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| Thread ID: 43231 | 2004-03-07 22:55:00 | Off Topic. Valve radio parts | Colpol (444) | Press F1 |
| Post ID | Timestamp | Content | User | ||
| 220962 | 2004-03-10 11:25:00 | Thanks Graham. I was getting a bit lost there. To tecko for lil ol me. Anyway you are right. Now that I look closer the plug is wired the way you say. Thanks for the warning about the vcoltages to the speaker. I will certainly watch that. A earlier Reply suggested that other things may cause the speaker to play up so I will take your advice and replace the volume control. I have checked the back of the control as you sugested. There is text there but it is almost impossable to read what it says and none that I can desypher. The unit has three terminals. Mesuring between the centre terminal ( Common)and each of the others gives readings of 0 at one extreme and 150 at the other for both sides( Does that make sense to you??) |
Colpol (444) | ||
| 220963 | 2004-03-10 21:07:00 | Hi Colin Those figures won't mean anything unless you tell us the meter range. It could be 150, 1500, 15000, 150,000 or 1.5 Megohms. None of those options sound like a standard pot, so set the control to the middle, measure between the two ends and see what that says, then measure from centre to either end. Post the readings & meter type (analogue or digital) and resistance scale (x1/x10/x100 etc). I suspect that it could be technology from the earlier days as per one of my earlier posts where the volume control is not at the input to the audio stage but controls RF/IF gain instead. Can you measure any voltages on the pot when the radio is going? If you only have zero or full volume available, as I said earlier your speaker may not be faulty at all. The problem could be gross overdriving of a tired audio output valve. Do voices sound like they are gargling in soup, or do you have rattles and scraping noises? Do you have a makers name or model number for the radio? Cheers Billy 8-{) |
Billy T (70) | ||
| 220964 | 2004-03-11 01:22:00 | Ok I got my son whos eyes are better than mine to check out the back of the pot. The numbers on the back are 79-006-1504. using my digital meter on Ohms ( 200k range) with the control in the middle the reading accross the two ends is 155. 55 in one direction and 115 in the other. I suspect that the end mesurements would be the same if all was ok. There is a small voltage present at either end of the pot 3-8 volts if I am reading the meter right. I had a rush of the smelly stuff to the brain and improved the position of the aerial. This has improved the quality of the speaker output to allmost normal :-)),(bearing in mind an earlier post about these early speakers not having the nice mellow tones of the later one which is what I was expecting) Turning the pot back from the full position by a very very small amount reduces the volume to a level that does not distort much so I suspect that the speaker getting full volume was more than it could handle The radio is an RCA. The only numbers apart from the patent numbers is C169778 which I think is the serial number. I tried to check for voltages on the speaker. I got some fairly large numbers and also some reverse readings?????? |
Colpol (444) | ||
| 220965 | 2004-03-11 01:47:00 | OK Colin Sounds like a logarithmic pot of 150-175K . Does it look original? Voltages should not be present on the pot, which indicates either an RF/IF gain control (pot carrying valve bias voltages) or an open circuit grid leak resistor on the output valve allowing signal rectification to take place if it is a conventional volume control . This causes the audio to provide variable bias on the valve through signal rectification, hence awful distortion . Other possible explanations are too complex to traverse here . If the distortion slowly clears after a signal strength change then it is probably an agc or audio bias problem . Putting your meter probe on the grid of the output valve (with the other probe grounded) should improve the audio if it is a bias problem but if a digital meter the impedance will be a little high and the effect may be limited . Getting better audio quality by changing the signal input also suggests either IF overload (if automatic gain contol is not present) or simply the variation in audio level with different signal strengths affecting sound quality per the previous paragraph . Remember, our local radio station field strengths may be significantly higher than the poor old thing was designed for . To test this theory, reduce aerial size or find weaker stations . Take a look at the diagnostic advice on the website I posted, find the audio output valve and check the signal grid and cathode voltages . The output valve will be the one wired to the speaker transformer via the plug & socket if the transformer is mounted on the speaker . Grid will be the one that goes to the wiper of the volume control and cathode will go to ground via a resistor of less than 500 ohms . Gotta go do some work now, but keep us informed . Cheers Billy 8-{) |
Billy T (70) | ||
| 220966 | 2004-03-11 05:06:00 | Further update: Valve types 80 - Rectifier 47 - Pentode Audio output 56 - Triode, possibly audio preamp, especially if right next to the 47 . 24A - Tetrode RF/IF amplifier . Most likely second and third IF/detector in this radio 35/51 - variable mu (gain controllable) Tetrode . Most likely RF (gain controlled) and first IF (gain controlled) Seven valves is quite a number so this could have been a reasonably up-market radio . At a guess, the 80 will be back in one corner, then the line-up might be 35-51, 35-51, 24A, 24A, 56, 47 . Of course I could be miles out here, and there is always a chance that the 24As and the 35-51s have been swapped around . Where is PF1's vintage radio expert when we need him? Cheers Billy 8-{) |
Billy T (70) | ||
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