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#16
10-26-2022, 10:10 AM
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Here's the schematic for anyone interested. Found this in Riders vol 6, and more in vol 7. Also covered in Sams 127-7, but I don't have a copy.
Looks like a pretty conventional design to me with 3 IF stages. They did economize a little with the use of a germanium detector diode. Also, one of the earliest sets to use a 6AV5 hor. output tube which came out in 1949. Notable because it does not have a plate cap. Note you can click for a larger version. 
Last edited by bandersen; 10-26-2022 at 10:14 AM. 
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#17
10-26-2022, 05:08 PM
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Capless horizontal outputs didn't come back untill the 6p0's or so, especially with those cute Japanese sets that combined it with the damper.
I'm guessing a ceramic socket cost more than a plate cap and some wire...
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#18
10-26-2022, 05:58 PM
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The design includes an interesting DC restoration circuit, identical to the RCA 621TS and 721TS. I found that black level was reasonably well clamped by grid rectification on the final 12AU7 stage. The sync is positive going at the 12AU7 grid which will cause it to draw current, charging the 0.05uF coupling capacitor to peak sync level. This will bias the 12AU7 with the sync tip close to ground. The plate of the 12AU7 is DC coupled to the CRT grid.
When I was restoring my 721TS I was surprised how well it seemed to work. Has anyone seen reference to this clever design?
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#19
10-26-2022, 06:25 PM
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Quote:
Last edited by bandersen; 10-26-2022 at 06:34 PM.
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#20
10-27-2022, 09:58 AM
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Quote:
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#21
10-27-2022, 10:40 AM
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I pulled the chassis last night and it appears it has never been serviced. I was hoping this was the case as all six chassis mounting screws were present and matched. Seems a few go missing whenever a set is serviced
 The cabinet is a bit crude. Just a lightweight plywood box. A bit odd that they used perforated masonite for the bottom. I'm not surprised it is damaged. The chassis is sealed on the bottom with a metal pan. Makes the perforated masonite seem all the stranger. The construction is a bit different than what I'm used to seeing. Many of the components are mounted directly on tube sockets rather than terminal strips. Electrolytic cans are mounted horizontally up front by the controls. The flyback is right out in the open. Overall, the underside it is in outstanding condition. No doubt due to the metal pan sealing it off.       
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#22
10-27-2022, 11:09 AM
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Quote:
Here's a look "under the hood" at the 1X2 HV rectifier tube. 
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#24
10-27-2022, 12:45 PM
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A specimen so nice that if it weren't for the white bumblebee caps and the micamold papers (that I know these use from my old 20" majestic) it would almost be worthy of a restuff.
__________________
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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#25
10-27-2022, 07:10 PM
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#26
02-17-2023, 01:18 PM
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Time for a long overdue update.
I tried a slow, variac power up and was able to get a bit of a raster plus a horrible hum out of the speaker.  The main filter cap was getting warm so I gave up on trying to reform and tacked in some replacement electrolytics for that can. The side panel of the chassis come off allowing easy access. The chassis is quite well made for an off-brand. Majestic was going through a bankruptcy around when this set was made. Wilcox-Gay bought up their assets so they may be responsible for the design.   Progress!  Replaced more a bunch of paper caps with little improvement. Finally, found a way out of spec resistor in the vertical hold circuit and got a lock.  Replacing the other electrolytic mostly fixed up that horizontal ringing.   I tested the caps afterwards and every section has very high leakage. Considering how baked the CRT is, I expect this is a high hour set.  Amazingly, I was able to boost emissions using 'Manual Restore 2' on a CR70. Enough that I was able to remove the booster and still have a good image. 
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#27
02-17-2023, 01:24 PM
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Some interesting discoveries during the project.
This hunk of solder was wrapper around the burned out 1/4 Amp fuse.  The original Majestic 6AV5 horizontal output tube is on the left. A newer RCA 6AV5A is on the right.  It really cooks!  The 12LP4 CRT is stamped Stewart Warner on the dag coating. I don't know if it is the original or if there was any connection between the companies.  The ion trap magnet is an interesting clip-on design.  A few of the original tubes are still good like this tiny 1X2 HV rectifier. 
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#28
02-17-2023, 01:34 PM
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Finally, detailing. Most of that corrosion on the top of the chassis cleaned up easy with CLR.
 A little Simichrome of the tuner tube shields. What a difference!  More of the same on the other side.  Had to remove the CRT to really get in there.   
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#29
02-17-2023, 01:40 PM
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Now circling back to the electrolytics. I picked up some "adapt-a-cap" boards to try.
 I gotta say I'm really digging it.  Out with this temporary mess  Very well made boards. All the holes lined up perfectly and the little square, triangle, ect. symbols match up.  One down, one to go. 
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#30
02-19-2023, 02:35 PM
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Here's a look with the opposite side of the chassis removed. I replaced the AC line caps, and made final adjustments with a test pattern generator. I'm calling the electrical portion of the project done.
  Now on to the cabinet. The leatherette on top is quite dirty. Gojo did a great job cleaning it up. I used some isopropyl alcohol on some paint splatters. A sharp blade helped loosen up some stubborn areas.    That was follow up by some Howards Restor-a-finish Mahogany to put some color back in. Finally, Howards Feed'n Wax.  
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