Electronics – Page 4 – AB4UG Radioblog

A new power supply

One of my acquisitions from today’s TARC swap meet was a variable power supply. The person I bought it from acquired a bunch of these from a school district surplus auction. I’ve been wanting to get a variable power supply for the workbench, so I bought one of them for $20. Seemed like a pretty good deal. Almost grabbed a second one from him.

Banana jacks provide AC and DC outputs, and voltage for both is controlled by the knob on the left. Two meters show DC volts and amps, but if you’re using the AC output, you’ll need to measure it yourself.

The panel indicates the power supply will do 0-20 VDC and 0-25 VAC. With no load, the power supply topped out at 35 VDC and 26 VAC. This is an unregulated power supply, so any load is going to bring the voltage down.

Getting inside the power supply requires removing a total of 18 screws (6 on each side, 6 on the top). Seems a bit excessive to me, but I didn’t design the thing. Once the screws are out, removing the top exposes the innards.

Not much to it inside. There’s a big beefy transformer which accounts for almost all of the power supply’s weight. Voltage control is performed by the variac. A large (and loud) 120mm fan (lower right) provides cooling.

Voltage and current meters — Variac autotransformer

The key just turns a metal plate that flips the actual power switch on or off.

The cooling fan gets its own power supply so that it’s not affected by changing the output voltage. It’s just a 12V wall wart that gets powered from the AC input I think (haven’t traced any of the wires to see what goes where).

Plenty of modification potential with this power supply. I’ve got a small list of easy ones that I think I’ll make:

Replace the key with a regular switch
Switch out the banana plugs for 5-way binding posts and Powerpole connectors
Replace the fan with a quieter one

This seems to be a pretty sturdy power supply designed for the harsh environment of a high school lab. Everything inside looks to be in pretty good condition.

Dead oscilloscope

My oscilloscope died the other day. <sad trombone />

I was using it to look at the output of one of my Etherkit Si5351 breakout boards when it made a high pitched “pffft”, the waveform disappeared and a bunch of indicator lights came on.

Haven’t opened it up yet to see if I can spot anything wrong. The scope still turns on, but there’s no display. The indicator lights behave like the scope is working, but there’s nothing on the CRT. That could be a good sign.

Will be a while before I have some time to poke around the innards of the scope.

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.