RCA Model 20 TRF

[philo]

Had it working many years ago.

As far as I can tell ,the problem is int he detector stage.


With an audio generator I've confirmed the audio portion is working.

All tubes are good and I've swapped with others.


The schematic calls for a 2 - 4 meg grid leak resistor


The installed 5 meg measures at 6


I did try a 2 meg but no difference


Any ideas?


Right now I'm stumped

[philo]

Post by philo
Had it working many years ago.
As far as I can tell ,the problem is int he detector stage.
With an audio generator I've confirmed the audio portion is working.
All tubes are good and I've swapped with others.
The schematic calls for a 2 - 4 meg grid leak resistor
The installed 5 meg measures at 6
I did try a 2 meg but no difference
Any ideas?
Right now I'm stumped

Detector OK


I should have figured this out sooner.

Though there was audio output when I connected my audio signal
generator, it was very low level

Problem is the first inter-stage transformer.


I replaced the other one 25 years ago



Got one from AES designed to go inside the original "can"


It was kind of a PITA removing the guts from the original tar seal

[Over]

Post by philo

Post by philo
Had it working many years ago.
As far as I can tell ,the problem is int he detector stage.
With an audio generator I've confirmed the audio portion is working.
All tubes are good and I've swapped with others.
The schematic calls for a 2 - 4 meg grid leak resistor
The installed 5 meg measures at 6
I did try a 2 meg but no difference
Any ideas?
Right now I'm stumped

Detector OK
I should have figured this out sooner.
Though there was audio output when I connected my audio signal generator, it
was very low level
Problem is the first inter-stage transformer.
I replaced the other one 25 years ago
Got one from AES designed to go inside the original "can"
It was kind of a PITA removing the guts from the original tar seal

pardon?

[philo]

On 8/12/19 5:27 AM, Over wrote:
<snip>

Post by Over

Post by philo
Detector OK
I should have figured this out sooner.
Though there was audio output when I connected my audio signal
generator, it was very low level
Problem is the first inter-stage transformer.
I replaced the other one 25 years ago
Got one from AES designed to go inside the original "can"
It was kind of a PITA removing the guts from the original tar seal

pardon?

The Atwater-Kent radios used tar-sealed inter-stage transformers inside
of a metal can

To keep the radio as original as possible, the transformer can be
removed from inside the can and replaced. To remove the tar (from the
last one I did) was difficult.

Looks like Peter (below) gave a great suggestion though.

[Peter Wieck]

I am glad that there are still some individual that are doing these sorts of repairs, and apparently enjoying it!

A few weeks ago, I was part of repair clinic at the Museum of Industrial History in Bethlehem, PA - and the three of us involved were busy from the moment the doors opened, to the end of the event. The oldest radio I "touched" was a Zenith 11-tube shutterdial, the newest, a 1963 Japanese 5-tube clock-radio.

Congratulations!

On the "tar" thing. Most of the time, if you freeze the original can, the whole shebang will just pop out with a sharp rap.

Peter Wieck
Melrose Park, PA

[philo]

Post by Peter Wieck
I am glad that there are still some individual that are doing these sorts of repairs, and apparently enjoying it!
A few weeks ago, I was part of repair clinic at the Museum of Industrial History in Bethlehem, PA - and the three of us involved were busy from the moment the doors opened, to the end of the event. The oldest radio I "touched" was a Zenith 11-tube shutterdial, the newest, a 1963 Japanese 5-tube clock-radio.
Congratulations!
On the "tar" thing. Most of the time, if you freeze the original can, the whole shebang will just pop out with a sharp rap.
Peter Wieck
Melrose Park, PA

I really enjoy working on vacuum tube equipment.

Years ago it seemed I had to shortage of them to work on but during the
past ten years or so have only had three or four repair jobs.


Thanks to Facebook I made an announcement that I was looking for radios
to work on and got quite a few to repair. For the most part they were
from friends who had a family heirloom they wanted to have working again.

I specifically told them: No transistorized radios.


Thanks for the tip about freezing the transformer to get the tar out.

I will probably re-stuff it one of these days but for now the similar
one I had from another Atwater Kent is doing the job>


I am fortunate in that over 40 years ago I found two shopping bags full
of old radio tubes in the basement of an apartment I lived in. Most of
them still good


Tubes such as 201's , 45's 80"s, 24A and 27's etc>

A very valuable find!




I told my wife that I bet I;m the only person in the world who repairs
vacuum tube radios AND computers. Often I will have a computer and a
radio on the bench at the same time>


My wife laughed and said:

But the computers you repair are 5- 10 years old...
Just as antiques as the radios!

[retiredinprescott]

I just finished restoring the guts of a 1925 RCA Radiola 24. That set has the 6 tube catacomb circuit where the entire circuit is in a can full of pine rosin.
The tests I ran showed several bad connections so I had no choice but to melt out all the pine rosin. I got a cheap small controllable toaster oven at a thrift store. I put the entire catacomb into a flexible 99 cent aluminum turkey roaster pan and shoved it all into the small toaster oven at 350 degrees. 3 hours later the pine rosin was like water (except for the smoke and the smell) and I was able to get to all the broken wires and a bad rf coil in the catacomb. A really tough repair at my age with all the human hair size wires but I managed to get it working. I had to do RC coupling around the bad coil which caused some loss in sensitivity but I can still get our local stations and I'm a happy camper.

[philo]

Post by retiredinprescott
I just finished restoring the guts of a 1925 RCA Radiola 24. That set has the 6 tube catacomb circuit where the entire circuit is in a can full of pine rosin.
The tests I ran showed several bad connections so I had no choice but to melt out all the pine rosin. I got a cheap small controllable toaster oven at a thrift store. I put the entire catacomb into a flexible 99 cent aluminum turkey roaster pan and shoved it all into the small toaster oven at 350 degrees. 3 hours later the pine rosin was like water (except for the smoke and the smell) and I was able to get to all the broken wires and a bad rf coil in the catacomb. A really tough repair at my age with all the human hair size wires but I managed to get it working. I had to do RC coupling around the bad coil which caused some loss in sensitivity but I can still get our local stations and I'm a happy camper.

Sounds like it was a real challenging project but glad you got it going.

I have been using a magnifying visor for years.


For now at least...my work bench is again empty