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 Post subject: Radiola 25 problems
PostPosted: Sep Thu 24, 2020 11:16 pm 
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Joined: Sep Thu 24, 2020 9:16 pm
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So I had this Radiola 25 laying around for years, was going to keep it for parts in case a ran across another one worthy of repair. However the other day I put a little stripper on the front panel, then one thing led to another. Anyway the only thing it will pick up is my AM transmitter if the transmitter antenna is right up to the Radiola antenna. When I check out the catacomb I found the audio transformers missing. So I put a couple of P-T156 audio transformers in it and not sure if they are causing a problem? I have a couple different ones that are more closely matched to the originals coming to try. But they may not be the problem. It was missing the antenna which is part of the tuning circuit, and to test it I made one using some wood and 80' of 18 ga. speaker wire, 13 loops 3/8" spacing. It just won't get any stations except for the AM transmitter. My AR-812 will pick up a lot of stations with just it's built in antenna. All the tubes check good, and I have tried other tubes and swapped some around. One time with a external antenna wire and the end coiled up next to the radio antenna and the end going to ground it faintly got a station for a bit. But not now, just the AM Transmitter. Maybe some of you have some ideas?


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 Post subject: Re: Radiola 25 problems
PostPosted: Sep Fri 25, 2020 3:51 am 
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Posts: 14
Here is my home made antenna.


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 Post subject: Re: Radiola 25 problems
PostPosted: Sep Fri 25, 2020 4:06 am 
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Here is a video with the radio playing from a AM transmitter. However like I said, it will not pick up a station day or night. Really it doesn’t play well at all on the transmitter, the antenna from the transmitter has to be wrapped around the radio antenna to work.
https://youtu.be/Ae9V_8lmPCc


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 Post subject: Re: Radiola 25 problems
PostPosted: Sep Sat 26, 2020 10:31 pm 
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Looks like I got this figure out. The plug in antenna connection is my problem. Since I had to fabricate the whole antenna I don’t have the base of the antenna that plugs into the radio socket. I bypassed the socket and connected the antenna directly to the tuning coil. It picks up 3 stations weakly and my AM transmitter very well. No longer have to wrap the transmitter wire around the antenna.

Video https://youtu.be/hmesX4IQd4s


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 Post subject: Re: Radiola 25 problems
PostPosted: Sep Sun 27, 2020 2:41 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 16985
Location: S. Dartmouth MA 02748-1225 USA
Welcome!

Good Job on the 25...

Some bits....

The loop must have a near exact inductance (turns) for the antenna station selector to resonate all through the tuning range to track the oscillator tuning. Add or subtract turns...

The A53 PT 156 has a low resistance winding and may not be providing an ample plate load resistance for the '99. There is a replacement transformer for the Radiola III made by Hammond.

https://www.tubesandmore.com/products/t ... ment-audio

Or the windings provided by ARBE-III folks which will probably do a better job.

https://www.arbeiii.com/Transformers/

The "A" voltage should be 4-1/2 volts the onboard battery rheostat knocks down the voltage the difference become part of the bias for the tubes. The output tube should be a 120 and there should be the correct bias.

It appears that reception is in a concrete block building, though the 25 is sensitive, there is considerable attenuation in such a building.

The 99 may not be testing correctly. Suggest set up a "B" supply, "A" supply and a filament voltmeter, plate voltmeter and a plate milliammeter and test the tubes for emission connecting the grid to the plate. If the plate current is low the tubes can be rejuvenated here:

http://www.nostalgiaair.org/References/ ... lash08.htm

The chart shows 1.1 volts filament for the plate load chart, it should be 3.3 volts...

If the intent is to continue operation of the 25 in the "shielded" area. wrap two to three turns of wire around the loop and connect one end of that coil to a long wire antenna and the other end to a earth ground.

Do not directly connect an antenna or ground to the 25 circuits. That, will shift the resonance of the 1st station selector.

RCA provided a "Loop replacement coupling coil" for such shielded environments. I have one for a Radiola 28 so the tuning inductance is somewhat different, center-tapped, but the input winding should be the same. That unit plugs into the loop socket and mounts to the inside of the cabinet. A similar antenna coupler could be fabricated, making the tuned circuit windings "adjustable" and mount the form to the inside of the cabinet, directly connected to the loop terminals at the rear terminal strip...

After all those checks, it could be that the catacombs IF is no longer in good resonance especially if the OEM mica stack tuning caps have been replaced without regard for resonance at the IF frequency... The 25 will receive down to the noise level. I had one operating and was amazed how sensitive it was, even in my lab which is partially foil shielded... GL Chas

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 Post subject: Re: Radiola 25 problems
PostPosted: Sep Sun 27, 2020 5:08 pm 
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Joined: Sep Thu 24, 2020 9:16 pm
Posts: 14
Thanks for the tips and suggestions Chas.
The antenna is pretty close to exact specs I think. Length of wire was 80' however I cut off just a bit more when connecting to the base so it's really close to 79'. It has 13 loops and spacing is 3/8" apart. I bought a roll of 18GA., speaker wire and stripped out one of the conductors. I didn't have a antenna base to build from so I had to make something. That's where my problem was. The radio was working on the AM transmitter so I thought the antenna was working. However to verify this I disconnected the tuning capacitor parallel with the antenna and the radio was still playing on the AM Transmitter. So I suspected the antenna was not working. I bypassed the plug connection for the antenna and put the antenna right to the tuning coil and it worked. I'm in Maryland and picked up a station in Boston last night. It just seems like I can't turn up the volume or battery voltage much without the radio loosing the stations. I'm hoping it's the Audio Transformers. Like you stated, the ones I put in to test it have a very low primary resistance, 400 & something ohms I believe. The one I have coming, same ones you suggested that are for the Radiola IIIA have a primary resistance of around 1,200 ohms. I'm thinking the original transformers would be like my AR-812 with a primary of about 1K and secondary about 6K? So maybe the volume will be better without trying to turn up the voltages and causing problems. The new Radiola Transformer should be here in a couple of days.
I would have looked into the windings as you suggested but the Radiola 25 that I have didn't have any audio transformers in it. It had been melted out and no audio transformers. I do have a spare catacomb for my AR-812 with 1 good and 1 bad audio transformer but I am saving that in case I need it for the 812 someday.
I have a 120 tube in the last stage as you stated. Now that it's at least picking up some stations I'm hoping that the audio transformer will give it a little more volume and maybe the front end of the radio will work better too? Thanks for the reply and tips.


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 Post subject: Re: Radiola 25 problems
PostPosted: Sep Sun 27, 2020 6:18 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 16985
Location: S. Dartmouth MA 02748-1225 USA
Quote:
Radiola 25 loop has 13 turns, uses 79 feet of wire, turns spaced 3/8" and is NOT center-tapped.
That is what I had measured in '99 when I had made some loop wire.

I suggest that you add 2" to account for the connections to the terminal strip...

With the radio off, the overall circuit continuity and the tuning range of the loop can be tested with a pocket portable transistor radio. Huh? Yep. Be sure the loop os connected and the 25 turned off. The reason to turn it off is there will hetrodynes appearing and that could lead to confusion.

Turn on the transistor radio and place it withing the loop frame. Tune the transistor radio to a station at of very near 1mhz like wins 1010 or WBZ 1030. Now tune station selector on the left (FWIR) it is the loop tuner. When the 25 tuner reaches the 1mhz the station on the transistor radio will become louder... Note the calibration on the 25 dial. The mat finished paint on the dial is intended for direct logging with a #2 or #1 pencil with a sharp point.. now explore the limits of the 25 loop/tuner with the transistor radio and log at each end of the dial the stations...

That calibrates the "front end", remove the transistor radio and turn on the 25. set the loop tuner to where it peaked for one of the stations and bring the oscillator tuner to near that same calibration. in the center of the dials the stations should align but can be +-40khz away. It is possible to get them to match. The issue is that the "Q" of the loop circuit is not all that good so it is possible to have "Two Spot Tuning", generally, reducing the input signal by turning the loop antenna one of the double station will fade more than the other. The stronger station is the correct one. The drum dial for the oscillator can be slipped to make the calibration or the universal joint to make the drums agree.agree. There may be a locking shim between the drum and the tuner shaft if so it cannot be moved... The side-plates of the tuning condensers are made of a thermoplastic and over time will warp, such warping will cause erratic friction setting to the felt pad the joins the two drums. What can be done is to remove the wooden front panel and file the tuning condenser mounting holes oblong to align the shafts, even shim the condenser if need be. The universals are not all that effective. May have to change the screws to binder or flat head or make a dent in the wooden panel (backside) to account for the screw heads. Further the electrical connections to the tuning condensers can also fail yet look O.K. be sure the screws hold the lugs and are electrically connected. Fair warning, the bushings molded into the tuning condenser side plates make break away if excessive torque is used...

The A53 PT 156 transformer has a following among restorers, but in radios using tubes like the 99 and the set is not regenerative the audio appears to suffer. I imagine because it is far less money. Another ARF member has an NZ-2 China winder and has been winding his own audios for his sets and has had very good results. Homemade bobbins and layers of Kapton tape, 28ga Teflon lead wires...

The audio transformer in the Radiola III, 24, 25 29, 30 (all the catacomb sets) are the same 3 to 1 in both the 1st and 2nd audio locations.

I no longer have spare loops I either use a replica ACME spiral loop or a loop coupler when I test cats. The ACME is wired with solid enamel so a clip lead gets through to tap the loop...

Please post, good or bad your results in changing out the transformers.

Chas

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Last edited by Chas on Sep Mon 13, 2021 11:46 pm, edited 1 time in total.

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 Post subject: Re: Radiola 25 problems
PostPosted: Sep Sun 27, 2020 7:56 pm 
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Joined: Sep Thu 24, 2020 9:16 pm
Posts: 14
I'll have to look around for a transistor radio, think I still have one. My AM Transmitter is 1mhz and the radio is very close to 50 or 55 on the dial, I marked it with a pencil so I'll have to look. Both dials are within 1 mark of each other when the radio is playing the strongest, seems to be really close to being correct. It might have been one of your post that I got the information from to make the Radiola 25 antenna? I googled it and came up with the 79', 13 loops, 3/8" spacing, 18ga. , and another said it was about 12X24X6. So it wasn't too hard to do that part, just getting the plug in connection to work well.
I was thinking of winding a audio coil myself. I did a Jewett Super Speaker and it worked really well. I use some double stick tape to hold the form on a spindle that I put in my variable speed battery drill motor which I clamped to the bench. I was afraid I would break the wire, but I didn't. It has two coils, one was good and the other was open. They were not the same, one had noticeably more wire than the other so I suspect someone unwrapped one until they got to the open and reconnected. Any way, I took a resistance reading on the one that looked original and rewound them both, figuring the other would go bad later so I might as well do both. When I got the first one close to the same diameter wrapping I took a resistance reading without cutting the wire until it was correct. Then I did the second coil to match. Not sure if it was 1k total or 1k per coil, but anyway it works fine now. So I was thinking of trying a audio transformer some day. Like you said the P-T156 is cheap compared to the Radiola IIIA replacement transformer. That's why I wanted to try the homebrew antenna and 156 transformers to get the radio working before I spent the money on the Radiola transformers. I think they are going to work better than the 156's I have in there now, I will post the results.


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 Post subject: Re: Radiola 25 problems
PostPosted: Sep Sun 27, 2020 10:17 pm 
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Looking at me radio dials and I notice that WBZ is the Boston Station I was listening to last night. Today however some other station is coming in at that setting. It’s not English Language so I’m not sure what station it is. Anyway I marked the dial last night for WBZ and one for my AM transmitter that I labeled WRB which is 1MHZ. I need to get that dried wax off.


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 Post subject: Re: Radiola 25 problems
PostPosted: Oct Fri 09, 2020 4:02 pm 
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Joined: Sep Thu 24, 2020 9:16 pm
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A little update on my Radiola 25. I replaced the P-T156 audio transformers with the Radiola IIIA transformers ( P-T118944) and the radio performs much better. With the P-T156 transformers I could not turn up the battery or loudness settings without the radio going haywire. It would only pick up one local station very poorly or my AM Transmitter if the transmitter was right up to the antenna.
After replacing the audio transformers with the ones produced for the Radiola IIIA, (P-T118944) the radio now is working much better. The volume can now be turned up to a good listening level without the radio feedback, static and oscillation noises. When I had the P-T156 transformers in I tried using about 20 ft of wire as a antenna with 13 loops and the end to ground to get better reception. That didn’t help. But after replacing the transformers with the Radiola IIIA transformers the 20 ft of wire with the 13 loops of wire place behind the Radiola 25 with the end to ground helps the radio a lot. The volume and quality improves greatly when I ground the wire and decreases when I lift the ground. Before it didn’t help any. By the way, the 20 ft I’m referring to doesn’t include the 13 loops is wire, that’s just the straight length stretched out in my garage. I’m sure the Radio would do much better if not in my garage.
When I got this radio it didn’t have any audio transformers in it so I had to mount these new transformers. This is how I mounted them. They fit in the catacomb nicely. Looks good from the outside.


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 Post subject: Re: Radiola 25 problems
PostPosted: Oct Fri 09, 2020 6:10 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 16985
Location: S. Dartmouth MA 02748-1225 USA
Kudos!

I have been skeptical for some time that the A-53 and its clones do not work well in all radios. I am very pleased the Radiola III transformers have performed as expected...

BTW apparently leaving the long hookup wires did not seem to ruin performance. If it were my repair I would have shortened the connection wires. But, it works... :D

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Last edited by Chas on Oct Tue 27, 2020 9:22 pm, edited 1 time in total.

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 Post subject: Re: Radiola 25 problems
PostPosted: Oct Sat 10, 2020 2:06 am 
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Joined: Sep Thu 24, 2020 9:16 pm
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I just wanted to make sure it worked before I cut those transformer wires back. Figured if they didn’t I would take the transformers back out an keep for another project. I’m going to shorten them up and cut the center taps back close to the coil. I’ve had that assembly out of the catacomb so many times one more is no big deal. Just glad it worked, those transformers are not cheap. I tried the P-T156 transformers in my spare AR-812 catacomb and it appears to have the same issue. Lot of noise and would only play at very low settings, was not good at all. I’m guessing the Radiola IIIA replacement would work, but I’m not going to buy more just to find out and the one in the Radiola 25 are in to stay.
Thanks for all the tips and advice.


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