Repair/restore my Sprague Tel-Ohmike help needed.

[dragonbat13]

I'm wanting to clean this up and get it in good working order. Someone gave it to me so spending a little bit of coin is okay. And I like it's coolness. I also have a bunch of uses for it. So I may as well fix her up.

My usual method on amps is to just replace the ECaps. Change all power supply section resistors to metal film, and do voltage tests.

However this thing isn't an amp. I need to order some components. I do have a manual for it so I can get an idea what I need with that.

So, once I get the power end safe, what else should be considered? It's old and those resistors are probably out of spec. Same with the non polar caps. But I can't stand doing a parts cannon on a piece and prefer to know what's going on. So I need an idea on a procedure or method to go with.

So if this were yours please chime in on what you would do to get it in good working order for our hobby.. Thank you in advance for your time.

Here's a couple pictures.

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17422597399317194609348285657994.jpg

[sluckey]

Nice piece of history. It will not make you a better technician.

Every FAA radar station had one. Nobody ever touched it except to pull it out of the cabinet once a year to get it calibrated. Then shove it back in the storage cabinet until next year.

It's neat but totally useless on a workbench for audio.

[dragonbat13]

It won't test for leaky coupling caps?

I kinda find it hard to believe that it won't serve a purpose. Seems to be quite a popular piece in every other area of vintage gear.

Can you elaborate on why it's not suitable for testing coupling caps to see if they need replacing in a 1965 bassman?

[nuke]

It's useful for testing capacitor leakage at full rated voltage. There was a model of those that had a ratio function, which you could use to check the turns ratio or impedance ratio of a transformer. I think it was deleted from your model, which is newer.

It wouldn't get enough use for me to warrant the space to have it on the bench, but it is a useful box and have fun fixing it.

[sluckey]

Can you elaborate on why it's not suitable for testing coupling caps to see if they need replacing in a 1965 bassman?

I never said it's not suitable for testing caps. I said, "It's neat but totally useless on a workbench for audio."

It's actually a quality piece of equipment. And it does what it claims. But it's been superseded by more modern equipment. Even back in the '70s it was not useful on a repair bench. If I want to check a cap I reach for my Fluke 87V meter. If I need to check for leakage, I use my Fluke to check in circuit.

I did play with it once, testing various caps from bench stock, learning how to operate it. After an hour my curiosity was satisfied so it went back in the test equipment cabinet. I worked on radar systems with the USAF and FAA for 40 years and never once used that TO-6A for anything.

[dragonbat13]

Okay. I get it.

So since you brought it up, how exactly does one test a leaking cap in circuit? For instance if I have a scratchy pot, what would I probe, and what setting would I use on my cheaper meter? Or does the fluke you have use a higher end feature for this?

I would like to have a nicer fluke, but it hasn't been in the budget for quite awhile.

I honestly would prefer testing in circuit.

I'll still fix up this one for all the old pulls I have laying around that I can use for prototyping and such.

[nuke]

It would be a decent tool for testing capacitor leakage at full voltage (or the limits of the tool) and it would be useful for re-forming capacitors.

The latter is probably what I'd do with it. Some of the e-lytics in my parts stash have been around for years. Although brand new and kept in indoor storage, they do experience some degradation simply from the effects of time. It is a good idea to reform them before use, as per the capacitor manufacturer's data.

Having high-voltage power supply, current limiting resistor and meter all in one unit like that, could be pretty handy. When I have the need for such, I have to jimmy something up. 500vdc adjustable regulated power supplies don't exactly grow on trees you know.

The other functions are less useful for me. I have a modern digital LCR meter that can measure inductance and capacitance and ESR, and other parameters in a handheld meter. it also wasn't expensive and it measures at 100hz, 120hz, 1khz, 10khz and 100khz and that's pretty useful.

Hence my comment on not having the space for it. That's really the kicker for me. My space is limited, more so than my budget.

A 0-500 regulated power supply - that would be grooving, especially if it had some nice additional features like current limiting and metering.

[sluckey]

To test for leakage in circuit... For example, you have a gain stage and the signal passes from the plate through a coupling cap, to a volume pot. The pot is scratchy and no cleaning helps. You suspect the coupling cap is leaking voltage from the prior stage plate and getting on the volume pot. Disconnect the downstream lead of the cap leaving the other lead connected to the prior plate. Set your meter to measure dc volts. Connect one probe to ground and connect the other probe to the dangling end of the cap. There should be zero volts on a good cap. A leaky cap will have some dc volts, the higher, the more leakage you have. Anything over .1V and I'll replace the cap.

[dragonbat13]

Oh ok. I was assumingg you meant leaving the cap completely in circuit.

That was my original plan with the bassman I mentioned. However afterwards I found the tester for free.

However, your spec of "over .1 volt leakage is the first time I've heard an actual number put out.

That's what I will do first with the bassman, however I will confirm with the tester for the hell of it.

Part of the tester restore is simply because I want to fix it, I do have some uses, and it gives me additional options when messing with stuff. Especially all these old caps o have all over.

Thanks for the help.

[dragonbat13]

The other functions are less useful for me. I have a modern digital LCR meter that can measure inductance and capacitance and ESR, and other parameters in a handheld meter. it also wasn't expensive and it measures at 100hz, 120hz, 1khz, 10khz and 100khz and that's pretty useful.


A 0-500 regulated power supply - that would be grooving, especially if it had some nice additional features like current limiting and metering.

What brand and model of LCR meter are you referring to?

[sluckey]

However, your spec of "over .1 volt leakage is the first time I've heard an actual number put out.

That's a personal opinion. Others may have different leakage tolerance.

Actually, I have zero tolerance for leaky caps. But keep in mind. 0.1V on a volume pot will most likely cause a scratchy sound. But put that same leaky cap between the PI plates and the output tubes and you will most likely never know it's leaky.

[nuke]

What brand and model of LCR meter are you referring to?

DER-5000. Just $137 on Amazon. Comes with accessories and a manual in Chinese. I found an english manual online from an american re-branded version of the same meter.

Honestly, it works quite well.

[dragonbat13]

Yes that's a decent price for a lot of usefulness. I may have to put the Sprague aside to purchase it.

I'm still gonna at least get the Sprague well enough to power up. That would probably be all I would really need to do instead of worrying about accuracy. The rest of the money could go towards the 5000.

[maxkracht]

I bought a used tel-ohmike years ago, fixed it up, used it once, has remained in the closet ever sense. I use one of those $10 chinese component testers nearly every day, the de-5000 whenever i need something more accurate or to read inductance. If you purchase one of those, might want to get some kelvin clips with longer leads.

[B Ingram]

Oh ok. I was assumingg you meant leaving the cap completely in circuit.

That was my original plan with the bassman I mentioned. ...

Even using the TelOhMike, you have to remove the cap from circuit (at least by disconnecting one side of the cap).

Otherwise, circuit resistances will give you false readings.

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Understand the motivations of the folks saying "this thing wasn't useful":

1. Fixing a Broken Amp: If you're just trying to get something working, you can usually discern whether a cap is leaking in-circuit, because there are "DC Volts" where there should be "Zero DC Volts." It's pretty obvious that a coupling cap needs replacing without removing it to hook to a special tester.

2. Fixing a Broken Amp for Money: If someone else is paying you, there's an upper limit to the amount of money they will give you (or maybe the amount of money they have). Since the price for the service is fixed, you increase your Dollars-per-Hour by working faster. Unsoldering & removing caps to test in an external box is time consuming, and so lowers your income. So the usual tech-recommendations you see on forums ultimately come from a motive of, "Don't run all the tests, just get the thing working as fast as possible!"

3. Restoration, or "Working for Yourself": If you're tinkering something for your own amusement, it doesn't matter if you take a long time. Maybe you even enjoy the process more if you take a long time. Now you have the opportunity to remove caps and check on an external tester. I promise that by the 3rd or 4th amp, most people will be annoyed by the extra labor (it's like building an amp 2 or 3 times over) and will develop some opinions about whether caps need testing and/or which caps need testing and/or which caps are "always bad" and can be replaced without testing.