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Author Topic: RCA Senior VoltOhmist Probe Question  (Read 4514 times)
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WA2POW
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« on: June 10, 2017, 07:35:28 AM »

I picked up an RCA WV-98C VTVM with the WG-299D DC-AC/OHMS Probe. It looks like the Probe cable was replaced since it seems too new and not very flexible. It is coax, has no markings, diameter is RG-8X /RG-59, and the length seems too short for actual use.

Can anyone tell me the overall end-to-end cable assembly length, meaning Probe tip to Connector Contact? Also, is the "correct" cable RG-58?

TNX es 73, Bob  WA2POW
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Jim, W5JO
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« Reply #1 on: June 10, 2017, 08:37:22 AM »

The one I have is 53.5 inches long and is made of Belden 8218 cable.  The probe is from a later 98C and the cable is quite small. 

The cable is marked 75 ohms.
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k4rhh
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« Reply #2 on: June 10, 2017, 11:54:20 AM »

MOST TEST LEADS USED ON TEST EQUIPMENT WAS VERY LOW CAP CABLE LIKE RG 62  93 OHMS.RANDY K4RHH
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WA4WAX
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« Reply #3 on: June 10, 2017, 12:42:26 PM »

Dear Bob:

Let me help:

The WV-98C is your typical 11 Meg input VTVM.

Short term ideas:

1) Check all of the voltage divider resistors and the probe resistor (1 Meg).  Replace those out of tolerance with 1/2 watt metal film resistors.  You want resistors to be within 1% of spec, or better.  Same goes for resistors in the ohms chain.  Meter performance will not be good if resistors are out of spec!  No way to compensate here.  It must be right.

2) Lose the mic connector.    Put a nice female BNC (UG-1094) in its place.  Silver plated desirable.  You will not have to ream or drill.  Hole size is the same for both.  BTW, if you have gear with phone tip jacks, remove and replace with female banana  post.  Much better! Again, same sized hole for both.

3) Remove the existing cable from the probe.  Get a 3 to 3.5 foot (length unimportant) piece of RG-223 with a male BNC on one end.  Strip the other end nicely, and put it in the probe.  Make sure you have strain relief.  The center conductor goes to the resistor.

4) Get a Fluke 85 or 85 II RF probe, along with a male BNC to female banana adapter.  When using the Fluke, put the adpater in the BNC jack.  Depending on probe purchased, you may measure RF up to 500 MHz!  Your limit is 30 volts RMS with these probes. BTW, the banana adapter is also good for measuring resistors out of circuit, especially in the lower ohms range.  OK, not as good as a 4 wire meter, but not bad!  Make a short bar (R/C battery bar good) with two male banana tips.  Set infinity and zero with short bar out and in. It may take two or three iterations before infinity and zero coincide with short out and in, respectively.  Now, put resistor across screw terminals.  Make sure zero is set.  Now. pull the short and read ohms.  :-)  BTW, you will run the Fluke RF probe with the set to DC +.

5) Some cat on Ebay sells a little book on VTVM upgrades.  You want to get this book.  It shows you how to lose the battery and the 6AL5.  The 6AL5 will be replaced by a pair of 1 KV (or higher) silicon diodes. The battery is replaced by a silicon rectifier and a decent sized electrolytic.  Good bye to 6AL5 contact potential and all of that stuff.

6) Replace all electrolytics with long lived Nichicon or Panasonic.

7) Make sure that bridge tube is well balanced. Find a guy with a Hickok or jackson if you do not have one.  Also, check match of the fixed cathode and plate resistors.  If they are anything but metal film, you might as well replace them.  Burn the bridge tube in for 75 to 100 hours before you calibrate.

Cool Give all switches the deoxit treatment, but be mindful of the plastic parts.  Replace any scratchy or leaky pots.

Have fun!
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W6MQI
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« Reply #4 on: June 14, 2017, 02:05:05 PM »


3) Remove the existing cable from the probe.  Get a 3 to 3.5 foot (length unimportant) piece of RG-223 with a male BNC on one end.  Strip the other end nicely, and put it in the probe.  Make sure you have strain relief.  The center conductor goes to the resistor.

I have the WG299E probe how does the probe come a part? I've tried to unscrew it, pull it etc. Nothing seems to work I'm afraid of breaking it gotta feeling this thing is glued together.

Dave
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WA2POW
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« Reply #5 on: June 15, 2017, 10:14:57 AM »

TNX fer the data es info. I was planning on replacing the precision resistors. Didn't think about solid-stating the rectifiers OR the resistor in the Probe, but will do everything at once.
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