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 Post subject: Need Help on Diagnosing a Sprague Tel-Ohmike TO-6A
PostPosted: May Mon 07, 2018 10:04 pm 
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Joined: Mar Fri 30, 2018 2:52 pm
Posts: 21
Hello All!

I recently got a Sprague Tel-Ohmike TO-6A after Cornell Dubilier was unable to offer me help on identifying mystery parts in my BF-50 cap analyzer.

I have been reading the TO-5/6/6A calibration procedure and I am currently trying to set my capacitance dial to match known high quality capacitors of the correct capacitance, as listed in the procedure. You can find the procedure on my website (same as username), in the documents round-up section.

I currently have a Tenma 72-1020 TrueRMS bench top multi-meter hooked into the test leads along with a .02 uF capacitor. The guide says it is far more accurate to use an oscilloscope or multi-meter. Procedure says the largest eye opening is equivalent to the lowest read voltage when rotating the main dial. Although the actual wording in the calibration guide is "lowest level (deepest AC null)". The guide says to start with a known Hi-Quality .02uF, I am using a Sprague Orange Drop that reads .022uF on my BK Precision 810C and my Extech 380193 LCR meter.

First question, what sort of voltage range should I be seeing during calibration of C3 (voltage from the test leads (w/ cap inserted directly to leads) -> VMM)? I am only reading 1.55V-1.388V (with the lever in the test position) across the entire range of the dial for C3. With 1.55V being with the dial rotated all the way CCW/to the left (lowest values on dial end) and 1.38V being turned all the way to the right/clockwise (highest values end of dial). I read the largest 6E5 eye opening with the voltage reading 1.45V.

Next question is about the Magic Eye 6E5 itself, the largest opening I see is about 1/8 of a circle. My old BF-50 was closer to 90 degrees (of a 360 degree tube) when it showed the largest eye opening. If the TO-6A should be showing the same eye opening size as the BF-50, any guesses on what might cause this?

I have more questions, such as what R37 is set to stock. R37 is a continuously adjustable 5000 ohm 1.5W resistor. You can see the TO-6A owner's manual on my site also. This part was knocked loose during shipping and I'm wondering if I accidentally moved the dial when re-securing it.

For now I'll start my questions on calibration and/or diagnosing bad parts with these first three.

Thanks in advance for any help.


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 Post subject: Re: Need Help on Diagnosing a Sprague Tel-Ohmike TO-6A
PostPosted: May Tue 08, 2018 3:07 am 
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Joined: May Wed 06, 2009 7:01 pm
Posts: 822
Assuming you're push button switches are clean, you should be seeing more AC voltage on the capacitor value test especially with a high impedance true-RMS voltmeter. Like almost all of the tube-era cap/ resistor testers, the Tel-Ohmike series use an AC bridge circuit for cap value. The bottom of page 2 of the manual has the simple circuit. For another simple explanation of cap measurement using a bridge, see
https://people.ohio.edu/postr/bapix/TSM/CapBridgeTSM_1-2015_pg5.htm

I prefer using a Sencore LC-75, but here are pics of several of my Tel-Ohmikes.
https://people.ohio.edu/postr/bapix/CapChkrs.htm

Check your transformer winding used for the bridge circuit (connected to the potentiometer ends) and find out why you are not getting the voltage to the proper test terminals. It could very well be the main pot or preferably some resistor or connection.

-- Rich


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 Post subject: Re: Need Help on Diagnosing a Sprague Tel-Ohmike TO-6A
PostPosted: May Wed 09, 2018 11:05 am 
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Joined: Mar Fri 30, 2018 2:52 pm
Posts: 21
Well one for sure issue was the fact that the seller of this unit put it in a box so small, that the test leads were shattered as was the faceplate of the 7-27H combo resistance and amperage meter (try finding that these days without having to buy an entire TO-6A (only TO-6A have that specific meter, even though the manual does say the TO-6's 7-27 is fine too, but no easier to locate). I am planning on getting a 3.5"x3" meter just to take the plastic face off it and transplant it to my meter. I never realized these meter's pointers are so sensistive that a simple fan running on low to med will make it move while no cover is on it. I'm afraid I'm going to need a new meter and I'm not sure what I will do. Even if I wanted to spend the cash I think it would cost ($50-$75), I can't find any even remotely similiar for sale. Is it possible for me to switch to 2 separate meters or is that just too much trouble?

Back to the test jacks, can someone confirm for how the insulators and conductor layers are arranged? I assumed you'd use plastic washers or the stock bakelite disks on both sides of the stud sticking through and then the cable coming from the tester would then be secured with a metal nut holding metal to metal for both jacks, but not making a connection to the metal of the enclosure for either one. Please correct me if I have done anything wrong!

Thank you for the information you have provided me, I will do some reading/looking and see what I can find. I will be putting pictures up on my website ASAP (same as my profile name).

Actually I already read your write up, it was very good! How much of TO-4/5 stuff can you carry into troubleshooting a TO-6A?

Edit: I think someone else has already been in this unit replacing caps before, the TO-6A bone stock should not have any Orange Drops according to the Rev 2 parts list (and Rev 1).

Pics of my unit here: https://hallmanlabs.com/2018/05/09/high ... ike-to-6a/


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 Post subject: Re: Need Help on Diagnosing a Sprague Tel-Ohmike TO-6A
PostPosted: May Wed 09, 2018 9:57 pm 
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Joined: Aug Sat 02, 2014 3:59 am
Posts: 1206
Location: Brooklyn, NY 11217
I tried to contact you on your site when I saw your prior cap tester issue. Anyway, the meter is just a 50ua movement. It's not hard to find one of those. The issue is really what the internal resistance. You should not have any issue measuring that on your current meter. disconnect it from the internal leads. Put a 10k resistor on end of your multimeter. Measure the resistance between the lugs. Subtract the 10K (or what the acutal resistance of your 10k resistor is). Then measure. Odds are, this meter is an off the shelf microameter from Simpson or another. Find one with about the same internal resistance (or less), swap the face panels and install it. You can then add a resistor in series to bring up the resistance if you need to. Hopefully it wasn't a special meter made as a one off.

I have a pristine TO-6A (it was missing a knob). Need to get it out of the back of a closet to look at the jacks, but they should not be grounded. I would pull out the rectifier and measure the AC from the transformer.

Edit: Meter is a Hoyt based on your pictures. Go look for a Hoyt 50ua full scale meter of the same size. Measure the internal resistance and move on.


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 Post subject: Re: Need Help on Diagnosing a Sprague Tel-Ohmike TO-6A
PostPosted: May Wed 09, 2018 10:12 pm 
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Joined: Aug Sat 02, 2014 3:59 am
Posts: 1206
Location: Brooklyn, NY 11217
I also would not use old caps in this. They are 30-40 years old. Use new ones. No one cares about black beauty caps in this unit. They care about it working. As for R37, it was set at the factory based on the specific unit. It would not have a nominal setting. The parts list also lists Poly Film as the cap type. That is what orange drops are, so that may be ok.


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 Post subject: Re: Need Help on Diagnosing a Sprague Tel-Ohmike TO-6A
PostPosted: May Fri 11, 2018 8:26 am 
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Joined: Mar Fri 30, 2018 2:52 pm
Posts: 21
Do all of the polarized capacitors need to replaced with polarized ones? They are the Sprague Atoms on the bottom side of the unit. I believe on the top-side of the unit there is only one polarized cap and it is a small scale 60D or something, blue in color.

Chucking a pristine Hoyte combination amp/resistance meter for a broken plastic face seems extreme. I am trying to salvage the original until I am sure something is wrong with it. I have found one candidate for the right size and style of face to replace the stock unit's broken one, but will take a $20 investment with no guarantee I could even get the face off in one piece. This meter is a 3.5"x3" style.

(If anyone reads this, I am willing to pay $60 shipped for an original Sprague TO-6 or TO-6A meter, or anyone with a 7-27, 7-27D or 7-27H meter (Sprague Description: Microammeter | 0-50 microamperes | 2% accuracy | special scale (<--- combo resistance/ampere)

With regard to the capacitors, I use what I have on hand that is testing well on my Extech 380193 LCR Meter and/or my MESR-100 ESR Meter (above 2uF). I use those in addition to a BK Precision 810C capacitance meter for precision (even with 50 year old NOS Sprague Black Beauties and Atoms (just like stock). The only reason I even give a d*mn about a vintage capacitor analyzer like this, is because vintage capacitor buyers care about it. People see these in ads and it brings a premium and lets me rest assured that 95% of my sales don't come back at me when I can check the internal resistance on a realistic scale (set for the cap under test).

Honestly I am surprised that someone hasn't taken Sprague's last design and done a modern spin on it. It's not a bad DIY project for my site. BTW, thanks for checking out my site! I have been slow lately, but I am moving.

Edit: Poly Film is a type with many subtypes on it, if you look the parts list actually gives you the exact part numbers in the description which allows us to literally know what caps were in at stock. None of those numbers are for Orange Drops from what I have read to this point. I also labeled the schematic with the part values, I am going to upload that to my site and I'll also post it here too, in case it helps someone.

Up to this point I have found one bad capacitor, probably due to how it was mounted. It was actually a small black beauty, the .05 uF one on the bottom-side of the unit. I also found 3 suspect resistors that had drifted outside their accuracy ratings. I replaced all of those. So far all the Sprague Atoms are doing good and still test well on all of my modern equipment.

I appreciate all input and please don't hold back!


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 Post subject: Re: Need Help on Diagnosing a Sprague Tel-Ohmike TO-6A
PostPosted: May Fri 11, 2018 11:00 am 
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Joined: Aug Sat 02, 2014 3:59 am
Posts: 1206
Location: Brooklyn, NY 11217
The problem is that sprague made that unit for years. They changed what they put in it over time. They didn’t update the parts list. I am not saying someone didn’t mess with yours. I am saying it could still be bone stock.

The meter cover can be obtained by buying a Hoyt of the same size to swap. Also, it is an ammeter. All dc meters pretty much are as they use ohms law to deflect. It is a 50 microamp ammeter. Regardless, find the Hoyt catalog online, look in it to figure out what the meter is, and look on eBay for one of the same size. The cover will fit and will be lesss if you buy a weird purposed one (which will likely be a 1ma ammeter).


Regardless, I get your point on sales, but the tester has to work right to be useful. If an internal cap has leakage, your tester may be useless. Replace the internal caps. Bipolar are fine. Cap technology has really changed over time. I would use caps from justradios or Sals. But, before doing that, check the voltages in the power transformer after removing the rectifier. If you are getting high voltage (unloaded) you know you don’t have a transformer issue. If that is the case, clean the push buttons (don’t use spray cleaner) and fix the binding posts. Check all the resistors. The carbon comp resistors drift up over time. Change out of spec ones with metal film. Then see where you are.


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 Post subject: Re: Need Help on Diagnosing a Sprague Tel-Ohmike TO-6A
PostPosted: May Fri 11, 2018 6:16 pm 
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Joined: Mar Fri 30, 2018 2:52 pm
Posts: 21
Are you 100% positive that I can use non-polarized capacitors in place of the polarized Sprague Atoms? I know there are definitely times you can not swap out polarized with non-polarized. I thought there might be a chance it was okay when this was built, since back then finding non-polarized 10uF+ caps was rough. If so I am going to use some 630V ERSE PulseX 12uF “defect free” aluminum metallized polypropylene film caps in place of the (still good) Atoms to ensure longevity of my repairs/restoration.

Link to ERSE PuiseX: http://shopping.na3.netsuite.com/s.nl/c ... gory.78/.f Their PEx series is also good, but only comes in 250V while the PulseX comes in 250V/400V/630V.

The two 12uF Atoms are reading about 0.333 ohm on my ESR meter. The MESR-100 measures at 100kHz to minimize the effect of impedance on measurement vs. the Extech 380193 that measures the ESR at only 1kHz or 120 Hz and it makes a big difference on the ESR measurement when the capacitance is high enough (Impedance = 1/(2*Pi*F*C) | where F = frequency used and C = capacitance of the capacitor.

Also, I noticed they didn't use much heat shrink or any sort of insulation on the majority of the component's metal leads inside the TO-6A. I am going to start heat shrinking every part I remove, especially those on the bottom that are less than 0.5" from the metal bottom plate (top metal plate when looking at the bottom).

BTW, that $15 meter I got had a face that fit perfectly on my TO-6A's 7-27H Hoyte Micro-Ammeter. It's just the standard 3.5"x3" from a Triplett 5A meter. Even the triplets screw adjust was at the right height on the face to fit in the Hoyte's adjustment loop. The only thing is I have to use something to secure it to the Hoyte meter. It can't snap over the Hoyte like it does the Triplett meter.

Edit: if curious, it was a model 320-G (meter) from Triplett.


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 Post subject: Re: Need Help on Diagnosing a Sprague Tel-Ohmike TO-6A
PostPosted: May Sat 12, 2018 11:57 am 
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Joined: Mar Fri 30, 2018 2:52 pm
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Here is a link (my site) to the labeled TO-6A schematic: https://zosoncsu.files.wordpress.com/20 ... n-labs.pdf


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 Post subject: Re: Need Help on Diagnosing a Sprague Tel-Ohmike TO-6A
PostPosted: May Sat 12, 2018 2:00 pm 
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Joined: Aug Sat 02, 2014 3:59 am
Posts: 1206
Location: Brooklyn, NY 11217
I know the schematic. You can use non-polar caps at the same or higher voltage. Non-polar caps are film caps. They just did not really exist or cost a ton then. I always go higher voltage because today’s [should have said electrolytic] caps are not quite as good with ripple current (but it doesn’t really matter, I am just overly cautious). I also pick 105 degree caps if I get electrolytic caps.


Last edited by sfox7076 on May Sat 12, 2018 8:25 pm, edited 2 times in total.

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 Post subject: Re: Need Help on Diagnosing a Sprague Tel-Ohmike TO-6A
PostPosted: May Sat 12, 2018 6:14 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 10748
Location: Mpls, Minnesota
sfox7076 wrote:
You can use non-polar caps at the same or higher voltage.

Just make sure the film caps also have a equal or higher current rating.

Dave


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 Post subject: Re: Need Help on Diagnosing a Sprague Tel-Ohmike TO-6A
PostPosted: May Sat 12, 2018 6:39 pm 
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Joined: Aug Sat 02, 2014 3:59 am
Posts: 1206
Location: Brooklyn, NY 11217
I have not run across a cap with current rating. Do you mean ripple current? That I understand.


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 Post subject: Re: Need Help on Diagnosing a Sprague Tel-Ohmike TO-6A
PostPosted: May Sat 12, 2018 7:43 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 10748
Location: Mpls, Minnesota
Yes, ripple current. Here is a good article on ripple current in different capacitors.
https://passive-components.eu/ripple-cu ... apacitors/

Dave


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 Post subject: Re: Need Help on Diagnosing a Sprague Tel-Ohmike TO-6A
PostPosted: May Sat 12, 2018 8:21 pm 
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Joined: Aug Sat 02, 2014 3:59 am
Posts: 1206
Location: Brooklyn, NY 11217
OK. That's what I thought. I agree. That is why I mentioned metal film. They have higher tolerance for ripple current than electrolytics generally.


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 Post subject: Re: Need Help on Diagnosing a Sprague Tel-Ohmike TO-6A
PostPosted: May Mon 14, 2018 4:44 am 
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Joined: Mar Fri 30, 2018 2:52 pm
Posts: 21
sfox7076 wrote:
I know the schematic. You can use non-polar caps at the same or higher voltage. Non-polar caps are film caps. They just did not really exist or cost a ton then. I always go higher voltage because today’s [should have said electrolytic] caps are not quite as good with ripple current (but it doesn’t really matter, I am just overly cautious). I also pick 105 degree caps if I get electrolytic caps.


Yeah, same as I said above, back then it was nearly impossible to find non-polar 12uF caps of any voltage rating worth a crap. Thanks for confirming though.

I didn't post the schematic for you, it was for everyone who runs across the thread when trying to find info on the TO-6A. I haven't seen many, if any, labeled schematics of the TO-6 or TO-6A. Trying to read from a parts list and a schematic together aren't the easiest thing to do compared to a labeled schematic. That was why I hosted and posted it. Didn't mean to imply you needed it! :lol:

With regard to ripple current ratings, that isn't even listed on the NIB packaging of Sprague Orange Drops or Black Beauties (I have a lot). Perhaps if you could find the datasheets on a 40+ year old capacitor?

Thanks to all for your input, I will update with new info when I have it or when I finish going through the unit. I finally finished the move for good, so I can focus on this again. Woot!


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 Post subject: Re: Need Help on Diagnosing a Sprague Tel-Ohmike TO-6A
PostPosted: May Mon 14, 2018 1:37 pm 
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Joined: Aug Sat 02, 2014 3:59 am
Posts: 1206
Location: Brooklyn, NY 11217
Many of the datasheets then didn't list ripple current for film caps. https://www.surplussales.com/capacitors ... e-225P.pdf is a good example of a 1970s Sprague Datasheet (though they likely had bulletins with a bit more detail). The great thing about this is that it tells you the nominal values of everything. Page 7 and 8 for the testing done on the caps.


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 Post subject: Re: Need Help on Diagnosing a Sprague Tel-Ohmike TO-6A
PostPosted: May Wed 16, 2018 3:17 am 
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Joined: Mar Fri 30, 2018 2:52 pm
Posts: 21
Thanks for the link, interesting to look at!

I have finished checking and replacing components on the bottom of the TO-6A that needed to go. I am almost ready to test it!

Here is a shot from my cell camera of one half:

Image


Full Size: https://www1.picturepush.com/photo/a/15 ... stored.jpg


Note: I found an error on my labeled schematic, R6 should be 500k ohm, not 560k ohm.


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 Post subject: Re: Need Help on Diagnosing a Sprague Tel-Ohmike TO-6A
PostPosted: May Wed 16, 2018 7:10 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3491
Location: Seattle WA US
That black .005/600v capacitor with the orange markings sure doesn't look like a part that I would trust.

-Chuck


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 Post subject: Re: Need Help on Diagnosing a Sprague Tel-Ohmike TO-6A
PostPosted: May Wed 16, 2018 10:33 am 
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Joined: Aug Sat 02, 2014 3:59 am
Posts: 1206
Location: Brooklyn, NY 11217
Have you measured those carbon comp resistors? I would have put in films. I see you did in a few spots. Did you stay with the same dissipation rating? Or figure out the sharing to know the series resistors you put in will work? And I would change that .005 as well.


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 Post subject: Re: Need Help on Diagnosing a Sprague Tel-Ohmike TO-6A
PostPosted: May Thu 24, 2018 1:00 am 
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Joined: Mar Fri 30, 2018 2:52 pm
Posts: 21
Right now I am trying to deal with this meter. I found a face and used silicone to glue it back on. I tested it without the decorative fascia seen in this photo and I came behind it with silicone. However, once I let it dry 24 hrs and I went to remove the tape, the meter needle jumped from 0 to ~900 (scotch tape, nothing hard) and now the adjustment doesn't work... I guess it needs to come back off again.

Although there is one thing, I have heard of people who have meters that sit between 0-1 from being pinged (knocked to max quickly) multiple times. Is it possible that is the cause? I am waiting on two parts before I am going to test the hardware again.

Full Size: https://www1.picturepush.com/photo/a/15 ... ing-sm.jpg


Image

Note: this pic is taken before trimming the excess since I know I am probably going to use a heated blade with my dremel torch to cut it back off.


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