Toolkit – AB4UG Radioblog

Heathkit HD-1250 clean up

The Heathkit HD-1250 grid dip meter (SN 05706) I acquired at the hamfest a few months ago looked in decent condition aside from some pretty major foam rot.

Heathkit HD-1250 grid dip meter
Nasty case of foam rot

Finally got around to getting it cleaned up, which turned out to be more of a mess than I anticipated. Unfortunately, the foam rot wasn’t as easy to clean up as I thought it might be.

The foam ended up disintegrating into an ugly sticky mess. I was able to brush most of it off, but it left a gluey residue on everything.

Sticky foam residue on the coils
Sticky foam residue on the coils
Corroded contacts

I was able to clean off the sticky goo on the coils, but unfortunately whatever it was that the foam disintegrated into also had the same effect as paint stripper. When I tried to wipe the residue off the case, the paint underneath was coming off as well.

Since it didn’t look like I was going to be able to get any of the goo off the case without taking the paint off as well, I decided to just scrape it all off. The case is in three pieces: face plate and two side pieces. Undo four screws at the bottom and one each in the front and back of the case and the case comes apart pretty easily.

The insides look in pretty good shape, aside from a leaky battery that also took the battery connector with it (easy enough to replace). A few more pieces of disintegrating foam that were originally to cushion the battery, but it was just dry and crumbly fortunately.

Scraping the goo off the face plate was easier once it was freed. Pretty much all of the paint on the top of the face plate ended up coming off. Some of the paint along the top of the side pieces where the foam stuck also had to be scraped off.

Goo and paint scraped off
Goo and paint scraped off

There was also a good bit of pitting in the lower part of the face plate from the goo.

The HD-1250 is a little more bare, but mostly cleaned up. The labels on a couple of the coils have faded away, so there’s no indication of what frequency range they’re for. I’ll have to hunt down a manual and see if the coils are described in it. Aside from some surface corrosion on part of the RCA plug shell (easily sanded off), the coils still look in good shape. The electrical repairs and testing will have to wait for another day.

New battery for the Voltohmyst

Finally got around to replacing the old corroded soldered-in D cell in the RCA Voltohmyst with a new battery today.

Snipped the wires off the battery and into the trash it went. Had to drill out a rivet to remove the battery holder clip.

The battery holder is a little on the big side and just fits into the space vacated by the old battery and clip. Soldered the wires onto the battery holder and fastened it to the Voltohmyst using some double sided tape.

Now it’s got a new battery that can be replaced whenever it’s needed.

Next thing to do is go through the manual and read up on how to calibrate the meter.

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.

An RF probe

I camee across N5ESE‘s site and started browsing around some of his projects. He’s got quite the list of them. In his Gizmos section is an RF probe which looked pretty easy to build.

I had a set of those springyy hook-y grab-y DMM leads that were broken, so I cut off the spring-y hook-y grab-y part to use for the DMM side.

On the work bench was a perfectly sized piece of PCB from when I was experimenting with cutting and scoring PCB. From the RXTX build, I just happened to have some extra 0.01 μF SMD capacitors which were perfect for the job. A 1N34A diode and 4.7MΩ resistor finished off the parts.

I used a small hacksaw to score the PCB and divide it into three sections, cut out a notch for the diode and cut the head off a brass nail to serve as the probe. Soldering everything in place was pretty easy.

Soldered on the leads and a ground wire with alligator clip and put everything into a shrink wrap tube.

RF probe covered with heat shrink tubing

The full probe, with all the leads ready for some RF to measure. I still need to find a decent enclosure to shield the probe with.

Dummy load enclosure

A little Sucrets tin turned out to be a good size for my QRP dummy load.

A short section of 1×4 sliced in half to about 1 cm thick serves as the base. The dummy load is screwed onto the board and the board glued into the tin. For extra security, I put three screws through the sides of the tin into the board.

On the antenna analyzer, it reads pretty close to 51Ω and 1.0 SWR between 3-10 MHz. Above that the impedance creeps up to as high as 55 Ω and 1.5 SWR below 28 MHz. SWR shoots up pretty high in the VHF range.

I don’t have an external connection for the power measurement yet. Still need to figure out how I’m going to do that. For now it will be an “open the lid” measurement.