Electronics – Page 4 – AB4UG Radioblog

RCA Senior Voltohmyst

The RCA WV-98C Senior Voltohmyst (SN 201001 1104)I picked up at the hamfest yesterday looks to be in pretty good working order.

No smoke, popping or other unusual sounds/odours when I plugged it in. Some initial testing with batteries and some resistors suggests a bit of recalibration or maybe a bit of repair might be needed though.

Fortunately I was able to find a manual for it at the Boat Anchor Manual Archive.

Removing four screws from the back lets you take the back cover off (a fairly substantial chunk of cast aluminum) revealing the innards of the meter.

The blue adjustment screws are trim pots used for adjusting the meter calibration.

One thing that surprised me was the presence of a soldered in D cell.

Top part of the Voltohmyst. Badly corroded 1.5V D cell and transformer

No idea what vintage the battery is or what it’s used for (at the moment), but my multi-meter showed it still had 1.4 V across it. The battery has definitely seen better days. I think I’ll see about replacing the battery with a battery holder so it can be removed and replaced in the future. Fortunately it looks like the wires have enough slack to work with.

The rest of the meter looks to be in pretty decent shape, and pretty clean. Without taking off the face of the meter, it’s a little hard to get in there to check out the middle. There are two tubes in the unit, a 12AU7 and 6AL5.

Left side with the range selector switch and 6AL5 tube

Lots of colourful wires at the range selector and mode selector switches.

The probe for the meter is a big chunky thing, about the size of a Sharpie marker.

Next step will be to go through the manual and read a bit more about the meter works before I try to dive in and replace things.

Power supply: It lives!

The power supply lives!

On my initial inspection of the power supply, the only obvious thing wrong that I had found was a blown fuse. After replacing the power cord, I noticed the power on lamp had burned out too.

Off I went to the last remaining Radio Shack in my area (a franchise store, also known as Hurricane Electronics) to see if I could find some fuses and a replacement lamp. Found some replacement fuses easily enough (35V, 20A), and much to my surprise, replacement bulbs that were the exact same style as what was already on the power supply.

Replacing the light was easy enough, but took a bit longer than expected. The original pair of wires for the light kept breaking when I tried to put the light back into place, so I ended up just replacing the two wires with some 18 gauge stranded wire I had. Once the light was back in place, I plugged the power supply in and on came the light. Yay!

Then I replaced the fuse, turned it back on and was greeted with the meter telling me there was 13ish volts. With my DMM, I read 13.3 V DC at the meter.

I tweaked the pot at the control board to bring it up to 13.7 V DC.

So it looks like the only problem with the power supply was the blown fuse. Now to see how it works with a load applied.

Power supply: Power plug replacement

With the power supply schematic mostly done (I need to make another pass through to check that I got everything right), I decided the power supply needed a new input plug.

The power supply came with a two prong plug and a smaller two prong socket for power input. At some point someone had soldered a regular two prong power cord to the plug, which I quickly removed (mostly to get it out of the way).

The smaller socket (the round one on the right in the image below) looked like it might have been something standard decades ago, but didn’t look like anything I recognized.

The plug and socket both came out, and I dug out an old three prong power cord from the junk box. After spending some time figuring out how to connect things, I wired in the power cord.

Inside I used a terminal block to connect the power cord with the wires that used to run from the original plug.

Ground on the power cord got tied to the chassis, while the neutral line went straight to the transformer, and the hot line to the power switch.

Plugging it in and pushing the power switch made the transformer produce the characteristic buzzing sound, but it looks like the power indicator lamp is burned out. 23.2 V AC measured on the output of the transformer, which seemed reasonable. No smoke released, which is a good sign.

Now that I can get power into the power supply, I can start figuring out what works and what doesn’t.

Power supply: Out to the control board

I’ve finished tracing the power supply schematic out to the control board. I’ll go through it once or twice more to make sure I’ve got everything right, but I think I’ve got a fairly complete schematic of the power supply now.

Here’s the schematic so far.

I did run into an anonymous transistor type part with a red case that I wasn’t able to identify. Whatever markings were on it have been rubbed off, so there’s no way to really identify it anymore.

The base of the transistor (I’m assuming it’s the base anyway) is connected to pin 10 of the 723 voltage regulator while the emitter is connected to the bases of the power transistors. The collector goes to pin 12 of the 723 and the collectors of the power transistors. Checking with schematics for my Astron RS-35, there’s a TIP29 transistor in roughly the same spot, so I’m reasonably confident this mystery part is an NPN transistor of some kind.

Tracing out the control board was a bit tricky, but I think I managed to get it right (I think I’m missing the diode on the schematic though). I’ll need to recheck where the wires coming off the control board go in the rest of the power supply.

Quite impressed with the way this power supply was put together. Everything is soldered or screwed together nice and cleanly, and the wires are nicely dressed and bundled together with zip ties and string.

Except for the blown fuse, I don’t see anything obviously wrong with the power supply. I think I will look into replacing that AC input plug though before I try applying power to this thing.

Power supply: Power transistors

I’ve traced the power supply schematic past the SCR and out to the power transistors that are attached to the big heat sink. This part of the schematic might be a little questionable because a lot of the wires are covered up or obscured by other components and a little harder to follow.

The schematic so far:

I’m up to the control board now. It’s small, but has a lot of wires coming in and out of it. Fortuantely it’s all point to point wiring on perfboard, so I don’t have to follow any PCB traces.