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The physical size of the plate structure in the China tube is noticeably smaller than the size within the old US tube.) I tried replacing the 12AU7 with the China-made tube, no change in meter behavior (in fact the meter deflection is a bit less. OK, I will work on replacing the capacitors which means yet another Mouser order, so you won't hear more from me on this for several days. The 6AL5 remains in its socket while these measurements are being taken.Īnyway, I would appreciate your comments and suggestions (pls refer to the schematic in the link provided in post #1, if you wish to follow along.) Not sure whether C1 is OK, but at least it is not shorted. #Replacing a selenium rectifier with a silicon diode seriesHow would the resistors in series with the capacitor affect the capacitance measurement? (Having trouble finding the side of C1 which is not grounded.) I also measured 0.22 uF capacitance from that pin to ground. I measured resistance from pin 5 of the 6AL5 to ground and got 1.94 megohm. If C1 is shorted then the AC voltage readings would be depressed. This caused me to think that maybe C1, 0.1 uF is bad since that filters the 6AL5 output before it goes to R1, 500 Kohm, the range-switching resistor network, then finally to R19, the 1 Mohm resistor in the path to pin 7, grid of the 12AU7. If the capacitor is OK, why does it show this steady-state resistance of 32 ohms? However, I then used the capacitance measurement function of the Fluke and measured 0.26 uF. (The way that I did this was to measure from pin 7 of the 6AL5 to the ACV and Ohms probe input while the selector switch was set to ACV.) The resistance was 32.4 ohms and it did not increase over time. Then I measured resistance across C2 which is 0.25 uF. The measurement started at 0.9 megohm and ended up at 1.2 megohm. ![]() I measured resistance from pin 7 of the 6AL5 to ground. I turned off the VTVM, opened it up, and started using my Fluke 87 ohmmeter function to probe around. Since the DC voltmeter and the ohmmeter seem to be operating OK, I am going to assume that the power supply (incl. The bad news is that after installing the RCA 6AL5, there was no change in behavior, where the AC voltmeter reading is ~80VAC when the correct value is ~120VAC house voltage. Well, the good news is that I received my vacuum tube order (from Triode Electronics) today. However, there was no increase to the DC voltage (in fact the ripple voltage increased slightly.)ĭo you think that the selenium rectifier is bad, since it shows a 30V drop? If so should I replace with a silicon diode or will that cause trouble with excessive voltage? (One problem is that the schematic does not show expected voltages.)ĭear friends and old-timer electronics troubleshooters: I tried replacing the 20 mFd capacitor with a 22 mFd, 160V capacitor purchased from Mouser. The voltage at the selenium rectifier cathode is 90.5 VDC and 1.04 VAC ripple as measured by the Fluke. Since there is 0.9V drop across the 33 ohm resistor, it appears that 27 mA current is flowing. The voltage at the selenium rectifier anode is 119.1 VAC. The transformer high voltage secondary voltage is 120 VAC. The voltage across the transformer primary is 118.3 VAC, which is house voltage. (See the schematic diagram per the above link.) Should I expect that a VTVM which is powered by house AC will be able to measure house AC voltage accurately, or is there some ground loop issue? I had hoped that the power transformer would isolate the VTVM circuitry from the AC line. However, when I tried measuring AC house voltage, the reading was way off. I used lab power supplies as the DC voltage source, and was able to calibrate DC voltage measurements fairly well. When I received it, I tried calibrating it using my Fluke 87 multimeter as the reference. #Replacing a selenium rectifier with a silicon diode manual(Here's a link that provides photos and the owner's manual including schematic diagram.) My dad recently gave me his Simpson 303 VTVM. This question is for electronics old-timers. ![]()
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