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 Post subject: Re: Tv-7d/u calibration problem. Need guidance!
PostPosted: Apr Wed 07, 2021 9:32 pm 
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When I received my recently acquired TV-10D/U is was not packed very well. As a result the 83 tube took a hit and lost its vacuum during shipping. Something else may have happened to it as well even though it looked intact upon inspection, and did not show the usual signs of whitened getter material. This resulted in the meter pegging in the reverse direction when the tester powered up with no tubes under test or buttons pushed. So there are more reasons that a needle could peg in one of these testers other than just testing a shorted tube, or testing a tube with incorrect settings. I haven't taken the time to study all of the other possible fault scenarios, but this is one of them as I have found out.

Sincerely,
Steve


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 Post subject: Re: Tv-7d/u calibration problem. Need guidance!
PostPosted: Apr Thu 08, 2021 8:54 am 
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Chappy wrote:
When I received my recently acquired TV-10D/U is was not packed very well. As a result the 83 tube took a hit and lost its vacuum during shipping. Something else may have happened to it as well even though it looked intact upon inspection, and did not show the usual signs of whitened getter material. This resulted in the meter pegging in the reverse direction when the tester powered up with no tubes under test or buttons pushed. So there are more reasons that a needle could peg in one of these testers other than just testing a shorted tube, or testing a tube with incorrect settings. I haven't taken the time to study all of the other possible fault scenarios, but this is one of them as I have found out.

Sincerely,
Steve


Steve,

See attached circuit for the Hickok meter bridge circuit. With no tube under test there is no way to create an unbalanced (DC) current to deflect the meter via the cathode circuit. In addition, even if the plates of the rectifier got shorted together and AC was applied to the meter, it would not deflect as the average value of the AC is zero. Plus its unlikely with no button press (not shown in my diagram) that the circuit would be complete either.

So the scenario you describe of meter deflection (from some apparent DC source) with no tube under test and no buttons pushed regardless of the condition of the 83 rectifier, seems barely plausible to me, but I will take your word it happened ...UNLESS somebody had in fact added rectifiers, of one polarity only across the meter, and this rectified the applied AC in a fault condition, resulting in meter deflection. As I pointed out, there are caveats making well intentioned modifications but without considering all the possible fault modes and the modification could, under some conditions, make the situation worse.

In addition, from what you have said your violently deflected meter survived whatever the fault was, confirming what I said that all of these meter movements I have seen, which had failed, it was not due to an over-current event.


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hickok.jpg
hickok.jpg [ 432.04 KiB | Viewed 755 times ]
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 Post subject: Re: Tv-7d/u calibration problem. Need guidance!
PostPosted: Apr Thu 08, 2021 2:03 pm 
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Looking at the circuit diagram I see what you are getting at but it did happen. I am fairly certain that I did not have any tube under test in the machine, or that I had pressed the gm button as it was on first power up after receiving it. However, I may be wrong & perhaps I noticed the needle movement after pressing the gm button during a preliminary functional test. I took pictures immediately after the incident as I had just purchased it and thought that I might need them for an insurance claim. The box it came in indicated that it had been dropped during shipping, and one corner of the aluminum case had received damage.

The 83 tube did not look unusual to me, and I didn't notice anything else unusual inside prior to initial power application. After the un-commanded meter movement I reopened the tester and then the 83 looked like this (see image). Upon removing it from its socket that is when I also discovered that the glass base was no longer firmly attached to it's base. So it is definitely possible that connections/electrodes within the 83 had been dislodged. I replace the 83 tube with a new one and the problem went away. Perhaps something else was dislodged inside the tester that I missed, such as a loose piece of solder, which sorted itself out during the tube replacement work.

I haven't modded this tester yet, and I don't see any diodes across either the meter or the bathtup cap (second image). I believe that the tester is still stock, and now that I have a copy of the D/U manual with the correct circuit diagram I will open it up again and have a really good look at all of the circuitry to verify that there are no additional diode(s) or other modifications that have been added.

Sincerely,
Steve


Attachments:
83 Rectifier Tube 2.jpg
83 Rectifier Tube 2.jpg [ 357.74 KiB | Viewed 748 times ]
Meter & Cap.jpg
Meter & Cap.jpg [ 407.24 KiB | Viewed 748 times ]
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 Post subject: Re: Tv-7d/u calibration problem. Need guidance!
PostPosted: Apr Sat 10, 2021 1:53 am 
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Chappy wrote:
Acornvalve:

Meter Diode Protection:
This was a common practice by several manufacturers, including Simpson and Triplett to protect the meter movement. Both my Simpson 260-6 and Triplett 630-NS have them. My military PSM-6B and USM-223s have them as well. So it is not just some guys coming up with ideas on a whim 40-50 years later. Also take into account that meters and bias pots for these testers are very difficult to find now, so protecting them from accidental damage is important to some, myself included. Did a bunch of people make a small error by installing just one pair of 1N4001s across the meter. Yes they did as it affects the readings (as myself and others discovered) but it did not put their testers at risk of damage.

Bias Fuse Installation Considerations:
I have a TV-3B/U, TV-10D/U and a TV-7D/U. I have spent countless hours studying the circuitry and design of these testers, as well as countless hours researching all of the available information on them. The basic configuration of the gm circuitry and the pulsed DC voltages that excite the tubes are all the same with only minor differences. The physical size of the testers are quite different. The TV-3 and TV-10 series were contracted for the Navy and are housed in large enclosures with the parts spaced out nicely including room for roll charts. The TV-7 series, contracted for the Army and Air-Force are packed into a much smaller enclosure with the parts all jammed together. These differences in designs make sense as the Army and Air Force which forward deployed into the field and foreign airfields needed to consider the space and weight for all of their gear that they bring. This is most likely the reason behind the compact design of the I-177 and then the TV-7. As a result there is not a lot of real estate on the face of the TV-7 series or extra space inside. All of these designs are based off of the Hickok gm design, and my TV-3 is basically a militarised version of the 600A which I also have. It is also in a large case. The 600A, TV-3, and TV-10 all have a bias fuse lamp on the face plate. Hickok put it there for a reason. People make mistakes and gm test tubes with shorts. The bias fuse lamp was probably left out of the TV-7 due to face plate real estate, and available space underneath the hood. Military organisations have extensive repair facilities so replacing a blown bias pot in a TV-7 was probably not a big deal, and probably not very costly back in the 50s and 60s.

I do not make modifications to my gear on a whim. I put thought into everything I do and I make sure it makes sense. Do a google search for replacement bias pots for Hickok testers. You will come up short on suppliers. Most recently Dan Nelson has been the only available source for these customised parts , and the spare I bought from him cost me nearly 90CDN shipped. The bias fuse mod that I made to my TV-7 is identical to the fuse circuitry installed in the other Hickok design testers with the exception that I used a normal in line littlefuse vice a 49 lamp. I verified the readings of a handful of tubes before and after the installation just to be sure that the negligible resistance of a fuse had no effect on the readings. It did not.


Great insight into the protection of Hickok testers!

I've seen folks using a pair of inversely paralleled diodes across tester meters to protect them against overload/short. But the photo you posted earlier seems to use 4 diodes, what's the difference?

Image


Also, would you share with us the specs of the in line fuse to protect the bias pot? :)

Thanks.


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 Post subject: Re: Tv-7d/u calibration problem. Need guidance!
PostPosted: Apr Sat 10, 2021 4:40 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1522
Location: Auburn, AL
One diode has too much leakage current at the voltage involved. I tested a few and they had close to 50ua leakage at .474 mv, which is the voltage across the meter when you’ve got a full scale reading. So 50ua of leakage at what should be full scale, and it won’t reach full scale.

Two diodes in series, and then parallel them with two also in series going the other way has no measurable leakage for me. I’m sure there’s some, but it was less than my DMM could read on the uA range.

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 Post subject: Re: Tv-7d/u calibration problem. Need guidance!
PostPosted: Apr Sat 10, 2021 4:44 am 
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Location: Auburn, AL
The bulb used for the fuse is the same as on a tv-3 or tv-10. If you don’t hear back what it is, look up bias fuse tv3 or bias fuse tv10

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 Post subject: Re: Tv-7d/u calibration problem. Need guidance!
PostPosted: Apr Sat 10, 2021 3:03 pm 
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Here is the data on the 49 bulb:

#49 Miniature Bulb Ba9S Base - 2.0 Volt .06 Amp 0.12 Watt T3-1/4 Miniature Bayonet (Ba9s) Base

I recall using a small 'Littlefuse' that I had in my spare parts that was in the neighbourhood of that value. It would have been rated at 100ma or less.

Steve


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 Post subject: Re: Tv-7d/u calibration problem. Need guidance!
PostPosted: Apr Sun 11, 2021 12:30 am 
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Joined: Jun Tue 02, 2020 11:47 am
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Chappy wrote:
Here is the data on the 49 bulb:

#49 Miniature Bulb Ba9S Base - 2.0 Volt .06 Amp 0.12 Watt T3-1/4 Miniature Bayonet (Ba9s) Base

I recall using a small 'Littlefuse' that I had in my spare parts that was in the neighbourhood of that value. It would have been rated at 100ma or less.

Steve


Cool, thanks a lot for the specs. :)


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 Post subject: Re: Tv-7d/u calibration problem. Need guidance!
PostPosted: Apr Sun 11, 2021 12:34 am 
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anchorman wrote:
One diode has too much leakage current at the voltage involved. I tested a few and they had close to 50ua leakage at .474 mv, which is the voltage across the meter when you’ve got a full scale reading. So 50ua of leakage at what should be full scale, and it won’t reach full scale.

Two diodes in series, and then parallel them with two also in series going the other way has no measurable leakage for me. I’m sure there’s some, but it was less than my DMM could read on the uA range.


Great, makes sense. I recall some TV-7 meters have a FSD voltage round .5mv, if a diode leaks too much around that area, definitely need 2 in series. :)


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 Post subject: Re: Tv-7d/u calibration problem. Need guidance!
PostPosted: Apr Sun 11, 2021 2:56 am 
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I’m pretty sure all of them are at .474V, ~ 2370 ohms resistance.

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 Post subject: Re: Tv-7d/u calibration problem. Need guidance!
PostPosted: Apr Sun 11, 2021 4:19 am 
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anchorman wrote:
I’m pretty sure all of them are at .474V, ~ 2370 ohms resistance.


I concur. I tested my meter out of the unit taking readings of different currents from zero to full scale deflection noting the voltage drop across the meter. The average resistance from the calculations of V/I worked out on average to 2365ohms for my meter.


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 Post subject: Re: Tv-7d/u calibration problem. Need guidance!
PostPosted: Apr Sun 11, 2021 8:36 pm 
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Some TV-7s have the meter resistance printed on the scale . . . .


Attachments:
TV7Meter2.jpg
TV7Meter2.jpg [ 146.52 KiB | Viewed 665 times ]

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Last edited by Ed Engelken on Apr Mon 12, 2021 4:42 pm, edited 1 time in total.
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 Post subject: Re: Tv-7d/u calibration problem. Need guidance!
PostPosted: Apr Sun 11, 2021 10:27 pm 
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Is there a manufacturer listed on your meter?

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 Post subject: Re: Tv-7d/u calibration problem. Need guidance!
PostPosted: Apr Mon 12, 2021 4:17 pm 
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anchorman wrote:
Is there a manufacturer listed on your meter?

That meter was manufactured by the Roller-Smith Company.

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 Post subject: Re: Tv-7d/u calibration problem. Need guidance!
PostPosted: Apr Mon 12, 2021 5:56 pm 
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The TV-7 series of tube testers for the military was designed by Hickok and manufactured under contract from the Military by many different contractors including Hickok, which only manufactured the TV-7B/U type. The meters and other components were also manufactured under contract by many different companies it would have been a logistics nightmare to have different meter specifications by manufacturer, so you can rest assured the meters specifications were 2365 ohms, 200uA and 473mV for all manufacturers.


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 Post subject: Re: Tv-7d/u calibration problem. Need guidance!
PostPosted: Apr Mon 12, 2021 11:03 pm 
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I’m curious about the manufacturers to see what other more common meters might be easily adaptable with a simple change of scale

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 Post subject: Re: Tv-7d/u calibration problem. Need guidance!
PostPosted: Apr Mon 12, 2021 11:06 pm 
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Phasotron!


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 Post subject: Re: Tv-7d/u calibration problem. Need guidance!
PostPosted: Apr Sat 17, 2021 12:52 pm 
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With as busy as I am in the shop I ended up sending my TV-7 to Dan Nelson. I plan to compare my TV-10 to it to see how accurate my work is on the TV-10. It must be the power amp time of the year as nearly everything that has come in is a stereo integrated power amp. Got a Nice Onkyo in I can't wait to see what is wrong with it.


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 Post subject: Re: Tv-7d/u calibration problem. Need guidance!
PostPosted: Apr Sat 17, 2021 2:56 pm 
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When you do make comparisons be sure to take note of the settings. Although the testers use basically the same circuit the Bias and Range settings are not the same between the two models for several tubes. Therefore you will get different gm readings back. One example is the 12AX7 series. On top of this the spacing between the index markings for the bias dial on TV-10 are not exactly the same as the markings on the TV-7 across the entire range of motion. I pointed this out in another thread on bias pots a few weeks ago.

One way to get around this is to use the same range settings (thus same grid signal levels) and bias values (actually same DC bias voltages) if you are trying to compare the results between these two testers. I have checked this out and when the bias voltages are the same, and the range settings are the same both of my calibrated testers give very similar results, and usually within 10% of one another.

How I did my comparison: I used high impedance DVMs with the leads inserted into an unused socket noting the test voltages just prior to testing a tube for gm. I took note of the actual DC bias and AC signal levels. I then ensured the other tester had the same levels, fine tuning the bias pot as required, and then tested the same tube for gm.

Why I did this: after calibrating my testers (ensuring the grid signal levels were set the same) I wanted to ensure that they were in agreement with one another. My methodology accomplished this task and takes away the error introduced by slightly different grid signal and DC bias differences which have a huge impact on the gm reading. Now I can use either tester, with the confidence that they are working properly even though I will sometimes get different answers when testing the same tube in both testers due to differences for the reasons mentioned above.

Cheers
Steve


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