Hello Experts! I'm new to this Hobby of repairing vintage tube radios and am in the process of assembling my work bench with the tools/testers needed. I have just purchased a Eico 905 capacitor-resistor bridge tester and was wondering if it will be sufficient. I have my eye on a Sprague TO-4 Ohmike capacitor analyzer but don't know if I will need it. Would it be OVERKILL to have both??? PLEASE ADVISE, and Thank you in advance!

They're mostly useful for checking and reforming capacitors at working voltage, something that modern capacitance meters can't do. Chances are, the Sprague is better, but I'm not familiar with the Eico.

I think you meant a Eico 950 which will work just fine.

Dave

Thanks Fella's! greatly appreciated. looking forward to being a member of this fine forum!

The Sprague TO-4 is a better unit, but it is not so much better that it's worth going after if you already have the Eico. The TO-4 is also more complicated and older, so it is more likely that you'd have to fix it before it would be useful.

Like most other electronic gadgets of the day, older Sprague capacitor testers offen suffer from dirty switch contacts, out-of-tolerance resistors, and bad internal capacitors. OTOH, the leakage voltage test pot on the Eico 950 was wired in such as way that it was vulnerable to burn-out if somebody shorted the leads or tried to test a badly shorted capacitor. They redesigned the circuit somewhat in the 950B to make it less delicate. It would be worth connecting a DC voltmeter across the test leads and seeing if the voltage control works properly in "leakage" mode. You should be able to vary the output voltage between approximately 50 volts and 450 volts if all is well.

Nevertheless, a capacitor analyzer is a worthwhile addition to your bench. But not so much for testing old paper and electrolytic capacitors, which are nearly always bad anyway--or will likely fail in short order even if they test good initially. Where these instruments come in handy is for testing new capacitors, since it's not out of the question that you'll get a dud every now and then, and for testing mica and ceramic caps which are not normally replaced unless actually bad.

Another good use for these is to trouble shoot a shorting tuning capacitor. First you disconnect any wires to it and the connect the cap checker to the frame and the different sections. You use the paper- mica range on the leakage test. Turn up the voltage to say about 100 and rotate the capacitor through its range. With the lights off you will see tiny sparks at the trouble spot. You may need to use more voltage depending on the situation. Sometimes that will actually burn off small metal particles causing the shorting. The paper -mica range limits the current to a few microamps so no damage will be done, at least on the Eico 950B or Heathkit IT-28.

The Telohmikes don't limit the current on the electrolytic capacitor leakage ranges. On my TO-6, I use a 20k-100k 2 watt series resistor to limit the current when I burn the "whiskers" out of a variable capacitor; the value will depend on the voltage being used.

It takes a lot of work to rebuild a TO-4 but the end result is amazing accuracy when you can match components to 1% or less using modern parts.
The one gotcha with that tester is it leaves an electrolytic charged if you remove it without setting all the buttons out...its in the manual.

I also have a prewar Model 16 (some call it a TO-16 which it is not) thats been rebuilt and I like its up to 1000V capability for testing oil caps as well as some micas and disc caps.. The postwar model was the TO-2 without the meter and TO-3 with it.

Using a 1M series resistor it funtions well as a megger to check transformer and other breakdown issues.

Carl

A few corrections to Carl's post

The model 16, 18 and the TO-1, TO-2 were the early prewar models and the schematics were about the same. Then came the TCA-1, TO-3, TO-4, TO-5, TO-6, and the TO-6A These were all built in the new style case with added features.

The model 16 is a TO-1 with a meter added. The TO-1 is the only one that originally came without a meter


Dave

Thanks guys, great info and ideas. Greatly appreciated!

A few minor corrections and additions to Dave's post.

The Model 16 and TO-1 both came out in 1940, the TO-1 being without the meter. Some claim the 16 refered to the 6 features offered. Im not familiar with the Model 18.

After WW2 the TO-1 continued and the 16 became the TO-2. The TO-3 of around 1949-50 was the first new postwar model.
The TCA-1 was a later model primarily for SS level testing and up to 150VDC for AC/DC type equipment. Strangely enough it had a rather elaborate all tube circuitry and its a nice tester for those that specialize in those areas. It also used Spragues difilm version of the Black Beauties and those are still good in mine as well as lots of early HP gear.

Carl

I am a big fan of, and really enjoy using, the TO-6 and 6A's. I think I have tried just about everything.

I have restored over a dozen in the past several years and have sold them on E-Bay. (Just for fun, too many hours and parts to make money!)

I carefully calibrate them using a Lab certified Sencore and provide a set of Calibrated Capacitors so that the calibration can be checked at any time.

I also add two test points in the back so that the balance can be made with a VOM and/or an O-Scope - not just the eye tube. This provides dead on accuracy as you just balanbe for the lowest AC vltage on the bridge.

Mostly, the Geetar Amp guys really like them to "restore" vintage caps for their amplifier. I shipped several to Europe.

However, if you are just doing radio work, the one you have should be fine.

If you go to the TO however, I'd suggest you go for the TO-5 or TO-6 or TO-6A. The features and accuracy are much better. Also, if you get one, you should be prepared to restore it.

I am glad to help if you need to know anything about the TO world.

Jim
Jim

I'm kind of partial to my EICO 965, which has a 4" meter and measures leakage down to .15microamperes, full scale, with applied voltage of up to 500vdc. It was available only as a factory-wired instrument.

I don't see where you have corrected anything, you only verified the information I posted.

I hear you Brian. I passed up on one of these Eico 965 years ago but now I am kicking myself. That was as real gem and still is.