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 Post subject: Re: Restoring an RCA WV-98C Senior VoltOhmyst VTVM
PostPosted: Dec Thu 17, 2020 2:59 am 
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Update #10

I’ve finished calibrating the run K114 unit.

Image
Click the image for a full-size view.

It meets its specs, but the 150, 500, and 1500 volt ranges were pretty close to the edge, and the AC zero and 50 volt range calibration were a little flakey. But after a couple of rounds of calibration and checking the lower ranges it seemed to settle in. I’ve put it on the bench to use as the main VTVM for a while, to see if the calibration is going to drift.

I’m reminded of the troubles that Calambres had with his RetexKit VV-1 VTVM (a Spanish clone of the Heathkit IM-11 VTVM). His upper AC ranges were also inaccurate. He solved the problem by replacing the 6AL5. I should give that a try.

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 Post subject: Re: Restoring an RCA WV-98C Senior VoltOhmyst VTVM
PostPosted: Dec Thu 17, 2020 8:28 am 
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Steve;
Your work is always such a pleasure to follow.

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 Post subject: Re: Restoring an RCA WV-98C Senior VoltOhmyst VTVM
PostPosted: Dec Thu 17, 2020 12:36 pm 
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Steve, back when I was building kit VTVM's for myself and several others, they all came with what was called, in various ways, "special factory aged tubes". So I can well believe the tubes might make a difference in the instruments today. I think they were aged as in burned in to minimize drift, but also probably selected for matched sections in each tube. Who knows. Makes some sense though.

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 Post subject: Re: Restoring an RCA WV-98C Senior VoltOhmyst VTVM
PostPosted: Dec Thu 17, 2020 1:47 pm 
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Barry H Bennett wrote:
...I think they were aged as in burned in to minimize drift, but also probably selected for matched sections in each tube. Who knows. Makes some sense though.

Barry, I have always been under that assumption for what it's worth. I have read several times that tubes that have similar (or identical if you are lucky) sections work the best.

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 Post subject: Re: Restoring an RCA WV-98C Senior VoltOhmyst VTVM
PostPosted: Dec Sat 19, 2020 7:29 pm 
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Do you have a list of the resistors you used to replace the out of tolerance resistors?

Also on my WV-98C (VIZ version) someone installed a D cell holder and wired the negative to the chassis. Not sure if grounding the - to the case was a later manufacturing change or if someone just did it wrong. I may just pull the battery since I use a DMM for ohms measurements.


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 Post subject: Re: Restoring an RCA WV-98C Senior VoltOhmyst VTVM
PostPosted: Dec Sun 20, 2020 5:48 am 
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Tube Radio wrote:
Do you have a list of the resistors you used to replace the out of tolerance resistors?

No, I have a large stock of precision resistors and used whatever I had that was the right value.

I buy mostly from Mouser but also from DigiKey. I try to stock Vishay/Dale CC60 resistors when I can, but Mouser doesn't stock every value, and Vishay doesn't make them larger than 1 Megohm. For the higher values, I use Vishay/BC Components HVR37 and VR37 series resistors. I'll use KOA/Speer MF1/2 series resistors if I can't get the others, or KOA/Speer MFS1/2 series if I can't get the larger MF1/2 ones, or at last resort Xicons or whatever else is available in the value I need.

Tube Radio wrote:
Also on my WV-98C (VIZ version) someone installed a D cell holder and wired the negative to the chassis. Not sure if grounding the - to the case was a later manufacturing change or if someone just did it wrong.

According to the schematic, if the negative end of the battery is grounded, the function switch will short the battery on the AC, DC+, and DC- function settings.

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 Post subject: Re: Restoring an RCA WV-98C Senior VoltOhmyst VTVM
PostPosted: Dec Sun 20, 2020 5:51 am 
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Maybe I looked at it wrong.

Or someone could have modded it at some point?


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 Post subject: Re: Restoring an RCA WV-98C Senior VoltOhmyst VTVM
PostPosted: Jan Fri 08, 2021 7:43 am 
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Hello Steve,
I do the same far as my resistors go but it seems like Dale brand is getting harder to buy.


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 Post subject: Re: Restoring an RCA WV-98C Senior VoltOhmyst VTVM
PostPosted: Sep Thu 16, 2021 1:24 am 
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I have heard that some people put back-to-back 1N4001 diodes across the meter movement. Is this advisable?


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 Post subject: Re: Restoring an RCA WV-98C Senior VoltOhmyst VTVM
PostPosted: Sep Thu 16, 2021 4:58 am 
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I don’t think it’s needed. The VTVM circuit inherently limits the voltage that can be applied across the meter and its series resistor.

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