RCA WV-98C Senior VoltOhmyst VTVM Restoration

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.

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.

Update #2

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

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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.

Does anyone know who made these precision resistors? I don’t recognize the logo, nor the initials RC.

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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.

Update #3

On the Antique Radio Forum, 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 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 over 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.

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. This is why you should gently tap a meter with your forefinger before taking a reading.

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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.

Update #4

I decided to complete the run L60 unit first. 

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I started by reconnecting the front panel Zero and Ohms controls. I made good use of the photos I had taken during disassembly. The photos combined with RCA’s use of color-coded wires made the job pretty easy.

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The next step was to reinstall and reconnect the Amphenol 75-PC1M chassis connector for the probe, and re-mount the lower support bracket. The kit instructions were somewhat helpful, although I was doing some steps out of sequence.

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I still have to construct and install a battery eliminator for the ohmmeter circuit. Unlike the Heathkit VTVMs, the RCA WV-98’s use a floating battery. Neither side of the battery is connected to the chassis.

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My design for the Heathkit IM-13 battery eliminator assumes the negative end of the battery is grounded to the chassis, and the PC layout achieves this through one of the mounting stand-offs. I’ll either have to design a new PCB for the RCA WV-98’s or hack up a mounting method that leaves the negative rail of the supply ungrounded.

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It looks like a VTVM again! And an initial checkout and preliminary calibration shows it to be working, with the exception of the ohmmeter portion due to the lack of a power supply.

One common issue with VTVMs is that the 12AU7A initially has a high grid current. This shows up as the zero drifting up as you switch to the lower voltage ranges. When I first turned it on, when I zeroed the meter on the 50 volt DC range, the needle would drift up to the “5” mark on the 0-50 DC scale when switched to the 1.5 volt range. That’s about 10% of full scale. 

The high grid current is due to gas in the tube, leaking in over all the years it sat abandoned and not in use. After leaving the VTVM powered-on for three days, the getter scavenged much of the gas from the tube and the zero drift on the 1.5 volt range went down to about 1/2 of a division on the 0-50 DC scale, which is about 1% of full scale.

Note that the zero drift is much larger on the 0.5 Volt DC range. The RCA manual instructs you to re-zero the meter when switching to the 0.5 V DC range.

Now that it has aged a bit, I can start to do the final recalibration. I’ll ponder the battery eliminator question for a while, and leave deciding on an approach until after I’ve reassembled the other two meters.

Update #5

I’ve completed calibrating the run L60 unit, except for the resistance ranges.

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Click the above thumbnail for a readable image. The VTVM easily beats its accuracy specification.

© Steve Byan 2011-2019