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  #136  
Old 02-12-2016, 09:36 PM
old_coot88 old_coot88 is offline
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That puppy was kickin' our collective keester for sure. Glad you finally nailed it to the wall.
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  #137  
Old 02-13-2016, 11:23 AM
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dtvmcdonald dtvmcdonald is offline
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The candohms I have taken apart were insulated with paper.
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  #138  
Old 02-13-2016, 04:08 PM
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Kamakiri Kamakiri is offline
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Still not enough contrast to the picture though. This thing should have amazing contrast, and it's too washed out for what I know this set should be doing.

Gonna have to yank the chassis for the 53rd time and start testing the resistors in the AGC. The contrast control is questionable too, as it has a dead spot and seems to not do a heck of a lot.
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  #139  
Old 02-13-2016, 10:45 PM
old_coot88 old_coot88 is offline
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Well, in lookin' at the schematic, the contrast control appears to be a manual (not automatic) gain control, varying the negative grid bias on the IF tubes. Low gain would indicate the control voltage is too negative.

The control pot is supplied with a negative voltage thru R44 (2.7K), and the other end of the pot goes to ground thru R42 (680 ohms).

The control voltage being too high could be due to: R44 being too low in value, or R42 being too high in value. Those would be the first things I would check. Then disconnect one end of the pot, and connect an ohmmeter to that end and to the wiper. Rotate the control, looking for dead spots. Also be sure the control element (10K) is not open.

Then go ahead and check R40, R41, and R43 (whose purpose appears to be to make the control's action more linear).
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  #140  
Old 02-15-2016, 01:22 PM
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ChrisW6ATV ChrisW6ATV is offline
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On my 8TS30, the contrast control has a wide range of action, from no video to way overloaded. It is indeed a "manual (IF) gain control"; there is no AGC.

Compare to your DeWald BT-100, how does that set's contrast control behave? Both sets should be nearly the same, I would expect.
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  #141  
Old 02-15-2016, 01:32 PM
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Well yeah.....I interchange the terms "contrast" and "AGC" on this set because they're one in the same.

It's definiitely washed out and has very little range.....from totally washed out to barely adequate. Plus, there's a dead spot on the control. The DeWald is the same as your 8TS30, which is the same as my 8TS30 (this isn't even my set!), so I know there's definitely something awry.
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  #142  
Old 03-22-2016, 08:43 AM
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Kamakiri Kamakiri is offline
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Gonna finally call this one done after I give the tuner a good cleaning

The screen shot is lousy, but the set regained contrast after I removed ALL the wires from the candohm, and used terminal strips to isolate everything. It looks like a bit of a train wreck, but hey, it works!

It's not *as* contrast-y as my 8TS30, but this set has zero horizontal or vertical lock drift....and mine does a little while it's warming up. I'll just chalk that up to their individual "personalities"
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  #143  
Old 05-09-2018, 09:31 AM
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Kamakiri Kamakiri is offline
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Well here's an ironic post-script to this thread!!!

I bought a restored 8TS30 at the ETF auction, and I remembered that I did the work to it, but it JUST dawned on me that it was THIS SET!!

There it is, under the table:

https://www.flickr.com/photos/420028...7668760424818/

The contrast issue ended up being a couple micas, it now works as good as new. And now it's back here!!!
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