Inoperable tone controls - Printable Version
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Inoperable tone controls - lasurveyor - 11-15-2016
I'm restoring a Philco 48-482 and have it sounding pretty good after replacing a gazillion caps and resistors and going through the complicated FM alignment. A remaining problem is the separate Bass and Treble controls. They are both totally dead with no noticable effect at all. At first I was puzzled by the fact that the ground terminals didn't seem to ever been used but I have since found a post here on the Phorum that the pots are sometimes grounded within. Still, I went ahead and wired them to the schematic. Still dead. I checked the resistance and the ohm meter gives perfect readings, the treble is 500k and the Bass is 1 M, and the tap seems to operate well, one is linear and the other has a taper. Logic is telling me that with the correct resistance the pot should be working. What am I missing? As a lot of you guys know, this radio is a lot of work. I hate to give up and go without the tone controls. Thank you for any knowledge that you can share.
Alan
RE: Inoperable tone controls - klondike98 - 11-15-2016
For those who might have some insight on this problem,
Nostalgiaair Schematic: http://www.nostalgiaair.org/pagesbymodel/637/M0013637.pdf
Audiophool Schematic (better quality): http://www.audiophool.com/Philco.html
R203 is the Trebel Control and R207 is the Base Control.
Unfortunately I don't have any thoughts on what could be wrong. If it were me, and it has been, I would double check the wiring ... I've driven myself crazy by mis-wiring something.
RE: Inoperable tone controls - Jim Koehler - 11-15-2016
Before you changed out the old components, were the controls working? I've found a difference between old caps and new when working in these sections....so much so that I went back to the old caps! And, you could always try installing the old caps and see if that makes a difference.
...Jim
RE: Inoperable tone controls - lasurveyor - 11-15-2016
(11-15-2016, 07:11 PM)klondike98 Wrote: For those who might have some insight on this problem,
Nostalgiaair Schematic: http://www.nostalgiaair.org/pagesbymodel/637/M0013637.pdf
Audiophool Schematic (better quality): http://www.audiophool.com/Philco.html
R203 is the Trebel Control and R207 is the Base Control.
Unfortunately I don't have any thoughts on what could be wrong. If it were me, and it has been, I would double check the wiring ... I've driven myself crazy by mis-wiring something.
Yeah, I'll take another look. When I needlessly added some ground wiring I did match up the schematic some, but it's worth a deeper look. Thanks.
Alan
RE: Inoperable tone controls - lasurveyor - 11-15-2016
(11-15-2016, 07:11 PM)klondike98 Wrote: For those who might have some insight on this problem,
Nostalgiaair Schematic: http://www.nostalgiaair.org/pagesbymodel/637/M0013637.pdf
Audiophool Schematic (better quality): http://www.audiophool.com/Philco.html
R203 is the Trebel Control and R207 is the Base Control.
Unfortunately I don't have any thoughts on what could be wrong. If it were me, and it has been, I would double check the wiring ... I've driven myself crazy by mis-wiring something.
Bob,
So you can agree that if the pot checks out with the ohm meter, then the problem is elsewhere? Also, I can't help but think that there's a clue in that both the bass and treble pots are dead. Too much coincidence.
Thanks,
Alan
RE: Inoperable tone controls - lasurveyor - 11-15-2016
(11-15-2016, 07:28 PM)Jim Koehler Wrote: Before you changed out the old components, were the controls working? I've found a difference between old caps and new when working in these sections....so much so that I went back to the old caps! And, you could always try installing the old caps and see if that makes a difference.
...Jim
I didn't power it up before I started re-capping. The thing was a mess. Quarter inch of dust on everything and a huge nest of some kind in the chassis.
Thanks so much for your reply.
Alan
RE: Inoperable tone controls - Radioroslyn - 11-15-2016
When I've tried to troubleshoot some of these tone control circuits I've used a tracer to hear what is going on the each control. Sometime treble/bass circuits can be a little confusing on paper to wrap your head around.
RE: Inoperable tone controls - Radioroslyn - 11-15-2016
After having a look at the schematic it's not too bad. On the treble control if you ground the end that isn't grounded it should cut the highs out. If not there is a problem with C-201 (open) I suppose C-200 could be bad but kinda doubt it it's probably a mica. Do you have 132vdc at the top end of C-201??
RE: Inoperable tone controls - klondike98 - 11-15-2016
Alan, I'm not the best guy for troubleshooting but Terry (Radioroslyn) has been doing this a long time and is much better at it.
RE: Inoperable tone controls - lasurveyor - 11-15-2016
(11-15-2016, 09:40 PM)Radioroslyn Wrote: After having a look at the schematic it's not too bad. On the treble control if you ground the end that isn't grounded it should cut the highs out. If not there is a problem with C-201 (open) I suppose C-200 could be bad but kinda doubt it it's probably a mica. Do you have 132vdc at the top end of C-201??
Whoa, C-200 is one of the few caps that I didn't replace. C-201 is a new cap.
I see on the schematic that the plate voltage at C-201 should be 132v, I have 267 volts !! It's getting late and I'm getting that look from the wife that I'd better get away from the radio right now. In the meantime, thoughts?
Thanks,
Alan
RE: Inoperable tone controls - Radioroslyn - 11-15-2016
(11-15-2016, 10:39 PM)lasurveyor Wrote:So if the short the ungrounded terminal on the treble control to the chassis does the tone change??(11-15-2016, 09:40 PM)Radioroslyn Wrote: After having a look at the schematic it's not too bad. On the treble control if you ground the end that isn't grounded it should cut the highs out. If not there is a problem with C-201 (open) I suppose C-200 could be bad but kinda doubt it it's probably a mica. Do you have 132vdc at the top end of C-201??
Whoa, C-200 is one of the few caps that I didn't replace. C-201 is a new cap.
I see on the schematic that the plate voltage at C-201 should be 132v, I have 267 volts !! It's getting late and I'm getting that look from the wife that I'd better get away from the radio right now. In the meantime, thoughts?
Thanks,
Alan
RE: Inoperable tone controls - lasurveyor - 11-15-2016
(11-15-2016, 10:56 PM)Radioroslyn Wrote:(11-15-2016, 10:39 PM)lasurveyor Wrote:So if the short the ungrounded terminal on the treble control to the chassis does the tone change??(11-15-2016, 09:40 PM)Radioroslyn Wrote: After having a look at the schematic it's not too bad. On the treble control if you ground the end that isn't grounded it should cut the highs out. If not there is a problem with C-201 (open) I suppose C-200 could be bad but kinda doubt it it's probably a mica. Do you have 132vdc at the top end of C-201??
Whoa, C-200 is one of the few caps that I didn't replace. C-201 is a new cap.
I see on the schematic that the plate voltage at C-201 should be 132v, I have 267 volts !! It's getting late and I'm getting that look from the wife that I'd better get away from the radio right now. In the meantime, thoughts?
Thanks,
Alan
No, no change grounded or not.
RE: Inoperable tone controls - Radioroslyn - 11-16-2016
c-201 is open replace it. Check for -11vdc on grid of 6SQ7 and a good gnd connection on pin 3. If this doesn't lower your plate voltage try another 6SQ7.
RE: Inoperable tone controls - lasurveyor - 11-16-2016
(11-16-2016, 12:07 AM)Radioroslyn Wrote: c-201 is open replace it. Check for -11vdc on grid of 6SQ7 and a good gnd connection on pin 3. If this doesn't lower your plate voltage try another 6SQ7.
There is -1 volt on the grid and the cathode has a good ground.
The tube being used is a 7B6 and seems to be a Philco swap. The pin assignments are different and the solder connections seem original.
C-201 is a new cap and checks at 0.01 mfd, but I can go ahead and swap it out. I have other 7B6's that I can try.
Thank you so much for your time!
Alan