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 Post subject: Sprague Tel-Ohmike Refurbishment
PostPosted: Oct Sat 12, 2019 10:58 am 
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Hi, long time reader, newbie to tube electronics & repair, but constantly learning...

I was curious if anyone refurbishes the Sprague Tel-Ohmike 6 to bring it up to spec with modern components. I realize the schematics and instructions are provided, but I don't feel comfortable taking on a project like this yet...

At the moment, the IR function is not working, and it's not V4. Plus, it's probably a fire hazard running it with all the stock parts...

Thanks,
Bud


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 Post subject: Re: Sprague Tel-Ohmike Refurbishment
PostPosted: Oct Sat 12, 2019 10:07 pm 
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I have just finished going through my TO-6A and it now meets all original specifications. It needed very little work. Your TO-6 is very similar except it does not have the 100 picofarad range. If the leakage test works the insulation resistance should work because IR is not really a different function, just an interpretation of results and modern capacitors will never show any insulation resistance. The calibration instructions tell you how to check the IR function this requires a several hundred megohm resistor to test iwhich you will probably not have. If your electrolytics are paper covered Sprague Atoms they are probably suspect but if they are the plastic covered Sprague Atoms they might be fine, I unnsoldered my plastic covered ones and a friend tested them on a TO-6A and they tested as new on capacitance, leakage and power factor. I put them back in and they do not get warm and the scope showed normal low ripple in use so no problem. The big problem with mine was that Range C was way off. The reference capacitor had drifted significantly and replacing it with a modern generic cap hand selected to be .02mfd on a quality digital capacitor meter brought the calibration back within the specified limits. I am a bad typist so contact me by private message and I will be glad to help. We can do phone if needed. Also, do you have the calibration document that is on the webb? If not, I can send it to you as it is not in the manual. The best use of this device today is measuring electrolytic leakage at actual working voltages, power factor and reforming old capacitors. It is accurate for capacity measurement and fun to use but not as convenient as a capacitor meter (including DMMs with that function) if all you care about is capacitor value. I consider it very useful for tube electronics and use it daily.
Dave W7VM

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 Post subject: Re: Sprague Tel-Ohmike Refurbishment
PostPosted: Oct Sun 13, 2019 3:40 am 
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Do a search here on the radio forum and you will find all the information you need to restore and calibrate your TO. Many of us have restored dozens of these.

DM


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 Post subject: Re: Sprague Tel-Ohmike Refurbishment
PostPosted: Oct Tue 15, 2019 12:57 am 
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Just a quick question..now that we have accurate 2uF polypropylene caps, do folks use the 2 - 1uF caps (C5 in the TO-4, C10 TO-6) in parallel for originality sake, or just the single 2 uF....

Steve


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 Post subject: Re: Sprague Tel-Ohmike Refurbishment
PostPosted: Oct Tue 15, 2019 1:08 am 
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I always parallel the bridge capacitors, that way you can get them to the exact reading you need. Or you could spend a few bucks for some 1% capacitors and pick through them to find one that is right on.

DM


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 Post subject: Re: Sprague Tel-Ohmike Refurbishment
PostPosted: Oct Tue 15, 2019 12:31 pm 
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Keep in mind that matching the 2, .2, and .002 caps is more important than making sure they are all exactly on value. Otherwise your dial calibration will be off for one or more ranges. In other words, if the 2uF is actually 2.0005, you'd need the other two to be off by that same amount to match. Then again, how much accuracy do you expect or need out of this 60+ year old instrument?

For measuring exact cap value, I just use a DER-EE. The TO-6 gets used for leakage, and in electrolytics, reforming and power factor measurements.

There is nothing wrong with selecting precise capacitors at precise value, but it's a lot less time consuming if you've measured and marked a bunch of them and just select ones that match each other, for this application.

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 Post subject: Re: Sprague Tel-Ohmike Refurbishment
PostPosted: Oct Tue 15, 2019 2:36 pm 
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Thanks..I use my DER-EE for values as well. I like using this vintage equipment for leakage. Barry, did you see I went ahead and built that reformer on the other post?

Steve


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 Post subject: Re: Sprague Tel-Ohmike Refurbishment
PostPosted: Oct Tue 15, 2019 2:53 pm 
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I recall the reformer but not that you built it... how does it work? I don't *usually* use old electrolytics, but when I do :

I tend to just do a "quick" reform with the TO6 or the IT28, and then let the equipment handle the rest of the reforming... it's powered up plenty while I'm troubleshooting or calibrating or just playing with it.

Older test equipment just gets a slow power up, over perhaps half an hour, via Variac...... been mostly successful so far.

Most of the high end gear, HP, Tektronix etc ... used such high grade electrolytics that they do not generally need replacement. I have to confess that occasionally I don't even bother with the longer Variac treatment ... sometimes just a few minutes. I get impatient. Only once has this resulted in a tube red plating .... I still haven't gotten back to that Tek 585A to see what's the cause... one of these days.

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 Post subject: Re: Sprague Tel-Ohmike Refurbishment
PostPosted: Oct Tue 15, 2019 5:26 pm 
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In the "freshening up" thread, wazz posted the popular electronics ad...and I saw you might build one... I went ahead and built it, and will use it to "top off" those electrolytics that have been in the bin a while..probably overkill, but entertaining nontheless! It works great, with little heat generation.

I still like these cap testers for their leakage testing!

Steve


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 Post subject: Re: Sprague Tel-Ohmike Refurbishment
PostPosted: Nov Thu 14, 2019 12:27 pm 
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For the Forum: I've just posted up on my website, the work done by Mike Higgins, edited by Steve Dingle, a complete workup on the TO-5. It includes Mike's work on refurbishing his unit, a description of how it works, and color coded drawings on each circuit function.

If anyone has other info on the other Sprague TO line, I'll be happy to include it. I'm going to add photos to the TO-5 page as soon as I get them, and I will be shooting my own TO-4 and TO-6 as soon as I can.

Hope this helps someone, somewhere, sometime. :)
Barry

https://www.bbtvtestequipment.com Navigate on the top bar to PHOTO GALLERIES and choose the CAPACITOR TESTERS button.

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 Post subject: Re: Sprague Tel-Ohmike Refurbishment
PostPosted: Nov Thu 14, 2019 9:22 pm 
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Thanks, Barry!

As noted above, I went ahead and picked up a DER-EE, and have done the calibration. I have noticed that the electroytics I test with the DER-EE come back with a lower capacitance value than my other DMMs or less expensive LCR meters. For example, a 47uF cap might read 46.5uF on my DMMs, but 42uF on the DER-EE.

Any ideas why?

Steve


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 Post subject: Re: Sprague Tel-Ohmike Refurbishment
PostPosted: Nov Thu 14, 2019 9:54 pm 
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The DER-EE is much more accurate for measuring capacitance than any of the Sprague, Heath etc tube bridges. An old GR bridge might give it a run for it's money, but not worth the weight, space, and expense to refurbish. ;-)

Also of note is that the DER does NOT test the capacitors at their rated operating voltages, the Heath and others of that type DO.

I think you'll find the same is true for ESR measurements as well

One note... make sure you have calibrated the DER-EE with whatever test leads you are using, even if it's just the short ones that comes with the DER.

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 Post subject: Re: Sprague Tel-Ohmike Refurbishment
PostPosted: Nov Thu 14, 2019 10:28 pm 
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The fact that the service oriented bridges measure at high voltages is their downfall when using with modern components. The 50 V or more used can destroy low voltage parts, both capacitors and resistors. And in many cases the applied voltage for leakage testing may be grossly inaccurate.

Of course, the intent originally was for rough testing, not laboratory accuracy, in spite of the glowing descriptions in the advertising and the manuals.


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 Post subject: Re: Sprague Tel-Ohmike Refurbishment
PostPosted: Nov Thu 14, 2019 10:32 pm 
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bob, bridges such as the Heathkit don't apply that kind of voltage.... it's switch selectable. Most of them are. 3v to 500v in this case I think ..

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 Post subject: Re: Sprague Tel-Ohmike Refurbishment
PostPosted: Nov Thu 14, 2019 11:40 pm 
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Barry, take another look. When measuring capacitance or resistance, the unknown receives a lot of stimulus. In many cases, I see around 50 or more volts applied to the bridge and hence to the unknown.

I agree with you when it comes to leakage testing, although my comment about poor accuracy still holds. In some cases, the leakage at low voltage is insufficient to be indicated on the eye tube. Short the capacitor terminals and measure the leakage of the jumper wire. You will need to turn the voltage knob a way before anything shows.


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 Post subject: Re: Sprague Tel-Ohmike Refurbishment
PostPosted: Nov Thu 14, 2019 11:57 pm 
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I'll take another look next time I get one on the bench, bob. Actually I think most are using these for reforming electrolytics and testing same, not the low voltage types. For the most part they use a DVM that has a capacitor tester function like mine, for instance, which set me back about $45. Or a DER-EE type device, which I got for high precision work, mainly to select some precision components for my own in house standards after consultion with several on the forum. There was a thread or two on this a while back.

Thanks for the heads up tho. Hopefully anyone testing a capacitor is not going to leave it hooked up too long. What's the worst thing that can happen?? ;-)

...don't answer that

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 Post subject: Re: Sprague Tel-Ohmike Refurbishment
PostPosted: Nov Fri 15, 2019 2:01 am 
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That DER-EE looks like a keeper. If I didn't already have plenty of bridges I would certainly consider it. Is it a bridge, or how does it measure reactance? I remember the old Hickok capacitance function on the VTVMs that was merely a 60 Hz source in series that measured the current. Not a capacitance meter at all.

Actually, there are many ways to do this but only a few do the job properly. A bridge is one way. My fancy GR 1658 is very sophisticated and accurate. The 1650A is good too but doesn't have the precision. And of course the Q meter is an excellent device for small values of capacitance.

I once cobbled up a setup to measure some 20,000 microfarad units (I think) by means of charge time but that's not really doing it right, as its accuracy is ruined by even a little bit of leakage. I could have measured the phasing between the voltage and the current but that would be getting too complicated. The source stability gets into the equation, and even a little bit of noise ruins the test.

One must factor in the reason for the measurement. Perhaps the best way is to build the circuit in which it's used and set up limits, just a go/no go scheme.


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 Post subject: Re: Sprague Tel-Ohmike Refurbishment
PostPosted: Nov Fri 15, 2019 11:37 am 
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Quote:
Is it a bridge, or how does it measure reactance?


Download the manual and give it a quick read. It's an amazing device for the price. I do believe it is actually a bridge, but a modern and fully automatic one, and with all the power that digital instrumentation brings to circuit analysis.

I think if they'd put one or two more components in there, they probably could have a spectrum analyzer. As it sits, it's apparently basically a network analyzer.

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 Post subject: Re: Sprague Tel-Ohmike Refurbishment
PostPosted: Nov Sat 16, 2019 5:10 pm 
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Location: SW WA state
devilsmist wrote:
I always parallel the bridge capacitors, that way you can get them to the exact reading you need. Or you could spend a few bucks for some 1% capacitors and pick through them to find one that is right on.

DM


I have both 1 uF and single 2 uF caps: I find that a lot of the 2 uF caps are slightly over the stated value, so I tend not to order them for TO-x rebuilds.
The 1 uF caps are both over and under, so I can usually match them, or go a little under and use a third, very small cap to come out dead nuts on value.
That being said, I can usually get the TO-x products to 2% or better as verified by using my reference capacitor set.

-Tom


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 Post subject: Re: Sprague Tel-Ohmike Refurbishment
PostPosted: Nov Sat 16, 2019 11:30 pm 
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Keep in mind that it is more important to match the .0002, .002, and 2.0 to each other than have them all be dead on. In other words if they all have the same % error, you can calibrate that out

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