1955 RCA 6-XD-5 Help Needed

[jstout66]

I picked up a cool twin speaker radio. My friend Walter said the resistor capacitor is bad. ( the thin deal with 6 legs) He said he can modify it, but does anyone know if the part is actually still available?

[Gregb]

I am sure that someplace in North America there is one sitting in a box someplace but good luck finding it.
Best bet is to have a look at the schematic and build yourself a new one. I have done this many times with great success. And if your not sure how to do it check out bandersontv on you tube, watch the Philco videos and he shows how to do it. Phil Nelsons web site also shows how to do it in his Philco restoration.

Gregb

[Bill R]

I see a lot of other paper caps in there that I would replace before condemning the resistor capacitor network. I would also replace the electrolytic filter caps first. Then if troubleshooting indicates it replace the network.

[walterbeers]

I replaced the paper capacitors, and still no stations come through. Audio works, and I get buzz if I touch the terminals on the volume control meaning that the audio output circuits are working. I am kind of thinking that one of the IF cans might be bad, or possibly the resistor/capacitor network. IF cans have continuity, but something is killing the radio 455 khz signal. If I put a 455 kHz signal from an RF generator, into the grid (through a small capacitor) of the 12BA6, I have nothing. The first IF can has four terminals, and the second IF can has 5. RCA numbers 970441-12 and 970441-11, also part number 540-542 and 540-548. The Sams is 309-14. It's almost embarrassing being stumped by a simple 5 tube radio. Where can I find 455 Khz IF cans? JW miller, Thordarson, had them, and others. Cross reference?

[maxm]

Maybe bad mica capacitors in the IF cans?

[Bill R]

Use a scope or signal tracer to check pin 7 of the 12AV6 tube. Good audio? If ok then check pin 2 of the 50C5. If no audio or very low audio there the network may be bad. Depending on the schematic for the network you may be able to disconnect pin 5 and 6 and put a coupling cap between pin 7 of the 12AV6 and pin 2 of the 50C5.

[Findm-Keepm]

Originally Posted by maxm

Maybe bad mica capacitors in the IF cans?

+1, any IF can of that vintage is suspect. I'll cross the drawing number to a part number for you tomorrow - that'll get a part number you can try with Moyers or Talonix.

[old_coot88]

Dumb question, but have you verified that there's plate voltage and screengrid voltage on the 12BE6 and 'BA6?

[jstout66]

Moyers had an IF can. Will keep you posted when it arrives and is installed.

[RJMiranda]

The 6-pin thing was called a "couplate" and it was maybe the first kind of IC ever. Passive, of course.
If you are in doubt about it, better take it off and put discrete components instead. The 5-tube superhet circuit was almost the same, no matter the make or model. See the attached pic.
The couplate not only includes components for coupling the volume control to the 12AV6 grid, and the 12AV6 plate to the 50C5 grid, but also the 150 pF IF filter capacitor between 47k/volume control and common negative. That´s why if it is defective you could have good audio but no radio reception.
I changed the phono-radio switch diagram, it had an error in your drwg.
Of course you could have any other cause of no reception in the converter/IF stages, and not on the couplate. Check first whether they are working: change capacitors, tubes, check the variable capacitor for shorts, you may even need to use a ´scope to isolate a defect in the IF cans without changing them.
But if the couplate is the guilty one, put in discrete components the good old Cuban way.

[walterbeers]

Well, problem solved. It was the 2nd IF can which I was able to get from Moyers. Replaced it, and aligned the coil to 455 Khz, and radio plays fine. Only problem that is still there is a low level 60hz hum even with the volume turned all the way down. Filter caps were replaced with new ones, and hum still remained. Even if I pull the 50C5 while the radio is powered up (with the volume control completely down), there is still 60hz hum from the speakers until the tube filaments cool off. The odd thing about this radio, is that part of the audio output transformer has a winding that I assume is also used as a filter choke for the power supply in series, between the 50uf and the 30uf with the 1.2K 1 watt resistor and the 2 capacitors. I don't ever recall a 5 tube radio using a tapped audio output transformer. (See schematic in post #4). I solved the problem though with replacing the 50uf with a 220uf 160 volt capacitor, which brought the hum down to a acceptable level. Apparently the higher capacitance does a better job of filtering out the 60hz ripple. Strange, but radio now plays fine.

[RJMiranda]

The idea behind the tapped OPT was to use a small filter condenser, and "cancel out" the hum in the transformer, because the residual ripple over the B+ flows in two directions in the primary: from the center tap UP to the 50C5 plate, and from the center tap DOWN to the other tubes´plates. If the 50C5 plate current is of the same order than the sum of the other tubes´plate currents, the flow in both sides of the primary would be about the same, and the two opposite flows of the hum components would cancel each other because they would create similar 60 Hz flows in the xfrmr iron but in opposite directions.
Somehow it is not working in your radio.
When you have hum in a circuit like this, the lower part of the primary could have a short (total or parcial), or somebody could have reversed the wires to the plate and the 1200 ohm resistor (both windings are usually different, with the tap off-center).
The 200uF cap may shorten the life of the rectifier tube.
By the way, without the 1200 ohm resistor, the second half of the primary would short to ground the audio frequencies coming from the plate. Looking from the 50C5´s plate, both halves of the primary work as an autotransformer (the plate winding would be the primary and the lower half the secondary) but both ends of the secondary would be grounded by filters, and the OPT tube would have a bad time trying to supply audio that was being shorted at the other end. The resistor separates one end of the secondary from the 30uF cap (and lowers the plate voltage to the earlier stages).