Old VTVM (Precise Model 909) - Page 1
Forum section: electronics / repair

author: Analog Kid
author_rank: Super Contributor
timestamp: November 11, 2024, 04:40:54 am
content: 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.

author: jonpaul
author_rank: Super Contributor
timestamp: November 11, 2024, 04:42:53 am
content: old dead caps, drifted res, dirty switch, broken meter galss...junker worth perhaps $1j

author: Analog Kid
author_rank: Super Contributor
timestamp: November 11, 2024, 05:06:03 am
content: 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 ...

author: TimFox
author_rank: Super Contributor
timestamp: November 11, 2024, 05:15:39 am
content: 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.

author: blue_lateral
author_rank: Contributor
timestamp: November 11, 2024, 10:02:58 am
content: 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.pdfI 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.

author: TimFox
author_rank: Super Contributor
timestamp: November 11, 2024, 06:45:04 pm
content: 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).

author: Analog Kid
author_rank: Super Contributor
timestamp: November 11, 2024, 08:07:56 pm
content: Quote from: TimFox on November 11, 2024, 06:45:04 pmAll the schematics I've found (3 so far) show a 1 megohm probe resistor for DC volts. (0 for AC and ohms, of course.)Quote from: blue_lateral on November 11, 2024, 10:02:58 amButbutbut ... 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.)

author: TimFox
author_rank: Super Contributor
timestamp: November 11, 2024, 08:38:02 pm
content: 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.

author: Analog Kid
author_rank: Super Contributor
timestamp: November 11, 2024, 08:45:59 pm
content: Two Rs but no second filter cap:R27 & R28 = 15K.(pretty badly drawn schematic, btw)

author: TimFox
author_rank: Super Contributor
timestamp: November 11, 2024, 09:08:12 pm
content: 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?

author: Analog Kid
author_rank: Super Contributor
timestamp: November 11, 2024, 09:09:50 pm
content: 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.

author: floobydust
author_rank: Super Contributor
timestamp: November 11, 2024, 09:32:39 pm
content: I found what might be a HV probe (x100 max. 60kV) 2,490MEG sort of implies 10MEG? for the 909.Precision Test Equipment 1953CatalogSometimes 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.

author: TimFox
author_rank: Super Contributor
timestamp: November 11, 2024, 09:35:19 pm
content: Quote from: Analog Kid on November 11, 2024, 09:09:50 pmTo 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?

author: Analog Kid
author_rank: Super Contributor
timestamp: November 11, 2024, 09:45:53 pm
content: 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.

author: TimFox
author_rank: Super Contributor
timestamp: November 11, 2024, 09:55:12 pm
content: Quote from: Analog Kid on November 11, 2024, 09:45:53 pmNo, 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. -  - [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.

author: Analog Kid
author_rank: Super Contributor
timestamp: November 11, 2024, 10:16:12 pm
content: Oh, before the cableat 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.

author: TimFox
author_rank: Super Contributor
timestamp: November 11, 2024, 10:35:11 pm
content: 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.

author: blue_lateral
author_rank: Contributor
timestamp: November 11, 2024, 11:42:02 pm
content: Quote from: Analog Kid on November 11, 2024, 08:07:56 pmThe 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.

author: blue_lateral
author_rank: Contributor
timestamp: November 12, 2024, 12:13:52 am
content: Quote from: floobydust on November 11, 2024, 09:32:39 pmThat'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.

author: Analog Kid
author_rank: Super Contributor
timestamp: November 12, 2024, 02:36:34 am
content: Quote from: floobydust on November 11, 2024, 09:32:39 pmLike 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?

author: blue_lateral
author_rank: Contributor
timestamp: November 12, 2024, 12:14:50 pm
content: Found a manual for the kit version:http://www.ko4bb.com/getsimple/index.php?id=download&file=06_Misc_Test_Equipment/Precise_Electronics/Precise_Model_909_VTVM_Meter_Construction_Manual.pdf

author: George Edmonds
author_rank: Frequent Contributor
timestamp: November 12, 2024, 01:30:38 pm
content: 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

author: TimFox
author_rank: Super Contributor
timestamp: November 12, 2024, 02:44:23 pm
content: 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.

author: TimFox
author_rank: Super Contributor
timestamp: November 12, 2024, 02:47:21 pm
content: Quote from: Analog Kid on November 12, 2024, 02:36:34 amNote that in this power supply, there is a substantial resistance from the 6X5 cathodes to the input filter capacitor.In the limitRC>> 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).

author: floobydust
author_rank: Super Contributor
timestamp: November 12, 2024, 05:54:36 pm
content: Later models or more expensive VTVM's had a regulated B+.Precise9071is worth comparing to the 909. It uses0A2for 150V.EICO-250, Precision Apparatus EV-10-S, HP 410B etc. had the0B2for 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.1N53xx5W 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 1952on the higher end around $4,200/year for a professional, 1.5-2 month's wages.VTVM's were quite expensive in their day.

author: Analog Kid
author_rank: Super Contributor
timestamp: November 12, 2024, 06:18:41 pm
content: Sorry to bug you, but you didn't really answer my question: is the little schematic snippet I gave above correct?Hell, I'd use an OA2 if I had one, but I'm fresh out.

author: TimFox
author_rank: Super Contributor
timestamp: November 12, 2024, 06:23:44 pm
content: With correct values, that should work.1. High-voltage Zeners have large temperature co-efficient, so it is better to use several lower-voltage units in series. With high voltage and reasonable current, self-heating is important.2. Typically, the current through the Zener string would be roughly equal to the expected current through the tubes in the voltmeter circuit. You can play with that current, but you don't want too much current, or for the Zener current to drop to zero, under the range of operation.3. The existing two resistors in your circuit are probably too large for the extra Zener current, and need to be calculated accordingly. Note that the voltage across the filter capacitor is considerably less than in a circuit where the rectifier connects directly to the capacitor.4. Looks like you need to do some math.

author: Analog Kid
author_rank: Super Contributor
timestamp: November 12, 2024, 06:43:00 pm
content: OK, help a guy out here if you would:To do the math, I'd need to know what the two triodes supplied by the HV draw, correct?How can I determine that? Would I want to break that circuit and insert an ammeter?Not saying I'm even going to do this; I'm fine leaving the VTVM stock ATM. But a guy would like to know anyhow.

author: floobydust
author_rank: Super Contributor
timestamp: November 12, 2024, 07:00:59 pm
content: You can see what the VTVM circuit is drawing for current by reading the voltage drop across R27 or R28.What do you have for their values? The 9071 is 10k, 4.7k and another 4.7k, they'd use the same transformer. So I would be lazy and copy that.Or for the math... you know min/max VTVM current draw and then give extra for teh zener at low AC line. The zener is a bit oversized so higher current through it is not a problem.But first in the restoration: C4 is surely dead, dried out and needs to be replaced. Note it has no bleeder resistor - don't let it bite. I'd add a mains fuse too.Measure the carbon resistors to see if they are on value.I couldn't remember what the cigar-tube probes usually have inside for a resistor for DCV.

author: Analog Kid
author_rank: Super Contributor
timestamp: November 12, 2024, 07:09:00 pm
content: Quote from: floobydust on November 12, 2024, 07:00:59 pmMaybe not; saw it yesterday and it says "dry electrolytic" on it, so maybe it's OK.Probly better to replace it, though.Going to build a capacitor tester one of these days ...

author: TimFox
author_rank: Super Contributor
timestamp: November 12, 2024, 08:09:07 pm
content: As discussed above, you can easily determine the plate current (if the unit is operating) by measuring the voltage across the cathode resistor R22 or R24, since the cathode currentIkequals the plate currentIpwith negativeVgk. The plate and cathode currents are defined as positive in normal operation. That current will depend on the plate-cathode voltageVpkand the grid-cathode voltageVgk. In turn,Vgk= -RkxIk.Calculating the tube's plate current is a good exercise in reading tube data sheets, if you are going to change the plate supply voltageVbbby using a Zener regulator that will reduce it to the Zener voltage.Take the characteristic curves, which is a family of curves forIpvsVpkat one value of grid voltageVgkfor each curve.You need to add two "load lines" to the graph:1. The actual plate-cathode voltage is reduced by the voltage across the cathode resistor:Vpk=Vbb- (IpxRk). Plot this as a locus of possible pairs ofVpkandIpfor your design values ofRkandVbband get a straight line on theIpvsVpkaxes.2. Since the grid is grounded (not connected to cathode), the grid-cathode voltage isVgk= -RkxIp, remembering that the cathode currentIkequals the plate currentIp. Plot this as a second locus (not necessarily straight) of one point on each curve of constant grid voltage that corresponds to the current that gives that voltage.Where these two added loci intersect, that is the operating point: the actual plate-cathode voltage, plate current, and grid-cathode voltage determined by the tube, the supply voltage, and the cathode resistor.In your case, the cathode resistance is the sum of R22 or R24 plus half the value of the pot R23, for each of the two triodes.The gold mine for tube datasheets containshttps://frank.pocnet.net/sheets/093/6/6SN7GTB.pdf. The curves are on page 4.

author: blue_lateral
author_rank: Contributor
timestamp: November 13, 2024, 12:07:40 am
content: Quote from: Analog Kid on November 12, 2024, 07:09:00 pmMy guess is he means is you could get a hell of a spark or shock even days later because there is nothing to drain the filter capacitor once the unit is shut off. That was considered bad design practice at the time, although it was occasionally done for various reasons. I see no good reason here but I guess I could be missing something."Dry Electrolytic" in the time this was made just translates to "aluminum electrolytic" today, in other words the default normal kind of electrolytic capacitor. The ones with a paste inside that seeps out and eats the traces off of circuit boards today are "Dry Electrolytic" too in the way capacitors were spoken of back then. There were, before 1935 or so, also "Wet Electrolytic" capacitors. Those were full of a solution of water and borax, and would leak this liquid borax soup out the vent holes and all over your bench if you dared turn the chassis upside down. You could have still easily run into these now and then still in service when the Precise 909 was new.An electrolytic capacitor from the 50s could still work, some do, but the survival rate is quite low. It would be way better to just replace it. The other caps are almost certainly some form of paper capacitor, and if so, need to be replaced. We would have to see them to know what they are. If any are ceramic disc or mica, those are probably OK.

author: Analog Kid
author_rank: Super Contributor
timestamp: November 13, 2024, 12:32:17 am
content: Quote from: blue_lateral on November 13, 2024, 12:07:40 amYeah, I got that; forgot to reply to that point above. I think that's not a problem here; when I measured the HV output (using a DMM, so very high impedance), when I turned the unit off the voltage went down, slowly, but was at a safe level before very long. Which tells me two things: one, the cap is probably at leastsomewhatOK (i.e., it's storing charge), and two, there's enough resistance to bleed off the charge at shutoff.But yeah, I'm pretty much HV-phobic, so I'm super-careful. Hand in pocket, dontcha know.QuoteNo paper caps there; ceramic disc, which I'm sure is OK.

author: Analog Kid
author_rank: Super Contributor
timestamp: November 13, 2024, 01:59:56 am
content: I got the cracked meter glass replaced. Luckily it was just a rectangular piece, not glued in. However, it was thin glass, and I wasn't sure if ordinary window glass ("single strength") might not be too thick. (The original glass is .06", 1/16" (about 1.5mm), while window glass is .087", ~3/32" (2.2mm)). But it fit perfectly; even used the original retainers under the screws. Plus fixed a break in a corner of the bezel (epoxy).The meter's now warming up; next step: calibration.

author: Analog Kid
author_rank: Super Contributor
timestamp: November 16, 2024, 02:19:18 am
content: Just wanted to report, to anyone who might care, that I've gone as far as I'm going to at this point with this old meter.I got it working reasonably well on two of its four functions: +DC volts and ohms.For some reason +DC works well, but -DC is totally flaky: with the meter out of its case, if you flex the case the needle jumps all over the place. I've been over and over the thing, cleaned it up, checked for shorts, bad solder joints, etc., and couldn't find anything.Similar thing with AC volts: I got it working on that scale, but then it just stopped working at all; couldn't zero it nor calibrate it. I swapped out the rectifier tube (6AL5) with another working one; no joy.I wasn't able to calibrate it on the +DC or AC scales; the reading was just shy of where it should have been. I ended up adding 1K resistors in series with the calibration pot, which fixed both scales.Part of the adventure was running across some wires that had been snipped out; there's a board inside that holds resistors with solder lugs, and I found a few lugs that had obviously been cut with dikes, and also 3 wires that obviously had been attached to one (?) of the lugs. I ended up not attaching any of these, as I had no clue where they went, and wasn't up for that particular wild goose chase.This unit was obviously not a kit but was factory-assembled; all the wiring that went any distance was neatly bundled and tied with twine every inch or so. And they used many different colors. Even so, tracing this circuit would have been a minor nightmare, so I'm leaving well enough alone.My theory ™ is that the snippage was done at the factory because of some modification to the circuit. Another weird thing is that that resistor board has a resistor on the back side that doesn't connect to anything; maybe that's where those snipped wires went.This is in contrast to that stupid capacitor (big ol' electrolytic) that was soldered to the wires that were supposed to go to the battery; this was apparently done by some dumbass. I ended up epoxying an AA cell holder on the chassis and connecting it to those wires to make the ohms range work.So I have a partially-working VTVM; woo hoo. Works okay on (+) DC volts; works so-so measuring resistance. Not terribly accurate; it wanted to tell me that a 10Ω resistor was 8Ω. The only really good thing is that it has a higher sensitivity than a DMM (up to 100MΩ, at least in theory, as opposed to my DMM's 2MΩ limit).It was obvious as I worked on this that this poor meter was either partially submerged in water at one point, or at least in some very wet environment. Lotsa rust inside the case and on all steel surfaces. It's amazing that it works as well as it does!