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 Post subject: RCA WV-77A
PostPosted: Mar Sun 04, 2018 3:48 pm 
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Greetings to the Forum:

I have been working on my RCA VTVM, model WV-77C. Yes, I know that the subject header says WV-77A, but they are very close... and the closest schematic available for down load is that of the WV-77A. Schematic here:

http://bama.edebris.com/manuals/rca/wv77a/

I have sort of gone through the thing (replaced paper caps and the electrolytic) and was setting out to cal it.

At first, the AC readings were about 10% low, so I went hunting for some out of tolerance resistors in the AC circuit and changed one... R21 on the schematic (470 ohms). This got things a lot closer, but when I was poking around, I noticed that I had omitted to change one cap...C4 on the schematic.... a .01 to ground from the input grid of the balanced amplifier. So, I put in one of the standard 630 volt yellow film caps there too.

Here is where my memory gets a bit fuzzy.... I would swear that the cal pot was somewhere near the center of its range before I changed C4.... but I can't remember for sure. However, now it is right at maximum resistance for correct calibration. I don't like pots at the end of their range, so I went looking for more out of tolerance resistors. I found quite a few and changed them out.... with no improvement whatsoever. The cal pot is still at the end of its range.

Referring to the schematic at the above link, I have replaced R17, R19, and R20. I have measured R13 to be about 16.8K and R14 to be about 13.6K. The two cathode resistors, R15 and R16 are a bit high but very close to each other at 1.7K. The cal pot measures 8K.

I started to go through the input range resistors but had to quit before recording any results. The highest range resistor was out quite a ways, but I wasn't worried about that because I don't intend to measure any high voltages. The rest of the scales track very well when changing ranges, though.... but I just can't get the cal pot off the end of its travel.

I haven't changed the rest of the resistors in the DC amp because they are very difficult to get at. I was hoping for some insight from youse guys.

BTW, the 12AU7 checks super good in my TV-7; it goes to 120... max scale. Both sides the same. It passes the gas test. It is marked RCA with white lettering... I am guessing that it is the original tube.

I put my megger across the new .01 cap on the grid (C4) and it is not leaking at 500 volts.

Also, I have lots of zero adjustment room.... I can almost get full scale with the zero adjust in the positive direction, and although I don't remember how much I get in the negative direction, it is plenty. No problem getting the meter to zero. The amplifier isn't quite balanced; there is a slight (about 1 or 2 small divisions) change from DC- to DC+. The DC+ zero and the AC zero are pretty much identical.

I am going by memory here.... the cathodes of the 12AU7 measure about 2.4 volts.... the plates are a more fuzzy memory but somewhere between 40 and 50 volts as I recall.

I will get more precise data on request but I have been frozen out of my garage at this point; the heater tripped the breaker again and I have called it a night.

I am tempted to add some series resistance to the cal pot and call it a day, but that is a band-aid approach. I may go that route, but I thought I'd get some ideas here first.

Thanks,

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Jim T.
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 Post subject: Re: RCA WV-77A
PostPosted: Mar Sun 04, 2018 6:38 pm 
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Jthorusen wrote:
I started to go through the input range resistors but had to quit before recording any results. The highest range resistor was out quite a ways, but I wasn't worried about that because I don't intend to measure any high voltages. The rest of the scales track very well when changing ranges, though.... but I just can't get the cal pot off the end of its travel.

By "highest range resistor" do you mean R6?

If R1 and/or R2, the highest value resistors in the input divider, are high, then all the rest of the ranges will be off, and you'll have to crank over the DC cal pot to get the voltage ranges in calibration.

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 Post subject: Re: RCA WV-77A
PostPosted: Mar Sun 04, 2018 6:48 pm 
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Jthorusen wrote:
I have been working on my RCA VTVM, model WV-77C. Yes, I know that the subject header says WV-77A, but they are very close... and the closest schematic available for down load is that of the WV-77A.

I don't have a record of where I downloaded it, but I have a good scan of a WV-77C manual. Google doesn't turn it up any longer. I put it up for download on my site:
http://www.byan-roper.org/steve/manuals ... V-77C+.pdf

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 Post subject: Re: RCA WV-77A
PostPosted: Mar Sun 04, 2018 9:29 pm 
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Hi Steve!

Thank you for pointing out my error. I was reading the schematic wrong, thinking the input was tapped down the divider string by S-2A. I knew the input was supposed to be a constant 11 meg ohms, which couldn't be in that case, but I just didn't see it. Thanks to your pointing this out, I now see the path and the divider string makes more sense to me now.

Having said that, I went back out to the garage and measured all the resistors in the divider string inside the instrument. The largest error is in the 5 meg resistor that connects from the input jack to the switch. It is off by 4%. However, the rest of the resistors are closer and the total error in the entire string is only 2.4%. I put the results in a spread sheet for easy reading; the spread sheet is here if you have Excel:

Attachment:
WV-77C Voltage Divider.xlsx [10.24 KiB]
Downloaded 14 times


So.... 2.4% is within the specified accuracy of 3% full scale.

Which means that I am back where I started from.

Thank You for the correct schematic. R18 which shows on the "A" schematic as a 100 ohm resistor is a 130 ohm resistor in my unit... which has escalated to 150 ohms. I'm sure there are some other differences, but I will have to print out a copy of your schematic and see if I can spot them.

In the meanwhile, Thank You for your input... you have been a big help already.

Regards,

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 Post subject: Re: RCA WV-77A
PostPosted: Mar Sun 04, 2018 10:05 pm 
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Yes, the input divider looks good.

Try putting back the original R21 (470Ω). What is its current value, and what is its color code? Perhaps RCA selected its value during production.

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 Post subject: Re: RCA WV-77A
PostPosted: Mar Sun 04, 2018 10:24 pm 
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Greetings to Steve and the Forum:

Sorry, R21 has been tossed... its leads were cut too short for re-installation and it had escalated to something close to 600 ohms as I recall. It was marked as a 470 ohm resistor as called for in the schematic. I don't believe it has any effect except on AC ranges, and after its replacement, the AC measurements were more accurate.

I had another brainstorm and I decided to see if the meter movement had a tired magnet... after all, it is 60+ years old. I had four meters that were of an appropriate range to verify the sensitivity of the RCA meter. I placed them in series with the RCA meter one at a time and here are the results:

One was an old Simpson panel meter which differed from the RCA meter by 15%, so I decided to discard that data point.

The remaining meters were my Simpson 260... on the 1 mA scale (lowest) it is hard to get an accurate reading, but it appeared that 200 uA on the RCA (full scale) was 208 uA on the 260.

A Micronta VOM had a 50 uA full scale range and it and the RCA agreed exactly at 50 uA.

Last, my Agilent DVM read 202 uA when the RCA meter was at exactly full scale.

So, the RCA meter movement appears to be in good shape.

At least, that is one more culprit exonerated.

Regards,

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 Post subject: Re: RCA WV-77A
PostPosted: Mar Sun 04, 2018 10:43 pm 
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Jthorusen wrote:
Sorry, R21 has been tossed... its leads were cut too short for re-installation and it had escalated to something close to 600 ohms as I recall. It was marked as a 470 ohm resistor as called for in the schematic. I don't believe it has any effect except on AC ranges, and after its replacement, the AC measurements were more accurate.

The way I read the schematic, it's in-circuit for AC and DC+, but not DC- :shock:

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 Post subject: Re: RCA WV-77A
PostPosted: Mar Mon 05, 2018 12:17 pm 
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General note on these critters ... sometimes resistors were selected in manufacture at the time of calibration to bring them into specs. Also make sure your pegged potentiometer is not itself off value. I generally don't do wholescale replacement of resistors in any measuring instrument without first doing a quickie test of all ranges to see where it sits, just for this reason.

I suppose in the final analysis, if it takes a resistor in series or parallel with your adjustment pot to bring all test ranges into compliance and center up the pot, that's not entirely a bad thing either. Bottom line is ... is the instrument accurate. But as you see, some changes might have an effect on AC, some on DC, some on Ohms, and some on various combinations of those. You've gone this far however, so at this point, "make it work right" trumps whatever the original values were.

Or, you can pick up another one at a flea market, hamfest, or auction for $25 or less and start over. Or a garage sale, where I scored one for $2 (not a "c" tho). There are lots of them out there.

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 Post subject: Re: RCA WV-77A
PostPosted: Mar Mon 05, 2018 1:55 pm 
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Jthorusen wrote:
--- snip --- So, the RCA meter movement appears to be in good shape. ---snip ---


Will it pass the "balance test" here:? http://www.crankyyankees.com/Analog%20Meter%20Balancing/Meter%20Balancing.html


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 Post subject: Re: RCA WV-77A
PostPosted: Mar Mon 05, 2018 10:46 pm 
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Greetings to Barry, Blast, Steve and the Forum:

Regarding Blast's comment, I did attempt to balance the meter as it was a ways out when I got it. I got it closer, but not perfect. There is about 1 small division difference between horizontal and vertical now... before there was a difference of about 4 small divisions.

I am pretty ham-fisted when it comes to delicate stuff, so I am going to leave well enough alone.

(I agree with another broadcast engineer: "I am not going to work on anything that is too small for me to walk into it.")

As far as picking up meters on the cheap, that's exactly what I did. I bought every one I could find at the local ham swap meet in February. I got the RCA for $5 as I recall, a Heathkit IM-18 with book and probe for $8 and an IM-28 for $4. Sadly, the latter has a bad meter movement, but I have another one coming.

My best score of the day was an HP 410B for $5. It had broken meter glass and the meter wouldn't move, of course, so I offered the guy $5 and he took it. I happened to have a spare 410B meter movement in my junk box, so I figured I could get it going. When I got it home, I cleaned the glass fragments out of the meter and replaced the cover with one from another HP meter that I had in stock. Lo and behold, the original meter movement survived and works OK... so I still have a spare 410B meter movement in stock. The meter itself had a leaky 3300 pF mica cap on the grid of the input DC amplifier which caused the meter to be very insensitive. I was surprised because HP normally sourced good parts and micas were supposed to be very long-lived. I even posted about it in Tubes and Parts. Anyway, with a new 3300 pF cap and new meter front cover, my $5 HP 410B works just fine.

I am trying to put together some meters for a lab for a basic electricity / electronics course. Hence the requirement for a fairly large number of meters on the cheap. I don't know if the class will ever actually happen, but if it does, I want to be prepared... but I don't want the preparedness to bankrupt me. :D

Because of the application, I may just add a couple of K ohms to the cal pot and let it go at that. In looking at the schematic, it would seem that the bridge type DC amplifier used here should be fairly critical as to gain based on the cathode and plate resistors, which are out of tolerance. What I am seeing is a lack of gain. The cal pot is essentially across the meter, so when it is at max resistance (where it needs to be to get the meter to read correctly) the maximum unbalance current is available to drive the meter. Hence, lack of gain.

I suspect that there is some coupling of signal from one side of the bridge to the other by means of the two cathode resistors. Both of these resistors are out of tolerance high (1.7 K instead of 1.5K) and this might reduce the amount of coupling to the opposite side of the bridge (the side with the grounded grid). It might be instructive to breadboard the circuit and see what components have the most effect on its gain..... and how far one can depart from the design values before linearity suffers.

I am not a good enough engineer to answer those questions without empirical evidence... and I don't know if I will ever be ambitious enough to build the breadboard test article to get the empirical evidence. If someone here wants to undertake a theoretical circuit analysis approach, I'll be glad to listen. :D

Thanks all,

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 Post subject: Re: RCA WV-77A
PostPosted: Mar Wed 07, 2018 2:46 am 
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Greetings to the Forum:

Fixed it. Steve Byan has been very kind in catching my errors reading the schematic, but even he cannot catch them all. :D

I made two errors which canceled each other out, leading me to the correct conclusion.

The first error I made was mis-reading the schematic so as to think the meter cal pot was across the meter. It is not, it is in series. The second error that I made was inverting the pot's rotation.... I thought it was at maximum resistance. It was not, it was at minimum resistance. So... the errors canceled out leading me to conclude that the DC amplifier was lacking gain, which it was.

Addition of a series resistance to the cal pot quickly pointed up my errors, so I had no choice but to fix the amplifier.

There were four out-of-tolerance resistors that I had not changed. The two cathode 1.5K resistors which had escalated to 1.7 K and the two plate resistors of 15K and 12K which had escalated to 18K and 14K respectively. In the interests of gathering data, I replaced the two cathode resistors first and then checked the calibration. This turned out to be the cause. Although the pot remains near one end of its rotation, there are now 30 to 40 degrees (estimated) of rotation remaining at the correct position. A 9 volt battery reading 9.5 volts can be made to read almost 10 volts at the end of the cal pot range; before there was only about .1 volt of headroom.

I went on to replace the two plate resistors. This made almost no difference at all. It would seem that the cathode resistor value is the most critical in the circuit. I don't have more specific data for you about the magnitude of the change due to replacing the plate resistors because Murphy was in full force as usual. Apparently, my 9-volt battery that I was using as a test voltage source had its terminals shorted by the clutter on my bench while I was replacing the plate resistors. When I went to check the calibration afterwards, the meter read only 9 volts whereas before changing out the plate resistors, it was reading 9.5 volts. A little experimentation and checking against my Agilent DVM revealed that the battery had lost half of a volt during the time I was working on the plate resistor change, but not before I lost any data that would accurately measure the improvement due to replacing the plate resistors. All I have was the initial observation that the meter was showing 9 volts after the change which was correct but I didn't know it. However, I believe the improvement from changing the plate resistors was minimal.... correct cathode resistors are the trick.

So.... I will button up the RCA and on to the Heathkit IM-28. Many thanks to all for the help here and especially to Steve who was very patient with my blundering about the schematic.... and who supplied me with the correct one.

Regards,

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Jim T.
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