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 Post subject: Moddled Sound after recapping
PostPosted: Nov Fri 04, 2016 3:42 pm 
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Joined: Aug Sat 19, 2006 9:40 pm
Posts: 105
Location: Fremont, California
I've worked on a Troy model unknown with push button tuning.
There is no model number but the chassis is similar to a model 57 revised so I've used that to work from.

I've replaced all of the caps and resisters except for the low value plastic pieces and tested all of the tubes. The radio plays well, has good volume and picks up a couple of weak station very well. The only issue is that the sound starts out clear and then after a short time becomes muddled. Then I turn off the set, count to ten , turn it back on and the sound is clear again. I have played it for many hours throughout the day with clear sound, but the next morning this routine starts over again. Any ideas??


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 Post subject: Re: Moddled Sound after recapping
PostPosted: Nov Fri 04, 2016 7:40 pm 
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Joined: Dec Sat 24, 2011 9:17 pm
Posts: 4108
Location: Vancouver Island, Canada
Have you seen this website?

http://www.geojohn.org/Radios/MyRadios/ ... Radio.html

He used a Model 57 schematic and redrew it to match his Model 75.

Did you discover a bias cell under the chassis? That component should definitely be checked.
Distortion could be tube related, or due to heat stress on some other part.

I am attaching some pics from that site of the schematic and the bias cell.
Funny we can't find that bias cell on his schematic?

OK, further scrolling down reveals he has added a voltage divider to eliminate the cell. He found that a negative bias of -2 volts works best. Had distortion problems before getting the bias right.


Attachments:
MODEL75.jpg
MODEL75.jpg [ 118.98 KiB | Viewed 1285 times ]
BiasCell.JPG
BiasCell.JPG [ 63.51 KiB | Viewed 1285 times ]

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 Post subject: Re: Moddled Sound after recapping
PostPosted: Nov Tue 29, 2016 4:19 am 
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Joined: Aug Sat 19, 2006 9:40 pm
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Location: Fremont, California
It's Fixed.
The source of the muddled sound turned out to be one of the .0005 molded plastic caps.


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 Post subject: Re: Moddled Sound after recapping
PostPosted: Nov Wed 30, 2016 7:13 am 
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Joined: Apr Sat 11, 2015 5:30 pm
Posts: 746
Location: West Point, PA
How did you find the trouble??


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 Post subject: Re: Moddled Sound after recapping
PostPosted: Dec Mon 12, 2016 6:43 pm 
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Joined: Aug Sat 19, 2006 9:40 pm
Posts: 105
Location: Fremont, California
I got a little ahead of myself in thinking the .0005 caps were the source of the trouble, although they were in bad shape. I've been playing the radio 11 hours a day, all week and have had less events of muddled sound once I replaced the 6A7 and 41 tubes. But the muddled sound gremlin still pops up after the set has played for a few hours. Turn it off count to ten and turn back on and the sound clears up. I sound add that the muddle sound is more subtle now than in the past.


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 Post subject: Re: Moddled Sound after recapping
PostPosted: Jan Fri 13, 2017 7:12 pm 
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Joined: Aug Sat 19, 2006 9:40 pm
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Location: Fremont, California
I replaced the 75 tube and that seems to have fixed the problem.
I've played the radio for a time now and the muddled sound seems to have cleared up.


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