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 Post subject: Re: Tv-7d/u calibration problem. Need guidance!
PostPosted: Apr Sat 03, 2021 11:57 am 
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Have you tested the meter for linearity? It may explain some of what's happening. IF not, try this:
https://www.bbtvtestequipment.com/testing-a-panel-meter

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 Post subject: Re: Tv-7d/u calibration problem. Need guidance!
PostPosted: Apr Sat 03, 2021 4:44 pm 
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Location: Auburn, AL
Barry H Bennett wrote:
Have you tested the meter for linearity? It may explain some of what's happening. IF not, try this:
https://www.bbtvtestequipment.com/testing-a-panel-meter


I woke up this morning and this was the first thing that popped into my head! I probably should have tried to calibrate before I took the meter apart ;) the good thing is that I know how to take the meter apart, and if I study “the meter at work” and maybe ask a few questions, I’ll be able to adjust the hair springs/magnet so that it’s more linear. I went out of my way not to touch the spring adjustment on the back, because, though I may be an idiot who can’t help but take things a little too far apart, , I’m not always full blown stupid ;) I did measure 203 uA FS, but did not check to see if it was linear across the scale or not. Meter resistance was slightly low, at 2365. I have read, but don’t know the veracity of the claim that it should measure between 2370 and 2390 ohms.

The fact that it’s linear enough at the low end is good, I suppose, as that’s where you determine if a tube is bad, not at the high end. And it was giving me the correct readings when switching between ranges from about 100 on down to 40 (didn’t check lower than that, but easy enough to set up and do!

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 Post subject: Re: Tv-7d/u calibration problem. Need guidance!
PostPosted: Apr Sat 03, 2021 4:50 pm 
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Location: Auburn, AL
I can’t tell you how much I wanted to muck about with the knob in the middle of the meter magnet, but again, not being totally stupid, I managed to restrain myself. Not sure of it’s purpose, but I think it’s for adjusting the intensity of the magnetic flux? Above my pay grade.

On to some other projects, though at some point after I acquire a spare meter (or find one hiding in my boxes of meters) I’ll Get back to this mystery.


Attachments:
D33022AE-D4EE-40F5-A776-E7D55272040D.jpeg
D33022AE-D4EE-40F5-A776-E7D55272040D.jpeg [ 605.75 KiB | Viewed 1857 times ]

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Last edited by anchorman on Apr Sun 04, 2021 4:44 pm, edited 1 time in total.
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 Post subject: Re: Tv-7d/u calibration problem. Need guidance!
PostPosted: Apr Sat 03, 2021 4:56 pm 
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…...don't do it. That knob is connected to the Hickock Police Hotline :mrgreen:

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 Post subject: Re: Tv-7d/u calibration problem. Need guidance!
PostPosted: Apr Sat 03, 2021 6:42 pm 
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Location: Centennial, Colorado
Some of these posts are simply amazing by providing the step by step instructions for how to destroy a nice tester.
Sorry, but sometimes I'm just unable to restrain myself. Bill


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 Post subject: Re: Tv-7d/u calibration problem. Need guidance!
PostPosted: Apr Sat 03, 2021 8:07 pm 
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Bill Eccher Jr wrote:
Some of these posts are simply amazing by providing the step by step instructions for how to destroy a nice tester.
Sorry, but sometimes I'm just unable to restrain myself. Bill


What has been destroyed? I surely didn’t destroy anything by carefully replacing out of spec resistors, and turning the adjustment/calibration pots back and forth. And the meter currently works as well or better than when I took it apart to clean out the goo. I don’t recommend people without the steady hand or tools to do so, that they take apart a perfectly good meter. Mine was not perfectly good, and is slowly getting better, as is the tube tester.

If it makes you feel better, Bill, here’s a picture of the meter after the goo is gone and put back together. I unfortunately didn’t remember to take one before I cleaned it.


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402BD4B1-4CE6-4210-A2ED-168F55332E1D.jpeg
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Last edited by anchorman on Apr Sun 04, 2021 7:06 am, edited 1 time in total.
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 Post subject: Re: Tv-7d/u calibration problem. Need guidance!
PostPosted: Apr Sat 03, 2021 8:14 pm 
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Bill Eccher Jr wrote:
Some of these posts are simply amazing by providing the step by step instructions for how to destroy a nice tester.
Sorry, but sometimes I'm just unable to restrain myself. Bill


If you have any constructive suggestions regarding why I was unable to get the 6L6 to read at 120 on scale B, I’d be most interested to know. Because it has been eating up a fair bit of brainpower and concentration trying to figure out why everything else checks out, but this part does not. Next step is check meter linearity, and if that is good, to acquire or borrow a guaranteed new 6L6 to test against what I’ve got, and see if that is the problem. But first I’m going to think about some other things, and hope someone with more experience has an answer.

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 Post subject: Re: Tv-7d/u calibration problem. Need guidance!
PostPosted: Apr Sun 04, 2021 12:01 am 
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Have you tried a brand new 6L6 yet? Or just some that appear to be new? Buy one from JJ Electronics, or any other brand spanking new made within the last few years NEW tube? Otherwise you're just guessing.

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 Post subject: Re: Tv-7d/u calibration problem. Need guidance!
PostPosted: Apr Sun 04, 2021 5:15 pm 
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anchorman wrote:

If you have any constructive suggestions regarding why I was unable to get the 6L6 to read at 120 on scale B, I’d be most interested to know. Because it has been eating up a fair bit of brainpower and concentration trying to figure out why everything else checks out, but this part does not. Next step is check meter linearity, and if that is good, to acquire or borrow a guaranteed new 6L6 to test against what I’ve got, and see if that is the problem. But first I’m going to think about some other things, and hope someone with more experience has an answer.


I have a TV-7D/U and spent countless hours analysing the circuit and then calibrating it. Reference your inability to reach 120 while performing the 6L6 Range B & C ratio test:

Have you tested and/or replaced the bathtub capacitor across the meter?
Did you install protection diodes across the meter?

The reason I ask is because a leaky capacitor or the protection diodes may be bleeding signal current and preventing you from getting to 120 on the meter. I had to replace my bathtub cap as it was affecting readings. When I added the safety diodes I did comparison tests with and without the diodes to make sure that they didn't affect the readings. For the diodes that I used I noticed that they started to conduct for currents that moved the needle past 80. The error increased as the needle went from 80 to 120 as the protection diodes started to turn on as the voltage across the meter increased. I kept the diodes for protection and made myself a small error chart which reminds me to compensate for the error for tubes that read over 80.

On another note your original calibration methodology was not so far off after-all. The USAF modified the original Army procedures for performance testing and calibration starting by setting the filament voltages (and thus signal voltages) first, then moving on to the static DC voltages. See the bottom of this site for the procedures:

http://www.jogis-roehrenbude.de/Roehren ... /TV7-2.htm


Sincerely,
Steve
TV-3B/U, TV-7D/U, TV-10D/U, 600A owner


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 Post subject: Re: Tv-7d/u calibration problem. Need guidance!
PostPosted: Apr Sun 04, 2021 5:31 pm 
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Location: Auburn, AL
Hello Chappy, and thank you for this information!

I did in fact install protection diodes across the meter when I replaced the bathtub cap. It was something that seemed so ubiquitously done, that I forgot try removing the diodes as a cure for the odd meter readings. I also have two 1n4007 diodes that I installed years ago to replace the failed copper rectifier, and was thinking about whether those might be involved in my issues. I will check these things later and report back with my findings!

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 Post subject: Re: Tv-7d/u calibration problem. Need guidance!
PostPosted: Apr Sun 04, 2021 5:55 pm 
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Just had a look at my old notes. I wound up installing 1n4007 diodes versus 1n4001 diodes since they had less impact. I first tested the diodes out of circuit and compared diode voltage drop versus Id.
A sample from my readings:
1n4007 Vd 0.3965V 10uA,
1n4001 Vd .3969V 13.6uA.

That amount of current draw is significant when compared to a 200uA meter movement such as the one in the TV-7. I then took measurements with my TV-7 using a 6L6 on range B varying the bias and noting meter readings. I had the tester out of the case so that I could access the meter lugs and quickly attach the diodes on the fly while taking measurements.

For readings up to 80 there was no difference
At 90 adding diodes dropped reading to 89
At 100 adding diodes dropped reading to 99
At 110 adding diodes dropped reading to 106
At 120 adding diodes dropped reading to 113

So as more current flows through the meter, its voltage drop increases and the diodes start to conduct, not by much but enough to pull current away from the meter thereby affecting the readings.


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 Post subject: Re: Tv-7d/u calibration problem. Need guidance!
PostPosted: Apr Sun 04, 2021 8:18 pm 
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I read the document that Dieter Wächter made about repairing and restoring the tv-7 for Jogi, and he says that putting two diodes in series in the forward direction of the meter should prevent the current leakage that causes this problem. Hopefully that will help.

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 Post subject: Re: Tv-7d/u calibration problem. Need guidance!
PostPosted: Apr Sun 04, 2021 9:23 pm 
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Yes but then you just doubled the forward voltage drop from 0.7V to 1.4V before the diodes really start to conduct and save your meter during an overload condition event. Based on how scarce and expensive these meters are I am accepting the slight degradation in accuracy using only one set of parallel diodes for the added protection of my meter movement.


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 Post subject: Re: Tv-7d/u calibration problem. Need guidance!
PostPosted: Apr Sun 04, 2021 9:29 pm 
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Location: Auburn, AL
It seems like the faceplate is scarce, but are 200ua movements really scarce? I haven’t looked hard to find out.

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 Post subject: Re: Tv-7d/u calibration problem. Need guidance!
PostPosted: Apr Sun 04, 2021 9:56 pm 
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I have a box of meters of many different shapes and types. I haven't looked in the box for some time; perhaps I should take inventory. Some are sealed and some have specialized scales. Most are US made but there are a few Asian ones. Some are zero center. Even a couple of VU meters. More than a couple, probably.


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 Post subject: Re: Tv-7d/u calibration problem. Need guidance!
PostPosted: Apr Sun 04, 2021 10:34 pm 
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anchorman wrote:
It seems like the faceplate is scarce, but are 200ua movements really scarce? I haven’t looked hard to find out.

There are replica replacement meters for sale on the auction site. I believe that they are made in Taiwan and may be a good replacement but I suspect that they are made out of plastic. As well, there are NOS 200uA meters still kicking around and with your meter repair skills you should be able to swap out the faceplate. I was able to get NOS Phasotron DC milliamp meter (200uA FSD) on the auction site a while back which I am keeping as a spare. When my TV-7 meter dies I will try to transfer the faceplate to the NOS milliamp meter.


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 Post subject: Re: Tv-7d/u calibration problem. Need guidance!
PostPosted: Apr Sun 04, 2021 11:21 pm 
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And here is another way to replace your TV-7 meter . . . .

http://www.arizona-am.net/misc_files/TV7MeterB.htm

--Ed

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 Post subject: Re: Tv-7d/u calibration problem. Need guidance!
PostPosted: Apr Sun 04, 2021 11:57 pm 
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Location: Auburn, AL
Science to the rescue:

https://www.antiqueradios.com/forums/vi ... p?t=118294

I think we’re probably still safe with the two in series. At 1.4v, we have a little under 600ua flowing, if I’m doing the math right. That’s only 3x the meter rated current for full scale, and the poster of said info was applying 20x or more and not having failure/problems. I don’t see a long duration overload being an issue, so much as a short pulse being more likely. We’ve all made a mistake and set things wrong, and pegged the meter at one time or another, and the natural reaction is to let up on the test button ASAP. If these were as delicate as folks sometimes think, they wouldn’t have survived ever in real world service. I’m not advocating blatant meter abuse, but I think they’re likely tougher than we worry they are. Mine survived its whole life until a week ago with no protection diodes. Adding a protection diode slightly larger than what is normally recommended is still better than none at all. I’ll probably try to use a couple different options and see what has the least leakage with the lowest forward drop.

I also have a little bit of faith in the judgement of dieter Wächter, after seeing his restoration skills, and reading his full account of re-winding the meter on one of these and restoring it to service.

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 Post subject: Re: Tv-7d/u calibration problem. Need guidance!
PostPosted: Apr Mon 05, 2021 12:15 am 
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Anchorman,

Thanks for pointing out that other thread. I had missed that one in my research while restoring my tester. Interesting tests and a big thank you to Ron for potentially sacrificing working meters and sharing his results. Next time I open up my tester I will replace the diodes with a pair of two in series, or maybe just a pair of back to back LEDs. IIRC one of the colours has roughly a 1.5V drop.

Did you remove your protection diodes and then retry the 6L6 B/C scale ratio test? Were you able to get it to read 120?

VR,
Steve


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 Post subject: Re: Tv-7d/u calibration problem. Need guidance!
PostPosted: Apr Mon 05, 2021 12:37 am 
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Location: Auburn, AL
My worry with an LED is that they really don’t handle much current. How much before one goes poof and the magic smoke gets let out? If you do LED make sure it’s protected from light sources that will cause it to conduct more leakage current. There was a good thread on the eevblog that I found also, that had some diode recommendations with low leakage, and option for bipolar transistor b-e junction as another possible low leakage option, but the site seems to be down at the moment. Putting the link here anyway so I can refer back to it: https://www.eevblog.com/forum/projects/ ... f-a-diode/

Ive not had the chance to snip out the diodes yet, but will by tomorrow night at the latest.

(Edited for clarity)

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