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Old 17th Aug 2019, 1:02 pm   #1
IsquaredR
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Default No AC measurement on HP 410B VTVM

My Hewlett Packard HP-410B vacuum tube voltmeter is quite a challenge.

It was working then after setting for a couple months stopped indicating on AC. It will not ZERO on AC nor will it indicate any AC voltage on any range. I checked the suggested items on the manual's troubleshooting chart. I tried replacing V2 and V3 (12AU7) with no effect. I did find that the outer shield of the AC cable had shorted which I cleaned-up and redressed. My megger indicates there is more than 5,000 megohms resistance between the outer and inner shield now so I am confident it is good. I substituted a replacement for the probe tube (2-01C) with no effect. I substituted a new 6-4 ballast tube with no effect. My voltages coming out of the 6X4 rectifier are right where they are supposed to be. The only voltage I can find that is off (compared to the schematic) is the plate voltage on V2A and V2B which is 17 instead of 22 volts. Conversely the plate voltage on V3A and V3B is high, 195 VDC instead of 165 VDC.

I've checked continuity through the SELECTOR switch which is OK. The meter balance potentiometer R19 was checked and its resistance 200 ohms full rotation.

Resistors R53 (330 ohms) and R42 (100 ohms) are within 5 ohms. This VTVM is in the 28000 serial number so, following the manual backdating suggestion I substituted a 2200 ohm resistor for the 330 ohm resistor but it had no effect.

The VTVM zeroes nicely on DC+ and DC- and works well on the resistance ("Ohms") function.

Can anyone give me any suggestings as to where to check next?
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Old 17th Aug 2019, 2:35 pm   #2
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Default Re: No AC measurement on HP 410B VTVM

Are the probes diode heaters working?

Lawrence.
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Old 17th Aug 2019, 4:33 pm   #3
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Default Re: No AC measurement on HP 410B VTVM

Hi Lawrence,

I just checked the heaters in the probe and they were receiving 5.0 VAC and the
tube was getting warm.
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Old 17th Aug 2019, 4:40 pm   #4
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Default Re: No AC measurement on HP 410B VTVM

All I can think of at the moment if the selector switch contacts are ok is something to do with the probe or the probes cable, can we assume that the probes resistor and capacitors are ok including C3?

Schematic etc for anyone that's following:

http://hparchive.com/Manuals/HP-410B...prefix-024.pdf

Lawrence.
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Old 18th Aug 2019, 1:29 am   #5
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Default Re: No AC measurement on HP 410B VTVM

Thank you Lawrence. I have checked C3 (and actually even substituted another one just in case) and it did not change anything. I'm not sure how I could test or replace any of those in-probe capacitors or resistors as they are built in. Maybe my best option would be to find another probe. By the way, I did do a continuity test on the SELECTOR switch segments and everything was good in the AC position. This one's really got me baffled.
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Old 18th Aug 2019, 2:26 am   #6
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Default Re: No AC measurement on HP 410B VTVM

I have a complete HP 410 EXCEPT for the meter movement.

You can have the leads for the postage.

I have all the internals as well. This unit was not stored too well, hence the movement dying.

Joe
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Old 18th Aug 2019, 6:38 am   #7
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Default Re: No AC measurement on HP 410B VTVM

The meter unit is an entirely DC machine and the probe rectifies AC. So you could try injecting a little DC at the probe output from a small battery and see if the meter senses it.

There's a 22 meg resistor in series with the probe output that might have gone high/open.

David
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Old 18th Aug 2019, 10:19 am   #8
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Default Re: No AC measurement on HP 410B VTVM

The AC probe on my HP 410C, which uses a probe that looks the same as the 410B, had a poor connection between the input capacitor C1 and the gold spike anode connection of the EA53 diode. I found that a tiny, very thin brass shim between the spike and the capacitor was sufficient to restore good contact. I had previously changed the EA53, which must have been down to air as it got hot but passed no current.

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Old 18th Aug 2019, 11:24 pm   #9
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Default Re: No AC measurement on HP 410B VTVM

Thank you Ron. I will look into that. But I am perplexed as to why I cannot even get the meter to zero on AC. The meter pegs full left and I must rotate the AC ZERO potentiometer full right to get the needle slightly (1 or 2 indices) above "0."

I did check C3 for value and leakage and it was fine. I also checked the 22 Megohm resistor in the probe and it read 25 megohms.The 500 pF capacitor in the probe measured 650 pF. So I am thinking all those components are not suspect.

I also checked the RANGE and SELECTOR switches for continuity on the AC circuit and they all checked good and responded properly when rotating the switches.

This is a real head-scratcher! :0
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Old 19th Aug 2019, 8:50 am   #10
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Default Re: No AC measurement on HP 410B VTVM

I think that the AC zero control is used to balance out the voltage offset caused by the contact potential of the EA53 diode. If the offset control is not centred, I presume that the contact potential that it is supposed to null out is missing. Although you have changed the EA53, is the replacement working correctly? Maybe you could make a simple test fixture to check that the diode has some emission and will conduct its rated current.

Ron
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Old 19th Aug 2019, 9:09 am   #11
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Default Re: No AC measurement on HP 410B VTVM

The coax cable on these probes breaks down and becomes conductive between the cores. It happened to mine. There was an Ebay seller in the US that sold replacements but in my case I found a similar sized twin coax and used that instead.

Al
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Old 19th Aug 2019, 1:33 pm   #12
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Default Re: No AC measurement on HP 410B VTVM

Thank you Ron and Alistair.

The EA53 (2-01C) that I bought a few years ago was sold to me as "new" but I am concerned that it might not be good after all. I would like to rig up some sort of test fixture to verify its operation and am going to solicit information on how to do that. I've got the EIMAC data sheet and perhaps I can match its parameters up and just alligator-clip it into my Hickok tube tester. I'm looking into that.

Alistair, although I am confident I have cleaned-up the braid-to-braid short I also have a couple unknowns which would be obviated by replacing the triax cable: internal breakdown or carbon tracking as well as something where the cable enters the probe that I cannot see. Once I verify the diode is working tthat will be my next stop.

Late last night I discovered something else that has left me wondering: The wire wound potentiometer (R50) used in the ballast/diode heater circuit shows a short to ground. When I removed that pot it looks like there is a small piece of cardboard separating the wirewinding from the metal body of the pot which is bolted to the chassis. There is only one set of wires connected to one of the contacts on the poteniometer so obviously the return must pass through the body of the potentiometer to ground. But in my simplistic way of thinking it seems like the windings should be above ground potenial with another wire going to the center pin. Although it appears to adjust the 5 VAC OK I am wondering if there might be leakage that is throwing things off.

The journey continues...
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Old 19th Aug 2019, 2:04 pm   #13
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Default Re: No AC measurement on HP 410B VTVM

Hi!

If you've got an ordinary radio–type signal generator, or a working valve set, you might be able to try one half of an EB91 on short wire leads to see if the meter responds to AC on this – if you use a low frequency of about 1 Mc/s or less the strays shouldn't cause too much of a problem!

Connect pins 1, 3 & 6 on the EB91 to the earthy/H.T.– negative rail of the probe, pin 2 to the probe–tip and 4 to the probe's 5V LT supply connection, then try the o/p from your signal generator or the osc. grid pin of a valve radio f.c., if you then get some meter deflection, you'll be able to get the correct EA53 coaxial u.h.f. diode n.o.s. with the assurance that the expensive cost of an EA53 isn't wasted!

EB91 pinout:– 1:– k', 2:– a", 3:– h, 4:– h, 5:– k", 6:– s, 7:– a'

If you get no reading with a temporary EB91 setup, there's still another fault, most likely in the cable and/or switching, that still needs tracing!

Chris Williams
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Old 19th Aug 2019, 2:45 pm   #14
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Default Re: No AC measurement on HP 410B VTVM

Thank you Chris, that is a grand idea! On this of the Atlantic EB91s are more available as 6AL5s which I am sure I have and can rig up in something or other.
It would be awfully nice to know if my "NOS" 2-01C is indeed good or am I chasing windmills (as in Don Quixote).
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Old 19th Aug 2019, 8:36 pm   #15
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Default Re: No AC measurement on HP 410B VTVM

Did the little spring contact fall out when you replaced the 2-01C or did you change from a EA53 to the 2-01C? The spring & contact is supposedly slightly different for each type.

EA53
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2-01C
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Quote:
From the manual changes section of the service manual;
Two different types of probe diode tubes are available for VI: the 2-01C (HP stock
No. 1920-0008) manufactured by Eitel-McCullough, Inc., and the EA53 (HP Stock
No. 1920-0010) manufactured by Amperex Electronic Co. These tubes are interchangeable;
however, each type requires a different plate contact and attaching spring, and a
different heater voltage. The proper plate contact and spring are supplied with each
replacement diode (HP Stock No. 410B-21P for the EA53; 410B-21N for the 2-01C).
The proper voltage for the 2-01C is 5 volts rms; for the EA53, 6.3 volts rms. When
the type of diode is changed, minor adjustment of R50 may be required to obtain exact
heater voltage.
David

Last edited by factory; 19th Aug 2019 at 8:43 pm. Reason: Added pictures
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Old 20th Aug 2019, 12:32 pm   #16
IsquaredR
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Default Re: No AC measurement on HP 410B VTVM

Greetings David...This one was originally furnished with the 2-01C and the little spring contact that attaches to the anode of the tube is present. It more or less "screws" onto the anode so it remains intact when removing the base of the probe. Wish finding my problem was that easy.

Chris, today I am going to try the 6AL5/EB91 "assessment." I'm anxious to see if the meter comes alive on AC and will post my results here.

As a follow-up to my previous posting in which I mentioned that the gain potentiometer (R50) showed a short to ground, it's only a 25 ohm pot and I substituted another one and there was no change. AC still dead. So THAT isn't the problem.
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Old 20th Aug 2019, 7:52 pm   #17
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Default Re: No AC measurement on HP 410B VTVM

Hi!

I suggested the EB91/6AL5 as it's a very cheap, plentiful and standard small valve diode actually used in countless pieces of valved T & M equipment, including a.c. probe circuits identical to those in the 410 – altho' the stray and self–capacitances of this valve and it's holder mean the h.f. response won't be as good as the OEM's u.h.f. coaxial EA53/2C–01 assembly, but for simply proving the point for fault–tracing it's quite adequate!

If you suspect the twin coaxial cable for insulation–leakage, simply disconnect it's ends (both outer screen and inner cores) at the hp410 end AND ALSO at the probe end, and connect the cable outer braid to the positive h.t. line of your meter, (the anode pin 1 of V3 (12AU7 or ECC82 or pin 7 (k) of V5 (6X4 or EZ90) will do for this test!) then join the coaxial cable inner cores to H.T.– (tag 2 of mains transformer h.t. secondary) via a 1M resistor and nothing more, then measure the voltage across the 1M connected to the inner cores – if the cable insulation is sound you should get negligible voltage reading across the 1M cable test resistor.

Any appreciable d.c. readings on the inner cores with the braids connected to H.T.+ means the coaxial cable's inner insulation has degraded and you'll need to source a new length – I've not looked up the exact make of cable, but any good–guality polythene–insulated type will do for replacement purposes – you might have to bundle two pieces of thinner cable if you can't locate twin co–axial!

Chris Williams
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Old 20th Aug 2019, 8:17 pm   #18
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Default Re: No AC measurement on HP 410B VTVM

Not cheap at $35 but this company is advertising the cable specifically for the 410B/C. http://www.kiss-electronics.com/products.htm

I think it was originally on this site that I read about the insulation breakdown issue.

Al
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Old 20th Aug 2019, 10:37 pm   #19
IsquaredR
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Default Re: No AC measurement on HP 410B VTVM

Thanks Chris5500. I did the 6AL5 assessment and AC remained dead; no reading, meter movement or meter zeroing possible. I am grateful for your guidance on checking the triaxial coax. Right now I am going back through the switching to make sure there's no sold solder joints or other anomalies. Once I am through with the continuity checks I am going to check that cable for sure.

Alistair I appreciate the lead on the coax. I am going to head over to that site to see what I can learn. That is pricey but I'd really like to get this old girl working. Thanks for your input.
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Old 20th Aug 2019, 10:50 pm   #20
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Default Re: No AC measurement on HP 410B VTVM

OK, time to ignore the diode head. Just inject some DC from a small battery into the cable that the diode would have driven.

Any effect? Try different ranges.

David
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