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 Post subject: Re: Radiola 20 troubleshooting
PostPosted: Jun Fri 21, 2019 4:04 am 
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Tube #3 (detector) is the mystery. The schematic indicates that for an open circuit the voltage should be +4.5. How can it measure 2.8V? I measured the voltage at the coil side of the resistor and it is +4.5V as expected. The grid side of the resistor is the 2.8V. I also observed that the voltage is 2.8V when operating. I'm really confused by this.
Yes, the grid of the detector should be +4.5, no tubes in the set, there is no current drawn through the rheostats and the meter used is a DVM with high impedance. If you are measuring 2.8 then there is a miss-wire some place. Also look for failed mechanical connections, wrong hardware causing a short. Solder blobs or a cut wire touching the frame of the radio. The fact that is is still 2.8 radio operating is not only and indication, that grid of the detector when in operation and signals present is not measurable, such an attempt will "kill" the signal.

Follow the schematic and do a discontinuity in and around the detector grid circuit. Looking for leakages where infinite resistance should be present.

GL

Chas

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 Post subject: Re: Radiola 20 troubleshooting
PostPosted: Jun Fri 21, 2019 5:26 pm 
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Jim, I meant left to right, looking into the radio from the front.
Yes, where I said 22 v should have been 45.
Those measurements look good to me.
What about my question about power supply hook-up to the #1 cable?

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 Post subject: Re: Radiola 20 troubleshooting
PostPosted: Jun Sat 22, 2019 1:24 am 
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Jeff,

Everything is connected to ground as it should be.

Chas,

What should the grid voltage be in normal operation?

Do you know the value of the grid leak resistor?

Do you know how much grid current is drawn?

Jim


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 Post subject: Re: Radiola 20 troubleshooting
PostPosted: Jun Sat 22, 2019 6:05 am 
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What should the grid voltage be in normal operation? Initial voltage is set by the tuning circuit return to the +A.

Do you know the value of the grid leak resistor? It can be 2 to as much as 8 meg ohms.

Do you know how much grid current is drawn? In the detector no garden variety instruments can measure it

In the grid leak detector the grid derives its negative voltage from the electron flow from the filament to the plate. The plate voltage is set so the tube is used in a non-linear portion of its curve. The grid leak resistor must be at a value that does not allow the grid to become too negative. If that happens the detector becomes cut off and stops working. If there is a slow leak-down the detector can function for moment then cutoff very rapidly. This is called "motorboating" and is a accompanied by a "Putt-Putt" sound. If the grid leak is far too low the detector will also stop or be very weak, insensitive.

In referencing other RCA radios with 199 gridleak detectors 8.2 megs is used at 45 volts plate. An attempt to measure the grid voltage with a common DVM even a vtvm will overload the minute current available. There is a an Eico vtvm that is 25 meg input than may give a representative reading. But none the less, because the tube is used in a non-standard manner grid voltage values referenced to published characteristics are meaningless. Beside when regeneration is brought into play the average grid voltage shifts again...

The detector should work with 2 megs, if it is weak, up the value but also check the grid blocking capacitor it too could be leaking and between the resistor and the cap the value may be too low.

Be sure all the mechanical joints have been loosened and re-tightened too.

Truth: Even in the audio amp section, if the negative grid voltage derived from a battery is lost, the grid will become more negative until the tube is cutoff too. It causes strange plate voltage readings, since no plate current is drawn the plate voltage will go up to the level of the battery despite passing through the transformer winding. Conversely, if the grid is positive like the "C" battery is reversed, the plate current will be strong and the plate voltage somewhat lower than normal by a greater drop in the primary of the audio transformer plate winding. This dramatically shifts the tubs curve and will cause a very obvious distortion. Raises hell with the filament as well...
--------------------------------------------------------------------------------------------------------------------------------------
Is the "Amplification" control set so the movable winding is in-line with the main tuning coil? In that position it is maximum regenerative feedback. This sets the detector into oscillation and it can be heard using a transistor portable radio tuned to the same frequency as the Radiola 20. By backing off this Amplification control to just after the oscillation stops, is the most sensitive position for the detector at that particular frequency.

If the radio cannot be moved smoothly in/out of regeneration little or no radio signal will be detected.

Is there any evidence that the connections to this movable coil are reversed? If there is try reversing them. If the connections are backward the detector will desensitize and not work...

GL

Chas

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 Post subject: Re: Radiola 20 troubleshooting
PostPosted: Jun Sat 22, 2019 8:58 pm 
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We are missing something somewhere. Maybe try this check list again but write in the actual ohms measurements.
Attachment:
Radiola 20 test.jpg
Radiola 20 test.jpg [ 274.12 KiB | Viewed 712 times ]

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 Post subject: Re: Radiola 20 troubleshooting
PostPosted: Jun Mon 24, 2019 2:44 am 
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Chas,

Thanks for the grid leak answer. I have also been doing some reading and now better understand what I am seeing. The voltage reading that I observed on the grid was due to the voltmeter impedance (10M) upsetting the reading. I didn't realize the grid leak resistor was such a high value. I actually thought the high reading I got on the resistor was indicative of some issue.

I used a transistor radio as you instructed and was able to adjust the Radiola to get a signal at 1000KHz.

Now to what I have now observed. I have once again coupled an AM signal into the input. 1000KHz with 400 Hz modulation at 100%. I followed the signal through the stages and show a 400 Hz signal, approximately 5 Vpp on the grid of the 2nd audio stage. However there is no signal at the plate. This gets me back to my earlier point of no audio output.

Considering the possibility of a bad speaker (despite all my other tests) I used a 2.5K resistor (I hope this is a valid value) in place of the speaker. Still no signal at the plate.

Jim


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 Post subject: Re: Radiola 20 troubleshooting
PostPosted: Jun Mon 24, 2019 3:15 am 
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Quote:
Now to what I have now observed. I have once again coupled an AM signal into the input. 1000KHz with 400 Hz modulation at 100%. I followed the signal through the stages and show a 400 Hz signal, approximately 5 Vpp on the grid of the 2nd audio stage. However there is no signal at the plate. This gets me back to my earlier point of no audio output.
Excellent troubleshooting method. Note results would be the same but, typical practice is to use roughly 35% modulation.

-- Does the signal noted on the transistor radio go away as the amplification tuner is turned, actually come and go? If so the detector is acting normally.. In normal Radiola 20 operation, "Amplification" is advanced but never quite goes into regeneration.

It is obvious either one of two things are going on in the first audio.

1 - The tube has no emission or some other other defect.

2 - There is some sort of contact issue with the tube socket and/or the sides of the pins of the tubes should be cleaned.

It is hard to clean a tube socket with conventional tools. Try this: Take any dead 4-pin tube, even a salvaged base with no glass. Burnish the pins on this base with very, very coarse sandpaper, leaving the pins very rough. Insert this tube/base into the audio socket say 10/20 times. The rough pins will effectively burnish the contact area of socket.

Do re-solder all the joints from the grid and plate of this 1st audio. In fact do the same for the second (last) audio too.

Look at the jack across the 2nd audio transformer primary, if that jack has a large solder blob, pinched or shorted wire across the contact tabs or other defect so it is shorted the will be no signal passing through the transformer. Lack of signal by measure at the first audio plate would be because this "short" sends the signal to the relatively low impedance B+.

Oh be sure there is about 4-1/2 volts on that 1st audio grid and a return of the grid to A- via C-. An open grid connection will cause the grid to go more negative to the point where it cuts off the plate current, ie no signal...

Jim, You are very close to having the radio work.

The 2.5K load resistor is fine for measurement of an output.

Do cut the test signal modulation level down..

Chas

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 Post subject: Re: Radiola 20 troubleshooting
PostPosted: Jun Mon 24, 2019 4:38 pm 
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Have you tried plugging the speaker into the left jack socket? I'm sure you have. A strong station should be heard there softly.

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 Post subject: Re: Radiola 20 troubleshooting
PostPosted: Jun Mon 24, 2019 10:57 pm 
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Jeff,

Yes, I can clearly hear from the headphone jack (1st AF amp). Actually this is the first time I have heard my AM signal.

Chas,
Quote:
Does the signal noted on the transistor radio go away as the amplification tuner is turned, actually come and go? If so the detector is acting normally.. In normal Radiola 20 operation, "Amplification" is advanced but never quite goes into regeneration.

YES

NOTE: You refer several times to the 1st audio amp. No problem with it. The problem has always been with the 2nd amp.

Quote:
1 - The tube has no emission or some other other defect.

I earlier detailed my emission checks. I have also swapped a known good tube from the 1st stage to the 2nd stage. I have done this with 3 different tubes.


Quote:
2 - There is some sort of contact issue with the tube socket and/or the sides of the pins of the tubes should be cleaned.

It is hard to clean a tube socket with conventional tools. Try this: Take any dead 4-pin tube, even a salvaged base with no glass. Burnish the pins on this base with very, very coarse sandpaper, leaving the pins very rough. Insert this tube/base into the audio socket say 10/20 times. The rough pins will effectively burnish the contact area of socket.


I have previously cleaned the 2nd amp socket per your directions.

Quote:
Do re-solder all the joints from the grid and plate of this 1st audio. In fact do the same for the second (last) audio too.

Look at the jack across the 2nd audio transformer primary, if that jack has a large solder blob, pinched or shorted wire across the contact tabs or other defect so it is shorted the will be no signal passing through the transformer. Lack of signal by measure at the first audio plate would be because this "short" sends the signal to the relatively low impedance B+.


Remember - 2nd stage is the issue.
I have inserted an audio signal at the plate pin of the 2nd stage socket and clearly have output at the speaker.


Quote:
Oh be sure there is about 4-1/2 volts on that 1st audio grid and a return of the grid to A- via C-. An open grid connection will cause the grid to go more negative to the point where it cuts off the plate current, ie no signal...

I verified the -4.5 V at the grid once again.

Are you sure that a 199 will work in place of a 120 when properly biased? Obviously, the grid bias is good and the plate is 90V. I have hesitated putting the 120 back in until I have everything working - don't want to introduce another unknown.

Jim


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 Post subject: Re: Radiola 20 troubleshooting
PostPosted: Jun Tue 25, 2019 1:36 am 
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I though I was carefully following your testing of stages and ended up with signal at first AF :roll: .

Nuts...

O.K. so 1st AF is fine. It is the grid of the last audio that has signal but none present at the plate....

Nuts again...

Go back to all the exercises that I had stated to do to the 1st audio and repeat on the 2nd audio stage, Oh wait, you have done that already...

Yes, I have a few radios that normally have a 99 as an output stage. In fact the Radiola AR-812 can use an adapter that accepts a 120 in the last audio to replace the 99. As expected 135 new B+ and 22-1/2 new C- It is a "direct" exchange...

O.K. on the regeneration test. That result stated is perfect...

Here is where I look at resistance and the fact that it is not reactance...

I want to substitute an inductor for the resistor...

If you have a defective 20's era audio transformer with say the grid side open, ignore the grid. replace the resistor with the good winding of that transformer. Connect a 1 to 4 mf 200 volt or better cap the the plate connection too. Connect high impedance headphones, one pin to B- the other pin to the open end of the "new" cap. The cap will block the DC, the winding will provide an excellent load for the plate of the tube.

See if that works.

Actually, the plate load can be the headphones but the tone will be better with the choke/cap connection.

I am wondering if the "phones were used earlier, the plate current could have been pulling the diaphragm down to the pole piece. Normally, this is correct polarity but if the diaphragms have a dent (both?) that could lockup against the pole piece, no sound.

No transformer? Up the plate resistance to 25K...

GL

Chas

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 Post subject: Re: Radiola 20 troubleshooting
PostPosted: Jun Tue 25, 2019 2:57 am 
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Chas,

Quote:
I want to substitute an inductor for the resistor...

What resistor are you referring to here?

I do not have any high impedance headphones.

Why would I have sound when connecting the speaker to the headphone jack but not when it is connected to the speaker jack if plate current is the issue? More current in the 2nd stage? Can signal level be having some impact here? - i.e. too much signal.

Jim


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 Post subject: Re: Radiola 20 troubleshooting
PostPosted: Jun Tue 25, 2019 4:46 am 
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Check the choke in the 100 speaker and the connections to the speaker jack such that they properly mate when attached...

Chas

73's

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 Post subject: Re: Radiola 20 troubleshooting
PostPosted: Jun Tue 25, 2019 3:35 pm 
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wa4ywm wrote:
Jeff,

Yes, I can clearly hear from the headphone jack (1st AF amp). Actually this is the first time I have heard my AM signal.

At this point, did you try to tune in any radio stations? Play with the amplification control?

What is the resistance measured from G5 to the -22 v connection wire?
When using the 199 in 2nd audio are you connecting -22 and -4.5 together?

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 Post subject: Re: Radiola 20 troubleshooting
PostPosted: Jun Tue 25, 2019 5:04 pm 
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way4wfm,

Think you're wasting a lot of effort with the signal generator. Just follow the schematics and see what's not
in specs. Chaz gave you the checklist of tests from RCA. Now are you getting any output from the 1st or 2nd
stage jacks thru headphones or a proper speaker? You need the high impedance phones or speaker unless
you convert a makeshift modern speaker to work.

You said you had open audios. Did you wire them correctly? Red=B+, Blue=Plate, Green=Grid and A-.
I'd hook up your eliminator for just A voltage, then B voltage and then C. You can also ohm them out
to make sure your set matches the schematic. Besides the radio, you need working tubes, speaker, antenna,
and ground. Don't think signal generators were around in 1925.


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 Post subject: Re: Radiola 20 troubleshooting
PostPosted: Jul Tue 02, 2019 4:03 pm 
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Guys,

Been busy and just came back from a trip.

Chas,

I checked the speaker connections using the troubleshooting in the 100 manual several weeks ago and again recently. All is good.

Also, remember that I have injected a signal at the 2nd amp tube plate pin (no tube in the socket) and clearly hear it in the speaker. I would think that would verify the connections.

Jeff,

I did try to tune a station - couldn't find one so I just used the AM generated signal to check things out.

I am not using the -22V. I connected the grid to the -4.5V.

brnhornet52,

If I hadn't been troubleshooting with the signal generator, I would have never reached the conclusion that I have a signal path all the way to the grid of the 2nd amp.

I used the checklists way back at the beginning of the troubleshooting process. This is how I discovered the open transformer windings.

Concerning the output, see my previous recent discussions primarily with Jeff.

Jim


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 Post subject: Re: Radiola 20 troubleshooting
PostPosted: Jul Tue 02, 2019 4:53 pm 
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Good morning.
To hear any stations on both of my 20s the antenna ground has to be hooked to an actual earth ground.
Can you measure the resistance from terminal 2 to P4 (power supply and speaker disconnected)?
Thanks.

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 Post subject: Re: Radiola 20 troubleshooting
PostPosted: Jul Tue 02, 2019 5:58 pm 
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Hmm,

As I currently understand:

Speaker is O.K.

Signal injected into the plate connection of the last audio (output) tube can be heard in the speaker.

In a previous post, signal is present at the grid of the same output tube. Not that it matters.

Tube is good that is used in this output location.

Power up with all correct voltages good tube installed in output, plug in speaker. Apply signal (somehow) Nothing, nada, dead...

Some conclusions come to mind:

- Something is going on with that socket. Ya FWIR it is an integral socket within the large Bakelite chassis.

- Something is overall output circuit that fails upon final connection and applying power...

- I am not understanding how this is possible, therefore will simply read and see how this gets resolved..

GL

Chas

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 Post subject: Re: Radiola 20 troubleshooting
PostPosted: Jul Tue 02, 2019 6:50 pm 
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More questions:
You said "Also, remember that I have injected a signal at the 2nd amp tube plate pin (no tube in the socket) and clearly hear it in the speaker. I would think that would verify the connections." How is the signal generator connected when doing this test?
With power supply all hooked up and turned on; speaker plugged in; 2nd audio tube removed; what is the voltage, measured from terminal 1 to P5?
Thanks.

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 Post subject: Re: Radiola 20 troubleshooting
PostPosted: Jul Wed 03, 2019 4:09 pm 
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Jeff,

The signal generator has the positive connection to P5. The ground side is connected to the speaker connection that is not connected to the plate. Thus, I am applying the signal across the speaker from P5. I have also applied the signal from P5 to ground with the result that the speaker audio is greatly attenuated.

The voltage measured on P5 is 90V.

Jim


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 Post subject: Re: Radiola 20 troubleshooting
PostPosted: Jul Wed 03, 2019 5:00 pm 
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Running out of ideas here.
"Can you measure the resistance from terminal 2 to P4 (power supply and speaker disconnected)?"

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