Stromberg Carlson 21TQ restoration

[ohohyodafarted]

Originally Posted by bandersen

I'm not quite sure. I tried cleaned a section and a lot of the yellow came off. That plus the baked on crud on the tubes leads me to think it's nicotine and tar.
Underneath it does appear to be treated with something other than cadmium though. Whatever they used it's corrosion free

The yellow is from yellow chromate coating over the base plating which could be zinc or cadmium.

[bandersen]

Originally Posted by Electronic M

Could the detector diode be plugged in backwards?

That was the first thing I thought of too No, switching it screws it up even more. I discovered that this problem has existed all along. When I injected a signal directly in the IF, I had the contrast up 100%. If I back it off a little, I get a reversed test pattern too. The service specifically mentions an inverted image in the troubleshooting section and indicates it's a problem in the video amp.

[bandersen]

I've been swapping tubes and tweaking AGC and contrast settings. Things are looking better but still don't seem quite right. Contrast is too harsh. If I back off, the image inverts or fades away.
Cartoons aren't so bad, but live action is hard to watch.

[Username1]

Wow that is a beautiful cabinet, and a nice looking chassis too....
I'm wondering if you have a bias issue around the video amplifier tube....
Possible bad resistor, or coupling cap in the area of the contrast control....
Maybe even a bad spot in the contrast control itself.....
I would love to see that set in person !

.

[bandersen]

It's a pretty set alright I reinstalled the full size CRT and get a nice bright picture but contrast problem remains. I've checked all the resistors but will recheck and try replacing the ceramic disc cap on the contrast control.

[old_coot88]

Any chance of posting the schematic of the video amp area? It kinda looks as if the G1 on one of the video tubes is biased too heavily negative. Maybe a cathode resistor too high in value(?).

[bandersen]

Bingo. Grid bias resistor on the video amp should be 560K but measured more than 10X that! It wasn't apparent until I had clipped one lead out of the circuit.


Much better

[old_coot88]

Hah. Semi-floaty grid. Which tube was it, the 6AU6 or 12BY7?

[bandersen]

It was the 12BY7. I think the last challenge is going to be fabricating a yoke cover using styrene

[Tim]

Originally Posted by bandersen

I think the last challenge is going to be fabricating a yoke cover using styrene

If you send me the dimensions I can try and design and 3D print one for you. I have printed several plain ones but not yet one with the centering ring supports. It may take a couple of weeks though as I only have access to a 3D printer one evening a week. If you just want to make one yourself and button this set up that is fine, just giving you an option.

[bandersen]

Thanks for the offer - that cover looks great. I think I will go ahead and make one using styrene though. I have the materials on hand and want to get the set all buttoned up. I will definitely keep you in mind for future projects

The set has really good bandwidth and there's no need to touch the alignment.


I managed to get the heavy chassis back into the cabinet and cleaned up the safety glass.


The back is in very good contition but only one screw was holding it on when I got it. That's been taken care of.

[bandersen]

I just finished fabricating a new yoke cover from some styrene stock. A circle cutter with a sharp blade makes the task much easier. I think it turned out quite well.

[Kevin Kuehn]

That looks excellent. Can you still paint it black, or just going with the white?

[Radiotronman]

That looks awesome! Great job

[bandersen]

Thanks. I'm just going to leave it white. I figure black paint would flake or scrape off when the centering rings are adjusted.