Zenith 10S690 Volume Control Issues

[vortalexfan]

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.

[old_coot88]

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.

[vortalexfan]

Originally Posted by old_coot88

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.

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.

[init4fun]

Originally Posted by vortalexfan

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.

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 ......

[vortalexfan]

Originally Posted by init4fun

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 ......

When I said that I was thinking of an actual chassis ground connection and yes I do know how to read a schematic but like I said, when he said "ground connection" I was thinking he meant a physical chassis ground connection, not B- connection.

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.

[init4fun]

Originally Posted by vortalexfan

When I said that I was thinking of an actual chassis ground connection and yes I do know how to read a schematic but like I said, when he said "ground connection" I was thinking he meant a physical chassis ground connection, not B- connection.

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.

Ok , so we've got the "ground" and "B-" thing worked out , great .

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) ...

[vortalexfan]

Originally Posted by init4fun

Ok , so we've got the "ground" and "B-" thing worked out , great .

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) ...

Thanks, I'll probably monkey around with it more tomorrow as I spent most of the evening at the doctor's office because I accidentally cut my right pointer finger knuckle on a 1907 GE Pancake Fan Blade when I was trying to turn it off after demonstrating it to a friend of mine, and wound up with 2 stitches in my finger and a tetnus shot.

[init4fun]

Originally Posted by vortalexfan

Thanks, I'll probably monkey around with it more tomorrow as I spent most of the evening at the doctor's office because I accidentally cut my right pointer finger knuckle on a 1907 GE Pancake Fan Blade when I was trying to turn it off after demonstrating it to a friend of mine, and wound up with 2 stitches in my finger and a tetnus shot.

Man that sucks , and I can totally sympathize with you . One of my other hobbies is RC planes and 10 or so years ago I almost lost a finger to a hungry propeller , by my luck it was a carbon fiber prop that disintegrated when it hit my finger after slicing what looked like a shark's gills into it , and it took about 5 hours for them to fish out all the carbon fiber chunks and sew up the mess (had like 4 different sets of X rays till they were satisfied they got all the propeller chunks out) .

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

[old_coot88]

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.)

[vortalexfan]

Originally Posted by old_coot88

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.)

The Candohm is still good, it measures a little high but still within tolerance, I'm getting about 53 Ohms there at the 40 Ohm section and about 215 Ohms at the 200 ohm section.

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.

[old_coot88]

Originally Posted by vortalexfan

... so that means that the negative side of the volume control pot is fine it seems.

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.

[vortalexfan]

Originally Posted by old_coot88

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.

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...

[old_coot88]

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?

[vortalexfan]

Originally Posted by old_coot88

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?

Well I figured out what it was, a piece of capacitor lead wire got down into the volume control pot and was shorting it out, so I flushed it out with some contact cleaner and now the volume control pot seems to be working fine now.

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.

[vortalexfan]

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.