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#1
03-02-2022, 10:50 PM
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I shorted out the 50 ohm resistor and saw little difference in performance.
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#2
03-02-2022, 11:24 PM
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But why have it at all? It's hacked in near the HV coupling caps like an afterthought.
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#3
03-02-2022, 11:49 PM
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Maybe it helped tame that atrocious audio hum.
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#4
03-03-2022, 12:41 AM
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If it wasn't "tacked in like an afterthought" I'd guess maybe it was a means of extending tube life assuming they weren't asking for full tilt emissions from them in the biasing design.
Barring that there's 3 thoughts that come to mind. IIRC they hadn't invented the GTB suffix tubes (controlled synchronized series warmup heaters) until the 50s... Lacking those they may have had issues with heaters on different tubes warming up at different rates and one tube getting 2-3x rated heater voltage on warmup and failing prematurely, maybe during warmup an OSC wasn't biasing an output or temporary heater voltage imbalance was causing frequent HK shorts. It's also possible they wanted the RF to warm up first so there'd be picture and sound as soon as there was light. But with that afterthought installation it almost HAD to be a quick fix for a post design or field issue. Any maker back then would say that you don't just tack in an expensive power resistor for sh!ts and giggles.
__________________
Tom C. Zenith: The quality stays in EVEN after the name falls off! What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4
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#5
03-03-2022, 08:50 AM
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That resistor would also reduce the filament current through the 7JP4 and the 6X5.
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#6
03-03-2022, 09:20 AM
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Yes, it would. That whole filament string would be running about 10% lean. The 7JP4 and 6X5 about 5% lean.
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#7
03-03-2022, 10:16 AM
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IIRC, I seem to remember seeing a complete working Sencore TVA92 on u-tube in the hands of Shango066, not 100% sure.
If he does have one with cable, getting him to assist with info about it is another matter. :/
__________________
=^-^= Yasashii yoru ni hitori utau uta. Asu wa kimi to utaou. Yume no tsubasa ni notte. いとおしい人のために
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#8
03-04-2022, 04:18 AM
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I don't know if this will help or not, but I think your TV is actually an XA-701, which is located in Sam's Set 61 Folder 16, maybe this might help you track down your issues easier if you have the correct service data for your TV.
  There was a few different versions of this set cabinet wise. See picture below, Picture Credit to the Early Television Foundation. Last edited by vortalexfan; 03-04-2022 at 04:59 AM.
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#9
03-04-2022, 09:25 AM
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That is the service info I have been using and it matches what I'm seeing. Same chassis, different cabinet.
Here are the three models using this chassis. 
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#10
03-08-2022, 11:42 AM
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Quote:
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#11
03-07-2022, 08:11 PM
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Sencore TVA92 has arrived and is working
 I injected composite video to the grid of the video amp with the tuner powered and it had the typical traveling distortion in the image, Then I killed power to the tuner filament transformer and it cleaned up quite a bit. Even better than when I ran the tuner on batteries. Next up, I'm going to unmount the transformer and attach long lead to the secondary. I'll power the primary from a separate wall plug. That should allow me to determine if the issue is AV on the tuner tube filaments or a radiated magnetic field.  
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#13
03-09-2022, 02:24 PM
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I dug up a real mumetal shield, attached extension leads to the filament transformer and experimented.
I can now 100% confirm the transformer does radiate enough to effect the image. Rotating 90 degrees helps quite a bit but does not eliminate it. Unfortunately, there's not enough room to mount it this way. Putting the shield over it helps even more. Running the tuner off pure DC is about the same as using the shield. There is still some 60Hz getting into the horizontal sync but it's not very noticeable. I'll see if I can mount the transformer under the chassis. 
Last edited by bandersen; 03-09-2022 at 04:56 PM.
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#14
03-09-2022, 05:22 PM
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Hmm. I didn't realize you could by a sheet of the stuff and make a custom shield.
There are smaller, cheaper scraps for sale on eBay. https://www.digikey.com/en/products/...004-8/13160834
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#15
03-09-2022, 10:59 PM
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Quote:
https://en.m.wikipedia.org/wiki/Mu-metal jr
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