Good Day to you all.
I have a RCA WV-98A on my bench that I am restoring for my own use. Schematic was pulled off the internet and I have used it as referance for my descriptions below.
Some quick background.
Unit did not work (short dim bulb lit up) and I found the electrolytic c6 10uF was bulged probably causing the short.
Replaced the electrolytic and all paper caps, as well as R10 3.3meg which was over 4.2meg. (thought this may be affecting the meeter when I could not calibrate voltages, more on that below).
Unit powers up and in DC+ and DC- and AC you can zero the meeter, also set the Ohms reading to Infinity.
I checkd the resistance reading and they are spot on. Multiple resistors on each scale, compaired to my fluke meeter as well as my B&K 175 VTVM
I am guessing that the Meetering section is functioning properly since the meeter can be zero’ed as well as resistance reading correctly.
The 1Meg in the probe is also right at 1Meg.
Here is the dig.
When Calibrating V+, V- and ACV are all low on the meeter. For example when calibrating the DC you are supposed ot calibrate at 50VDC and with the trimmers turned all the way up the best I can get is around 40VDC, on both DC+ and DC-. The exact same thing for the AC volts, reading 40VAC with trimmer turned all the way up @50VAC.
I tried a different 12AU7 and same thing. The Cathode resistors R12&R19 both read 1.6k (schematic calls 1.5k)
Here are some voltages I read around the circuit. All Voltages are DC.
53.3V @ + side of C6 schematic says 50V
-75.9V @ – side of C6 schematic says -84V
40.2V @ pin 1, 40.1V @ pin 6 of 12AU7, schematic calling +46
1.7v @ pin 3, 1.8v @ pin 8, of 12AU7, schematic calling +2.2
-1.7V pin2 ref. to pin 3
-1.8V pin 7 ref. to pin 8
So I have scratched my head and I am open to any suggestions.
Thanks in Advance
Bob KD8DJC.
Did you measure/test the AC voltage your power socket (in the house) where you’ve plugged your Instrument ? If the AC power is lower than usual it could be the reason that all voltages are too low. Also measure all values for any calibrating pots and any fixed resistor in series with them Just an idea. I would also triple check the value of C6 even if is new.
Thanks for quick reply
123VAC mains
C6 measures good and passed leak test
I will check the calibration pots but I can’t believe that the AC, DC+, and DC- are all bad exactly the same.
Unless I am missing something the upstream resistors are OK since the meeter is functioning in resistance and the 0 and ac0 all work. I’m guessing it’s somewhere in circuit that is shared by voltage checks in all scales and ac and dc.
Thanks again for the reply
Finally got around to finishing up the meeter. I found the problem was in there were several resistors that were just on the edge and replaced them with ones that were “matched” to their nonimal value.
Love the meeter and works great.
Thanks for the help
Bob KD8DJC
I have a C variant and it appears the meter is blown. I put a small voltage thu a pot and resistor and i cant get the needle to move. Can the movement be taken apart?
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Repaired the meter and replaced the electrolytic cap but the meter pegs full right when put on DC+ so I am going to check the resistors off of the 12au7 tube. Any other thoughts?
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