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Old 05-16-2018, 05:09 AM
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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 havenít checked the tubes yet because my tester went out and I donít have a spare, but luckily a stereo store near me lets you test tubes for free and there is an upcoming hamfest iím going so i can hopefully get a new tester.
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Old 05-16-2018, 05:11 AM
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Not in bad shape

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Last edited by Celt; 05-16-2018 at 10:37 AM.
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Old 05-16-2018, 10:25 AM
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Don't bother testing the selenium, just replace it with a 1n4007.
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Old 05-16-2018, 10:38 AM
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Agreed...I do that on all old tweed Fender amps.
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Old 05-16-2018, 11:40 AM
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Quote:
Originally Posted by LADR View Post
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 havenít checked the tubes yet because my tester went out and I donít have a spare, but luckily a stereo store near me lets you test tubes for free and there is an upcoming hamfest iím going so i can hopefully get a new tester.
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.
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Old 05-16-2018, 09:47 PM
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Don't bother testing the selenium, just replace it with a 1n4007.
Thats what iím going to do.
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Old 05-17-2018, 10:07 AM
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Thats what iím 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.
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Old 05-17-2018, 10:13 AM
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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.
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Old 05-17-2018, 08:42 PM
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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.
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Old 05-18-2018, 09:15 AM
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Originally Posted by dieseljeep View Post
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.
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.
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Old 05-18-2018, 02:59 PM
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Quote:
Originally Posted by dieseljeep View Post
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.
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.
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Old 05-19-2018, 03:40 AM
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This set was made August 1954
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Old 05-19-2018, 01:51 PM
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Top and bottom view of the chassis
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File Type: jpg CB8E2959-C6B9-4F5F-A712-71162FBE0C3F.jpg (83.5 KB, 32 views)
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Old 05-19-2018, 02:18 PM
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Looks really nice, I look forward to seeing how it works!
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Old 05-19-2018, 02:51 PM
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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.
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