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 Post subject: Re: 6E5 tube not closing when station tuned
PostPosted: Nov Thu 09, 2017 11:28 pm 
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Hi, Thanks Chas, Leigh and Norm for your advice. I appreciate it!

Before I asked for help, I was too focused on the eye tube socket voltage on Pin #4 being too high. You have pointed me in a different direction-- check the AVC.

I have taken some time to read up on AVC. I have read about it before but needed a refresher. Antique Radio Repair by Alfred Corbin on page 234 has a good diagram on exactly where the AVC circuit is. Elements of Radio Servicing on pages 152-170 is very good on describing AVC. I believe my 91-C-4 has a "delayed AVC". I highlighted the AVC circuit on my schematic. This helped me find out exactly where to put my leads.

I did the 9V battery test per Leigh. The eye tube closed about 1/8". Per Leigh, the test result shows the problem is "above AVC, toward the detector". The tuned in station was blocked during this test, and the battery got very hot. This is a very useful test procedure-- I will save it for future use! Could be the 2nd IF coil.

Chas: I was not sure where to put my ohmmeter leads. This is what I did measure:
eye tube #3 pin to #41 resistor ... 860K ohm
eye tube #3 pin to chassis ground ...800k ohm
vol contrl to #41 resistor ...470k ohm
chassis to diode plates of 6H6 (pin 3 then pin 5) ... both 400k ohm
after vol contrl taken apart and cleaned: across whole vol contrl in circuit 360k ohm (10k ohm better). with ground removed 450k ohm. maybe vol control is ok since nearer to 500k ohm with this last measurement.
I want to check resistance on IF coil per page 161 in "Elements", but need to remove coil cover. Not sure how to do that without breaking something. I did see 2 mounting nuts on the under side of chassis. I will try to check this soon.

Joe


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 Post subject: Re: 6E5 tube not closing when station tuned
PostPosted: Nov Fri 10, 2017 1:03 am 
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Hi Joe

A quick comment. You mentioned the eye closed about 1/8" using a 9 volt battery. 9 volts should close a 6E5 all the way. Could the tube actually be 6U5? They look the same but close at different voltages. Don't understand why the battery got hot unless a resistor from AVC to ground is low in value??

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 Post subject: Re: 6E5 tube not closing when station tuned
PostPosted: Nov Fri 10, 2017 10:41 pm 
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Norm Leal wrote:
Hi Joe

A quick comment. You mentioned the eye closed about 1/8" using a 9 volt battery. 9 volts should close a 6E5 all the way. Could the tube actually be 6U5? They look the same but close at different voltages. Don't understand why the battery got hot unless a resistor from AVC to ground is low in value??

That sounds like there is a low resistance to ground and the battery was mostly shorted out. So the voltage would have been less than 9 volts, possibly much less. And the high current would make the battery get hot. The resistance to ground should be quite high and should not draw much current at all.

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 Post subject: Re: 6E5 tube not closing when station tuned
PostPosted: Nov Fri 10, 2017 10:53 pm 
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roguethistle wrote:
I did the 9V battery test per Leigh.
...the battery got very hot.
Hi Joe,

That's a huge red flag.

There should be zero current drain on that battery.
Shorted AVC capacitor?

- Leigh

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 Post subject: Re: 6E5 tube not closing when station tuned
PostPosted: Nov Fri 10, 2017 11:40 pm 
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Hi all,

I ran the 9v battery test like this:
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.

I connected the battery positive to the chassis. Was that the right place to connect it to?
All paper and electrolytic caps replaced and almost all resistors replaced. I have physically checked all solder connections for loose solder joints and continuity.


I also swapped out these tubes with good spares to see if the eye tube would close: 6F5, 6E5 (total of 3 swapped) and 6H6 ----- still no eye tube closing.

Quote:
Shorted AVC capacitor?

- Leigh

what is the best way to check for this?


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 Post subject: Re: 6E5 tube not closing when station tuned
PostPosted: Nov Sat 11, 2017 1:49 am 
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roguethistle wrote:
Quote:
Shorted AVC capacitor?
- Leigh
what is the best way to check for this?
Replace the capacitor.

That should have been done already.

In any vintage radio,...
ALL electrolytic and waxed paper capacitors must be replaced before testing begins.

Doing any tests before those replacements have been made wastes everybody's time.

- Leigh

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 Post subject: Re: 6E5 tube not closing when station tuned
PostPosted: Nov Sat 11, 2017 2:51 am 
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Leigh,

All paper and electrolytic caps have already been replaced and almost all resistors replaced. I have physically checked all solder connections for loose solder joints and continuity.

I connected the 9v battery positive to the chassis. Was that the right place to connect it to?

How else can I check for shorts and a low resistance to ground?

Joe


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 Post subject: Re: 6E5 tube not closing when station tuned
PostPosted: Nov Sat 11, 2017 4:36 am 
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roguethistle wrote:
I connected the 9v battery positive to the chassis. Was that the right place to connect it to?
Hi Joe,

Yes, that's the proper connection.

Perhaps your new cap is bad.
Such has been reported by others. Not common, but it does happen.

Remove the battery and check the resistance across the AVC capacitor.
It should be quite high, like several megohms.

- Leigh

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 Post subject: Re: 6E5 tube not closing when station tuned
PostPosted: Nov Sat 11, 2017 5:53 am 
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roguethistle wrote:
I ran the 9v battery test like this:
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.

With that setup, the most current you could have drawn from the battery would be 0.09 ma. That is nowhere near enough current to heat up the battery. Is there any chance you got a hold of the wrong value resistors?

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 Post subject: Re: 6E5 tube not closing when station tuned
PostPosted: Nov Sat 11, 2017 6:05 am 
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Well, the brain cell just woke up and I put it in gear.

Given two 100K resistors in series connected across the 9v battery, and
connecting a clip lead to their junction...

There is no place you can connect the other end of that clip lead that would cause the battery to heat.
NONE.

The maximum possible current drain is 90 microamps.

There's something very very wrong here.

- Leigh

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 Post subject: Re: 6E5 tube not closing when station tuned
PostPosted: Nov Sat 11, 2017 6:41 am 
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Leigh,

I made a diagram of how I hooked up the 9 volt battery test for AVC. I hooked it up, then turned the radio on. Is that right sequence?

Eye tube closed only about 1/8". Also, schematic for this radio.

I am not so sure now that AVC cap #16 is a AVC cap. (The #16 was just off photo).

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

resistance across cap #16 (.05 mf) = 330k ohm
resistance across res #48 (1 megohm resistor) =690k ohm

Joe


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 Post subject: Re: 6E5 tube not closing when station tuned
PostPosted: Nov Sat 11, 2017 12:00 pm 
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roguethistle wrote:
I made a diagram of how I hooked up the 9 volt battery test for AVC.
I hooked it up, then turned the radio on. Is that right sequence?
Hi Joe,

The diagram and sequence are correct.

roguethistle wrote:
resistance across cap #16 (.05 mf) = 330k ohm
resistance across res #48 (1 megohm resistor) =690k ohm
Was the resistance across cap #16 measured with the 9v battery connected?
That would result in an invalid reading.

If the battery was disconnected, the resistance is too low, but not low enough to cause battery heat.

To cause battery heating I would expect resistance of less than 100 ohms.

- Leigh

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 Post subject: Re: 6E5 tube not closing when station tuned
PostPosted: Nov Sat 11, 2017 3:23 pm 
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Leigh,

9v battery not connected during resistance reading.

My gut feeling is that there is a short somewhere in the radio. I am guessing the IF 2 transformer area. I did not open that up yet. I am not sure how to best do that. I watched some you tube and checked this forum- both had good info.

Joe


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 Post subject: Re: 6E5 tube not closing when station tuned
PostPosted: Nov Thu 16, 2017 9:44 pm 
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I did take off the cover of the IF2 Transformer. It was dirty, but it had no obvious short. It had continuity, and I took resistance readings.

The good news is when I used my Show & Tell AM Transmitter, the eye tube closed 3/8" from each side- the most ever since I have had this radio. Maybe there is something wrong with the radio's RF stage.

Joe


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 Post subject: Re: 6E5 tube not closing when station tuned
PostPosted: Dec Sat 02, 2017 3:21 am 
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Joe, did you ever figure out what was going on with your eye tube?

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 Post subject: Re: 6E5 tube not closing when station tuned
PostPosted: Dec Sat 02, 2017 8:37 am 
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No, I did not ever get that to close fully when a station tuned. It actually closed the most (3/16") from either side when I used the Talking House AM Transmitter I got from Terry at the Louisville Radio meeting. I put the radio chassis back in the cabinet for now.

Do you think it could be the RF section? I do have a long antenna on both the radio and AM Transmitter.

Joe


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 Post subject: Re: 6E5 tube not closing when station tuned
PostPosted: Dec Sat 02, 2017 4:54 pm 
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Joe

Did you try changing the ratio or eliminating the voltage divider to pin #3 of the 6E5 eye tube?

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 Post subject: Re: 6E5 tube not closing when station tuned
PostPosted: Dec Tue 05, 2017 8:21 pm 
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If you really think there is a short then you need to track that down first, otherwise I'd follow Norm's suggestions.

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