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Post by elorens on Jan 20, 2020 17:52:04 GMT 12
In the Gulbransen 7L I'm working on, the chassis is coming along nicely, but there's one quirk: the volume control works, but does not take the audio down to zero before the switch-off point is reached. It seems to be a problem with the pot itself: at the end of the track, just before switch-off, the slider still has a bit of resistance to ground, where it should be zero. This means that on strong signals, e.g. National Radio, there is still quite strong audio at this point. Just wondering if I am missing something here? Is there a fix for this kind of problem?
Thanks!
Lawrence --
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Steve
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Post by Steve on Jan 21, 2020 18:04:43 GMT 12
Hi Lawrence, I don't have the schematic in front of me, but I have seen this before with a faulty first audio tube with internal leakage. If you definitely have resistance to ground on the pot when its fully rotated though I'd try another pot. You could pull the pot apart and see why the wiper isn't getting to the end lug though. Is it wirewound or carbon?
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Post by elorens on Jan 21, 2020 20:22:38 GMT 12
Hi Lawrence, I don't have the schematic in front of me, but I have seen this before with a faulty first audio tube with internal leakage. If you definitely have resistance to ground on the pot when its fully rotated though I'd try another pot. You could pull the pot apart and see why the wiper isn't getting to the end lug though. Is it wirewound or carbon? Thanks Steve, will check.
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Post by elorens on Jan 22, 2020 17:35:25 GMT 12
Hi Lawrence, I don't have the schematic in front of me, but I have seen this before with a faulty first audio tube with internal leakage. If you definitely have resistance to ground on the pot when its fully rotated though I'd try another pot. You could pull the pot apart and see why the wiper isn't getting to the end lug though. Is it wirewound or carbon? Hi Steve, here is the circuit for the 7L, kindly supplied by Daniel. I'm a bit wary of removing the pot since it is connected to the dial display via string, pully and spring. How does the internal leakage mechanism work? Cheers Lawrence Attachments:
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Post by Steve on Jan 24, 2020 1:50:54 GMT 12
Throw another 6Q7 in it and see if that fixes the problem, if not I'd be pulling the pot. What is the resistance between the wiper and ground when the volume control is fully down?
Cheers, Steve
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Post by elorens on Jan 26, 2020 20:42:50 GMT 12
Throw another 6Q7 in it and see if that fixes the problem, if not I'd be pulling the pot. What is the resistance between the wiper and ground when the volume control is fully down? Cheers, Steve Tried some other 6Q7s but no better. Wiper to ground, just before the on/off switch operates, is 70 ohms. After the switch has operated, the resistance gets down to 18.5 ohms, but is never zero! However, I measured several other pots in the spares box, and they all seem to have a bit of resistance left at the end, though perhaps a bit less than this one. I’m wondering if there is an AGC problem linked to this. The AGC line at the front end has a resistance to ground of only about 50 kilohms, which seems way too low. As a result the AGC voltage drops to about -0.3V max, from a possible -10V max. At the moment I am tackling this problem by separating the AGC feed to the front end from that going to the IF amp, by a 1 megohm resistor. This gives a bit more control and allows the magic eye to work, but it would be good to find the source of the leakiness. It’s not the line decoupling capacitor. Coils? Trimmers? Thanks, L —
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Post by Steve on Jan 28, 2020 21:47:12 GMT 12
Hi Lawrence... there are bound to be many people here better able to diagnose this than me - but here is my rough plan if it was mine. I'd replace the 12uF cathode bypass cap on the 6Q7, and check the bias resistor (C26 and R12) to make sure the bias is correct. I'd also lift the grid cap - does the volume die then? If not the audio is getting through somewhere else. I'd experimentally lift the volume control tap (betwen C28 and 29) and isolate that to see if that solves it (then probably blame C27 and 28) - I'd also check R11 and C25 to make sure they are ok, and then all the caps around the volume control. The audio signal is getting through somewhere and I can't see where else it could be other than possibly a dodgy magic eye maybe - although that seems unlikely.
Others will hopefully chime in.
Cheers, Steve
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Post by DHockey on Jan 29, 2020 8:47:07 GMT 12
Hi Lawrence
You possibly have two problems with the set. The volume control issue and the low resistance of the AGC line to ground. 70 ohms seems too high a resistance for a pot at the end of its travel. Carbon pots are like any other carbon resistor and do go high. What is the overall resistance of the pot? Is it well over 2meg? One thing you could try is shorting the on/off switch out so the radio stays on when the switch is in the “off” position. Then you will be able to see if that decrease from 70 to 18 ohms provides any further reduction in volume.
50k is way too low a resistance for an the front end of the AGC line, it should be infinite, you may have some leakage to ground that is pulling the AGC line up closer to 0 volts. AGC lines are very high impedance and it doesn’t take much to load them down. The other thing worth checking for is the presence of positive voltage on the line. To do this disconnect the line after the 2 meg resistor R9 and power up the radio, measure the voltage on line, there should be nothing, if you see positive voltage (even quite low) there is probably B+ leaking onto the line somewhere in the front end of the radio. This can happen on a wavechange switch where you have AGC and B+ sitting on adjacent terminals, leakage can occur through the wafer.
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Post by elorens on Jan 29, 2020 17:54:25 GMT 12
Thanks Steve and Daniel - all good suggestions, which I’ll follow up.
Note that the pot appears to be a replacement, which is untapped, and C29 and R14 are missing. I have an unused tapped pot which I may try in its place. It is 0.5 meg instead of 2 meg, but may be worth a try.
The cathode resistor R12 measures a little high at 2.6 k. Cathode voltage looks about right. However, the capacitor across R12 looks to be original (it’s in a little rectangular box), so is probably due for replacement.
Been distracted by the loss of one of the tiny dial escutcheon screws. The plastic ziplock envelope I had put them in turned out to have a hole in the bottom, and one escaped. Furious with myself. L
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Post by elorens on Jan 29, 2020 21:20:43 GMT 12
Bingo - the problem with the volume control was solved by changing the cathode bypass capacitor C26. Thanks, Steve!
I checked the AGC line when disconnected, as suggested, and it reads about -0.0001 V, so no suggestion of positive voltage getting in. The resistance to ground drifts around, and is sometimes as high as 100k, but even this seems far too low. Not sure what the mechanism might be, but it doesn’t seem to affect the receiver operation very much, so I can probably live with it.
Cheers
Lawrence —
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Post by Steve on Jan 29, 2020 21:28:29 GMT 12
Thats great news Lawrence... and I'm in the same boat having recently misplaced 2 of those little screws myself... mine were on the edge of a bench and I spent 10 minutes scouring the floor looking - all to no avail. There is a black hole that those screws disappear to, along with one sock from each pair you own!
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Post by DHockey on Jan 30, 2020 13:10:06 GMT 12
Glad you sorted the problem and that it was a straightforward fix! Regarding the AGC line resistance to ground I would check out the .05uF cap C3 that is off the aerial coil. If you have not already changed it out. It could be leaking and causing those variable resistance readings on the AGC line.
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Post by Steve on Jan 30, 2020 13:18:35 GMT 12
Just an addendum to what Daniel said: change all the paper caps and all the electrolytics as a matter of course. All of them. Every single one will be leaky and every single one will be causing some problem that it was originally put in there to solve because its physically incapable of doing its original job any more (be that blocking DC or forming part of a filter network or something else). Those early caps had a finite lifespan, and that lifespan ended many many years ago.
If you're worried about how it looks, restuff the original capacitors with new 'guts' - but replace them all. And importantly, never just tack a new one across the old one - cut the old one out of the circuit. Particularly important for the electrolytics. If they're leaky, adding a new one might increase the capacitance and possibly even look like its solved the problem, but it does nothing for the leak.
You may already know this, but I thought it was probably a point worth making for anyone else who stumbles across the thread.
Cheers, Steve
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Post by elorens on Jan 31, 2020 8:37:33 GMT 12
Thats great news Lawrence... and I'm in the same boat having recently misplaced 2 of those little screws myself... mine were on the edge of a bench and I spent 10 minutes scouring the floor looking - all to no avail. There is a black hole that those screws disappear to, along with one sock from each pair you own! Reassuring to know I’m not the only one to lose tiny screws. The remaining three are now triple-wrapped in ziplocks. Your screws are probably hobnobbing with mine in the fifth dimension - and yes, I’ve a couple of socks in there too!
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Post by elorens on Jan 31, 2020 8:53:14 GMT 12
Glad you sorted the problem and that it was a straightforward fix! Regarding the AGC line resistance to ground I would check out the .05uF cap C3 that is off the aerial coil. If you have not already changed it out. It could be leaking and causing those variable resistance readings on the AGC line. Thanks, Daniel. Yes, I made a beeline for that capacitor (actually C5, C3 is one of the trimmers). However, I was unable to find it! I eventually concluded that it must be located inside the can holding the aerial coils, despite what the circuit suggests. To remove the can would be a major piece of surgery in which lots of other things would have to come out, so I have held back from that. There is a pin coming out of the bottom of the can which is earthed, and which may come from C5. I tried putting a capacitor in series with the pin to ground, but it didn’t help the problem. So I’m still baffled by that, but as I said the radio goes pretty well now regardless.
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