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 Simpson 715 AC Voltmeter is finished. The selenium rectifier has been replaced, the capacitors are all new, and a few resistors have been changed out for good measure. So…does it work?
Well:
Yes…but no. It does work, but the resistors in the divider ladder are bad. I knew that going in. It also still has a hard time zeroing. Some checking suggests that pretty much everything in that ladder is out of tolerance, so if I want to use this device it’s going to need to go under the knife a second time.
I knew that going in, so it’s no surprise, and that will be a project all on it’s own - possibly. Stay tuned!
The selenium rectifier in the Simpson 715 isn’t going to replace itself, even though I wish it would. Therefore, it’s time to start the task of doing that. I started by making some assumptions about the current flows in each of the B+ sides…which were wrong. I knew that ~2mA flowed in the B-side of the power supply, but I had no idea what the A-side would be. This really didn’t make sense to me as there’s supposed to be 10V dropped across the B-side resistor, and 2mA would not do that with a 10k resistor - you’d need 20V dropped. Regardless, I left the 10K resistor in the B-side circuit for the first part of testing.
I originally started with my assumed parts, and quickly set those aside. I then went for a 1k resistor with a 200Ω pot, but found out that wasn’t enough. A 5k pot was chosen, and I was able to dial in the necessary 130VDC. I did the same thing with B-side, and after adjusting both pots I was able to come up with values.
After testing, I settled on 1.27k for the dropping resistor (selenium replacement) feeding the A-side, and 5.168k for the dropping resistor feeding the B-side. With that, I now know that (approximately):
Input voltage = 123VAC
Rectifier output = 173Vp, or (Vin*1.414)-Vd
If we want 130V on the A-Side B+, we need a drop of 33V across the resistor being used to simulate the selenium device. That means in 1.27k, abot 26mA is flowing:
I=(Vdrop/Rdrop) or .0259A=33V/1270Ω.
10 volts needs to be dropped by the B-side dropping resistor, so that means in 5.168k, about 2mA is flowing:
I=Vdrop2/Rdrop2) or 0.0019A=10V/5168Ω.
So we now know that the A side is requesting ~24mA and the B side is requesting about 2mA, for a total current flow of about 26mA. Note that I’m using approximat values because these will fluctuate slightly with line voltage and with the tubes themselves actually doing something.
The actual values were slightly different, 1270 and 5170, but I used what I had - a 5100 and a 68 got me close enough, and I had a 1k and 270Ω in stock. The testing of these parts resulted in some organized chaos…
For the diode, I chose a 1N4007, for the specific reason that’s what I have a lot of in my parts stock. Mounting required removing the old selenium rectifier which was just screwed down:
And building it’s replacement on a terminal strip. That j-hook didn’t turn out quite as nice as I’d liked, but it’s solid.
For the B-side B+, I used two smaller resistors since the power was negligible. They routed around the pilot lamp, but I would have liked to have higher power parts for this. Perhaps we’ll look into that later.
(Note the LED bulb in the pilot lamp socket. We’ll talk about that in a minute.)
B+ came right into the slots allotted for it.
Just on a whim, I tried a LED bulb instead of the #47 pilot. The circuit actually relies on the load from this part:
So it got replaced. A quick check, and on to the final checks…
I’ve had this big USB hub in my rack for some years. It’s mostly provided power for USB devices, but had a few actual USB devices that needed data plugged into it.
I came home last Friday to a “BEEP” - something rebooted. As I was putting things away from work, “BEEP.” Ok, something’s wrong. The hub was power cycling continually…well, the switching supply inside probably decided to go to lunch and not come back.
There’s not really that much inside of the thing - a power supply and some USB hub boards. Here’s the supply, it’s a fairly beefy 5V, 14A unit.
It claims to be made by UMEC:
The supply itself doesn’t look damaged, so it probably just finally popped. Maybe something died on one of the USB stacks…I don’t know.
I looked up the part number. It’s referenced - in China. I suppose I could put a different supply in there, or even an external brick…but do I really want to? Not really, I can just tap off the 5V line that’s on the system already running the few RPi units I have left. This will probably wind up put back together, in the “to go” pile.
When we left this project in February, the unit was working but the selenium rectifier quit before I could do much with it. I believe I had enough data to make an educated guess about what size resistor is needed to replace it. Seeing as how the SICo 76 is done, it’s time to put this one back on the bench.
The suspect part will eventually come out. I think I have enough room in there for a terminal strip for the new diode and resistor(s).
This is an EICO 950A Resistor-Capacitor Bridge. It offers the things you’d expect from such a device. It’s odd in that it slots in-between the 950 and 950B, which were traditional EICO silver-face units. Not this one:
This one features EICO’s colorful scheme, much like the 145 signal tracer I wrote about a couple of years ago. It has comparator and leakage functions in addition to the standard R-C measurements. Leakage is provided by a neon bulb under a pilot jewel in the upper right corner, an unusual bayonet bulb that someone replaced with a standard #47 pilot, probably because they didn’t realize that it wasn’t working because it only lights when you’re using it…not when you turn it on.
The cabinet has seen some use and abuse.
Rusty Bottoms is playing this show tonight:
Even the inside is rusty.
The chassis shows a similar amount of rust. This thing had a wet, hard life.
That aside…let’s look at the inside.
Jay Hooks makes a lot of appearances here.
The underside, however, is “Mah boi, what did they do to you?”
Everything has j-hooks. Everything. Parts. Wires. Everything. ~~Why???
Exactly what was the previous owner trying to resolve here? Was the thing on fire? It amazes me this thing worked at all, especially with those capcitors flying around with no insulation. With all the wires being j-hooked to other wires…it’s like the person just replaced everything for no reason than to replace it.
I’m going to take this challenge, but I need to get a schematic first. I’ll look for one…stay tuned. Hopefully we can make this device a bit happier.
This was an interesting chassis. Not because it’s anything unusual in what it does, but because it definitely shows it was the lower end of the spectrum. Heathkit, Knight, EICO - all of those showed some concern and care in how parts laid in the chassis. This one? Not so much. Things everywhere, parts flying from one side to the other, such a mish-mash of parts and styles and type. I’m still not sure if this one was factory built or kit built, seeing as how it has pop-rivets for everything mounted on the chassis. Seeing as how some of the parts interfere with others, it probably was kit-built.
What do I think about this device?
There wasn’t really anything different about this rebuild except that it required a lot more thought on how to place things, I couldn’t simply move strips around a little or remove parts without drilling them out. That was quite the pain, but it was worked through and eventually everything was re-installed.
I took the opportunity to use some new sleeving I’d purchased, since many of the parts had spacing far longer than anything you could purchase without special dispensation. These parts had a lead j-hooked on, and run to their connection points. I tried to stick with more modern parts like 1% films, good capacitors, and the like - except for the one used in the leakage circuit. This 1.8MΩ @ 2W was hard to find, so I just chose another carbon that was closer to the marked value. It’s not really like it matters, but it is what it is.
I couldn’t get the unit to do much of anything. The eye would close when a part was attached, so something was happening. I’m not sure if the instrument is just that low on the scale, or if the parts I have are bad to the point of being unable to be tested by this device. Regardless, it didn’t seem to do much, and that’s pretty much what I expected. This wasn’t about an accurate instrument, it was about the rebuild process.
That’s all. This is just a footnote shelf queen. Next up is the Simpson 715, finishing up the rectifier section. After that, an EICO VTVM finally gets it’s time on the bench, and perhaps an EICO 950A - assuming I can find schematics for it because that one is a mess. Stay tuned!
Unfortunately, when hitting some of these parts with high heat, it tends to drive them (temporarily) back towards their actual values - especially in the case of carbon resistors where the heat may drive out some of the water the part has collected over the years.
In this case, the manufacturer provided a lot of surplus RN-type components, as the device is mostly WWII surplus. These held up well over the years, and probably were fine to leave in place - had they not been worked over when installed. Carbons…yeah, those probably weren’t anywhere near what they are now due to the 80W iron drying them out. Capacitors, for the most part, were well out of any tolerance, assuming there was a tolerance marked - or even a value marked at all.
Everything except the 68.1KΩ resistor is in this picture. That part was much like the 750KΩ, some small RN-type that I’m not familiar enough with to identify. It’s probably on the floor or behind some other item on my bench, hidden away due to size. Everything else is right here.
How did they test?
Capacitors, for the most part, are well out of anything I’d call tolerance unless you’re -20/+100. Resistors were mostly pretty good, being mil-spec parts. Carbons were probably better than expected because they were hit with a lot of heat - perhaps I’ll set the 820KΩ parts aside and see how they perform in a year.
In the chart, the following notations are used.
RN - Resistor qualified to MIL-R-10509
WW - Wirewound
DB - Dogbone, probably also RN
CC - Carbon Comp
A part of note
There were two weird metal can capacitors in the unit. There was no label on them, any paper label having come off probably sometime before I was on this earth. Placement suggested they were 0.02μF but they didn’t read anything like that:
I hooked it up to my Olson C-R bridge, but didn’t get any eye opening at all. I decided to run leakage and see what it did.
150V, and almost 8mA of leakage? This thing is shot.
Even at 50V, it leaks.
So…whatever these are supposed to be, they aren’t doing it anymore. I’ll toss them in the bin as known leakage test parts.
It’s time to power up the unit. I brought it up with a death cheater, and B+ settled in around 130VDC.
That’s probably about right for this, so I replaced the death cheater with a normal cord:
And brought it up assembled and stuck a capacitor in it.
Yeah, it doesn’t do much. Either all of the parts I chose are unable to be tested by this device due to bad components or something else, it didn’t do anything. The eye does close upon part connection, so something is home inside. Just…who knows what it is.
But never right after you did the work. You have something called confirmation bias that that point, that means that you just put it there so it must be right and you’re not going to catch mistakes. Take some time away from the unit, maybe even the next day. You want to kind of forget what you just did.
Here’s the chassis, completed so far:
It’s still messy, but less messy than it was, and all of the parts are of known values.
Soldering
Soldering a chassis like this is always an adventure…I do each part in sections, so multiple components might get mounted before I solder things. Sometimes, I don’t solder things right away because I’m thinking that terminal may need something from a later section…regardless, sometimes things do not get soldered. There were a couple here:
Misplaced parts
There was one part I put in the wrong spot. The 180kΩ resistor that’s behind the capacitor in the foreground attaches to the lower terminal of the potentiometer in the background. That should have went to the top terminal. This was an easy fix, unsolder, trim the lead, and re-solder.
Missing parts
Here’s the jack on the back of the unit.
You can see there are two wires on the terminal. There’s also supposed to be a capacitor on that terminal to ground, and that part was laying right behind the unit in the tray I used to hold components. Here it is:
I was going to take it back to a terminal near the AC input, but eh. I took a terminal strip, made it into a 1 lug unit, hit it with 180W of iron and made that my new tie point. It’s a bit messy, but that solder isn’t going anywhere.
Checkout is complete. Everything is now where it should be going, and is soldered properly. Time to move on to the power-up.
