[Paul Philco322]

Good day all:

I have the little Zenith, a nice player, sensitive too. while playing I noticed a fade out on FM, came back to play a bit, then faded to nothing. no AM or FM. 
I opened it up and noticed when power applied, the 12 DT8 would get very bright and then fade to darkness, and would repeat. So I replaced it and got great AM no FM. 

Could be bad tube? It checks out good on tester. Any suggestions on what other tube based on function could yield this result? Probably one I do not have 14GT8 FM detector?

Appreciate suggestions. 

Chassis is 7M05. Could not find in Nostalgia Air. 

https://www.radiomuseum.org/r/zenith_t2518_7m05.html



Paul

[Paul Philco322]

Well I must have had 2 tubes bad or weak, the 12DT8 restored the AM, a new 14GT8 which I got as a Christmas gift got the FM up again so I guess the set is fine otherwise because with the 2 replaced tubes all is well.

Paul

[David]

Welcome to the wonderful world of AM/FM Z sets. 12DT8, 14GT8 , 19T8 they are like rice cripes snap crackle and pop.

[Jayce]

I really like the late 50's through 60's Zenith AM-FM radios. Really solid sets all around and most seem to avoid the IF can issues of the late 40's and early 50's Zeniths. Not to say it can't happen. Also, most will actually give you some service right away, even before a recap. One thing I always expect is to at least replace the FM tubes in these as they are always either weak or dead when I find these radios. I have had at least five of these Zeniths from this era and only one has actually smoked itself when I brought it up on power. It's amazing how long these things have lasted before a recap.

[Phlogiston]

Welcome to the wonderful world of AM/FM Z sets. 12DT8, 14GT8 , 19T8 they are like rice cripes snap crackle and pop.

Stress on the filament string?

[David]

Most of the failures I have seen have not been from the heaters open. The 12DT8 has a internal shield grounded to pin 9 from some reason the shield fails and will short the tube out. The 14GT8 and 19T8 just seem to fail without any warning. The 12DT8 can be replaced with a 17EW8 without  changes to the tube socket. A 12AT7 can be used as a sub for the 12DT8 IF YOU MAKE A MOD TO PIN 9 of the tube socket or remove pin 9 from the tube. 12AT7 does not have a internal shield to ground on pin 9 but a tap to the heater so you have a dead short to ground your nice new tube will go poof!!! I have done the mod to the tube socket on 2 sets been happy with the results

[mikethedruid]

Hi Paul. Glad your set has come back to life. You may want to invest in a CD I own, https://www.ebay.com/itm/Riders-Perpetua...0005.m1851 . I own it, and it includes Beitman's , which covers a lot of the sets made after Rider's stopped. The schematics are clearer than Nostalgia Air, both for the Rider's and the Beitman's.

[Paul Philco322]

Only 58,000 schematics for 16.99 Mike, it should have 20,000 more 

Sounds like a good sensible investment thanks for the tip! Appreciate the feedback gang, both tubes that were weak/bad were Zeniths so maybe the originals. I will see how long these go for, maybe 50 more years .

Paul

[Eliot Ness]

Mike's right Paul, those scans by KE3GK are by far the highest resolution Rider's available. 

Where the high-res scans really shine are when you want to magnify (zoom in) an area to read a component value, etc.  Some of the Rider schematics were poor quality to begin with so these scans won't correct that, but with the better quality of scans I can get by over 90% of the time now without digging out my big manuals.

Another issue is what happens on large fold-out pages.  Every other scanned copy I've seen, including Radio Data, chopped it into two pages with gaps of information missing.  I think KE3GK must have had access to a very large bed scanner because his come out as one large (and complete) page.

[Paul Philco322]

Thanks for the feedback, maybe buy the CD before another set!

Paul

[Ron Ramirez]

I'll ditto what John (Eliot) said about the Rider's DVDs. After John recommended these here on the Phorum some years ago, I bought the set - and gave away my hard copy Rider's Manuals.

Having a 27 inch all in one computer at the workbench now, I find it much easier to use schematics such as this instead of paper schematics. As John said, you can zoom in as/when needed. I'm scanning factory service info now, converting to PDF, and using that at the bench instead of paper also.

[Philco444]

A wise choice Ron as the paper won't get holes burned in it from the soldering iron, torn from a sliding chassis, or splattered with tar from the Bakelite tar tubs.

[Arran]

I work from a more old fashioned, but reliable method, I make a photocopy of the original service info, since I already have most of the manuals I need, and own a B&W copier, then I can do as I like with them, and the originals stay safe. There are some online sources with good scans, such as Pacific TV, and occasionally Electrotanya, which are good for Canadian and European sets, either because I don't have the diagram or because I'm lazy and just want to look at something without going through a book.
Regards
Arran

[KCMike]

+ One Arran. I have the riders on cd plus most of the old large riders books that I use once in awhile.
I Like to print a copy or two of the schematics and have next to me on the bench so I can make notes them as I work on the chassis.