Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: [1]   Go Down
  Print  
Author Topic: Brain Teaser  (Read 394 times)
AC5UP
Member

Posts: 3875




Ignore
« on: December 11, 2002, 12:19:18 PM »

For the truly 'O' OM's out there, visit this URL

http://oldtech.net/Zenith/12H090sc.gif

and take a close look at the schematic.

This model has a feature that I'd consider rare, as I've never come across a consumer receiver like this. Can you find what's unusual about this set?

Hint: Bandswitch!

- AC5UP
Logged

Never change a password on a Friday                
NB6Z
Member

Posts: 550


WWW

Ignore
« Reply #1 on: December 11, 2002, 12:38:26 PM »

I'm still looking for the first LO stage. Should be between the RF amp and converter (mixer) tube. Or maybe the "converter" is the 1st LO?
I still have shoe boxes of some of these old tubes...
Logged
KB6TRR
Member

Posts: 32




Ignore
« Reply #2 on: December 11, 2002, 02:15:56 PM »

How about the 45MC FM feature. I can not ever recall a consumer radio with that coverage.
Logged
NI0C
Member

Posts: 2406




Ignore
« Reply #3 on: December 11, 2002, 02:49:21 PM »

It is my understanding (based on the PBS special, "The Men of Radio") that early FM broadcasting used frequencies in the 40 MHz range, and that David Sarnov of RCA lobbied the FCC to change FM broadcasting to the 88 to 108 MHz range we use today.  This was apparently a deliberate attempt to ruin Armstrong (the inventor and early promoter of FM).  
Logged
AC5E
Member

Posts: 3585




Ignore
« Reply #4 on: December 11, 2002, 03:21:55 PM »

Chuckle: Now that brings back memories! And while Zenith did things pretty much their own way there's nothing there that's all that unusual. Even the "padding wire" to trim the inductance of a tuned circuit; instead of a padder capacitor (to set the maximum or mininum capacity in a tuned circuit) was pretty common practice in the days just before and just after WWII.

As I remember it, that chassis was supplied in a large wooden console cabinet, either with or without the "cobra" turntable. There was a very similar chassis with a single output tube for a very large table model as well.

And yes, the original FM band was just below the Amateur 5 Meter band, where TV channel 2 is today. When FM was moved from there to it's present 88 to 108, we were moved to the old Channel 1 slot. Which is why the numbers on the TV dial ran from 2-13 instead of 1-12. Before UHF, of course!

73  Pete Allen  AC5E
Logged
AC5UP
Member

Posts: 3875




Ignore
« Reply #5 on: December 11, 2002, 03:44:10 PM »

You guys are good!

This chassis must be late-40's vintage as the 40 Mc commercial FM allocation was short-lived and only the larger cities had any stations on that band. Zenith selling a dual-band FM rig is interesting as it was definitely a niche product with little appeal after the local FM station(s) moved. As NIØC mentioned, the FCC re-allocated FM to 88 - 108 Mc at the urging of RCA to effectively obsolete every FM receiver sold by Armstrong. Sure, the official reason was to accomodate VHF TV, but when the handful of early FM buyers had to retrofit or replace their receivers it gave RCA (and others) a second chance at a sale. When the last station your Armstrong receiver could hear QSY'd, would you buy another Armstrong box?

RCA had a huge investment in the AM band and Sarnoff viewed FM as a threat to NBC's audience in the larger markets. Anything that slowed its adoption gave RCA more time to build competitive receivers and they had almost all their R&D cash promoting TV, with most of that coming from NBC and their big-gun AM stations... For RCA to drop any serious money into FM would have had them competing against themselves on AM and they pretty well had a lock on that band with their Red and Blue networks.

If TV had been slow to gain acceptance in the 50's, and if FM had siphoned off significant advertising revenues from NBC, RCA could have been in a tough spot for a few years... Let's just say it was a good thing for RCA that Jack Benny hadn't signed with CBS and that Milton Berle knew his dress size...

- AC5UP
Logged

Never change a password on a Friday                
AC5UP
Member

Posts: 3875




Ignore
« Reply #6 on: December 11, 2002, 04:24:04 PM »

Note to AC5E:

Sharp Eye, Pete... I noticed the gimmick caps in the VHF front end and LO stages myself, and you won't see any of that today. Near-Zero alignment makes for higher productivity and gimmick items meant the folks at the tail end of the production line back then needed a certain amount of finesse to tweak 'em in.

Did you also note the FM IF was at 8.3 Mc? And take a look at the part numbers... There are two .01 uF caps straight below the 7W7 tube marked '27' and '26', right above the 2200 ohm resistor marked '85'.

Betcha' that whoever who came up with the idea of marking things like C-27 and R-85 got a bonus as it reduced the cost of all those Secret Decoder Rings worn by everyone on the production line. (Hi!)

Also, I've seen plenty of tubes, but a 7W7 Huh Rare... And probably with a Loctal base, too!

- AC5UP
Logged

Never change a password on a Friday                
AC5E
Member

Posts: 3585




Ignore
« Reply #7 on: December 11, 2002, 04:59:01 PM »

Yes, the 7XX series tubes were 6.3 V heater Loctal, designed by Sylvania initially for Philco and Motorola, designed for auto and other mobile use.

Due to the many corduroy roads and the influence of the harsh "ox wagon" suspension of the mid to late 1930's the 6XX series octals would jar out of their sockets and roll around - causing customers to wail and gnash their teeth about their crummy radio.

Particulary where the radio maker designed the radio for "tube down" mounting to make changing the tubes easier. Just crawl under the dash and plug in a new 6SA7!

Studebaker finally figured out that tuning the chassis and suspension to 120 cycles per minute would allow much softer springing, allowing the use of miniature septal and noval (7 and 9 pin miniature) tubes. Of course, better roads helped as well.

73  Pete Allen  AC5E

Logged
K0BG
Member

Posts: 9868


WWW

Ignore
« Reply #8 on: December 11, 2002, 06:49:37 PM »

Chuck Guenther, NIØC is dead on! There is a PBS program (now out of print) called "Radio Pioneers" which among other things, covers the struggles between Armstrong and DeForest.  If you can find a copy of it, it is indeed worth watching. It is two hours of "must-see" material for every amateur interested in the "old days", and even mentions "amateur" radio.

I have a copy I recorded off the air, and I suspect I could make a copy of it for those who would want one. If you REALLY do, send me an e-mail, and I'll see what I can do.

Alan, KØBG

k0bg@aol.com
Logged

KG6AMW
Member

Posts: 616




Ignore
« Reply #9 on: December 12, 2002, 10:58:50 AM »

You can purchase this video at Wall Mart for $14.99.
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!