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Success Stories If you have successfully repaired or restored a piece of equipment, why not write up what you did and post details here. Particularly if it was interesting, unusual or challenging. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 21st Apr 2020, 8:53 pm   #1
The General
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Default HP 410B Vacuum Tube Voltmeter

This 410B was an ebay purchase from the USA. I was a little wary of it getting all the way here with the meter intact but the seller packed it well & it arrived in good order.
HP introduced the 410B in 1951 & produced it into the 70s. The early models have a flat front panel & at some point in the mid 50s the case was changed to a more elaborate style with a raised edge like the one I have. I found a date on the can electrolytic indicating it was built in 1959.
I have seen these described as the Cadillac of VTVMs, well it's certainly well made & feels solid.
Well first job is to uncase it & see what we have. First check is to test the meter movement, thankfully this was OK. The chassis is a flat L shape which carries the power transformer, the valveholders & two tagboards with the passive components & cal pots while the range switches are mounted on the front panel. There's not really enough cable slack to unbolt & pull the chassis back far enough for a resto but it was OK for what I needed to do.
Everything was really clean inside, the AC probe with its integral diode valve seemed fine, the only visible issues were the broken solder joints inside the multiway connector which connects the probe cables to the front underside of the main unit. So I sorted this first, cutting back & reterminating the two shielded cables (the AC probe one being triaxial) & installing new ground & ohms range cables. The mains power cord was a bit scabby so I replaced it with a new US spec 3 core cable & plug. It is possible to rewire the transformer primary for 230V but as it is possible to store the cable inside the case & the hole in the removable back cover isn't big enough for a 13A plug to go through, I left it wired for 115V. This isn't an issue for me as I have a 115V power strip on my bench fed from a stepdown transformer underneath.
Now to start on the electronics....
To be continued...
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Old 21st Apr 2020, 9:06 pm   #2
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Default Re: HP 410B Vacuum Tube Voltmeter

The HP manual was downloaded from the internet & printed out, & now the electrical restoration begins.
The circuit has two 12AU7 long tail pairs; the voltage to be measured is applied via the range selector to the first triode pair acting as a differential amplifier, this is DC coupled to a second pair in a balanced bridge circuit with the meter reading the potential difference between the cathodes. The first 12AU7 is run with low anode voltages of 22V & also a reduced heater voltage to minimise gas current effects in this very high impedance stage. The AC probe incorporates a diode rectifier valve & a 6-4 ballast tube supplies its regulated heater current.
Well there are no nasty waxies, the 4uF can electrolytic showed evidence of leaking so I fitted a tubular replacement under the chassis & left the can in place for appearance. The resistors (other than those in the range selection circuit) are all carbon composition, I checked these for drift but all were OK.
The AC probe with its tiny diode was undamaged & seemed OK & the valves all tested OK on my AVO Mk.4.
So it was time to apply power. No nasty surprises, the 0B2 voltage regulator lit, the meter drifted up then down & it was possible to zero it. I set the heater voltage adjustment for the AC probe's diode, checked the DC voltages were OK according to the manual & made some preliminary checks for accuracy.
Accuracy wasn't far off but the DC calibration control - which affects all modes except Ohms - was a bit too close to the end of its range for comfort. The manual lists all the production changes by serial number & I discovered there is a later modification in which four resistor values in the bridge circuit are changed in order to increase the drive to the meter. Ahhh.... I made the mod, using 1% 2W resistors in place of the 5% carbon comps, this brought the DC calibration control back to somewhere near the centre of its rotation. Now I could go through calibration & adjustment; after setting the DC calibration everything else was pretty close apart from a couple of cal pots which needed a squirt of Deoxit.
I cleaned up the casing, not much needed here as it seemed to have been well cared for, I found a suitable black knob to replace the aluminium AC zero knob someone had fitted & left it on soak.
Well after a while, the AC ranges became a little flaky. When the unit was cold they were fine, but after about 15 minutes the readings would fall away & then return to normal after a minute or so. DC ranges remained rock steady. Odd, as almost all the circuit is common to both ranges. I thought I might have an issue with the triaxial cable, maybe a fatigue break near the probe but no, when I checked it for continuity & flexed it, it was fine. Then I twigged it - the heater in the probe's diode valve was flaky. Cold, it measured OK but when hot, it became intermittent. I suspect the probe had suffered from being dropped on the bench a few times over its life. The valve is a tiny 2-01C which was apparently originally manufactured specifically for this VTVM (& maybe others). Luckily, Antique Electronic Supply had some so I ordered one & when fitted, it fixed the problem.
Now the VTVM sits on my bench, rear & centre, used fairly regularly. The only slight issue is there are four test leads, keeping them untangled is an extra job.....
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Last edited by The General; 21st Apr 2020 at 9:08 pm. Reason: put the same picture in twice... D'oh.
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Old 21st Apr 2020, 9:19 pm   #3
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Default Re: HP 410B Vacuum Tube Voltmeter

Gosh, I meant to put this in the Success Stories thread. That was a less than successful move on my part.
Must be this working from home business... Mods, I'd be grateful if this could be moved please.
Thanks, Mark.
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Old 21st Apr 2020, 9:38 pm   #4
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Default Re: HP 410B Vacuum Tube Voltmeter

HP did make some handsome and effective instruments in that era. At the same time they and their competitors were getting very inventive at finding ways round all the limitations they discovered. Ideas like chopper-stabilisers allowing AC amplifiers to amplify precision DC.
Thermistor bridges to allow Audio and later DC to power to be substituted by RF heating. Keeping the temperature stable, just measure how much you back of the DC heating and you have an accurate measure of the RF power. Works all the way up to daylight and beyond.

PAECO written on the top of your mains transformer was Palo Alto Engineering Company. Another firm set up by Bill and Dave to make transformers and common chassis parts.

David
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Old 22nd Apr 2020, 8:03 pm   #5
The General
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Default Re: HP 410B Vacuum Tube Voltmeter

David, thanks for the info. I'd idly wondered who Paeco were/are, I didn't know they were an HP company.
I admit to having a soft spot for HP test kit ever since I used some many years ago during my industrial placement at Control Data. While there, I was given a small HP printer to repair, which printed test results for Winchester disks on a paper roll. It had suffered from several bodge repairs over the years but I was impressed by its gold plated PCBs & general build quality. I decided to remove all the bodges & build it back up. I had all the print hammers & solenoids in a row on the bench while I cleaned up the rest of the mech. The boss walked past, looked at the parts I'd laid out & said: ing good job, I like to see someone who takes a ing pride in their work'. Made me smile.
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