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 Post subject: TV-7D/U Question
PostPosted: Dec Sun 27, 2009 6:07 pm 
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On my recently acquired Tester, Calibration is well within tolerance.

Noticed Problem testing 6L6 Tube vs 6V6 Tube

If I use the setting I have for the 6L6 Setting at Range Switch = D

All 6L6 test Bad Around 22 on the meter.

If I Set the Range Switch to C as for the 6V6 The 6L6 test as they are, Good or around 90 on the meter.

What am I doing wrong or missing here?
Is the Tube Chart wrong for the 6L6 settings?

Thanks
Bob T.


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 Post subject:
PostPosted: Dec Mon 28, 2009 1:17 am 
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If the tester is in cal then you have weak tubes. Using the 6V6 range of course will read much higher but of no use for a 6L6. You have to have the unit checked out for proper voltages. If those are new tubes then there is a problem with the tester.


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 Post subject:
PostPosted: Dec Mon 28, 2009 1:24 am 
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Lou deGonzague wrote:
If the tester is in cal then you have weak tubes. Using the 6V6 range of course will read much higher but of no use for a 6L6. You have to have the unit checked out for proper voltages. If those are new tubes then there is a problem with the tester.



Hi Lou

After much more searching , I believe there is a problem with this tester.

The only tube I tested with a problem was the 6L6 which is a Cal tube from my 600A

Tested 01A-24-27-45-47-80 all satisfactory results.

To get to the Bottom of it I will attempt a full Cal with voltage adjustments.

Seems much more compact than my 600A.

Will give it a go.

The test reading for the 6L6 I have should be 40 on the meter, I am getting 22.

Did the Filament and Plate settings so far. No Problem found there.

Onward and upward.

Thank you for the info. My first TV-7


Bob T.


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 Post subject:
PostPosted: Dec Mon 28, 2009 1:29 am 
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Did you check the 83 or try another one known to be good?


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 Post subject:
PostPosted: Dec Mon 28, 2009 1:56 am 
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Lou deGonzague wrote:
Did you check the 83 or try another one known to be good?


Yes first thing I did was check them. The original 83 was very weak on one side.

Of the 3 #83 tubes I have, I put the closest matched in.

The same with the 5Y3. Original was good , but I had a better matched one.


Bob T.


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 Post subject:
PostPosted: Dec Mon 28, 2009 3:36 am 
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joybird wrote:
The test reading for the 6L6 I have should be 40 on the meter, I am getting 22.


Bob,

Are you sure about this? I checked my copy of T.O. 33AA21-5-31 and TB 11-6625-274-12/1 and they both indicate a minimum value of 25 for the 6L6.

Bob


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 Post subject:
PostPosted: Dec Mon 28, 2009 4:47 pm 
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radious wrote:
joybird wrote:
The test reading for the 6L6 I have should be 40 on the meter, I am getting 22.


Bob,

Are you sure about this? I checked my copy of T.O. 33AA21-5-31 and TB 11-6625-274-12/1 and they both indicate a minimum value of 25 for the 6L6.

Bob


Yes. The 6L6 I have is 4900 mMhos so should read about 40 on the Meter. 25 is the min reading of a useable 6L6.
I am getting a reading of 22 for my known good 6L6.

Bob T.


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 Post subject:
PostPosted: Dec Mon 28, 2009 6:30 pm 
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It may be "4900" on another tester, but that is for a specific operating point. Unless the TV7 uses that same point, you can't compare the readings. What is the plate current in your TV7, and what plate current was specified for the standard 6L6??


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 Post subject:
PostPosted: Dec Mon 28, 2009 6:39 pm 
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Alan Douglas wrote:
It may be "4900" on another tester, but that is for a specific operating point. Unless the TV7 uses that same point, you can't compare the readings. What is the plate current in your TV7, and what plate current was specified for the standard 6L6??


Hi Alan

I have started going through the Calibration.

I do not have bias voltage Checking between pins 6 to 5 of the octal socket. Set up for 6L6. Rotating the bias does nothing. I check at the Bias pot itself an I have bias voltage and can vary it through its range without a problem up t0 40 volts.

Looks like I have a chore of finding why. In process of studying the Maint Manual.

So far it is suggested a switch may be the problem, WOW a rats nest of wiring.

Open to any suggestions at this point.

Looking for those 47 ohm resistors. (fuse) ????

Thanks
Bob T.


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 Post subject:
PostPosted: Dec Mon 28, 2009 6:56 pm 
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Measure the AC filament supply voltage to make sure it corresponds to the setting. The 6L6 filament draws twice more current than the 6V6 (0.9A vs. 0.45A). It's possible that the filament supply voltage is loaded down by heavy current because of high contact resistance or some other reasons.


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 Post subject:
PostPosted: Dec Mon 28, 2009 7:01 pm 
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Found the 47 ohm resistor on the Grid Stack Switch to be 580K ohms.

Crossing fingers and in process of getting at it to change out.

The Filament voltage is good for the 6L6 setting.

Bob T.


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 Post subject:
PostPosted: Dec Mon 28, 2009 9:06 pm 
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Replaced said 47 ohm resistor. Now have my bias volts.

Ran through Voltage cal. Plate screen Grid , ALL very close.

Still have the mentioned problem with the 6L6 test.

I know the Tube is very strong I have always used it as an indication of Cal for my Hickok 600A It test strong on the 600A

Should not be reading 22 on the TV-7.

Alan I am not set up as yet to do the current measurement you suggest. I have a socket on the way which will help in that.

Plate Current for a 6L6 @ Zero Sig is 72ma Max Sig = 79ma
TV-7-D/U Plate current is 150ma max.


Off to Lunch

Bob T.


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 Post subject:
PostPosted: Dec Tue 29, 2009 12:37 am 
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Well back to the Bench.

Continued on with the Simulated Tube Test

Apply 50 VAC through a 10K resistor. Pins 3 and 6 of Octal socket

Meter should swing to 40 during this TEST. It swung to 36.

So Following the directions I adjusted R115 POT.

Had no response at first. Wiped it over and over got it to set at
40 nice and stable.

Got good result after on Function positions Pos C reads 20 on meter

B,D,E,F, all read 40 on meter.

Now my test of the 6L6 reads 39 on the meter. Just about what it ought to.

I really do not know what did the fix. Hard to believe wiping R115 cleared the problem.

Possible the problem will return.

As of now it is working great .

Need a much finer Calibration. Will do that when my 1000 ohm /Volt meter arrives. Want to do it as compared with my Fluke Digital.

Very happy with this Tube Tester I acquired from Alan.

I like the setup vs the 600A little RED dot Match up.

Now I can sleep better tonight.

Bob T.


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 Post subject:
PostPosted: Dec Tue 29, 2009 2:00 am 
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Quote:
I really do not know what did the fix.


That's very common. Just using the various switches may clean up dirty contacts, and you'll never know which one it was. But once it's working, it will probably stay that way.

By the way the 47 ohm resistor ought to be a small carbon comp, not a metallized film or anything good. It's there as a fuse, and I've seen metallized resistors go dead short when they fail, and glow white. Carbon comps just smoke and open up. Lots cheaper to replace a resistor, than a bias pot.


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 Post subject:
PostPosted: Dec Tue 29, 2009 2:39 am 
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By the way the 47 ohm resistor ought to be a small carbon comp


Hi Alan

Yep you are correct . (As Always) :lol: I only had a 1 W Metal.

The other 2 are also wrong. someone put 1% wirewound in 1W

Will see if the Shack has any carbon tomorrow. Will change out all 3.

I have a I-177 here with a bad bias pot, so I know what a bad feeling that is.

They WILL be replaced before this tester is put into service.

Thank you for you input Alan. It is ALWAYS appreciated.

Bob T.
Wilmington, NC


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 Post subject:
PostPosted: Dec Tue 29, 2009 4:31 am 
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It's easy to accidentally test a grid-to-plate shorted tube. I'm in a hurry and don't bother to run the shorts switch through all its positions, or I'm doing a bunch of identical tubes...

I'm sure I have 1/4w carbons if you need them.


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 Post subject:
PostPosted: Dec Tue 29, 2009 8:08 am 
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Location: Medford,Or USA
Same thing here! If I'm testing tubes for over an hour, I'm screwing up the tests and calling them all good, shorts and all. :(
To properly test tubes, you have to CONCENTRATE!! You have to have your thinking cap on, as my gradeschool teachers used to say, back in the early '50s.

_________________
Steve McDonald


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 Post subject:
PostPosted: Dec Tue 29, 2009 4:45 pm 
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Found 5 old Carbon 1/4 W in my junk stash.

They read 50 ohms, good enough for this.

Replaced all 3.

In line for a final close tolerance Calibration.

Now on to my 3 I-77B testers.

Got motivated so have to push on.

Thank you
Bob T.


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