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Old 18th Jan 2020, 6:18 pm   #21
PETERg0rsq
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Default Re: HP 1725 Oscilloscope HT issue

After a few hours on this I am still having a problem with R8 blowing open.

What I have done so far in investigating this:-

I have lifted R83 (the 1000R resistor in the regulator feedback line going to Q15)

I connected a PSU to the lifted end, so I can feed a controled voltage to Q15 base through the resistor.

I connected a voltmeter to the pad where R83 was lifted to see what feedback voltage I get. I have disconnected the plug from the cathode, and the lead to the EHT is disconnected. I also disconnected the focus POT, so nothing directly connected to the -2950HT.

This results in me being able to manually regulate the voltage and measure feedback voltage. In theory regulation will occure when these two voltages match.

I am also monitoring Q1 base for oscillation with an oscilloscope.

Powered it up, and at -0.3V to Q15 the PSU oscillates and I get a stable output of about 1800V. Feedback voltage is about +50mV as HT is very low.

However after about 30 seconds, R18 blows open again!

I have removed C6 (0.1uF (its an error on the drawing showing 0.01uF)) and the same thing happens.

My diagnosis is there is a resistance to ground, but I can not find it. An ohm meter shows infinate resistance, and a 1kV megger shows infinate resistance (after disconnecting a winding of the transformer)

I am running out of ideas!

Still can not work out CR7 (400V 1 amp diode) (strapped between 300V AC and -2950V DC) which effectively shorts out the two rails!
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Old 18th Jan 2020, 8:58 pm   #22
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Default Re: HP 1725 Oscilloscope HT issue

Think I have worked out CR7

If I understand correctly, the grid needs to be more negative than the cathode, so this diode is adding the -2950V rail to the -300V rail for the grid.

Starting to make sence now

So the fault could be on either rail so I need to lift CR7 to isolate this rail.
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Old 19th Jan 2020, 9:20 am   #23
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Default Re: HP 1725 Oscilloscope HT issue

"If I understand correctly, the grid needs to be more negative than the cathode" Yep, just like a normal triode. "Starting to make sence now " These scope power supply's can take some figuring out, glad to see your making progress.

Andy.
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Old 19th Jan 2020, 8:01 pm   #24
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Default Re: HP 1725 Oscilloscope HT issue

Replace CR3 and C6 and try again.
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Old 21st Jan 2020, 12:48 am   #25
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Default Re: HP 1725 Oscilloscope HT issue

Thanks

C6 has been removed from the circuit, and R4 still burns out.

I have some 20kV 5mA rectifier diodes on order, and will replace when they arrive.
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Old 24th Jan 2020, 10:51 pm   #26
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Default Re: HP 1725 Oscilloscope HT issue

I was hoping to have some good news.

Yesterday sorted out the HT PSU.

Found a polystyrene capacitor in the feedback from the HT in the regulator circuit was intermittantly giving 1.5kOhm resistance, causing the HT to "runaway".

Replaced it, tested the PSU and everything was good, getting -2950V and the grid adjustable to about 300V below this.

Today tested the scope, and nothing on the screen. Flood guns work, but no trace, not even with beam finder.

Testing shows the heater is open circuit in the tube

I did all my testing with the connector off the tube, but one of the initial flashovers must have taken it out.

So the scope is scrap

I will put it into storage and keep an eye out for a tube, but this is the 275MHz version, so not expecting to find one.

Thanks for all the support on the way, I learnt a lot, sometimes just writing about, or expressing issues can be a big help to get ideas together.

On to the next project
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Old 25th Jan 2020, 8:46 am   #27
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Default Re: HP 1725 Oscilloscope HT issue

That's a kicker, still you learned something. Perhaps write what you learnt down lest you forget.

Andy.
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