Monday, April 6, 2026 at 08:39:44
This is a device I picked up at Dayton 2025, mostly because it was cheap, and was being ignored because it was very dirty and wasn’t a desirable EICO unit.
This is how I found it:
And how it cleaned up. It was missing a knob so I pulled a gray PACO knob from my stock. Same knob, different color. Yeah…the selector control should have had a different color on it, that’s the way it is…
This one might have been a factory build. There are names on everything as if multiple people had their hands in this.
The tube compliment, save the eye, is all Precision or PACO branded. Either this has it’s OEM tubes from the original sale, or the owner replaced like-for-like.
The bottom of the unit is quite lovely. All those old Dumont capacitors, tho…
Notice all of the holes are grommeted for wires. Quite a different level than other manufacturers.
Even the eye tube is held in with a band made of wire.
But…
Fortunately, the chassis is return, but that still probably caused some noise.
Some basic checks on the unit before applying power…
That one is no good.
That one is ok. Kind of.
I’ll have to do some other checks with voltage later. I’m curious as to how well the Dumont capacitors held up.
There is one part, however, that needs to go, and that’s this across the line capacitor:
A new safety rated part was installed.
So…does it work? Sure does:
The eye tube is just about as dead as can be without being unusable, so this thing has a lot of hours on it. The gain control has a thunderstorm in it after multiple cleanings, so that will need to be replaced. Other than that, if may be a use as-is assuming the electrolytics don’t go popboom. I’ll still do some diagnosis on it to see if any resistors are way out of tolerance, or any of the other capacitors leak. Some will need to go, but the rest? Who knows.
This model seems to be somewhat unusual. The PACO Z-80, which looks like this one save it doesn’t have the chart diagram on the left side, appears to be identical. I can’t find a manual listed as the ST-22, so I’ll compare to the Z-80’s schematic and see if they are the same.
Stay tuned, this one will be on the bench maybe later this year if I have the time.
Next part of this series: Coming soon.
Thursday, April 2, 2026 at 08:04:00
Another selenium-laden device. Why do I do this to myself?
The EICO 249 (And it’s small-form-factor cousin the 232) are VTVM units that offer both a P/P and RMS scale on their wonderfully large, but not parallax corrected face. It offers VDC, VAC, and Ohms (via an internal 1.5V battery.) These date to the 1960s, and as such, are probably one of the last-gasp devices that had both tubes and selenium rectifiers for the power supply.
The EICO 249 - The repair candidate
This unit came from the Cuyahoga Falls ARC hamfest along with the PACO G-30 that was on the bench earlier. It was the same vendor, so it has similar repairs inside - including replacement parts that don’t meet certain specs of the unit and will need to go if this wants to be in service.
The inside of the unit shows obvious work. It’s organized chaos.
Is that a polystyrene part in there?
The capacitors have all been replaced, as have some of the non-divider resistors. The orange parts near the bottom aren’t capacitors, but big resistors chained together to make some value. Front and center, however, is the 0.1µF @ 630V capacitor. This one is called out as 1000V in the specs, as this is an input blocking capacitor and may see a higher voltage on the input. The person doing the replacement just used a regular part without considering why that voltage was chosen. Of the most interest, however, are those blue resistors on the switch assembly. This is the voltage divider ladder, and we’ll come to that shortly.
The wiring seems to be thick and rubberized, and some is breaking down. It will need to be replaced as well, although not all of it.
The bottom of the chassis has the input jacks - the input was replaced with a BNC connector - the tubes, transformer, battery holder for the ohms function, as well as an enclosed 35mA selenium rectifier. This particular unit uses a split rail supply, so it’s going to be interesting seeing how it reacts with a modern silicon diode instead of this non-linear rectifier.
B+ total is about 150VDC between the positive and negative rails, so it’s not going to be difficult to work with. I question the use of a 150V filter capacitor in this circuit, however…those old paper caps did things differently and I’d probably chose a 200-250V part these days.
How does it perform? It’s…accurate-ish. Some scales are ok, others are not.
While the voltages were close after dialing in a test point with the adjustments on the side, they’re still off. That’s because the resistors in the divider ladder are all out of tolerance. These were 1% parts when new, and have simply drifted some over the years. That it’s not much is a testament to the quality of the parts themselves.
150kΩ
900kΩ
7MΩ
Others in the ladder were similar. They’re all bad. While 2% may not seem like much, it’s all of them in series with one another being off that causes drift and deviation. The unfortunate thing here is that most are unusual values and will require some chaining to replicate. Fortunately, these values can be made with standard values either in series or parallel.
The EICO 249 - The parts unit.
Setting that one aside for now, I have a second unit. This unit was purchased on fleebay, and the person packing it didn’t pack it well. The box was destroyed and the meter movement was damaged. That’s kind of a shame because this movement was the later, clear style. At least, I think it’s later…the date stamped was 1968. The golden color on the face is the glue from the original paper protector material, the paper apparently not having been removed until too late. There was still evidence of it under the nuts on the controls.
This one still has a lot of it’s original parts inside, with some old replacements. This one no longer had it’s selenium device, someone had put a diode in it without considering the effects of the much lower voltage drop on the power supply. I wonder how well that worked? I won’t know because it wasn’t functional when I got it, the meter being jammed to one side and coming unstuck broken.
This one is a mishmash of garbage and sadness.
These capacitors didn’t measure anywhere near the marked values.
Wiring was burnt. Almost every wire. Every. Single. One. Even the big 1000V capacitor in front of us is burnt.
How are the resistors in this one’s divider ladder?
About the same. There’s a couple in there that are right on, so…maybe I’ll take them off and see if they can be reused.
However…this one is not a unit that I can save, so it’s parts. Literally, a pile of parts.
But, all is not lost. The range switch may be a candidate for out-of-unit rebuild. I took some time to remove a hold-down mod the person put in for one of the contacts, and cleaned the metal rings. It seems to work ok, so I’m going to try it.
This thing may have a good chance of living again. Stay tuned!
Next part of this series: https://wereboar.com … d-resistors-therein/
Wrapup and final thoughts: Coming at some point.
Wednesday, April 1, 2026 at 07:28:26
The Simpson 715 is still somewhat of a mess, but I knew that going in. With almost any meter of this age, the resistors in the divider ladder are going to be way out of tolerance. While I didn’t measure any of them, voltage testing suggested that it starts at the bottom - so yes, literally all of them are probably bad.
I’ll revisit this device later, but right now I have other things I want to work on, and determining what resistors are going to be needed will require some thought.
This device didn’t have any real data about it, so I wasn’t sure what anything was supposed to be - plate voltages, tube voltages, etc. Fortunately, Simpson made this unit for Heathkit as the AV-3 kit, and Heathkit’s manual for the device was their normal, packed full book. All of the information was there and that helped considerably.
The original problem I was trying to correct.
The original issue with this device is that of no zero. While the device still doesn’t necessarily zero like I think it should, it’s much better than it was. I suspect there’s both some resistors bad in the unit itself, and that the meter movement may be a bit flaky from being beat around over the years.
As this was a piece given to me by a now-departed friend, I wanted to at least make sure it did something, and that it does. For the most part, I simply replaced all of the capacitors in the unit as they were old wax paper, bumblebee, and other assorted relics from a bygone age.
That was very straightforward and didn’t incur any issues.
The Selenium Rectifier
The selenium rectifier didn’t really present any challenges save that it stopped working midway through my testing of currents in the circuit. This required some guesstimations, which were incorrect. I was eventually able to determine was the device wanted after some physical experimentation.
That experimentation involved going back and making a new guess based on some other real-world devices, and getting it correct. I wound up with a 1.27K resistor replacing the selenium stack, bringing both of my B+ values to within 0.5V of the rated value.
I had posted about this device elsewhere, and had someone tell me that I was doing this wrong and making it harder than it needed to be. They read online that a selenium device only has this much drop, and that I should simply use that information to design a regulated supply, hope that helps.
No sir, that didn’t help. Sorry. But if you have a regulated 130VDC/120VDC power supply that fits in a selenium stack space, please let me know.
Final Thoughts
As I said earlier, this one isn’t done - especially if I want to actually use it. It’s a cute little device, so I’d like to - but the meter movement itself raises some concerns. I’ll definitely revisit this later, but for now - it’s mostly operational, and it’s good to sit around for a while.
Next up is an EICO 249 that has a similar issue, that of the divider ladder being bad. I have a spare unit and can rebuild the ladder assy out of parts, on the bench. Stay tuned for that one.
The complete Simpson 715 rebuild series.
A Simpson 715 AC VTVM https://wereboar.com … simpson-715-ac-vtvm/
Fixing the no-zero issue? https://wereboar.com … x-the-no-zero-issue/
Capacitors, of course. https://wereboar.com … acitor-replacements/
How did this work? https://wereboar.com … t-4-filters-and-wtf/
Testing the repair. https://wereboar.com … -testing-the-repair/
Intermission. https://wereboar.com … part-6-intermission/
Selenium-ectomy https://wereboar.com … -rectifier-replaced/
Final checks. https://wereboar.com … part-8-final-checks/
Wrapup: You’re reading it now.
Previous part of this series: https://wereboar.com … part-8-final-checks/
