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Zenith 10S690 Volume Control Issues
Hello Everyone, last night and early this morning was spent recapping my Zenith 10S690 Radio and after I finished recapping it and struggling for about an hour trying to get the chassis back into the cabinet (I was having a hard time getting the chassis screw holes to line up with the cabinet holes for the screws to go into to secure the chassis to the cabinet).
I plugged in the radio and turned it on to test out my work and sure enough my volume control wasn't working right, instead of raising and lowering the volume like its supposed to it instead acts like a tone control, and my tone control switches don't work except for the one marked "Normal". I was trying to be careful to observe where each capacitor went when I was removing the old ones and installing the new ones and only replacing one capacitor at a time. I've looked over the schematic for this radio and I can't seem to figure out what could be causing this issue, the radio was working fine prior to the recap (just had an intermittent AVC circuit that was more than likely due to a leaky AVC Bypass Cap). Any ideas as to what might be causing my problem? The schematic for this radio is posted below. Any help with this matter would be appreciated. |
#2
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An open at the ground end of the volume control would cause the problem you describe. Either an open at the end of the resistive element, or at the external ground connection. Hopefully the latter.
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#3
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Ground end of the volume control? There's no ground connection indicated on the schematic for the volume control pot, and the volume control pot isn't connected to ground anywhere according to the schematic, there's a reference to ground on the tone control keyboard itself but not not on the volume control pot.
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#4
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I think ol Levi here could use some schematic reading lessons , the side of the volume control pot at resistor R12 is at B- potential , which is a HELL of a lot closer to "ground" than it is to B+ , that's for sure . Old coot 88 knows his stuff , so don't be SO quick to jump on people trying to help you especially when you don't appear to have a clue as to how that circuit actually works ...... |
#5
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Also how would of that ground connection of just suddenly broke when it was working just fine previously? All I touched in this radio was the capacitors I never touched any of the wiring. |
Audiokarma |
#6
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Now , just know that while you didn't purposefully touch any wiring that doesn't mean a wire couldn't have got bumped or nudged as you were changing your capacitors . Your best troubleshooting clue that you've given us here is that the volume control worked before you changed the capacitors , that in itself is telling you to check the installation of those new caps very carefully , it's all too easy to make a mistake and sometimes it's not easy to spot a mistake we have ourselves made (a great reason for having at least one electronics savvy friend who might spot something you've missed) ... |
#7
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#8
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Just for the heck of it , post up a few closeup pics of some of the caps you replaced , who knows something may look familiar (or unfamiliar) enough to spot the problem |
#9
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Sorry, I shoulda been more specific. 'Ground end' simply means the lowest potential or no-signal end. In this case, there's a 40 ohm resistor (R22) from there to chassis ground. See if there's 40 ohms from the low end lug of the control to chassis.
(R22 may be physically distant from the control since it's part of the negative bias supply circuit.) Last edited by old_coot88; 06-24-2020 at 05:42 PM. |
#10
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I have some pictures of the radio underside showing the capacitors I changed, there was one little spot that looked like it could of been from a component that was cut out at some point in time but there wasn't anything near by that matched it so I chalked it up to maybe a sloppy part placement job from the factory. See pictures below. And yes there's 53 Ohms on the low side of the volume control pot to the chassis, so that means that the negative side of the volume control pot is fine it seems. Last edited by vortalexfan; 06-24-2020 at 08:22 PM. |
Audiokarma |
#11
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Not necessarily. There could still be an open in the resistive element itself. If you float the negative lug (disconnect everything from it), see whether the element measures called-for resistance, or if it's open.
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#12
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yikes that's going to be a pain in the butt to do, as there's like several things hooked up to that volume control pot that will need to be disconnected as there are 5 lugs on that volume control pot and all 5 lugs have something hooked to them...
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#13
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Disconnect only what's going to the negative (low end) lug. Measure from that lug to the wiper lug, and rotate the control stop-to-stop. The meter should show a smoothly-changing ohms value, from very low to very high. (The wiper is the middle lug between the bottom and top-end lugs.)
Or does the meter show open circuit (infinite resistance) thru the full range of rotation? Last edited by old_coot88; 06-25-2020 at 01:10 AM. |
#14
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The biggest give away was that the volume control pot wasn't measuring open but it was being rather difficult to turn so I took some contact cleaner and sprayed it into the volume control pot and sure enough that piece of capacitor lead wire flew right out of the pot and it now rotates perfectly. |
#15
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OK I got the volume control issue fixed but now I've got a new problem, I'm not getting any stations in on any of the bands on this radio, and it was working just fine before I worked on the volume control issue.
It seems like this radio is developing one problem after another... I guess I should of just left it alone seeing as it was working fine before I replaced the capacitors in it. I don't think I've ever repaired a radio that was this touchy before. |
Audiokarma |
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