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 Post subject: 6E5 tube not closing when station tuned
PostPosted: Nov Sun 05, 2017 7:53 pm 
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Hi,

I have been working on this for some time now. I have done much reading, subbing parts and testing. I have tried 2 good working 6E5 tubes that actually close when channel is tuned. I know they work because I used them in my Packard-Bell. It is time to ask for some help.

These known good 6E5 tubes, when used on my Fairbanks-Morse 91-C-4 will not close when a station is tuned. The tube is very bright. (The original bad 6E5 that came with the radio has long ago been discarded.) I have replaced all out of tolerance resistors and all paper capacitors. The volume control pot measures 350k ohm in circuit. It should be 500k ohm. I have aligned the radio, and the radio sounds very good. All tubes test good. I have checked the following voltages:

tube Pin Voltage should be Actual voltage
6E5
2 6.5 ..... 28.8
3 -.1 ..... -.77
4 250 ..... 327
6K7 (RF)
5 3.6 ..... .1
6L7
4 130 ..... 11.1
5 -.3 ..... -3.4
8 5.5 ..... .3
6J7
3 175 ..... 199
4 300 ..... 326
5 -2.2 ..... .032
5Z4
2 310 ..... 332
4 -90 ..... -56
6 -90 ..... -56
8 310 ..... 332
6K7 (IF)
3 250 ..... 326
4 130 ..... 10.3
5 8.5 ..... .3
8 8.5 ..... .3
6H6
3 -.35 ..... -.52
5 -.35 ..... -.53
6F5
4 120 ..... 140.9
6 .2 ..... .01
6L6
3 275 ..... 308
4 265 ..... 258

The resistor between pins 2 and 4 on the eye tube is 1meg ohm and brand new. (not open)

I prefer not to replace the volume control pot if possible. However, if anyone thinks it is the source of the problem, I will replace it.

Thanks,

Joe


Last edited by roguethistle on Nov Sun 05, 2017 9:35 pm, edited 2 times in total.

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 Post subject: Re: 6E5 tube not closing when station tuned
PostPosted: Nov Sun 05, 2017 8:49 pm 
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If appears that you have subbed two other 6E5 tubes into this radio and they work OK. Yet the one that you are concerned with does not work.

Why wouldn't you conclude from this that the non-responsive tube itself is defective and move on from there?

If the radio is working (which you seem to have proven with the two other 6E5 tubes) then the most likely problem with your non-working 6E5 tube is a broken grid connection.

You could try re-flowing the solder in Pin 3 (grid) of the inert tube to see if it restores function; it that doesn't work, the tube is internally defective and should be discarded.

You can also get a nonresponsive tube (lights, but with no shadow sector response) if you swap the B+ connection to pin 2 instead of pin 4. But, since the unit works with other tubes that doesn't seem to be relevant to this problem.

Put the nonresponsive 6E5 tube into your Packard Bell and see if it works. If it doesn't, toss it. If it does, there is something inconsistent about how you have described the problem.


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 Post subject: Re: 6E5 tube not closing when station tuned
PostPosted: Nov Sun 05, 2017 9:16 pm 
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lorenz200w thank you for the response. lol You did not understand my post, both 6E5 tubes worked well (closed when station tuned) in the Packard-Bell, but the same tubes (when swapped) did not in the Fairbanks-Morse. They both failed to close when a station tuned in the Fairbanks-Morse. The original bad 6E5 tube has long been discarded.


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 Post subject: Re: 6E5 tube not closing when station tuned
PostPosted: Nov Sun 05, 2017 9:31 pm 
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The lower value volume control will effect the final value of AVC voltage available to the eye tube as well as alignment AND and electrical leakage. For example, a defective tube, leaking grid circuits. If the AVC source is disconnect an ohmmeter is used to measure the AVC circuit to the IF, RF and the eye tube, reference to chassis negative. The measurement must be infinity. If there is leakage, lift additional leads in the circuit to isolate. A tube can leak too, draw grid current if it is defective and not show on a tube tester. Try substitute tubes, the grid cap can be leaking to the metal shell.

Do look at the values of R-54 and R-55, This circuit will have to be disconnected to evaluate the AVC too.

The band switch and all the RF coils are involved. A dirty band switch can be part of the problem.

Any IF coil that does not demonstrate a true peak when aligning is questionable as to a short in the coil or moisture... This will have an effect on the AVC too.

All else fails altering the value ratio of R-54 and R-55 could improve the performance.

The AVC voltage can be measured with a sensitive vtvm 10 - 25 megohms input.

GL

Chas


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 Post subject: Re: 6E5 tube not closing when station tuned
PostPosted: Nov Sun 05, 2017 9:38 pm 
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I may not be seeing this correctly but isn't pin 2 supposed to be on B+ and pin 4 at the terminus of the 1M resistor. Why does your pin 4 seem to reflect B+ and pin 2 displaying significantly reduced voltage? Are the pins reversed in the socket?

EDIT: I stand corrected.

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Last edited by Bugman on Nov Sun 05, 2017 10:22 pm, edited 1 time in total.

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 Post subject: Re: 6E5 tube not closing when station tuned
PostPosted: Nov Sun 05, 2017 9:49 pm 
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Location: Minneapolis, MN USA
No.
Pin four is the target and must connect directly to the source of B+
Pin two is the plate and connects at the terminus of the 1meg resistor.
Eye tubes are different from non eye tubes in that B+ goes to target not plate.

Joe:

In the Fairbanks radio, do the eye tubes not move at all, or do they move but fail to completely close ?


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 Post subject: Re: 6E5 tube not closing when station tuned
PostPosted: Nov Sun 05, 2017 10:05 pm 
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thanks for all replys.
Dale Saukerson, the eye tube trys to close less than 1/16" from each side when the radio is first turned on, then the small closed area disappears as the radio warms up.  

Chas, what you say makes sense about the vol control and coils. I did clean all of the switches with cdc. I fear bad coils. The radio receives well on all bands. However, the alignment acted funny and did not want to peak easily when aligning the bc trimmer. I do not have a vtvm. I have a Simson 262 vom. Is this a problem?

Chas, I will try this: "If the AVC source is disconnect an ohmmeter is used to measure the AVC circuit to the IF, RF and the eye tube, reference to chassis negative. The measurement must be infinity."

I have some more work to do on this radio.

Joe


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 Post subject: Re: 6E5 tube not closing when station tuned
PostPosted: Nov Sun 05, 2017 10:24 pm 
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If the set did not peak properly, there is a danger that you inputted to much signal from the generator and perhaps its not on frequency? If the signal is strong enough to activate the AGC, you will never peak the IF transformers properly.

Marc


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 Post subject: Re: 6E5 tube not closing when station tuned
PostPosted: Nov Sun 05, 2017 11:03 pm 
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Voltage on pin #3 of 6E5 determines closure of the eye. Eye will close when this voltage gets to around -8 volts.

This voltage is determined by signal strength and alignment. Are you using an antenna?

Tune in a strong station. You should see some eye movement. While looking at the eye tube peak IF Transformers.

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 Post subject: Re: 6E5 tube not closing when station tuned
PostPosted: Nov Sun 05, 2017 11:04 pm 
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Chas, I am not sure exactly what you mean by this: "If the AVC source is disconnect an ohmmeter is used to measure the AVC circuit to the IF, RF and the eye tube, reference to chassis negative. The measurement must be infinity."

I am still kind of new to radio work, and the lingo is hard for me to understand. I was not sure exactly where to place my ohmmeter.

I did my best with these measurements:
from middle post on vol control to pin 4 eye tube: 350k ohm
from grid on 6k7 IF tube to pin 4 eye tube: 320k ohm
from grid on 6k7 RF tube to pin 4 eye tube: 50.3k ohm

I did check once again just now the continuity on pin 4 of the eye tube to the lug-- it had continuity.
Also, just now with the radio on, I duplicated the attempt of the eye tube to close. When I hit the workbench hard with my fist, the radio has a sharp static and the eye tube closes a bit more only for a brief moment. Hmm, there must be a lose connection, short?, or bad solder joint somewhere.


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 Post subject: Re: 6E5 tube not closing when station tuned
PostPosted: Nov Sun 05, 2017 11:15 pm 
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Norm: there is no conventional eye tube socket on the 91-C-4 Fairbanks-Morse. see photo before recap and new resistors:
download/file.php?id=172198
The resistor between pins 2 and 4 on the eye tube is 1meg ohm and brand new. (not open)
I did use a long antenna when aligning. Also, at or below 30% signal input when aligning.


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 Post subject: Re: 6E5 tube not closing when station tuned
PostPosted: Nov Sun 05, 2017 11:24 pm 
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For the eye to close pin #3 of 6E5 needs to be negative 8 volts. This in turn causes pin #2 of the eye to be more positive.

The negative voltage is generated by the detector. Voltage is most negative when RF and IF stages are aligned.

You have a voltage divider to reduce input voltage to this eye tube. For a test try putting a jumper across resistor #55. This will give greater voltage to pin #3 of the 6E5 tube.

http://www.nostalgiaair.org/pagesbymode ... 005759.pdf

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 Post subject: Re: 6E5 tube not closing when station tuned
PostPosted: Nov Sun 05, 2017 11:30 pm 
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roguethistle wrote:
lorenz200w thank you for the response. lol You did not understand my post, both 6E5 tubes worked well (closed when station tuned) in the Packard-Bell, but the same tubes (when swapped) did not in the Fairbanks-Morse. They both failed to close when a station tuned in the Fairbanks-Morse. The original bad 6E5 tube has long been discarded.

OK, then the problem has to be insufficient AVC voltage, an open isolation resistor between the eye tube grid and the AVC bus, or an incorrect B+ hookup to the eye tube.

About -7.5V is needed at the 6E5 grid to achieve full closure. This voltage can only be accurately measured with a very high-impedance device (such as a VTVM or CRO) as the AVC bus has miniscule current drive capability.

Generally eye tube grids are isolated from the AVC bus by a large (megohm-range) resistor. If this resistor opens up, the eye tube will be nonresponsive.

As I mentioned earlier (but discounted because I had misinterpreted your statement) it's possible to connect the B+ lead to the wrong pin and kill the tube's ability to respond to grid voltage variations. B+ should connect directly to Pin 4, with the 1 megohm resistor joining pins 4 and 2. The resistor can be in place correctly between pins 2 and 4 but if B+ is wired to pin 2 instead of pin 4, the tube will light up but not respond, just the same as if the 1 meg resistor were open.


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 Post subject: Re: 6E5 tube not closing when station tuned
PostPosted: Nov Sun 05, 2017 11:36 pm 
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I placed a jumper across #55 resistor. measured voltage on pin 3: -.77 no change. no eye tube close.
radio sounded more mushy at higher volume.

I think chas right, the vol control at 350k ohm bad


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 Post subject: Re: 6E5 tube not closing when station tuned
PostPosted: Nov Sun 05, 2017 11:52 pm 
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lorenz200w thank you for you reply. I did check continuity on pins 3 + 4 of the eye tube-- there is continuity. The b+ is going to the correct pin-- pin 4.

There is the question--there is no conventional eye tube socket on the 91-C-4 Fairbanks-Morse, and no arrow indicator of where pin 1 goes, is the eye tube actually oriented correctly? I believe it is installed correctly. however...I could be wrong
However still, the pins on the 6E5 can only be installed one way because pins 1+ 6 are fatter, so it is installed correctly.
I will post photos soon


Attachments:
FM 91-C-4 eye tube socket from inside chassis.jpg
FM 91-C-4 eye tube socket from inside chassis.jpg [ 145.69 KiB | Viewed 577 times ]


Last edited by roguethistle on Dec Mon 04, 2017 5:02 am, edited 6 times in total.
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 Post subject: Re: 6E5 tube not closing when station tuned
PostPosted: Nov Sun 05, 2017 11:58 pm 
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Could resistor 54 be low in value? Believe Rider has this one at 50K. That may be an error and way too low with 55 being 1 meg. Increase 54 to 1 meg.

With 54 being 50K and 55 being 1 meg you have a 20:1 ratio. AVC would need to be -160 volts to get -8 volts at pin #3 of 6E5, impossible... Also notice 54 isn't in line with others resistors listed. May have originally been left off?

Difference in volume control resistance isn't a problem.

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 Post subject: Re: 6E5 tube not closing when station tuned
PostPosted: Nov Mon 06, 2017 12:03 am 
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Norm, the original #54 resistor was 2meg ohm. I tried 50k ohm like in schematic. Then I went back to 2 meg ohm.

I figure volume control at 350k ohm would at least cause the eye tube to close some. It is not that far off tolerance.


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 Post subject: Re: 6E5 tube not closing when station tuned
PostPosted: Nov Mon 06, 2017 3:44 am 
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I did find one cap (cap 17) to have a bad connection. I resoldered it, and still no closing of eye tube. There is something loose on the radio somewhere. I will keep checking.

Norm, I made a mistake and placed jumper across wrong resistor. :oops: I retryed jumping resistor #55 and it did seem to keep the eye tube closed a little more.

Chas - I understand now why my voltage readings of pin #3 will be wrong. I do need a VTVM with very high impedance. I see the EICO 232 vtvm has 11 Megohms internal resistance. That may be the one to get.

lorenz200w -- there must me something wrong on the AVC bus- most likely the loose thing causing the sharp static and then loud volume when I hit the work bench with my fist.


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 Post subject: Re: 6E5 tube not closing when station tuned
PostPosted: Nov Mon 06, 2017 4:46 am 
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roguethistle wrote:
I see the EICO 232 vtvm has 11 Megohms internal resistance. That may be the one to get.
The EICO 232 is an excellent choice.

The 249 is even better (IMO). It's a 232 with a larger meter.

I've used both for many decades. Still have a 249 on the service bench.
roguethistle wrote:
there must me something wrong on the AVC bus...
Take a 9v battery and two 100K resistors.
Connect the resistors in series across the battery terminals (alligator jumpers).
Connect the battery positive to ground (B-).
Connect the junction of the two resistors to the junction of the AVC resistor and cap.

The eye tube should close, and receiver gain should drop.

If that happens, the problem is above the AVC resistor, toward the detector.
If that does not happen, the problem is downstream toward the front end.

- Leigh

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 Post subject: Re: 6E5 tube not closing when station tuned
PostPosted: Nov Mon 06, 2017 12:40 pm 
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The EICO 221 is 25 megohms on DC...


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