Electronics > Repair
Old VTVM (Precise Model 909)
Analog Kid:
Oh, before the cable at the tip end. Gotcha. That's like the probe I made for my Simpson VTVM, with a switch to bypass the R in the probe.
TimFox:
Yes, if that unit used the same probe for DC voltage and DC current and ohms, then the switch to bypass the probe resistor is mandatory.
blue_lateral:
--- Quote from: Analog Kid on November 11, 2024, 08:07:56 pm ---Butbutbut ... you say 280 V; is that across the whole secondary or across each half? If the latter then my readings would make sense. But I still find it hard to believe that a unit like this would specify a 500-plus volt CT transformer.
But I guess if it works, it works. Time to calibrate! (And just snip out that spurious cap put in place of the battery.)
--- End quote ---
The center tap is "ground" or at least the lowest voltage anywhere in the circuit. Half of the secondary winding is used on one half-cycle and the other half of the secondary is used on the other half-cycle. It was standard practice in the vacuum tube days. Sometimes bridge rectifiers were used, but those are more expensive and extremely clunky to implement in vacuum tubes. It wasn't done much, in fact you hardly ever see it. Bridge rectifiers became popular after silicon diodes became cheap and common, although you still see this old method once and a while with a center tapped transformer and 2 silicon diodes. A transformer like yours would have been more often specified as 280-0-280 as Tim Fox wrote, but it's really 560 with a center tap.
blue_lateral:
--- Quote from: floobydust on November 11, 2024, 09:32:39 pm ---I found what might be a HV probe (x100 max. 60kV) 2,490MEG sort of implies 10MEG? for the 909.
Precision Test Equipment 1953 Catalog
Sometimes in old gear like this I will add a couple 5W zeners in the B+ filter to regulate it. Hate it when VTVM needs moves around due to mains fluctuations.
--- End quote ---
That's an easy goof to make. I've done it too. Precision is not Precise. More exactly, Precision Apparatus of Elmhurst or maybe Glendale, NY is not Precise Delevlopment of Oceanside, NY.
Precision Apparatus, AKA B&K-Precision, AKA PACO when they were making kits is the company we know as BK-Precision today. B&K, who were also Dynascan on some level or another, were in Chicago I think, not New York, so they must have absorbed Precision Apparatus at some point, or maybe Precision Apparatus absorbed B&K. This is all unrelated to Precise Development as far as I know. I don't know what happened to Precise. They must have just gone under at some point.
I like the zener idea. It would be interesting to see how this particular VTVM does while varying the supply voltage with a variac. I don't need to think about it too much anymore as line voltage is pretty stable where I live. Some VTVMs have a split power supply and let the bridge float around the second grid a little, so aren't affected by line voltage much. It appears Precise did not do that, and it might really benefit a lot from your regulator idea.
Analog Kid:
--- Quote from: floobydust on November 11, 2024, 09:32:39 pm ---Sometimes in old gear like this I will add a couple 5W zeners in the B+ filter to regulate it. Hate it when VTVM needs moves around due to mains fluctuations.
--- End quote ---
Like this?
If "a couple", where would the other one go?
And the zener voltage should be the HV output voltage, right?
Maybe I need a couple in series?
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