to TR681 and the -20V regulator will not work. Remember that capacitors and diodes in the high voltage supplies may look OK with the few Volts from a DMM but fail under high voltage stress.

Roger
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Old 30th Oct 2017, 6:44 pm   #127
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

I can confirm that the only thing that I did today is to remove the yellow secondary wire from the transformer.

I'll get on with the tests that you mentioned soon.
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Old 30th Oct 2017, 8:58 pm   #128
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

Roger,

When I disabled the 5KV line by lifting one side of C607 the transformer let out a little bit of smoke.

When the 5KV line was reconnected and the 1.6KV disconnected I did not have transformer trouble (no smoke or burning smell).

This test tells me that the fault lies in the 1.6KV section.
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Old 31st Oct 2017, 7:56 am   #129
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

Waiting for the transformer to overheat is clearly not a good diagnostic, so while the -1.6kV is disabled check that you can measure several hundred volts AC between the yellow wire from the transformer and ground. You can use this to check within a couple of seconds of applying power whether the transformer is overloaded.

Reconnect C601 and R601 and lift one end of D603. If you still have your several hundred volts AC on the yellow wire then C601, C602, D601 and D602 should be OK.

If you pass this test then reconnect D603 and lift one end of R607. At this point you are testing the remaining parts of the voltage multiplier - D603, D604, C603, 4, 5, 6 and R603, 4, 5, 6.

Do take a lot of care while doing these tests and don't ever rush when working with these high voltages.

Roger
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Old 3rd Nov 2017, 10:42 am   #130
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

Roger,

I have made the the measurements you suggested. Because of the voltages involved I used my AVOMeter 8.

The voltage between the yellow wire and ground (with the 1.6KV line disabled): 600VAC.

Reconect C601, lift one end of D603 and measure the voltage between the yellow wire and ground: 600VAC

Reconnect D603, lift one end of R607:
D603: 730V
D604: 760V

C603: 1100V
C604: 800V
C605: 500V
C606: 500V on one side, 0V on the other side.

I measured the voltage on each side of the resistors.

R603: 1100V, 1500V
R604: 1100V, 700V
R605: 700V, 350V
R606: 350V 0V

Do these numbers look all right?
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Old 3rd Nov 2017, 12:14 pm   #131
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

This all looks like the -1.6kV supply is OK with R607 disconnected, so the fault is 'after' R607. Unfortunately there is a lot of circuitry to look at!

In terms of single component falures, C615, C617 and C618 each appear to have the full 1.6kV across them. You can check them by lifting one end of each and measuring leakage but you may miss a failure that only comes in at high voltage. You can also check the Zener diodes D619 - D624 for high reverse resistance (not a forward diode check), I suspect more than one would need to fail to overload the -1.6kV. You can visually check the whole of the high voltage area for signs of tracking and damage and also do a visual check around the base of the CRT.

Needless to say all this is with the power well and truly off!

What you really need at this point is some means of safely applying a high negative voltage, but current limited, to the 'top' end of R607, the end nearer D604, and then tracing through with another meter to see where the voltage suddenly drops. Is there anyone in your area who might be able to help, maybe with the loan of an insulation tester that runs to 1kV?

Roger
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Old 2nd Feb 2018, 9:40 pm   #132
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

Out of respect for those who posted I thought that it would be appropriate to give closure to this thread.

I did not continue with the repair because of the lack of high voltage test equipment. The scope was relegated to the corner of the room where it sat for a while. During an end of year clear out I decided to strip the 'scope of any usable components. I decided to buy a Hameg oscilloscope.

I would like to give a big thank you to those who contributed as it was a fantastic learning experience for me. Especially when it comes to reading and following circuit diagrams.

Thank you
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