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 Post subject: SPRAGUE Tel-Ohmike TO-6A
PostPosted: Feb Sun 25, 2018 8:11 pm 
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I recently picked up a TO-6A and I am trying to make it work. The meter is not traking correctly, it way off. I want verify that the meter is good. Before spending time on calibration. Does anyone know what the internal resistance should be and the full scale amperage is on the meter? Thanks Joe.


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 Post subject: Re: SPRAGUE Tel-Ohmike TO-6A
PostPosted: Feb Sun 25, 2018 9:32 pm 
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Location: Maryland 20709, USA
The meter is 50 microamps full scale.

The manual does not give its resistance, but you can calculate it.

The resistance of the shunt on the 60uA leakage scale is about 9300 ohms.
The voltage across 9300 ohms with 10uA flowing through it is 0.093 volts.
The resistance required to drop that voltage at 50 uA is 1860 ohms.

Simpson datasheets for several models of their meters show 1800 ohms nominal.
I expect that is the resistance of your meter.

- Leigh

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 Post subject: Re: SPRAGUE Tel-Ohmike TO-6A
PostPosted: Feb Sun 25, 2018 9:34 pm 
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Location: Albuquerque, NM 87123
The manual is on BAMA.


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 Post subject: Re: SPRAGUE Tel-Ohmike TO-6A
PostPosted: Feb Mon 26, 2018 3:24 am 
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I measured the meter at 1800 ohms. So I think the meter is ok. I also measured the metering resistor string of R14 thru R17 I get 120k ohms. So I don't understand why the meter is tracking way off, any ideas?


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 Post subject: Re: SPRAGUE Tel-Ohmike TO-6A
PostPosted: Feb Mon 26, 2018 4:01 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Albuquerque, NM 87123
As stated, the meter is 50ua. If you have 50ua going through it the reading should be full scale; half scale with 25ua.

If you get these results then the meter is good.


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 Post subject: Re: SPRAGUE Tel-Ohmike TO-6A
PostPosted: Feb Mon 26, 2018 5:59 am 
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Is the meter off on every range? Is it high or low? Has the unit been rebuilt? What repairs have been done to it?

Dave


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 Post subject: Re: SPRAGUE Tel-Ohmike TO-6A
PostPosted: Feb Mon 26, 2018 12:32 pm 
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Location: Long Island
Measuring the coil resistance is not enough. Unfortunately, magnets in meters can and do weaken with time and with storage conditions. If this happens, the meter will read low by some percentage, and it may not be exactly linear across the scale. Most meter techs would check at least the full scale and half scale deflections by passing 50-uA and 25-uA through the meter. This should always be done with a good voltage source (regulated low voltage power supply) and enough series resistance to protect the meter.

Ohm’s Law will guide you to the right value; for example, 5-VDC / 0.00005 A = 100,000 ohms. Then if the meter coil is 1,800 ohms the resistor would be 98,200 ohms if one is going about it precisely. A slightly higher voltage, or slightly less than full scale deflection might be good enough to show that the meter is within a few percent.

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 Post subject: Re: SPRAGUE Tel-Ohmike TO-6A
PostPosted: Feb Tue 27, 2018 10:30 pm 
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I checked the meter and found its within spec. At 50% scale, I calculated 25uAmps and 100% scale 50 uAmps. So the problem is elsware. There is to much voltage that the meter is reading, the needle flys to full scale instantly. The metering resistors string of R15, R14, R17 and R16 add up to 120k ohms as per calibration instructions. Why is the voltage high? Any experts out there.


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 Post subject: Re: SPRAGUE Tel-Ohmike TO-6A
PostPosted: Feb Wed 28, 2018 12:03 am 
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Location: Maryland 20709, USA
If the needle pegs you need to disconnect it to avoid damage.

You must measure the voltages in the tester.
The voltmeter must have a 1000-volt range to do so.
Be very careful. This gear contains lethal voltages.

- Leigh

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 Post subject: Re: SPRAGUE Tel-Ohmike TO-6A
PostPosted: Feb Wed 28, 2018 12:46 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 17435
Location: Albuquerque, NM 87123
I suggest putting a shunt resistor across the meter temporarily while you do your troubleshooting, to protect the meter, or remove it and use another meter with higher rating than 50ua (maybe 1ma).
You may have a bad switch or open resistor that is causing too much current to flow.


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 Post subject: Re: SPRAGUE Tel-Ohmike TO-6A
PostPosted: Mar Sat 03, 2018 3:09 am 
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Location: Simonton, Texas
I have a TO-5 on which I had to replace all the resistors in the meter circuit to get it to read correctly.

Note they ran parallel resistors to get the exact values required. This is not shown on the schematic

but is explained in the parts list. After cap and resistor replacement the unit was dead on.

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 Post subject: Re: SPRAGUE Tel-Ohmike TO-6A
PostPosted: Mar Sun 04, 2018 2:35 pm 
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Location: Beaver Falls, PA. USA
When I reworked my TO-6, I found that the schematic didn't completely agree with the unit; one resistor in the insulation test circuit was never installed, obviously a factory "tweak". When working on one of those, it is best to use the manual as a guide, but replace parts with the values which were originally used. Evidently, Sprague hand-tweaked each Telohmike as it was assembled and calibrated, to make sure that it was as accurate as possible.

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 Post subject: Re: SPRAGUE Tel-Ohmike TO-6A
PostPosted: Mar Wed 28, 2018 6:36 pm 
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Posts: 7
I have been calibrating a tv-7D tube testers and will get back to troubleshooting the tel-ohmike soon. But lots of good advice here.


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