Author Topic: Old VTVM (Precise Model 909)  (Read 1784 times)

0 Members and 1 Guest are viewing this topic.

Online Analog KidTopic starter

  • Super Contributor
  • ***
  • Posts: 1109
  • Country: us
Old VTVM (Precise Model 909)
« on: November 11, 2024, 04:40:54 am »
I posted this purchase in the "What have you bought recently" topic. Old VTVM, apparently complete:




I checked it over a bit, cleaned the switches (filthy!), vacuumed out the spiderwebs, made sure the tubes were well seated. Today I plugged it in, crossed my fingers and powered it up. Tubes lit up, seemed like it might be working: the zero adjustment moved the meter needle.

I couldn't tell if one of the tubes was lit (5692, a 6SN7 clone) because of the getter silver on top, so I took out, checked the heater: it was OK.

I wanted to see what voltage filter capacitor I'd need to replace the old one, so I measured the voltage across it: only a couple hundred mV. WTF???

It gets weirder: next time I put my DMM on the transformer secondary. It showed 560 volts!

So what the hell is going on here? It's a regular little 3-tube VTVM with a small power transformer, so I figured the HV would be in the 150-200 V range. Could the voltage shoot up that high if the rectifier was somehow open circuit? But even that doesn't make sense to me.

This isn't a serious project--yet. It'd be nice to get this working, but I already have 2 working VTVMs, so it's not urgent at all. But it bugs me and I'd like to solve the puzzle.

Analog VOM shows 580 VAC.
« Last Edit: November 11, 2024, 04:47:17 am by Analog Kid »
 

Offline jonpaul

  • Super Contributor
  • ***
  • Posts: 3656
  • Country: fr
  • Analog, magnetics, Power, HV, Audio, Cinema
    • IEEE Spectrum
Re: Old VTVM (Precise Model 909)
« Reply #1 on: November 11, 2024, 04:42:53 am »
old dead caps, drifted res, dirty switch, broken meter galss...junker worth perhaps $1

j
An Internet Dinosaur...
 

Online Analog KidTopic starter

  • Super Contributor
  • ***
  • Posts: 1109
  • Country: us
Re: Old VTVM (Precise Model 909)
« Reply #2 on: November 11, 2024, 05:06:03 am »
OK, some progress: turns out that electrolytic cap that's hanging there isn't the power supply filter cap: someone was messing around in here and I think that's where the battery is supposed to go. Why would someone do that?

I found the filter cap and got ~190 volts across it, so that looks OK. Now I just have to figure out where the battery is actually supposed to go ... this thing might work after all.

Still weirded out by that extremely high voltage across the transformer secondary ...
 

Offline TimFox

  • Super Contributor
  • ***
  • Posts: 8995
  • Country: us
  • Retired, now restoring antique test equipment
Re: Old VTVM (Precise Model 909)
« Reply #3 on: November 11, 2024, 05:15:39 am »
Note that the unbalanced microphone connector on the front panel for DC volts mates to a shielded cable to a probe with a series 1 megohm resistor near the tip.
 

Offline blue_lateral

  • Contributor
  • Posts: 34
  • Country: us
Re: Old VTVM (Precise Model 909)
« Reply #4 on: November 11, 2024, 10:02:58 am »
It is a full wave rectifier, and the transformer secondary is double the expected unfiltered voltage, so it's really 280v going into the filter. A little higher than I would expect, but the maximum rating of the 6X5 is 350v, so whatever. 190v on the filter cap? Probably fine. Manual is here:

http://pacifictv.ca/schematics/precise909manual.pdf

I see Precise have committed the almost unforgivable sin of not listing the full scale current and internal resistance of the meter movement. Full scale current may be in small print in a corner of the meter face. Since there are no mA ranges, maybe the internal resistance doesn't matter?

It will work without the battery except for Ohms. IMHO it is often better to leave the battery out and just measure ohms with something else. Fumes from a leaking alkaline cell can easily destroy the meter movement, and they all leak today. The old carbon zinc cells were a little better, but still destroyed a lot of VTVMs. The battery is soldered in most VTVMs, to avoid unpredictable connection quality I guess. Lots of us who still use VTVMs add a small power supply to replace the battery. It was cost prohibitive back in the days of vacuum tubes compared to a flashlight battery but easy now.

The DC probe resistor (R7) is listed in the manual as 15 Megohms instead of the typical one Megohm. It can't be 15 Megohms, as the DC divider stack in the VTVM is the expected 10 Megohms as used by most other VTVMs. I'll bet the resistor is 1.5 Megohms.
 
« Last Edit: November 11, 2024, 10:49:42 am by blue_lateral »
 

Offline TimFox

  • Super Contributor
  • ***
  • Posts: 8995
  • Country: us
  • Retired, now restoring antique test equipment
Re: Old VTVM (Precise Model 909)
« Reply #5 on: November 11, 2024, 06:45:04 pm »
I wonder if the 15 megohm probe value is correct.
I was used to 1 megohm probe resistors, as in the RCA W77E "Voltohmyst" VTVM, giving a total input resistance of 11 megohms (with internal 10 megohm divider) and a minimum full-scale DC voltage of 1.10 x (internal full scale).
On the W77E, the minimum full-scale is 1.5 V DC, implying an internal full-scale voltage of 1.36 V DC.

The front panel of the Precise voltmeter shows 5 V DC minimum full-scale voltage.
With 15 megohm in the probe and 10 megohm internal divider, for 25 megohm total resistance, that implies input voltage = 2.50 x (internal full scale), or internal full scale = 2.0 V DC, somewhat higher than the RCA unit.
Perhaps the Precise meter was designed to use a larger probe resistor to reduce signal loading even more than the typical 1.0 megohm probe.

I have an -hp- 410A VTVM with a 22 megohm probe resistor and 100 megohm total input resistance, with minimum DC full-scale of 1.0 V.
The 410A vacuum-tube DC amplifier is much more complex (using two triode-connected 6AG7s and a 6SN7) than the simpler units (typically a 12AU7 or 6SN7 dual triode only).
 

Online Analog KidTopic starter

  • Super Contributor
  • ***
  • Posts: 1109
  • Country: us
Re: Old VTVM (Precise Model 909)
« Reply #6 on: November 11, 2024, 08:07:56 pm »
I wonder if the 15 megohm probe value is correct.

All the schematics I've found (3 so far) show a 1 megohm probe resistor for DC volts. (0 for AC and ohms, of course.)

It is a full wave rectifier, and the transformer secondary is double the expected unfiltered voltage, so it's really 280v going into the filter. A little higher than I would expect, but the maximum rating of the 6X5 is 350v, so whatever. 190v on the filter cap? Probably fine.

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.)
 

Offline TimFox

  • Super Contributor
  • ***
  • Posts: 8995
  • Country: us
  • Retired, now restoring antique test equipment
Re: Old VTVM (Precise Model 909)
« Reply #7 on: November 11, 2024, 08:38:02 pm »
With a full-wave center-tap transformer and two vacuum diodes (in 6X5), 280-0-280 or 560 V CT is not unreasonable, since the diode drop is considerably higher than with silicon.
Also, the filter input capacitor is not huge, so the ripple across it is large.
Is there a resistor from the input capacitor to a second filter capacitor?  That would drop the DC voltage to a reasonable value for a 12AU7.
 

Online Analog KidTopic starter

  • Super Contributor
  • ***
  • Posts: 1109
  • Country: us
Re: Old VTVM (Precise Model 909)
« Reply #8 on: November 11, 2024, 08:45:59 pm »
Two Rs but no second filter cap:



R27 & R28 = 15K.

(pretty badly drawn schematic, btw)
 

Offline TimFox

  • Super Contributor
  • ***
  • Posts: 8995
  • Country: us
  • Retired, now restoring antique test equipment
Re: Old VTVM (Precise Model 909)
« Reply #9 on: November 11, 2024, 09:08:12 pm »
That gives a fairly large drop from the 6X5 cathodes to the load at a few mA triode current.
That drawing seems to have R7 internally with an un-referenced (external) probe resistor to its left.
Does your manual list the probe-mounted resistor value?
 

Online Analog KidTopic starter

  • Super Contributor
  • ***
  • Posts: 1109
  • Country: us
Re: Old VTVM (Precise Model 909)
« Reply #10 on: November 11, 2024, 09:09:50 pm »
Oops: R7 = 15MΩ. Dunno where I got 1M from.
Also not sure why they show 2 resistors there.
Another weirdness: looking for the battery connection, I looked up R8 in the manual to locate it.
It listed it as "5M or 6M Res.". Ackshooly it's 5.6M. Typist got a little wonky there? or maybe it really doesn't matter, and depends on what they had on hand? In any case, it's listed as a non-precision resistor.
« Last Edit: November 11, 2024, 09:15:42 pm by Analog Kid »
 

Offline floobydust

  • Super Contributor
  • ***
  • Posts: 7678
  • Country: ca
Re: Old VTVM (Precise Model 909)
« Reply #11 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.
 
The following users thanked this post: blue_lateral

Offline TimFox

  • Super Contributor
  • ***
  • Posts: 8995
  • Country: us
  • Retired, now restoring antique test equipment
Re: Old VTVM (Precise Model 909)
« Reply #12 on: November 11, 2024, 09:35:19 pm »
Oops: R7 = 15MΩ. Dunno where I got 1M from.
Also not sure why they show 2 resistors there.
Another weirdness: looking for the battery connection, I looked up R8 in the manual to locate it.
It listed it as "5M or 6M Res.". Ackshooly it's 5.6M. Typist got a little wonky there? or maybe it really doesn't matter, and depends on what they had on hand? In any case, it's listed as a non-precision resistor.

To isolate the system under test from the cable capacitance, the resistance should be in the DCV probe before the cable.
Did they split that into two resistors?
 

Online Analog KidTopic starter

  • Super Contributor
  • ***
  • Posts: 1109
  • Country: us
Re: Old VTVM (Precise Model 909)
« Reply #13 on: November 11, 2024, 09:45:53 pm »
No idea, as it didn't come with a probe. So you're saying the resistance should be right at the attachment point (at the microphone connector) before the cable? I'll do that when I make a probe for it.
 

Offline TimFox

  • Super Contributor
  • ***
  • Posts: 8995
  • Country: us
  • Retired, now restoring antique test equipment
Re: Old VTVM (Precise Model 909)
« Reply #14 on: November 11, 2024, 09:55:12 pm »
No idea, as it didn't come with a probe. So you're saying the resistance should be right at the attachment point (at the microphone connector) before the cable? I'll do that when I make a probe for it.

No, the large resistor should go inside the probe handle, as close as possible to the tip, before the voltage goes through the shielded cable to the microphone connector.

[Tip] - [resistor] - [coax cable] - [cable connector] - [panel connector] - [internal switched voltage divider] - etc.
----(probe)----

Any safe probe-tip and handle can be used. so long as the resistor is close to the pointy end.
 
The following users thanked this post: Analog Kid

Online Analog KidTopic starter

  • Super Contributor
  • ***
  • Posts: 1109
  • Country: us
Re: Old VTVM (Precise Model 909)
« Reply #15 on: November 11, 2024, 10:16:12 pm »
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.
 

Offline TimFox

  • Super Contributor
  • ***
  • Posts: 8995
  • Country: us
  • Retired, now restoring antique test equipment
Re: Old VTVM (Precise Model 909)
« Reply #16 on: November 11, 2024, 10:35:11 pm »
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.
 

Offline blue_lateral

  • Contributor
  • Posts: 34
  • Country: us
Re: Old VTVM (Precise Model 909)
« Reply #17 on: November 11, 2024, 11:42:02 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.)

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.
 

Offline blue_lateral

  • Contributor
  • Posts: 34
  • Country: us
Re: Old VTVM (Precise Model 909)
« Reply #18 on: November 12, 2024, 12:13:52 am »
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.

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.
 

Online Analog KidTopic starter

  • Super Contributor
  • ***
  • Posts: 1109
  • Country: us
Re: Old VTVM (Precise Model 909)
« Reply #19 on: November 12, 2024, 02:36:34 am »
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.
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?
 


Offline George Edmonds

  • Frequent Contributor
  • **
  • Posts: 412
  • Country: gb
Re: Old VTVM (Precise Model 909)
« Reply #21 on: November 12, 2024, 01:30:38 pm »
May I offer a word of warning.

In truth this is a class 1 instrument without a protective ground, it also appears that the negative terminal is directly connected to the metal case work.

Under no circumstances should this meter be used to measure the  incoming mains supply or any voltage where the negative supply as above ground.

G Edmonds

 

Offline TimFox

  • Super Contributor
  • ***
  • Posts: 8995
  • Country: us
  • Retired, now restoring antique test equipment
Re: Old VTVM (Precise Model 909)
« Reply #22 on: November 12, 2024, 02:44:23 pm »
To my knowledge, all VTVMs of this vintage use transformer power supplies with the negative end grounded to the metal case.
For DC measurements, there is a polarity switch for + or - full scale.
AC measurements are also with respect to the case ground.
Most have a two-prong mains cord, so the case itself does not go to PE.
 

Offline TimFox

  • Super Contributor
  • ***
  • Posts: 8995
  • Country: us
  • Retired, now restoring antique test equipment
Re: Old VTVM (Precise Model 909)
« Reply #23 on: November 12, 2024, 02:47:21 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.
Like this?

(Attachment Link)

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?


Note that in this power supply, there is a substantial resistance from the 6X5 cathodes to the input filter capacitor.
In the limit RC >> 1/[ 2{pi}f ], the unloaded voltage across the capacitor goes to the average (mean absolute value) voltage of the AC waveform, rather than to the peak value found with a direct connection to the capacitor (neglecting diode drops).
 

Offline floobydust

  • Super Contributor
  • ***
  • Posts: 7678
  • Country: ca
Re: Old VTVM (Precise Model 909)
« Reply #24 on: November 12, 2024, 05:54:36 pm »
Later models or more expensive VTVM's had a regulated B+.
Precise 9071 is worth comparing to the 909. It uses 0A2 for 150V.
EICO-250, Precision Apparatus EV-10-S, HP 410B etc. had the 0B2 for around 108VDC B+ I believe.
It's just that I disliked the (Heathkit) VTVM's needle moving due to mains fluctuations which can look like a bad part/connection and it kind of threw me around, back in the day. Also mains voltage today is higher than decades ago so there is extra voltage.
To clarify- a VTVM mod for regulated B+, I'll add a zener shunt reg.
1N53xx 5W like 1N5383B 150V/8mA or for higher voltages over 200V use a 1N5388B 200V plus 1N5368B 47V to make a 250V zener, this is the "two" I was talking about.

Typical VTVM prices 1952: $25.98 (kit), $44.98 (wired) or $309 kit, $535 assembled in today's dollars. Average income 1952 on the higher end around $4,200/year for a professional, 1.5-2 month's wages.
VTVM's were quite expensive in their day.
« Last Edit: November 12, 2024, 05:56:45 pm by floobydust »
 
The following users thanked this post: blue_lateral


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf