Ballantine 320A TRMS VTVMWe have great news to report at Vince's old junk lab !
Well to be honest it was last evening's news but it was 00H45 so I went straight to bed. As always I lose track of time quickly as soon as I am doing something at the bench.
Anyway. So after the first power up the other day, and cleaning the glass ware, it was looking half decent inside, and I knew it had life in it. The needle of the meter movement would move, although it would do all sorts of crazy shit depending on what range you selected. So it was promising but not quite there...
So last evening did some more cleaning : the front panel. Looks much engaging now. You don't feel the need to wear gloves to operate it any more. Looks quite nice actually, it's in good nick.
Then electroniks-wise, tried to check supply voltages to see if there was a gross failure anywhere. Manual covers only the type 320 which is not the same as my 320A sadly. It's close enough topology-wise, but not close enough in the details to be used for accurate troubleshooting. Number of tubes is different, types of tubes used is different for the most part, layout on the chassis is different....
So the schematic could only give me a rough idea of how things are put together, but I could not rely on it for the details. Plus, it's hard to read /decipher anyway, the quality of the scan being not that great...
Also the schematic makes zero mention of component values, only designators, which is a major pain.
So I had no clue what actual voltages levels I should get, nor where to measure them... so I could only do basic sanity checks.
I measured DC and AC/ripple voltage across all the big filter can caps in the power supply section. Found tube-compatible voltage levels, and low ripple.
Checked the voltage on all pins of all tube sockets to make sure I had some high voltage on all of them... I did.
Also, all tubes do light up / glow, which is a good start.
Then powered it off.
Removed all tubes one by one, squirting some contact cleaner into the sockets, then exercising the tubes / pins in the sockets a few times.
Some contact cleaner in the wafer switches, only two, the range switch and "mode" switch. Exercised them extensively.
Then tested the instrument.... I was well rewarded. Needle does not do any crazy shit any more. It does what you expect it to do.
Using my HP 33XXX something, I tested the instrument on all the ranges that the HP could manage, i.e not much
I tested starting from the 100mV range because the HP can't go lower than 35mVrms or something, ridiculous. So that's 70mVrms really, because 35mV is into 50ohms... but the DUT is 10Meg impedance..
Then the HP can go only up to 3.5Vrms / 7Vrms high impedance, so I could test up to the 10V range full scale, no more.
I don't even know if the HP output is leveled or not ? I bet it's just "open loop", and you get what you get... and you do'nt get upset ?
Anyway, I don't have anything better so I went with that.
The result is incredible.... the VTVM read within spec in all the ranges that I tested !
It read consistently a tad low, but still within spec. ..... this thing is 60 years old, sat in storage for the past 25 at the sellers house, and the 10 or 20 years before that, most likely also in storage at Aerospatiale where it came from.... I power it up... and not only does it work, but it's within spec. That's quality engineering and build for you !
So imagine what it could do if I played with a trimmer or two here and there to calibrate it hmmm....
Anyhow much of that error is maybe due to the HP sig gen itself ? I don't know.
So here are the numbers. I fed a 1kHz sine wave, on ranges above 0.3mV, in which case the stated accuracy is 2%.
Depending on voltage levels and selected range, I get it looks like, accuracy between 1% and 2%... so it's in spec.
Range Applied Measured
voltage on VTVM
- - - - - - - - - - - - - - - -
100mV 100mV 98mV
300mV 300mV 295mV
1V 1V 0.99V
3V 3V 2.96V
10V 7V 6.9VSo basically I now have a cleanish instrument, that basically works out of the box... zero caps replaced, zero tube replaced, zero anything. Zero cost.
So if I can flog it for 50 Euros it would be all profit. If nobody wants it, might keep it a for little bit now that it looks nice / clean, and works fine...
Oh forgot, look how it's made underneath the chassis...it looks incredibly modern. This thing is old enough to have a shouldered rectifier tube, yet on the underside it has circuit boards, and state of the art fiber glass boards at that !
One of them even has a card edge connector.
OK so now onto the next piece of TE... what will I pick... place your bets....