| Electronics > Repair |
| Old VTVM (Precise Model 909) |
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| TimFox:
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 current Ik equals the plate current Ip with negative Vgk. The plate and cathode currents are defined as positive in normal operation. That current will depend on the plate-cathode voltage Vpk and the grid-cathode voltage Vgk. In turn, Vgk = - Rk x Ik. Calculating the tube's plate current is a good exercise in reading tube data sheets, if you are going to change the plate supply voltage Vbb by using a Zener regulator that will reduce it to the Zener voltage. Take the characteristic curves, which is a family of curves for Ip vs Vpk at one value of grid voltage Vgk for 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 - (Ip x Rk). Plot this as a locus of possible pairs of Vpk and Ip for your design values of Rk and Vbb and get a straight line on the Ip vs Vpk axes. 2. Since the grid is grounded (not connected to cathode), the grid-cathode voltage is Vgk = - Rk x Ip, remembering that the cathode current Ik equals the plate current Ip. 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 contains https://frank.pocnet.net/sheets/093/6/6SN7GTB.pdf . The curves are on page 4. |
| blue_lateral:
--- Quote from: Analog Kid on November 12, 2024, 07:09:00 pm --- --- Quote from: floobydust on November 12, 2024, 07:00:59 pm ---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. --- End quote --- Maybe 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 ... --- End quote --- My 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. |
| Analog Kid:
--- Quote from: blue_lateral on November 13, 2024, 12:07:40 am ---My 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. --- End quote --- Yeah, 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 least somewhat OK (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. --- Quote ---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. --- End quote --- No paper caps there; ceramic disc, which I'm sure is OK. |
| Analog Kid:
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. |
| Analog Kid:
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! |
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