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.

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.

:scratch2: 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 ...... :sigh:

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) ...

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

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.

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... :yikes:

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.

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... :sigh:

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.

Do you get any noise when turning the bandswitch, or is it silent?

All I get is the electrical sound of the band switch making contact at each position and when I tune through the tuning dial all I get is a hissing noise across the dial.

I disconnected the B- Wire Lead from the volume control like you told me to when I was testing the continuity of the volume control and I also disconnected a capacitor that went to that terminal as well, but I reconnected them just like they were originally and I wouldn't think that disturbing the B- connection to the volume control pot would mess with the radio's reception capabilities.

The aforementioned disconnecting of the wires from the negative terminal of the volume control pot was the only thing I disturbed in the radio this time around, so I'm kind of confused as to what could of happened to make the radio lose reception.

Check that there is plate voltage on the 6A8 and 6K7 (pin 3 of both tubes).

Also check screen grid voltage on both tubes (pin 4 of both).

Geez this radio is turning out to be a pain in the butt, because I have to put the radio in the cabinet to test it (the antenna is in the cabinet and the radio won't work without that antenna attached to it) and the speaker is too big for me to take out of the cabinet and plop it on my bench next to the radio, so I have to keep removing and installing the radio from the cabinet to do all of these tests. :sigh:

So what are you thinking may have happened if you don't mind me asking?

Also I rechecked the radio and it is indeed dead silent except for the popping noise it makes when changing the band switch, there is absolutely nothing on the radio reception wise when tuning through the dial not even a little bit of static or hiss.

I also checked all of the tubes and they all tested fine (they are all the original Zenith branded Tubes to this radio except for the 6A8 tube and the 5Y4 Rectifier tube which are RCA and GE Branded tubes respectively.)

Do you have a spare 6A8 to try for a sub? If not, try wiggling the 7G7, 6A8 and 6K7 in their sockets (with power on, volume high). Also try tapping them to check for intermittents.

This is a dumb question, but is there any chance the 6A8 and 6K7 coulda got interchanged?

Some 40s Zeniths would go stone deaf if the wave magnet wasn't connected. Don't forget to connect it for your tests.

:scratch2: What tube is the parallel caps shown in this picture connected to ? If it's anything "front end" related (RF amp /osc / or mixer) and it's in a grid circuit , the parallel caps may have thrown the circuit out of alignment ?

those are the coupling cap for the 6V6s (the original was a .05 MFD but the closest I had at the time that I replaced that was 2 .033 MFD caps which I paralleled to make a .06 MFD cap.

So nothing to do with the front end.

I never touched any of the tubes when I was recapping this radio or when I undid the B- wiring from the negative leg of the volume control.

The only time I touched the tubes was to test them in my tube tester to make sure there weren't any dead ones, but I disconnected them one at a time so that there wasn't a chance for them to be put into the wrong socket.

I don't have any spare 6A8 tubes unfortunatly, and the weird thing is that the original owner of this radio replaced the original 6A8G tube with a metal 6A8 tube so because of that they ended up throwing out the original tube shield that went in that spot and unfortunately the metal 6A8 tube doesn't fit into the tube socket very tightly like the original 6A8G did so I have a feeling that the replacement 6A8 tube may not be making very good contact with the tube socket because the tube pins don't seat all the way into the tube socket.

I also checked the 7G7 tube and the pins looked kind of crusty looking but I wasn't sure if that would of caused the issue I was having or not (I know Loctal tubes are notorius for making bad contact between the tube pins and the socket).

I know, I double checked to make sure the connection was good between the wavemagnet and the radio chassis and it still seems unresponive.

OK so now I'm getting some static sounds out of the tuner at the spots where the stations would be coming in at but still no audio or reception.

I tried wiggling the tubes you suggested I wiggle (the 6A7 and the 7G7 tube) and it didn't make much difference.

So another question, can having the chassis not being completely tightened down to the cabinet cause reception issues?

Can't think of any reason how/why it should. Question- is there any way the cabinet can be laid on its side, and the chassis slid back far enough to take voltage readings in the RF and IF areas? Or is this a large floor-standing console?

(Back in the day, this was a common accessability practice in TV service, even with console sets. It was usually done with the heavy end of the chassis down.)

This is a large Phono/Radio Console it's kind of hard to tip it on its side.

This is a long shot, but.... in post# 10, fourth photo, near the center of the shot, there is a 330K (orange-orange-yellow) resistor. To its left is a yellow capacitor labeled 333K 630V. A bare lead from the resistor looks to be lying directly across a bare lead from the capacitor when you blow up the image 300%. It may just be the camera angle, but would be worth checking.

I think that's an illusion, because the resistor you're referring to isn't actually touching that capacitor because it's about a 1/4" higher than the capacitor.

Well. I'll try and take the chassis out of the cabinet again and attempt to get the speaker out of the cabinet to put it on my bench and see if I can get some voltage measurements, the only reason why I didn't want to monkey around with taking it out of the cabinet is because with my cut on my knuckle it's hard to unscrew the screws without hitting my knuckle against the cabinet.

OK. so question, what is this coil for?
See picture attached below.

I'm asking because when I was cleaning out my volume control pot earlier with the contact cleaner I accidentally got a bunch of contact cleaner on this coil assembly and I'm wondering if me getting contact cleaner on this coil assembly might of messed something up with the radio's reception.

This is the only other thing I could think of that might of been messing things up.

Thoughts?

OK, so I don't know what I did to it, but now the tuner is working again. :scratch2:

OK well, I tried it out again now that I got the tuner working again, and the volume control quit working again and it started working like a tone control knob again.

I double checked EVERYTHING and there isn't anything wired up wrong and the volume control pot isn't open, so it seems like I'm chasing down some sort of weird electrical phantom in this radio... :sigh:

Any other ideas as to what could be causing my volume control issues?

OK so the only thing I can think of is that one of the capacitors that is attached to the negative end of the volume control pot might of went bad or were faulty, in the physical radio itself there are 2 capacitors that are going off of the volume control pot negative leg, which is weird because looking at the schematic there's only one capacitor coming off the Negative leg, which is C14 unless I'm missing something... :scratch2:

In the actual radio chassis, there's a mica capacitor that's going between the negative leg of the volume control pot and one of the taps on the volume control pot, which that mica capacitor isn't marked on the schematic (at least not that I can see of anyways) there's 2 capacitors marked on the schematic in that area that seem like they might fit the description of a mica capacitor value wise but the way they are drawn on the schematic they don't seem to connect from the tap of the volume control to the negative leg of the volume control pot, C12, which is a 350 pF capacitor is one possibility, the other is C9 which is a 25 pF capacitor, but like I said none of those capacitors appear to have a physical connection with the volume control pot negative lead and the tap on the schematic.

When you had the bottom lug of the control disconnected, did you actually verify with an ohmmeter that the element isn't open?

yes I did, it didn't measure O. L. which usually means its open, it actually gave a value of 2.5 Meg like the schematic said it should of, as for whether or not I could get a smooth up or down movement of the numbers when rotating the knob, I couldn't get it to do that because I couldn't hold the meter probes onto the terminals and turn the knob at the same time, I tried to use an alligator clip to hold one of the meter probes onto one of the terminals while I held the other one in place with my left hand and turned the knob with my right hand but the alligator clip kept coming loose and the negative lead of the probe kept falling off the negative terminal of the volume control pot so then I couldn't get any sort of readings, one way or the other.

Well, since it showed the called-for 2.5 meg, it ain't open. Since turning the control affects the tone, does it change the volume at all? And what is the actual volume level? Is it full volume, or partial, or... ? For the heck of it, try disconnecting those two tone taps. That way, the control becomes just a plain potentiometer with no junk hanging off it.

The volume level is between 1/8 and 1/4 volume level, and the volume control knob doesn't affect the volume at all, all it does is affect the tone and that's it, and the weird thing is that the tone control push buttons don't do anything to the radio except for the one marked "Normal" the rest of the tone control push-buttons don't do anything to the audio on the radio.

I'm wondering if one of my capacitors that I installed in the tone control/volume control circuit wasn't faulty or defective from the factory.

Have you done any signal injection testing to isolate the problem to a single stage yet? ISTR you recently bought RF generators (and they presumably have audio generator functionality too).

Inject audio (put a .1uF in series with your generator to protect it from DC) on the audio side of the detector if the set passes audio test the IF/RF system ( injecting IF frequency from the detector to every grid and plate back to the antenna) if it doesn't pass audio then trouble shoot the audio system by moving audio injection point to every grid and plate between the detector and speaker. If you know where the buck stops in the signal chain then you have isolated the problem to the components associated with 1-2 tubes....that is a heck of a lot easier than guessing where it is out of the entire radio.

I got it to receive radio signals again, it was the Phono/Radio Switch that was causing the problems, I got switched to phono mode rather than radio mode.

The main thing causing problems right now is the volume control still giving me troubles, and signal injection isn't going to help me troubleshoot a malfunctioning volume control pot.

I'm still wondering if it isn't something with the capacitors I installed in the volume/tone control circuit.

I really wish I would of taken a picture of the underside of this radio before I worked on it so I could of seen how things were wired up originally so I could make sure I didn't accidentally install a capacitor in the wrong spot.

does anyone have a picture of the underside of this radio that they could post so I can make sure I didn't accidentally install a capacitor in the wrong spot?