Stromberg Carlson 21TQ restoration

[bandersen]

A customer recently dropped off a 1954/55 21" Stromberg tabletop along with a nifty mid 30s Airline tombstone radio.

I figure I'll tackle the TV first.



The cabinet is solid maple - no skimping on veneer with this set.
It's also very heavy. No way could I life it inside so I pulled the chassis out on the porch.



Sneak peek inside.

[bandersen]

The 21XP4A CRT tested very good so I lugged the chassis up onto the workbench.


Much to awkward to work on like this so I unmounted the CRT.


Unfortunately, it's got a case of yoke cover rot. I'll try to fabricate a replacement using styrene.


Finally, here's a look under the chassis. It's had some work done, but not much. For sure, that big Sprague Atom isn't original. It also isn't connected to anything
Those can lytics will be super easy to work on


I popped out the 5U4 rectifier and fired it up. All the tubes lit and I get around 650 VAC on the secondary. Full steam ahead!

[Steve D.]

That S-C is one very clean and beautifully constructed set.

-Steve D.

[bandersen]

Definitely the easiest electrolytic replacement I've done. Skinny caps make it even easier. All four new caps fit nicely within the base.
I was just going to leave the new caps exposed, but figure it might be helpful to a future restored if I tape the can top back on. That way the original cap values, voltages and symbols will be preserved.

[bandersen]

I finished replacing the electrolytics and a few paper caps in the audio circuit and tried a power up. My hope was that I could at least get some sound.


I got sound alright - the sound of a 100uF cap popping !
It wasn't a wiring mistake though. I screwed up with the restuffing and had mixed up a 100uF 50v cap with a 47uF 450 volt cap.


I got that fixed up and this time got a nice bright raster but no sound.


I injected a composite signal right in the grid of the video amp and got a test pattern of sorts. Seems to be a problem somewhere between the 3rd and 4th IF stage....

[DavGoodlin]

I bet there were not many sets with a 4th IF stage by the mid 50s.

[bandersen]

I was thinking the same thing. This is a well designed set for sure. I did a little more recapping on the horizontal and sync circuits and finally got a stable image.
Vertical is still very touchy and no signal is getting through the tuner.

[bandersen]

The detector diode is very easy to get at. I wonder if they anticipated a lot of failures ?
This one happens to test good and swapping in a 1N34A made no difference.
Seems I have no choice but to dive into the tuner.

[Eric H]

Bob, is it the pictures or does that chassis have that gold anodized look that the Japanese stuff had?

Much better than the oxidized Cadmium we usually find.

[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

[Electronic M]

I've been working on a Fisher 101R tuner that has a similar diode mounting scheme for the ratio detector.

[bandersen]

The tuner was pretty easy to get out. Four wires for power and an RCA jack for the IF out.


Popping two pieces of spring wire out frees up the drum to reveal this mess.


I quickly found a 100K that measured almost 900! A couple others were off but not as much. I'll replace them, clean up the contacts and give it a try.

[Kevin Kuehn]

Originally Posted by Eric H

Bob, is it the pictures or does that chassis have that gold anodized look that the Japanese stuff had?

Much better than the oxidized Cadmium we usually find.

It looks like a Chromate conversion coating, which can be done over cadmium. That color was also common on some automotive parts.

I'm guessing that tuner chassis is the common cadmium, so you can see the contrast between that and the underside of the main chassis.

[bandersen]

Replacing those tuner resistors didn't seem to have any effect so I went back to square one. It seemed to be that although I could feed a signal into the IF and get an image, I was having to use a really strong signal.
So I started swapping and wiggling tubes even though I had already tested them all.
Eureka! An image and sound Only problem though is that it's inverted
I swapped out the video output tube while the set was running and briefly got a normal image as the tube warmed up. I'll investigate the video amp circuit more tomorrow.

[Electronic M]

Could the detector diode be plugged in backwards?