1955 Motorola 17T20
 

I got this set for free, just had to drive 75 miles both ways to get it but for free why not. I got this set home and powered it up, the tubes light up but no HV or sound, i knew then this set had no B+ so i pulled the chassis and found many a problem like falling apart sand resistors, fried and cracked resistors, split bumblebees and i think the selenium is bad but not sure at this moment. All the caps and some resistors will need to be replaced obviously. I haven’t checked the tubes yet because my tester went out and I don’t have a spare, but luckily a stereo store near me lets you test tubes for free and there is an upcoming hamfest i’m going so i can hopefully get a new tester.

Not in bad shape

http://videokarma.org/attachment.php...4&d=1526465440

Don't bother testing the selenium, just replace it with a 1n4007.

Agreed...I do that on all old tweed Fender amps.

The tubes light so the glo-bar resistor must be ok. Just check the sand resistors to see if they're still good for now! I've seen a few of them with cracked resistors. You probably need a schematic to determine resistors that are burnt beyond recognition. Of course, you have to change the split caps.
No B+, check the plug in resistor on top of the chassis, usually a 7.5 ohm, 5 watt. Make sure the speaker is plugged in, as the field coil is the B+ choke.

Thats what i’m going to do.

I always do a preliminary assessment of the condition of the set, by just replacing the obvious defective parts and checking the CRT.
A schematic is generally needed and makes it easier. I don't do wholesale changing of parts until I know what's involved. I still do it in stages.
Motorola made about three years of this chassis. They were always easy to get going, but that was when they were less than 10 years old. :thmbsp:

This looks like the American cousin of the Marconi I'm working on right now, 17" series string set, with a half wave selenium rectifier? What picture tube does this one use? Look forward to following this project.

Even though, it's not my set but I know it enough, from experience. It uses a 17HP4 CRT and a voltage doubler for higher B+. The tubes are in a series-parallel configuration, where the set you have is a newer model, using only one 600ma series string. That's the way, the US designed sets progressed to.
Yours is very similar to the US designs. :thmbsp:

You nailed it there, I don't want to throw this thread too far off topic, but it would sure be interesting to understand the progression in circuit design from the RCA 630 to say, the Philco Predicta or maybe a bit later.

The Crt is a rebuilt 17HP4 and it checks way into the green and has 10 original tubes and 6 replacements, so this set is sorta kinda low hour.

This set was made August 1954

Top and bottom view of the chassis

Looks really nice, I look forward to seeing how it works!

I parted out a similar vintage moto monochrome with a weak CRT and a ruined cabinet...I still have most of the chassis and may be able to supply parts if needed.

It looks like the 25L6 went to air!
That tube furnishes the lower voltage to many of the signal circuits. If it is bad, you'll have to replace it before going any further.

Why in the world did Motorola put the tuner tubes under the chassis?? Seems stupid to me.

Some Motos keep the fly and 1B3 under chassis with an access door. Dumont used to put 6AL5 detector tubes and 6X4 rects under chassis in some of their RA-103 sets...There are probably more bad examples in the TV field but at least they were socketed...In the tube hearing aid era, the subminiature types they used (pencil diameterX1.5" long) were often soldered directly into the chassis. :eek::D

^Yet another attack from boring-trivia-man, AKA me. :D

It didn’t, it was just dirty. Don’t know what the white stuff is.

I’m starting to paint the cabinet and i think its turning out nice!

That was the last Motorola that the chassis had to be removed to replace the tuner and high voltage rectifier. The slightly newer larger screen sets had the tuner on the right side with the tubes on top of the chassis. The HV rectifier had the trap door. UHF equipped sets had the 6AF4 under the chassis, but there was an access cover on the bottom.
Then there was the Sylvanias that had the solder-in 5642 HV rectifiers. IIRC, the tube location chart didn't even list them. :sigh: