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 Post subject: Maguire 500BW Problems
PostPosted: Jan Sun 15, 2017 4:12 pm 
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Joined: Jan Mon 05, 2015 4:55 am
Posts: 80
Location: Chattanooga TN
I recaped this set and added an auxiliary input awhile back and it worked great but it had this strange problem where it would run quiet with a dial bulb in place. Last night I thought I'd test the tubes in it. I found two so-so tubes and the 12BA7 had an intermittent Cathode short, I also found I'd not replaced a .05uf paper capacitor. I replaced the capacitor and the offending tubes then tested it. The volume was fairly lackluster even without a bulb then over the next 30 seconds it faded away as if someone was turning the volume down. I've tried at least three different sets of tubes,looked over the chassis for any shorts and even replaced that capacitor with a different one. If it's warm I have no audio no matter what I do. It does it on the tuner and the aux (which is a switched mono 3.5mm jack spliced between the tuning section and volume pot) I have no idea what I've done to it to cause this,any ideas?


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 Post subject: Re: Maguire 500BW Problems
PostPosted: Jan Sun 15, 2017 5:20 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 10781
Location: San Jose, CA USA
This sounds like drifting grid bias in either the first audio or audio output tubes. On the Sams schematic (available at Radiomuseum), check resistor R21 (10 megohms to ground on grid of 1st audio tube), R23 (470 K to ground on grid of output tube), and R24 (150 ohms to ground on cathode of output tube). See if any of them have drifted to a high value and replace. Also confirm they are wired properly.

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Tom K6VL


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 Post subject: Re: Maguire 500BW Problems
PostPosted: Jan Sun 15, 2017 11:44 pm 
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Joined: Jan Mon 05, 2015 4:55 am
Posts: 80
Location: Chattanooga TN
Well,I believe I've fixed it. I opened it up to check resistors and thought I'd check the tubes again,making sure they were good and hot this time. 35Z5 pegged the meter to zero. I replaced it with a good tube and that appears to have solved it. I've been listening without issues for about a half hour now. The bulb weirdness is gone too,working great.


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