Posts Tagged ‘schematics


On the bench.. VTVM Test probe kit.

This a continuation of the repair and restoration of a Simpson 303 VTVM.

The Blue Mat Group Member Series

The final decision was to purchase this kit. $21.35 seemed pricey but the alternative was to scrounge around for parts and/or order the balance of parts I didn’t have. Considering what i would have to do and I wanted to get this job up and running to have as my main meter, I placed the order (eBay).

After receiving this kit I realized that this was the better choice. The coax cables were just the right size and flexible. The other included test lead wire quality matched to boot. All the parts needed were there , no need to raid the parts bins. Plus I would have some parts left over for the parts bins.

The kit, great packaging, recyclable for parts storage or use as seller packaging.

The meter waiting to be completed, My combo LED fluorescent bulb lamp in the reflection.

un boxing or un bagging

attention to detail even a Velcro loop is included.

A quick view of the eBay listing, the seller had pictures that answered all my assembly questions. The finish for this project had been on the back burner for weeks. Retrieved the parts bin, now where did I put that ? (mushroom container) for the repair to get the male parts of the connectors. Now where’s the schematic?

It was the normal start at the bench , prep the soldering iron ,replacing a piece. Then locating the schematic to the meter to verify the placement of the inline resistor to the correct probe and available for troubleshooting if required.. Assembly is straight forward, first I assembled the grounding lead, no problem soldering. The seller specified by pictures , 3 inches of stripping for the coax cables. The hardest part is the fishing of the inner conductor down through the probe handle and out through an angled bore at the end of the probe handle in the metal tip.

R29 1 megohm resistor in the probe, supplied by the kit.

schematic , most of it is the multiple decks of the multi layer rotary wafer switches

replacing the barrel sleeve on the iron.

cross section of the supplied wire from the kit

Once through, some of the stripped wire is pulled through and the probe trim ring is screwed to the top of the probe tip, locking the wire in place and making the connection.

The second time around for the second probe, was problematic, The center conductor was tinned,. After repeated ( time consuming) failed attempts, I fished a length of thin wire solder up from the tip and tacked the center conductor lead to it. Acting as a fish tape, I simply pulled the solder wire through. The wire followed !. Done with the probe set , nice. It’s a bit frustrating and time consuming fishing wires but the probe handles are permanently attached (as i first tried to unscrew them) so they won’t unscrew and loosen as another set I have does frequently. The completeness of the kit even includes labels for the probes and transparent heat shrink . Well Done !

The 1 meg ohm resistor soldered to the center conductor

the angled bore at the top of the probe tip.

using the solder wire as a wire puller

No problem, a time saver I should have used at the start.

tinned lead

folded back for connection to the male coax connector

I soldered the shield braid to the metal bushing

The completed connection less the outer part of the connector

The outer part just screws on with the nut, backed up the small cable with a few layers of shrink tubing.

The completed male end of the probe lead

On the other end the wires had to be stripped to connect the lead to the coax connector, supplied by me as I replaced the original connectors on the meter when rebuilding it. those male connectors are not available and not included when I bought the meter.

.At first the leads did not work ! Guess what ? The first tech help question “is the unit plugged in” applied. Duh..

Another problem was that on the ACV/ohm , the ohms reading was shorted and not reading. The problem was that the cable braid was shorting to the center conductor after the connector was assembled. I trimmed back the braid, it;s not necessary to connect it at the coax end of the lead.

A final check of the meter was done now and revealed that any AC voltage readings were not properly registering . Ouff! Troubleshooting time. Was the rectifier tube faulty? The tracing reveled an error either in the wiring or a crazy schematic.

Now the time began to fly. Mistakes by tracing the schematic wiring from one page to another lead me down a path that cost time. I pulled the stapled schematic paged apart folded them and joined the pages to indicate the correct path from one page to another. One problem solved. Now a broken wire to nowhere was found from one of the wafers . With the proper tracing, it was discovered that one of the capacitors replaced was in error, going to ground instead of going to the probe connector and to the tube.. Most of the tracing of this device is done by tracing layers of selector switches connected to banks of resistors. Keeping my fingers crossed the meter was plugged in and set to measure AC line voltage. Voila ! Success. Luckily the mistake did not damage the meter when I checked it with 120vac previously.

All in all, including making a cup of coffee break , the bench time was from 13:10 to 18:03 . Taking my pleasurably time with soldering and troubleshooting was worth it. The meter is now ready to go as the humidifier repair is next on the bench.

Now with the purchase and rebuilding of the meter and the probe kit the best part, of course is the use of an analog meter, a Simpson. I would do the required Happy Dance, but I don’t do dances.

1 megohm reading

Final AC line voltage test , before calibration

The coax connections offers better shielding when you need a accurate reading of low ac voltages and the proper isolation for high voltage readings .

5 stars for the kit, those who frequently order parts know the time that’s involved if one had to do all the working ordering all the parts included in this kit.


February 2023

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