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Old 25th Nov 2019, 12:32 pm   #1
line sync
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Default AVO CT160, testing PZ30 rectifiers.

I wonder if anyone could help me with a problem I`m having with this valve tester.
I was testing some PZ30,s recently and they all make the buzzer sound and meter light up red.
I was going to throw these valves away but thought before I do I`ll try a new boxed one and the same happened as with the used ones.
I`ve since tried another new PZ30 and get the same result.
If I reduce the anode volts from 120 to 60ma all the valves pass the test but on checking the valves specifications each half of the rectifier is rated at 200ma so why is the tester behaving like this ?
The selector switch numbers I`m using are 029183210.

Robin
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Old 25th Nov 2019, 3:59 pm   #2
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Default Re: Avo CT160, testing PZ30 rectifiers.

Hi,

Did you test the valve as per the text in paragraph 10 on page 21 of the Instruction manual (the one from 1966 has the diode/rectifier test on page 21)?

I've included a screenshot of the text here.

/Martin
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Old 25th Nov 2019, 6:14 pm   #3
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Default Re: Avo CT160, testing PZ30 rectifiers.

Sorry, that screen shot was from the Mk IV manual, here is the correct one.
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Old 25th Nov 2019, 10:26 pm   #4
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Default Re: AVO CT160, testing PZ30 rectifiers.

I'm not familiar with the tester in question, but to my way of thinking if you're testing a rectifier with 200mA-per-anode rated ability you really need to see evidence of emission to deliver 200mA peak current on the tester's meter.
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Old 25th Nov 2019, 11:19 pm   #5
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Default Re: AVO CT160, testing PZ30 rectifiers.

Would many valve testers be capable of supplying that sort of current anyway? They could do the typical leakage, gas and so on but 200mA forward current sounds like a tall order. Perhaps a test lead could be made up to allow the tester to power the heater, and use a typical LV bench power supply to see what forward voltage is necessary to drive 200mA through one A-K assembly as a guide to valve health?
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Old 26th Nov 2019, 12:11 am   #6
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Default Re: AVO CT160, testing PZ30 rectifiers.

The CT160 and Mk2 provided up to 120mA per anode. The Mk3, Mk4 and 163 all went up to 180mA per anode.

And this http://www.roehrentest.de/RoeTest10.html does up to 300mA.

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Old 26th Nov 2019, 10:50 am   #7
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Default Re: AVO CT160, testing PZ30 rectifiers.

Silly question maybe, but have you tested different rectifiers that also use the 120mA range? I ask this because my ct160 can occasionally trip when testing a perfectly good valve. Sometimes just changing the d1 to d2 switch will cause this to happen. Never seen it happen on the 60mA test though.
I think maybe the trip is a little bit too sensitive at full load.
Rob
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Old 26th Nov 2019, 11:38 am   #8
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Default Re: AVO CT160, testing PZ30 rectifiers.

Hello Robin, testing rectifiers such as the PZ30 on valve testers will only give very variable and unreliable results.
These valves were prone to flashover and inter electrode shorts. The best test is in a known working chassis, even better if half the PZ30 is employed as a reclaim rectifier.
Let them run for maybe half an hour then give the bulb a very gentle tap to promote cathode flaking. If all is OK then it should be a reliable valve.
It's worth an internal examination looking for white flecks loose in the valve. These are particles of the cathode emissive material, a sure sign of internal flashover.

I have quite a collection of duff ones including new examples with the dreaded yellow colour markings from around 1964. The very early ones with a different anode assembly had a very short life and TV repair guys that were working when the Pye B18T, Ekco TS88 and the Bush TV12 were popular told me that they replaced large numbers. The later production were a lot more reliable. John.
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Old 26th Nov 2019, 7:17 pm   #9
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Default Re: AVO CT160, testing PZ30 rectifiers.

Thank guys for your comments.
Martin , I am testing the valve the way it says as per your attachment.
I`ve had the tester over 25 years and probably tested over a thousand valves on it , so I know it quite well.
The PZ30 is a full wave rec and each section is capable of suppling 200ma , so that's 400ma total.
I cannot understand why it would fail a test on 120ma !
I found the data on this valve yesterday to check if the selector switch numbers were correct in the avo book and they were , I have found a few wrong over the years.
Rob , good idea to try testing other rectifiers but those are not very accessible at the moment .
(hks) johns idea is the best and that is to try the valves in a working tv set.

Robin
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Old 26th Nov 2019, 9:09 pm   #10
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Default Re: AVO CT160, testing PZ30 rectifiers.

Ok, good to hear that you test it correctly.

Apart from other reasons explained here for the overload to trigger the voltages used in the AVO to test rectifiers means that this rectifier quickly reaches overload conditions. Just look at the datasheet and check the schematic for the CT160 to see what voltages are present and what load the AVO represents. You can also use a DMM or a scope (via an isolation transformer) to check the voltages present across the valve or measure the current flowing through it to see when it reaches overload conditions.

/Martin
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