1951 AMC 16" TV Restoration

[David Roper]

That's a Muntz design with a handful of parts thrown back in. Unless I'm blind, I see no mechanism for blanking the screen when the phono input is selected.

[Crist Rigott]

Originally Posted by David Roper

That's a Muntz design with a handful of parts thrown back in. Unless I'm blind, I see no mechanism for blanking the screen when the phono input is selected.

The switch for TV/Phono input is a simple SPDT slide switch. It does look like the TV picture is not turned off. When I get it up and running, I'll check its function.

[dieseljeep]

Originally Posted by David Roper

That's a Muntz design with a handful of parts thrown back in. Unless I'm blind, I see no mechanism for blanking the screen when the phono input is selected.

There's a capacitor in the bottom with the Bendix name on it. That's probably who made it. Only two IF stages.

[Crist Rigott]

Originally Posted by dieseljeep

There's a capacitor in the bottom with the Bendix name on it. That's probably who made it. Only two IF stages.

Oh yeah, it's made by Bendix that's for sure. Why do you say that 2 IF stages are boring?

[Phil Nelson]

Better quality TVs had three IF stages (like my Dumont RA-103), or even four (like my RCA CT-100). More IF stages means more amplification of the incoming signal, resulting in greater sensitivity (ability to pull in weak, far-away stations). A TV with only two IF stages would only have worked well in a metro area with nearby stations.

Sensitivity per se is not such a big deal nowadays, when many collectors feed their restored TVs from a DVD player or whatnot, with a nice strong signal. The number of IF stages can be interpreted as a sign of overall quality, however. If the manufacturer skimped in that area, they may have skimped in others, too.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

[Crist Rigott]

Originally Posted by Phil Nelson

Better quality TVs had three IF stages (like my Dumont RA-103), or even four (like my RCA CT-100). More IF stages means more amplification of the incoming signal, resulting in greater sensitivity (ability to pull in weak, far-away stations). A TV with only two IF stages would only have worked well in a metro area with nearby stations.

Sensitivity per se is not such a big deal nowadays, when many collectors feed their restored TVs from a DVD player or whatnot, with a nice strong signal. The number of IF stages can be interpreted as a sign of overall quality, however. If the manufacturer skimped in that area, they may have skimped in others, too.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

Phil,
Thanks for the explanation. I can see where if they scrimped here they could scrimp there kind of a deal.

When I'm done I'll for sure include some videos of the picture and sound quality.

Again thanks and I enjoy reading over your restorations both radio and TV.

[Crist Rigott]

Here some more pics. I cleaned the chassis with some mild dish washing detergent. It wasn't real dirty and it cleaned up pretty well.

I checked the tubes and some are weak and some are very weak!

I then got to work on making and ID on every capacitor and resistor and comparing it with the schematic and parts list. There were some discrepancies but nothing real major. I use a small block of masking tape and mark each component. While doing this I check the schematic and parts list. I then make the document changes in red ink.

I also drew out the 2 tube configuration for the listed 6T8 tube. Besides using 2 tubes, there were a few resistors and capacitors in the chassis that weren't in the schematic. I'll update my parts lists accordingly so I get my order right.

I'll draw up the new configuration and I'll post it side-by-side with the schematics so we can see what the changes were.

[dieseljeep]

Originally Posted by Crist Rigott

Here some more pics. I cleaned the chassis with some mild dish washing detergent. It wasn't real dirty and it cleaned up pretty well.

I checked the tubes and some are weak and some are very weak!

I then got to work on making and ID on every capacitor and resistor and comparing it with the schematic and parts list. There were some discrepancies but nothing real major. I use a small block of masking tape and mark each component. While doing this I check the schematic and parts list. I then make the document changes in red ink.

I also drew out the 2 tube configuration for the listed 6T8 tube. Besides using 2 tubes, there were a few resistors and capacitors in the chassis that weren't in the schematic. I'll update my parts lists accordingly so I get my order right.

I'll draw up the new configuration and I'll post it side-by-side with the schematics so we can see what the changes were.

Looking at your under-chassis view and the schematic, I noticed a low current selenium rectifier in the horiz yoke circuit. That's a new one on me!
I wonder what the reason was?

[dtvmcdonald]

CT-100 (and TRK-12) have FIVE IF stages!

Yes, more gives more gain.. BUT ... it also allows a more sharply
tailored bandpass curve with a very sharp 4.2 MHz cutoff.
Doing the sharp bandpass edges cuts overall gain.

[Crist Rigott]

The last day or so was spent taking inventory of what parts are needed and making a list and where to get them from. Some I had "in stock" while others will be ordered from Just Radios and Digikey. I've decided to bump up the 1/2W to 1W, the 1W to 2W and the 2W to 3W. Some of the 2W resistors seemed a bit discolored and heat stressed so those 3 will be replaced with 5W (R70, R71, R73). There was one resistor clearly bad. It had split open due to excessive heat. It is R76 a 470 ohm 2W resistor. This will be replaced with a 470 ohm 10W resistor the local parts house had in stock. It will still fit nicely in the chassis.

I'm using the taller, slender Nichicon 105c E-Caps so they'll fit into the cans. There are 2 cans with 3 each caps in them

[dieseljeep]

Originally Posted by Crist Rigott

Oh yeah, it's made by Bendix that's for sure. Why do you say that 2 IF stages are boring?

Even the Muntz had three IF stages. Boring was just a poor attempt at humor.
It seems, at the time, just about every set manufacturer used at least three stages of IF.
Bendix also designed the 12" set, using the 6BG6 reflex circuit horizontal osc and output and the damper less hv sweep circuit.
Muntz copied that design, not the other way around.

[old_coot88]

In order to get fine detail resolution in the pic, the IFs have to be stagger tuned. And stagger-tuning reduces overall gain. Add another IF stage for more gain, and you can stagger-tune for a wider IF bandpass, allowing finer detail resolution. So more IF stages equals better fine detail in the pic.

(Then into the 1960s, high gain IF tubes were developed (e.g., 6EH7, 6EJ7) which allowed good bandpass with just two IF stages.)