I decided the go ahead and semi restore the Eico 950 Resistance-Capacitance-Comparator Bridge. I was doing some cleaning and came across some heavy-duty electrolytic caps that I really wanted to test so I could keep them or not.
I got everything done for the recapping and noticed before powering up I ordered a wrong cap. I ordered a .2uF cap and meant to get a 2uF cap. I haven’t powered it on so im stuck until I get the right cap.
I have noticed a few things when I was recapping it. There is a .01uF wax/paper cap that goes from the AC Mains Live side to chassis ground. If you see this then remove it ASAP and replace it with a proper safety cap. I used a .01uF X1/Y2 cap. I also replaced the Line cord with a proper polarized line cord so there isn’t a chance to end up with a “hot chassis”.
I guess for now I’ll strip down the front panel and use some 2000 grit sand paper to get rid of the corrosion, put it back together and shelf it until payday.
Started on restoring the VTVM. I stripped down the whole thing and wire brushed the chassis using my Drill Press with a wire wheel. I also painted the chassis with some Semi-gloss black high heat exhaust enamel from Duplicolor since it prevents rust and holds stronger than the rustoleum stuff. Granted this does not count as in a “restore” of bringing it back to its new state but in my opinion better and safer.
Before painting it I had to drill out the rivets for the tube sockets and a few standoffs. I also numbered and wrote a legend sheet out to keep track to what wire is what. I may have to end up rewire the whole thing since there is a mix bag of wire types and gauges that don’t seem right since this uses a high voltage. Some of the wires are 22 gauge telephone wire and some have the cloth covering or with typical rubber coating. I don’t know if these meters were offered as a kit or not but it seems to be in bad shape.
I have measured the fixed Carbon comp resistors and they seem to be out of tolerance. Good thing I have a wide assortment of 1/2watt Metal Oxide resistors.
I used some plastic polish on the meter face and now im stuck with a static issue that moves the needle. I think if I rub it down with a anti-static sheet it will dissipate it. I even polished the front panel and has a chrome look to it. When im finished I’ll polish it one last time.
The Capacitors are on order. I even ordered the Capacitors for the Eico that I plan to restore after this one.
I breadboarded both battery eliminator circuits and there are some pros and cons.
I tried the Buck Converter first and I can indeed get 1.5 volts on the output. However when I put on the scope it was a freak show. This was with four 1N4007 diodes as a rectifier and a 2200uF filter cap. I even tried tossing on a couple of 100nF caps on the input side and the output side with no change.
With the zener diode approach it is more stable and cleaner DC output. However this is with a full bridge rectifier. So this is the updated schematic I will use for the battery eliminator.
With other things such as the strap all I could find was black nylon strapping. Guess one of these days I’ll go buy a cheap brown leather belt.
I also got some Naval Jelly attacking the rust. I wish I could take the whole thing apart and soak the chassis in vap-o-rust. The transformer looks fine so hopfully I can get this going after a recapping.
I recently acquired a RCA Volt Ohmyst Vacuum Tube Voltmeter. It looks like it was made in 1954. There are a few things that need to be fixed and replaced before I can use it.
When I opened it I was shocked to see a battery that was at least forty or fifty years old that was crudely soldered in. The battery leaked out at some point an there is a bunch of dried up battery acid that has made the chassis rust up a bit and killed the Zero Adjust pot. Besides that it is the typical things that need to be taken care with such as recapping the unit and replacing the line cord with a modern polarized cable.
The modifications I plan to perform are adding modern banana jacks and a mini power source to replace the battery since it is only used for the Ohms reading mode.
The meter came with the leather strap but it has seen better days so I might replace it with a military grade nylon strap or go buy a brown leather belt to cut up and replace it.
I didn’t get any probes with it but you can find all over the schematic for them. I may or may not just build the AC and DC switch into the unit or just build a small adapter that I can use pre made modern probes with it so I don’t have to make any.
So first thing is first. Remove rust or stop it with naval jelly and slap on some paint to seal it. Find a 15K ohm linear taper 1/2watt pot. The rest should be easy.
For the battery eliminator circuit I plan to tap into the filament power source. Add a diode, filter cap and a small Buck converter. The circuit should take up as much room as a D cell battery.
Note: The filter cap is going to be around 2200uF. Now I never used a Buck converter so if I can’t get 1.5Volts I’ll just use a Zener diode. This would make the circuit much smaller. Honestly I could be more crude and do this instead.
This could work. I would have to bread board it and give it a try. The diode on the end of the output adds protection and a voltage drop to get around 1.5 volts.
Since I have been getting more tube equipment I guess I should buy a tube tester soon.