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 Post subject: I am stumped or missing the obvious
PostPosted: Dec Fri 08, 2017 3:32 am 
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Location: Hamilton, Ontario
I am currently working on a Canadian Westinghouse 780 schematic below:

http://radio.codegods.ca/node/8547

I replaced all caps , resistors and electrolytics and had the radio up and working. I took the variable condensor out for cleaning and then reinstalled it. Now all I have is a slight background hum. The speaker responds to touching the wiper on the tone control with appropriate clicks. I have remetered every single coil and both IFs and all are good. All tubes have been retested and they are good. I rechecked my ground connections to the variable and they are good. None of the plates in the variable are touching. I have also double checked the tone and volume pots and they are good. I also don't see any problems with the bandswitch connections.

I have included the voltages below (mine are black and the targets are red). The only ones that stand out are the plate on the 6SQ7 is high and the G1 of the 6SA7 is low. I realize that the rectifier plate voltage is low but it was that low to begin with when the radio was working. Any ideas??

Image

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 Post subject: Re: I am stumped or missing the obvious
PostPosted: Dec Fri 08, 2017 4:04 am 
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So, the variable condenser is C-13 in the schematic, which also includes C-2, C-3, C-4, C-12 and C-14.

Since the radio was working before you removed it, the problem is right there, somewhere, in the way it was reconnected.

It looks to me (from the schematic) that there are three wires connected to it. Could you have them swapped?


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 Post subject: Re: I am stumped or missing the obvious
PostPosted: Dec Fri 08, 2017 4:17 am 
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I have retraced those wires ( 3 wires and 2 ground wires) many , many times to be sure and I had also taken pics of the chassis before hand. I am sure its something goofy small but I just can't seem to get it and I have been farting around with it for 4 nights. :lol:

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 Post subject: Re: I am stumped or missing the obvious
PostPosted: Dec Fri 08, 2017 4:48 am 
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...once my radio went silent. A few hours later I found out tubes went into wrong sockets...

It may be more efficient at this point to just go through step-by-step troubleshooting and isolate the problem as if it's a customer's radio that just came in for service.

Check audio section and inject a tone to see if you can get sound to come out. Then move on to IF alignment with a signal generator. If both check out, then check the RF and oscillator sections.

Use an adjacent radio to verify oscillator is working.

It may not be something you touched. There's always a chance of a loose lead or bad solder joint that's not visible to the eye. Sometimes "poking around" with a stick would reveal this type of fault.


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 Post subject: Re: I am stumped or missing the obvious
PostPosted: Dec Fri 08, 2017 6:14 am 
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Problem solved.

I found a lead from an old cap that I had clipped out had lodged itself between leads in the 2nd det tube. :roll:

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 Post subject: Re: I am stumped or missing the obvious
PostPosted: Dec Fri 08, 2017 7:34 am 
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Location: Powell River BC Canada
A story about radio techs. Most (those making a living) have very acute eyesight.
Often they translate a situation into looking for something odd. Almost subconsciously .

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 Post subject: Re: I am stumped or missing the obvious
PostPosted: Dec Fri 08, 2017 5:56 pm 
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I made a habit of always securing the end of a lead that I am cutting for this reason. The other is to chase down any solder drip or place a piece of paper towel to catch a drip before it becomes a problem.


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 Post subject: Re: I am stumped or missing the obvious
PostPosted: Dec Sat 09, 2017 2:25 am 
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jrs wrote:
Problem solved.

I found a lead from an old cap that I had clipped out had lodged itself between leads in the 2nd det tube. :roll:


Better it ended up there and not in your eye.
When I get done working on a chassis I turn it right-side up and shake the crap out of it. Literally.

I love it when a mystery is solved! Nice work.


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 Post subject: Re: I am stumped or missing the obvious
PostPosted: Dec Sat 09, 2017 2:34 am 
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Joined: Apr Sat 11, 2015 5:30 pm
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Location: West Point, PA 19486
AJJ wrote:
There's always a chance of a loose lead or bad solder joint that's not visible to the eye.

Boy, you sure called that one! In this case, the loose lead was REALLY loose.


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