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Post by elorens on Apr 3, 2020 17:37:10 GMT 12
I’m using this time to bring several radios up to scratch, including my 1937 Philips V7A Theatrette. This is now working pretty well, and I managed to correct an odd fault in which the dial pointer moved in the wrong direction!
There’s one fault that I’d like some help with, though. When left running for a while, the radio shows abrupt changes in sound output level, both up and down, at apparently random intervals of several minutes. These are quite noticeable, and subjectively might be a couple of ‘S’ points.
My own theory is that this is some thermal effect occurring inside the rather ancient valves, and not something I can do much about, but I’d be grateful for any suggestions on this one.
Thanks, Lawrence
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Steve
Society Members
vintageradio.co.nz
Posts: 727
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Post by Steve on Apr 4, 2020 10:21:09 GMT 12
Hi Lawrence, I have a couple of V7A's in my stash awaiting restoration but I've never worked on one - I know someone who has done a few though so I'll email him and ask if he's heard of this. The only thing I'm aware of with these is the IF coils fail - could it be windings shorting in and out due to heat?
Cheers, Steve
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Post by Richard on Apr 4, 2020 11:31:25 GMT 12
my immediate thought is a faulty valve, if the levels change after being on for a while
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Post by elorens on Apr 5, 2020 22:18:10 GMT 12
Hi Lawrence, I have a couple of V7A's in my stash awaiting restoration but I've never worked on one - I know someone who has done a few though so I'll email him and ask if he's heard of this. The only thing I'm aware of with these is the IF coils fail - could it be windings shorting in and out due to heat? Cheers, Steve Thanks, Steve. Maybe I could test this theory by monitoring the AGC line? Presumably this would change when the problem occurs. My hunch is that the problem is around the detector/first audio amp. Good luck when you come to work on your Theatrettes. As you are probably aware, they are a bit of a nightmare to service! cheers, Lawrence |
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Post by elorens on Apr 5, 2020 22:20:17 GMT 12
my immediate thought is a faulty valve, if the levels change after being on for a while Thanks, Richard, yes, I suspect the double-diode triode. Unfortunately I don’t have another TDD4 to try! cheers Lawrence |
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Post by Richard on Apr 7, 2020 11:31:37 GMT 12
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Steve
Society Members
vintageradio.co.nz
Posts: 727
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Post by Steve on Apr 7, 2020 12:58:43 GMT 12
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Post by elorens on Apr 9, 2020 22:14:41 GMT 12
Thanks, Steve. Would be good to have a spare TDD4. Cheers, Lawrence -- |
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Post by elorens on Apr 9, 2020 22:34:32 GMT 12
Thanks, Richard, no I hadn't seen these, and was interested to read them. Peter Lankshear is such a good writer! |
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Post by radioholic on Apr 17, 2020 15:55:06 GMT 12
I have had similar problems with intermittent fading in several radios. Usually it has been due (a) valve pins making poor connection ,(b) a dry solder joint or (c) a capacitor or resistor with intermtitent open circuit. My approach would be first , clean and tighten all the pin sockets. Second, try to reproduce the fault by prodding/tapping each component and solder joint in turn. If this fails, measure all socket voltages and see if anything changes when the fault occurs. It can be irritatingly difficult to localise this type of fault. Good Luck! James
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Post by elorens on Apr 21, 2020 18:59:36 GMT 12
I have had similar problems with intermittent fading in several radios. Usually it has been due (a) valve pins making poor connection ,(b) a dry solder joint or (c) a capacitor or resistor with intermtitent open circuit. My approach would be first , clean and tighten all the pin sockets. Second, try to reproduce the fault by prodding/tapping each component and solder joint in turn. If this fails, measure all socket voltages and see if anything changes when the fault occurs. It can be irritatingly difficult to localise this type of fault. Good Luck! James Thanks, James. I started on this approach yesterday. I did after all find a TDD4 in my spares box, and it does work, but didn't solve the problem. I now suspect the front end FC4 circuit: when I screwed the back on, the level changed, and it turned out that the back was nudging against part of the circuitry, which in the Theatrette flies all over the place. I cleaned and oiled the valve pins and base, and tightened one of the pin sockets, but so far I haven't been able to get rid of the problem completely, although perhaps it does settle a bit when it has been on for a long time. Maybe the next step is to monitor the cathode voltage on the FC4 and see if it changes when the sound level jumps. As the man said, I may be some time... |
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Post by elorens on May 17, 2020 7:31:46 GMT 12
Managed to buy these at a good price, and was delighted to find that both worked in the Theatrette. Thanks, Steve. However, the random level-shifting continues... Great, though, to have a couple of spares now. |
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