RCA 65X1 finally works great-except...
12/13/2009 6:22:45 PMTerry Decker(82284:0)
Well, I finally got around to replacing bad wires, capacitors, and the stupid 500 ohm volume control. I also removed a bunch of resistors the last goober had tacked onto the VC. Now the set works great- except... when I turn the volume control all the way up it actually gets lower at the top end. This is an audio taper that I got from AES. What's wrong? Should this be a linear taper, is there a bad cap I missed? I replaced the .02 to the wiper, but not the .005 to the 50L6. That should only remove the RF component, right? Has the 4.7meg from the grid of the 12SQ7 changed value? Any ideas or suggestions will be appreciated.
Terry
12/13/2009 6:49:09 PMThomas Dermody(82289:82284)
Should probably be an audio taper. I don't like the volume controls that AES and Radio Daze sells because they seem to cut in rather sharply at the low end (as do the ones sold by Radio Shack...they're made by Alpha). The capacitors serve to isolate direct current. If you were to connect the volume control directly to the grid of the 1st audio tube, the AVC voltage would be applied to the grid as the control was advanced. This negative voltage would throw off the grid's proper bias (and might very well make the audio get quieter, though more likely will simply introduce distortion). The capacitor between the 1st audio plate and the output grid also serves to isolate DC voltages. Replace them if they have any signs of leakage. It is best to test capacitors with high voltage, though an ohmmeter can reveal many faults, or you can simply substitute a different capacitor.
You might consider also looking at your grid leak resistors (the resistors that go from each audio grid to B-). If any of these has opened, the negative component of the audio signal could build up in the capacitor feeding the grid due to rectifying action of the grid, thereby driving the grid heavily negative, and cutting off most, if not all, tube conduction. This would make the audio get quieter as the volume was turned up, though the symptom is often accompanied by quite a bit of distortion.
T.
12/13/2009 9:58:29 PMTerry Decker(82297:82289)
:Should probably be an audio taper. I don't like the volume controls that AES and Radio Daze sells because they seem to cut in rather sharply at the low end (as do the ones sold by Radio Shack...they're made by Alpha). The capacitors serve to isolate direct current. If you were to connect the volume control directly to the grid of the 1st audio tube, the AVC voltage would be applied to the grid as the control was advanced. This negative voltage would throw off the grid's proper bias (and might very well make the audio get quieter, though more likely will simply introduce distortion). The capacitor between the 1st audio plate and the output grid also serves to isolate DC voltages. Replace them if they have any signs of leakage. It is best to test capacitors with high voltage, though an ohmmeter can reveal many faults, or you can simply substitute a different capacitor.
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:You might consider also looking at your grid leak resistors (the resistors that go from each audio grid to B-). If any of these has opened, the negative component of the audio signal could build up in the capacitor feeding the grid due to rectifying action of the grid, thereby driving the grid heavily negative, and cutting off most, if not all, tube conduction. This would make the audio get quieter as the volume was turned up, though the symptom is often accompanied by quite a bit of distortion.
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:T.
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Thanks, great suggestions and I'll follow up on them, especially checking the resistors. I think it's the control though. Like you indicate, a bad cap or resistor should introduce distortion, and the signal is clean. This poor radio has taken weeks to finally get going. I considered it a personal challange. I found bad solder joints, globs of solder shorting tube sockets and wrong value parts. I kind of hope the idiot that botched the repair of this little radio didn't have an isolation transformer.
12/14/2009 9:39:57 AMEdd(82306:82297)
Sir Terry . . . . . (and the Pirates ? )
OK we have kept up with your conundrum right on past the point of your having discovered the presence of the
"great 500 ohm volume control fiasco".
And now we fully undertsand the improbability of consulting the pricing of the volume control and finding it to be
$1.25 and its 35 cents postage and placing the order into an envelope to RCA parts, Indianapolis, with its 3 cent stamp affixed. Then your patiently waiting the week for the arrival of that exact RCA volume control.
In the interim I have called up your RCA schematic . . . . made a reasoning logic shift into Full Eagle Eye mode . . . and am now seeing the handling of circuit design at the origin point of the sets AF signal.
Taking a look at the low end of the secondary of the 2nd I.F. transformer one usually is expecting to see a pi network filter. . . .
Cap Res Cap . . . ."tweet" filter at the low end of the winding. Its function there is to strip any 455 RF presence from that pristine originating AF signal and take away some of the very high audio spectra above 5KC.
In this situation I see the first C element as being the 120 pf capacitor C11, and the 120 pf capacitor C13 as being the final element.
usually there is a fixed 37-47K resistor as the central R element, but in this situation they have opted to incorporate it as a sealed off portion of the volume control.
What is happening to you . . .now. . . .is that you DID get the proper log taper vol-you-me control, but are not now having that critical 50K tap included.
Cause and effect:
The volume control is acting normal in its control effect of the lower portion of the audio spectra, up until towards the end of its range where the .02 capacitor starts heavily shunting across the audio path due to the sub 50 K resistance value NOW starting to be present in that circuit.
That now severe incremental cramping of the treble audio portion down into a lower deep basso range has now fooled your ear into thinking that the volume is going down. In actuality, you were just experiencing a severe tonal shift on moving past that critical resistive threshold.
If you will now take one each, fresh . . .47 K reee-sistor . . . .gotta be fresh, otherwise the ohms might have gone bad . . .and
place it in series at the high side of the volume control. (Between that terminal and its tie in wiring). You then have just effectively remoted a stop onto that new controls mechanical range.
Then, you can proceed forward with testing for a full aural re-evaluation.
73's de Edd
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12/14/2009 3:07:27 PMBill G.(82320:82306)
Wow Edd,
Great analysis! Knowing that Sir Terry had to replace the volume control meant that he had to replace the stop, too.
Best Regards,
Bill Grimm
12/14/2009 4:36:45 PMDennis Wess(82324:82306)
Ah yes..good ol' Terry & the Pirates. Here's a memory helper for our younger members:
12/14/2009 5:40:19 PMTerry Decker(82327:82324)
:Ah yes..good ol' Terry & the Pirates. Here's a memory helper for our younger members:
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My great-grandmother used to read the comics to the family on Sunday afternoon. I was born in '43 and apparently there was something in the Terry and the Pirates story line that clicked with my parents. So, yes, I'm actually named after a comic strip. My father always wanted to be a writer-so I am also named after Engene O'Neil.
Terry Eugene Decker-now you know my shame!
Anyway-the RCA is just for display and my personal listening, so I'm not going to mess with the control for now.
Thanks, though, to everybody who responded.
T.E.D.
12/14/2009 9:05:43 PMThomas Dermody(82329:82327)
Keep in mind, too, that the 50K resistor isolates the IF transformer from any effects of capacitances and loads in the audio input circuit. Without the 50K resistor, as the volume control is advanced so that the .02 MFD cap comes closer to the IF circuit, it may very well throw off tuning of the IF circuit, though it would have to do this through the capacitance of the tube's grid, so I may very well be wrong on this.
12/14/2009 11:36:13 PMTerry Decker(82333:82329)
:Keep in mind, too, that the 50K resistor isolates the IF transformer from any effects of capacitances and loads in the audio input circuit. Without the 50K resistor, as the volume control is advanced so that the .02 MFD cap comes closer to the IF circuit, it may very well throw off tuning of the IF circuit, though it would have to do this through the capacitance of the tube's grid, so I may very well be wrong on this.
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