| Electronics > Repair |
| Old VTVM (Precise Model 909) |
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| Analog Kid:
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. |
| jonpaul:
old dead caps, drifted res, dirty switch, broken meter galss...junker worth perhaps $1 j |
| Analog Kid:
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 ... |
| TimFox:
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. |
| blue_lateral:
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. |
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