[KCMike]

I powered the radio up on the variac. you have to turn the volume control to about 2/3 up to get audio. From there on up it goes to full volume.
If you turn it down below the upper third you lose all audio and have to turn it back up. I tested all tubes. ck. good. Checked vol. con trol. it calls for 30 ohms at full on and 50000 at volume all way down. I got 90ohms and 50000. there was a note that said some vol. controls will be higer than the 30ohms. It does change all resistance readings when going from low to high. This control is odd to me it is connected to the ant.on one side and simultaneously the grids of the rf tubes.
When I touch the 24 det. tube grid cap when on station it loses the station and you have to slightly move the tuning cap. to get the signal back.
I checked some of the resistors all are with in 25% or closer.
This will be a challenge to change the caps in. The tar cans are rivoted to the chassis. Any ideas on what I should start with as far as replacing or checking? Thanks for any help.[Image: http://i1336.photobucket.com/albums/o654...25665a.jpg]

[jerryhawthorne]

Mike, nice to post a link to the schematic for help. Many early radios used the "volume" control to attenuate the antenna and work some wonders on the rf gain. Basically, the audio section ran wide open with the level controlled through the front end of the radio. It sounds like your pot is fine. You need to do some recaping and resistor checking.

Here is a link to what I think is some information on the radio including a lot of wiring etc. Perhaps this will help you and others in your problem. It looks to me as though there are two pots on the same shaft, one for controlling the gain in the RF and one for the antenna.

http://www.myvintagetv.com/Apple%20PDF%2...48&R15.pdf
Jerry

[KCMike]

Thanks Jerry, I found that manual the other day and was studying it. I did put up a schematic just now. I was just a little hesitant to jump in to those 4 metal tar cans. Each have 2 to 3 by-pass caps in them. But I will give it a try.

[Mondial]

The operation of the volume control seems normal to me. As Jerry mentioned, this is not your usual audio volume control. One section attenuates the antenna signal, while the second section adjusts the voltage on the screen grids of the RF amp tubes which determines the amplification.

Together, the resulting effect of the dual control is to set the sensitivity of the receiver. There is no automatic volume control, so audio level will depend on the signal strength and the setting of the control itself. Takes a bit of getting used to if you are more familiar with later radios equipped with AVC and an audio volume control.

When you touch the grid cap of one of the 24 tubes, you are detuning the resonant circuit associated with that grid with the extra capacitance of your body. So the tuning cap needs to be adjusted slightly to compensate for the added capacitance to maintain resonance of that stage. Again this is normal and not indicative of a problem.

Be very careful if you decide to open up the large tar filled filter cap can. It also contains the audio choke which drives the output tube grids. You may be better off adding caps external to the can rather than risk damaging the delicate windings of the choke inside. Some things are better left alone....

The Nostalgia Air schematic shows the contents of the can:
http://www.nostalgiaair.org/PagesByModel...040334.pdf

[KCMike]

Thanks Mondial. I guess the reason I thought there might be something wrong is you only get audio at the upper 1/3rd of the volume control. The first 2/3rds is complete silence.
I don't plan to get into the big filter cap. can. It does not have any hum whatsoever and it will only be played once in awhile.
I did consider changing the by-pass caps and hooking them up externally. But I don't know if that is nesessary.
I do remember the dial scale is off by 40 kilocycles. 980 comes in at 1020. etc. down the scale.
There is instructions on alignment in the service manual. I may do that and leave it alone.

[Mondial]

One thing I would definitely do is add two DC blocking caps in series with the existing ones in the filter block. These are the .025 uF audio coupling caps between the tapped audio choke and the output tube grids. Any leakage here will throw off the bias and cause the output tubes to draw excessive current. If you connect new .22 uF caps in series with the green leads from the can, the resulting capacitance will only be slightly less than the original, and any leakage will be eliminated. I would probably leave the other bypass cans alone, as slight leakage will be of no consequence. You can jump across each bypass section with a new cap and see any change in performance, just to insure they are not open.

You might check the voltage from screen grid to cathode of the RF tubes and see if the screen voltage varies from a few volts to about 90 as the volume control is rotated. If so, then the volume control action is as it should be. If you connect a longer antenna, then the volume control may respond over a greater range of rotation. Also, anything which reduces the RF gain of the receiver (bad tubes, coils, alignment etc.) will require you to advance the volume control more than otherwise would be necessary.

[jerryhawthorne]

Mike, quick and easy to measure the resistors. There are several that help "maintain" the effective balance of the antenna and RF gain. If these have drifted it could account for your problem with volume. Look closely at those involved in the screen control provided by the second pot. Much easier to confirm than caps and they seem to be prone to drifting high with age.
Good luck, Jerry

[codefox1]

Easier to just replace ancient carbon resistors whilst doing recap job. Of course these critters behave differently in/out of circuit, warm/cold, powered/not, humid/dry, arthritic joints, if you want your radio to be in better shape than me, replace 'em.

[KCMike]

Thank you guys for how to procede with this radio. I will start on it tomorrow. I will have to recheck which resistors were reading high. Just looking at them they look like the size of 2& 3 watts based on a dogbone resistor size chart I have. I only have a large assortment of 1 watt resistors.

[jerryhawthorne]

Mike, nice to match them against the chart on Chuck's site. I really doubt most are in the 2 to 3 watt size but I'm often wrong. Those puppies were large compared to modern ones. It does seem you have large number of low resistance wire wound if I understand the schematic. Those should test fine or open.
Jerry

[Arran]

I hate the diagram they posted on the Nostalgia Air site, when I see sets built like this, and so poorly documented, my first inclination is to #1 Sell or trade it off #2 Clip the power cord off, shove it into a corner and put a potted plant (or another radio) on top. The only other option is to pray that someone has the original factory service folder for this insult to good electrical engineering, the RCA/G.E/Westinghouse folder will have a chassis wiring diagram with the parts locations shown.
The only good news is that it looks like you can bypass all of the capacitors in that can while leaving the inductor portion connected, if the connection on one end of that choke (marked solder) that is connected to the .025 uf cap is accessible externally. One to start with is the .005 uf cap that connects across the primary of the output transformer, which is also connected to the plate pin of each #45 tube, use a 1 Kv rated cap here, or connect two 630 vdc .01s in series. Of course if that choke is shot you might as well melt the can out and start over.
Regards
Arran

[gregb]

Hey Mike, I have the RCA Red books and will have a look for a parts layout for you. Get back to you soon.

Gregb

[gregb]

Sorry Mike, no parts layout in the RCA Red Book. It does show what caps are in the bypass cans but does not show where they are in the chassis.

Gregb

[Arran]

I just found the factory folder on this page under Radiola 48/R 15:

http://www.myvintagetv.com/manuals_and_documents.htm

This may prevent a lot of hair pulling. If this doesn't work send me a PM with an email address and I will forward it to you.
Regards
Arran

[Eliot Ness]

I just found the factory folder on this page under Radiola 48/R 15:

http://www.myvintagetv.com/manuals_and_documents.htm

Wow, that is a goldmine of information, great find Arran.