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 Post subject: Restoring an RCA WV-98C Senior VoltOhmyst VTVM
PostPosted: Jun Wed 03, 2020 5:29 pm 
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I’ve got three WV-98C’s on my bench. Even on the newest one, at least one of the range resistors are out of spec and so need to be replaced. It looks like the only way to work on the range switch is to remove it from the PC board, including disconnecting a few zillion wires. Fortunately Jim (rja2907@comcast.net here on ARF) provided a copy of the assembly manual for the kit version of these meters:
https://antiqueradios.com/forums/viewtopic.php?t=361114

So it looks like I’m going to have to partially re-kit these meters. Has anyone done this before? Any tips?

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Last edited by stevebyan on Aug Thu 13, 2020 8:48 pm, edited 2 times in total.

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 Post subject: Re: Any tips for restoring an RCA WV-98C?
PostPosted: Jun Wed 03, 2020 9:31 pm 
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Wowzzers Steve. Only yesterday I dragged all 4 of mine home, from the file drawer from whence they came. Someone here might want one so I figured I'd do all 4.

I have no hints ... but probably will lol. We can compare notes here. I have two HiFi units ahead of these though .. you may beat me to it.

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 Post subject: Re: Any tips for restoring an RCA WV-98C?
PostPosted: Jun Thu 04, 2020 7:39 pm 
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Location: Portland Oregon
Hi Steve,

I just restored a WV98B - which I believe is very similar to the WV-98C.

Replacing components from the board is easy peasey - for today's resistors are much smaller than the ones on the board and the odd sizes can be created by series/parallel resistors. Using a solder sucker made removal of the out of spec resistors very easy.

As far as the resistors located on the switches, I didn't really do it right, but I got it done by clipping off the old resistors and leaving a short end to wrap and solder my new resistor. I had to replace probably 80% of the resistors (although I replaced anything that was over 5% out of tolerance - even though they may have been a 10% tolerance resistor - and replaced with 1% or better Metal Film), the 10 uF capacitor, and clean up the switches, etc. I had to put on a new cord and I put 1N4007's across the main meter. I left the germanium diode in place because it seemed to be working fine. I originally replaced it with a silicon 1N4007, but I had difficulty zero'ing the meter. I didn't look into it very much but just reinstalled the germanium diode and everything seems to be working great.

I calibrated the 98B according to the manual spec and found the meter to be surprisingly accurate and well within spec. I was very fortunate to find a new standard probe, the High Frequency probe, and the High Voltage probe too. I think I will keep this the RCA 98B VTVM and sell/give away my Eico 249. I have my Heathkit IM-28 as a second VTVM.

Dave


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 Post subject: Re: Any tips for restoring an RCA WV-98C?
PostPosted: Jun Thu 04, 2020 8:47 pm 
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TechyMechy wrote:
As far as the resistors located on the switches, I didn't really do it right, but I got it done by clipping off the old resistors and leaving a short end to wrap and solder my new resistor.

Thanks for the tips, Dave. I don't mind using quiggs where necessary, but there's enough wiring in the way that I don't think I could replace the dividers on the range switch without melting the insulation on the wires. I notice they don't all have the same wire routing, so some examples might have reachable range dividers.

Attachment:
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IMG_8637.jpg [ 830.93 KiB | Viewed 2135 times ]

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 Post subject: Re: Restoring an RCA WV-98C
PostPosted: Jun Thu 04, 2020 10:02 pm 
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I have a batch of three RCA WV-98C VTVMs on my bench for restoration.

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They aren’t the easiest VTVMs to work on. The three meters are from three different production runs: K114, K117, and L60.

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I started by removing the meter and the front panel.

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This leaves the lower support bracket still blocking access. I removed it by loosening the control nuts on the range and function switches.

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There are still a number of wires between the lower support bracket and the PCB.

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I removed the nuts on the Zero and Ohms controls and then unsoldered the wires from the Amphenol connector.

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I fed the ground clip wire through the bracket to get enough slack to set it aside. In this example, I had already cut off the line cord as the insulation was cracked at the Heyco strain relief clip.

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For the other units, I used some cheap Heyco pliers to remove the strain relief.

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I then fed the line cord through the hole to make slack for the bracket.

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I could have left the potentiometers mounted on the bracket, as they were still in the way and so I unsoldered them from the PCB.

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This layout diagram was really useful in locating the wires on the solder side of the PCB. The top of the PCB has silk-screened numbers where the wires connect to the board, and the diagram annotates the solder side with those numbers.

I don’t remember where I got this diagram. It’s much more legible than any of the PDF manuals for the WV-98C that I have. I might have scanned it from my paper copy of the manual.

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I loosely remounted the pots on the lower support bracket, so I wouldn’t get them mixed up.

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The range and function switches are mounted directly to the PCB. There’s a good bit of wiring from the switch decks to the PCB, making it difficult to replace the resistors in the range divider.

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I plan to remove the switches from the PCB. Fortunately Jim (rja2907@comcast.net on the Antique Radio Forum) provided a copy of the assembly manual for the kit version of the WV-98C.

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One interesting difference between the earlier production runs, K114 and K117, and the later run, L60, is in the connection of the ground clip lead. In the photo above of the L60, the ground clip lead is soldered to the ground connection from the function switch at the ground lug on the Amphenol connector.

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Whereas on the K114 and K117 runs, the ground clip lead is soldered to the ground connection on the function switch at the lug on the switch.

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 Post subject: Re: Restoring an RCA WV-98C
PostPosted: Jun Fri 05, 2020 10:16 pm 
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Location: Florida
I have a 98-C that had it's insides contaminated by alkaline battery goop and fumes. The goop cleaned easily, the fume damage didn't. Things would look clean and all would be well but then leakage paths would return after a few days. This happened several times.

I had to re-work the movement so that the metal scale plate is insulated form the movement frame and take the function selector switch completely out and clean it. The switch looked clean but the fumes had contaminated the phenolic, causing goofy things to happen. The cure was a couple of hours in a homemade parts washer filled with water and tide. After all this fun the meter was happy and still is.

RRM


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 Post subject: Re: Restoring an RCA WV-98C
PostPosted: Jun Mon 08, 2020 10:30 pm 
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Update:

I started reversing the steps in the WV-98C kit assembly instructions. I started at Section C, “Assembly of Range Switch, S-2”, step 50 and worked backward.

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I had been looking at the unit from run K114 as I planned out removing the range switch.

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However, about half-way through disconnecting the range switch wiring, I noticed that the range switch mounting nuts on the run K117 unit had been soldered to the mounting screw. Oops.

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The mounting nuts on the run L60 unit are also soldered.

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Fortunately after removing as much solder as I could with my trusty Soldapult solder-sucker ...

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… I was able to remove the nuts and washers.

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This is the run K114 unit. The wire colors don’t match those in the kit assembly manual. I took notes of the actual wire colors as I worked backward through the assembly instructions.

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Here’s the K114 range switch.

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It’s be much easier to replace the range resistors now.

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It’ll be a little easier to work on the PCB, too, with the range switch out of the way.

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This is the run K117 PCB …

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… and the K117 range switch. The K117 wire colors also don’t match the kit assembly instructions nor do they match run K114.

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Here’s the run L60 PCB ...

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… and range switch. Again, the wire colors match neither the kit assembly instructions nor any of the earlier production runs.

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I’ll need to refer to this spreadsheet when I reassemble the range switches.

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 Post subject: Re: Restoring an RCA WV-98C
PostPosted: Jun Mon 15, 2020 9:37 pm 
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Update #2

I’ve completed measuring the resistances in the three VTVMs. Click the image for a readable version of the table.

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Image
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The precision carbon film resistors in the older units, runs K114 and K117, have aged very poorly. Most are out of tolerance. The resistors in the newer run L60 unit have faired better, but some of the higher values are out of tolerance.

The 5% carbon film resistors have held up remarkably well, much better than the precision 1% carbon film resistors, in all the production runs in my sample. Most are still well within spec, except for the highest value, R11, 16 MΩ. It’s typical for higher valued resistors to have more drift over time.

The carbon composition resistors in runs K114 and K117 have aged surprisingly well, with the exception of R30. The schematic shows R30 as 1/4 Watt while the parts list says 1/2 Watt. I haven’t checked what is actually installed. Perhaps it is undersized for the power dissipated in it. The newer run L60 has much more drift in the high values, R25, 3.3 MΩ, and R26, 6.8 MΩ. I guess they used cheaper resistors in the newer run.

Don’t believe the values for the 91 MΩ carbon composition resistors, R10. I can’t accurately measure resistances that high.

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 Post subject: Re: Restoring an RCA WV-98C
PostPosted: Jun Mon 15, 2020 10:37 pm 
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Does anyone know who made these precision resistors? I don’t recognize the logo, nor the initials "RC".

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 Post subject: Re: Restoring an RCA WV-98C
PostPosted: Jun Tue 16, 2020 1:09 am 
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A general question. Have you checked the linearity of the meter movement? Linearity is important for all ranges but especially for the Ohms function.

RRM


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 Post subject: Re: Restoring an RCA WV-98C
PostPosted: Jun Tue 16, 2020 1:27 am 
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stevebyan wrote:
Does anyone know who made these precision resistors? I don’t recognize the logo, nor the initials "RC".

Nor I. A haphazard guess would be Royal Crown? Based on the crown, and the RC of course. ;-). Web search turned up nada. A bunch of dead ends including a current manufacturer of that name in Thailand.

Have to find it in some ancient catalog I guess.

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 Post subject: Re: Restoring an RCA WV-98C
PostPosted: Jun Tue 16, 2020 2:08 am 
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Barry H Bennett wrote:
Nor I. A haphazard guess would be Royal Crown?

Royal Crown Cola used to be my favorite soda treat as a kid growing up in Baltimore :-)

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 Post subject: Re: Restoring an RCA WV-98C
PostPosted: Jun Tue 16, 2020 2:13 am 
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Retired Radio Man wrote:
A general question. Have you checked the linearity of the meter movement? Linearity is important for all ranges but especially for the Ohms function.

Hi RRM. No, I haven't checked the meter movement linearity. Thanks for the reminder, it might save me a good bit of work if a meter movement is bad.

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 Post subject: Re: Restoring an RCA WV-98C
PostPosted: Jun Wed 17, 2020 9:12 pm 
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If, anyone needs parts----I have 2 units restored accept meter movement bad?????

Jim


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 Post subject: Re: Restoring an RCA WV-98C Senior VoltOhmyst VTVM
PostPosted: Aug Thu 13, 2020 9:03 pm 
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Update #3

Retired Radio Man asked if I had checked the linearity of the meters before embarking on the restoration. I hadn’t, but fortunately hadn’t started replacing the bad resistors. It would have been disappointing to put all that work into a unit only to find that the meter movement was kaput. So, I went about checking out the meter movements.

My first task was to zero the movements.

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RCA didn’t provide a zero-adjustment screw for the WV-98 meters. Instead they provided a small hole in the front panel, covered by a plug, that provides access to the zero adjustment tang on the meter movement.

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You have to use a small tool such as a tiny screwdriver to reach in and nudge the adjustment tang. You also have to be very careful to not put the tool in too deep, as that could damage the meter armature.

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As a final obstacle, the meters came from RCA with a piece of cellophane tape covering the hole in the meter bezel that gives access to the adjustment tang. You can try puncturing the tape, but if you do you run the risk of the tool going too deep and damaging the movement.

The tape had been removed from one of my meters, so that was easy to zero. On the other two, I had to remove the meter from the VTVM front panel. Then I discovered the first meter was out of balance, so I had to remove it from its front panel as well.

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To remove the meter from the front panel, remove these four nuts and lockwashers. (The other two screws held the upper support bracket to the front panel.)

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Then push the top of the meter out from the front panel. The clearance is tight on the two lower screws, but with a bit of jiggling they will come free.

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I carefully removed the meter cover to get access to the weights to rebalance the meter. To remove the cover, carefully release the two tabs at the top. These tabs fit into two recesses at the top of the meter housing. You can use a 3/16” bladed screwdriver to gently pry the top of the cover away from the meter case to release a tab.

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I now had access to the quad. The quad is the cross-shaped piece mounted to the meter pivot. The needle attaches to one arm of the quad. In the photo you can just see the tail weight on the arm of the quad opposite the needle. It’s the helical spring just above the solder joint for one end of the armature winding.

The balancing process is described well in several posts here on the Antique Radio Forum, so I’ll just provide links to them instead of repeating the process here:


The next step was to check the linearity of the movements, using my HP 6920B meter calibrator. I collected data on the current at each cardinal point on the meter scale. I started the run at zero and increased the current to reach each point, up to full-scale. The error is calculated as a percentage of the measured full-scale for that particular meter.

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The error of the meter from the K114 run (red plot) was a bit high on the upper half of the scale, although the total variation is within ±1%. The meter from the L60 run (yellow plot) looks better. But when I ran the meter from the K117 run (blue plot), the error was very high at mid-point, and the meter seemed sluggish getting past the half-way mark. I ran a curve starting from full scale and going back down to zero, to see if there was a lot of hysteresis (green plot). Some hysteresis is evident, but it’s not very bad.

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I suspected there was some foreign matter interfering with the movement of the armature, so I removed the meter card and went fishing with a sliver of Scotch tape in the gap. I didn’t find anything. so my next guess was some slight corrosion of the meter pivot. This VTVM showed evidence of battery leakage at some point in its life.

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I remembered a post by Chris H here on the Antique Radio Forum. He had suggested exercising a meter with a function generator to clear up minor corrosion in the bearings.

I set my B&K function generator to produce a 0.125 Hz triangle wave with a DC offset so the waveform went between zero and about 2 Volts. I connected each of the three meters to the function generator output through a 10K Ω resistor, giving a full scale of about 200 µA at the peak of the triangle wave.

Here's a video of the process:
https://www.byan-roper.org/steve/_Media/img_8730.m4v

I exercised the meters for a few hours one evening. Afterward, the problematic meter ran smoothly through mid-scale.

I then took a series of measurements on each meter, both running up the scale and running back down. Again, the error is calculated as a percentage of the measured full-scale for that individual meter. I took four sets of measurements in each direction for the run K114 and run L60 meters, and two sets of measurements in each direction for the run K117 meter. I plotted the average of these sets of measurements, along with the initial, pre-exercise, measurements.

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This is the plot for the run K114 meter. Comparing the linearity of the initial (red plot) and final (blue plot), you can see a small improvement. The hysteresis looks pretty bad though, nearly a half of a percent in the worst case.

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Here is the plot for the run K117 meter. Exercising it with the function generator greatly reduced the sticking at the half-scale mark. The overall linearity is now pretty good.

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This is the plot for the run L60 meter. The initial (red plot) and final (blue plot) have about the same total error (about 1.2%), but the post-exercise error is shifted up. There is also quite a bit of hysteresis evident.

L60 is a fairly late run, with the new-style RCA logo and the new blue paint scheme in place of the old gray hammertone. I suspect the meters in these later runs are a bit crappier as a result of cost-reduction.

Now that the meters checked out as usable, ...

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… the next step was to proceed with replacing the out-of-spec resistors. Here I’ve completed reworking the range switch for the unit from run K114.

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And here is the completed PCB for the unit from run K114.

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The “before” photo for the K117 run.

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All of the DC range resistors had to be replaced.

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Another view of the K117 run range switch with new resistors.

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And here’s the K117 run PCB. Sorry it’s out of focus.

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Here’s the range switch from the run L60 unit. Only a few of the resistors needed to be replaced.

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The run L60 PCB with its replacement resistors.

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I reworked the power supplies by replacing the original electrolytic with a modern radial-leaded part, and the rectifier by a 1N4006.

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I’ve reinstalled the range switch and am about to reconnect all the wires to the PCB.

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Reconnecting the range switch wasn’t too bad. I just followed the kit instructions. It was very helpful to have recorded the wire colors for each step, back when I disassembled it.

I’m undecided whether I’ll complete the L60 unit before moving on to the others, or do it assembly-line style and reinstall the range switches in the other units before reconnecting the controls and connector on the lower support bracket.

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 Post subject: Re: Restoring an RCA WV-98C Senior VoltOhmyst VTVM
PostPosted: Aug Thu 13, 2020 9:14 pm 
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By the way, that hysteresis in the meters between moving up and moving down is why you should gently tap the meter with your finger when taking a reading.

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 Post subject: Re: Restoring an RCA WV-98C Senior VoltOhmyst VTVM
PostPosted: Aug Thu 13, 2020 9:20 pm 
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stevebyan wrote:
I’ve got three WV-98C’s on my bench. Even on the newest one, at least one of the range resistors are out of spec and so need to be replaced. It looks like the only way to work on the range switch is to remove it from the PC board, including disconnecting a few zillion wires. Fortunately Jim (rja2907@comcast.net here on ARF) provided a copy of the assembly manual for the kit version of these meters:
https://antiqueradios.com/forums/viewtopic.php?t=361114

So it looks like I’m going to have to partially re-kit these meters. Has anyone done this before? Any tips?


are you the guy that paid almost $300 on ebay for that the other day.

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 Post subject: Re: Restoring an RCA WV-98C Senior VoltOhmyst VTVM
PostPosted: Aug Thu 13, 2020 10:07 pm 
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ttx450cap wrote:
are you the guy that paid almost $300 on ebay for that the other day.

No. $300 for a non-restored RCA WV-98C? Sounds like some shill bidding to me, no sane person would spent that for an unrestored WV-98, and even a restored one is unlikely to sell for that high a price. Unless it's in the original box or something.

Edit: I found the auction. It is one of the cleaner WV-98C's I've seen, especially for being from one of the earlier runs (old-style paint scheme, logo, and knobs). And an intact WG-299D probe is worth a bit by itself. Also it is guaranteed to be "working". I doubt it's been recently calibrated, though.

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 Post subject: Re: Restoring an RCA WV-98C Senior VoltOhmyst VTVM
PostPosted: Aug Thu 13, 2020 11:40 pm 
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stevebyan wrote:
ttx450cap wrote:
are you the guy that paid almost $300 on ebay for that the other day.

No. $300 for a non-restored RCA WV-98C? Sounds like some shill bidding to me, no sane person would spent that for an unrestored WV-98, and even a restored one is unlikely to sell for that high a price. Unless it's in the original box or something.

Edit: I found the auction. It is one of the cleaner WV-98C's I've seen, especially for being from one of the earlier runs (old-style paint scheme, logo, and knobs). And an intact WG-299D probe is worth a bit by itself. Also it is guaranteed to be "working". I doubt it's been recently calibrated, though.


I'll post the ebay link later. yes, one guy was in the 100 range and last min someone paid almost 300. It was in very nice condition and I was dreaming. I suspected it was a youtube blogger wanting it for there youtube show. who knows.. i hate seeing prices being driven up to high. then the next person see what it was sold for and the price gets $$$$. I wanted to get it, but at $250 out of my range. it was clean looking..

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 Post subject: Re: Restoring an RCA WV-98C Senior VoltOhmyst VTVM
PostPosted: Aug Fri 14, 2020 12:07 am 
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at the last minute bids didn't go from 100 to 300, at 2 were bidding bids to drive it that high, with one going 299 to drive a 300 bid.
One a shill and the other an idiot.

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