Videokarma.org

Go Back   Videokarma.org TV - Video - Vintage Television & Radio Forums > Vintage TV & Radio Tech Forum

We appreciate your help

in keeping this site going.
Reply
 
Thread Tools Display Modes
  #16  
Old 08-10-2013, 11:43 AM
bob91343 bob91343 is offline
VideoKarma Member
 
Join Date: Nov 2010
Posts: 745
Try my idea of adding a resistor from the screen to B+ and I think you have a good chance to make it work. It's obviously not oscillating because the tube has no gain without screen voltage. Of course, with a big enough audio signal there may be parts of the cycle where the screen voltage climbs enough to start oscillation but in any case you will have terrible distortion.
Reply With Quote
  #17  
Old 08-10-2013, 12:02 PM
Einar72's Avatar
Einar72 Einar72 is offline
Chasin roundies since '79
 
Join Date: Jan 2006
Location: Federal Way, Washington
Posts: 936
I have a circuit for you to try. I built this in 1968,and had marvelous results. Circuit #13. I'd still have it, but a friend lent me one of those battery eliminators that looked like a 9V battery with a power cord attached. It was hot to the power line and non-polarized at the AC plug. I had the circuit grounded, so there was a big bang when I plugged it in with hot on the wrong side!

http://cdn.ka6wke.net/AmateurRadio/S...e_Hobbyist.pdf
Reply With Quote
  #18  
Old 08-10-2013, 12:26 PM
dtvmcdonald's Avatar
dtvmcdonald dtvmcdonald is offline
VideoKarma Member
 
Join Date: Nov 2012
Posts: 1,191
My idea for such a circuit would be to put the screen at B+ voltage
and use the grid and cathode for the oscillator (screen as plate
of oscillator, grounded-plate type circuit) Then modulate from
a low impedance source (i.e. speaker impedance) using a
plate transformer in reverse to plate modulate the tube. Gain
would be adjusted using a say 25 ohm wirewound pot before
the primary of the modulation transformer.

the basic idea is at

http://nmwilliam.tripod.com/el84.html

though this uses a crystal rather than a coil. However ...
that circuit generates several watts and needs watts of audio.
Reply With Quote
  #19  
Old 08-10-2013, 03:07 PM
jr_tech's Avatar
jr_tech jr_tech is offline
VideoKarma Member
 
Join Date: Sep 2009
Posts: 4,510
Is it time to throw out the bathwater and the baby?

I don't think so... before giving up and embarking on a re-design or totally different circuit, a couple of questions perhaps should be answered.

1.As designed can the transmitter work?

a. There are several arguments here that indicate that the tube possibly could operate (perhaps poorly) outside of its normal operating parameters.

b. Calculating from the voltage drop across the 10k resistor to the plate of the 117L7 (pin 5) indicates that the tube is drawing about 2 ma current which is about 300 mW of power. This would seem to be consistent with a typical power output of 100mW that is generally considered
permissible for un-licensed operation on the AM band if a very short antenna is used. More power might incur a visit from the FCC. A good reason, perhaps for this low-power configuration.

c. Phil says it works... there is no doubt that what he built indeed does work fairly well!

2.Does it work?

a. Perhaps, but not on the frequency tested... you might want to test at the high end and low end of the broadcast band just to make sure that it is not transmitting.

b. I still favour testing with a sensitive little portable held near the transmitter. Sony for example, sells a sensitive little analog portable for around 12 bucks that would be ideal for this testing. http://www.amazon.com/Sony-ICF-S10MK...y+pocket+radio

3.If not, can it be made to work?

a. The circuit is pretty simple, and appears to be wired correctly... a big question involves the coil phase and/identification of windings... perhaps the primary and secondary are swapped, or one winding is simply out of phase with the other.

b. I suggest disconnecting all 4 leads to the coil and measuring the resistance between all the pins... perhaps we can figure out what is happening here.

OBTW, your test radios are very cool!
Not affil with Sony,
jr
Reply With Quote
  #20  
Old 08-10-2013, 08:45 PM
Geist Geist is offline
VideoKarma Member
 
Join Date: Dec 2012
Posts: 350
Hi All;
I have a copy of what I think is the origional article for this Transmitter, it is copied in jpeg, but I don't know how to make it show up here..
The Schematic is slightly different, but maybe it may help you find what the problem is with your unit.. It was origionally for a phonograph with a crystal pickup..


http://www.videokarma.org/attachment...1&d=1376186080

THANK YOU Marty
Attached Images
File Type: jpg 117N7Xmitter_0001.jpg (129.2 KB, 24 views)
File Type: jpg 117N7Xmitter_0002.jpg (125.2 KB, 18 views)

Last edited by Geist; 08-11-2013 at 08:43 AM.
Reply With Quote
Audiokarma
  #21  
Old 08-10-2013, 09:01 PM
Geist Geist is offline
VideoKarma Member
 
Join Date: Dec 2012
Posts: 350
Hi All;
Winky, I have this file in PDF format, but the file is too large for me to upload here..
If you want I can email it to where ever you want If you cannot see the details or fine print..
THANK YOU marty
Reply With Quote
  #22  
Old 08-10-2013, 11:01 PM
bob91343 bob91343 is offline
VideoKarma Member
 
Join Date: Nov 2010
Posts: 745
Yes R2 is the critical component missing from your unit. It's from the screen to B+.
Reply With Quote
  #23  
Old 08-11-2013, 01:14 PM
wa2ise's Avatar
wa2ise wa2ise is offline
VideoKarma Member
 
Join Date: Dec 2002
Location: USA
Posts: 3,147
speaking of AM transmitter circuits, I have some at my web page http://www.wa2ise.com/radios/amxmit.html, and
__________________
Reply With Quote
  #24  
Old 08-11-2013, 01:28 PM
jr_tech's Avatar
jr_tech jr_tech is offline
VideoKarma Member
 
Join Date: Sep 2009
Posts: 4,510
Sorta works !

For grins, yesterday afternoon I dug most of the parts out of my junk box to build a simple version of the Li'l 7. I do not have the proper oscillator coil, but found small antenna coil (likely out of a junked AM radio) that had 2 separate windings. The windings both had fairly low resistance (under 10 ohms), but I decided to give it a try. I did not use a power transformer for isolation, but made sure that ground on the board was always cold. I could not find a 1 meg pot, so I substituted a 1.2 meg resistor to ground at the input (pin 5).

Since the coil was not identified in any way, I connected the lower resistance winding (untuned) to the plate (pin 3) and B+ and used the higher resistance winding (tuned with a 270pf cap) in the feedback circuit (pin 4 and ground). A 6.3 VAC 60Hz "signal" from a bench power supply was connected to the input (pin 5) and ground.

I had to reverse the two leads in the feedback circuit to get oscillation, but when I did, I was able to find a loud buzz with a portable radio very near the transmitter. Since my "oscillator coil" does not have an adjustable slug for tuning, I merely scanned the band with the portable radio... found the buzz "signal" at about 700kHz. The range was only about 5 feet with no antenna connected to the transmitter.

Now the "sorta" part... when the signal was removed, it appeared that the transmitter stopped transmitting! In fact, there seemed to be a threshold... if the signal was reduced below a certain point oscillation stopped. I suspect that a real audio signal would be transmitted, but with much distortion, as bob91343 suggested.

Perhaps with the correct oscillator coil, the circuit *might* work ok, I don't know... today if I get a chance, I will try to "bias up" the G2 connection to a positive source to see if I can get undistorted modulation. As can be seen in the 'scope photo, alternate parts of the 60Hz test signal (likely the negative portions) are missing from the modulated output of the transmitter.

jr

Last edited by jr_tech; 05-30-2017 at 04:35 PM.
Reply With Quote
  #25  
Old 08-11-2013, 02:32 PM
Geist Geist is offline
VideoKarma Member
 
Join Date: Dec 2012
Posts: 350
Hi All;
Jr tech, If you put a tuning cap in parallel with your coil, you could tune it to where ever you want..
That is in the schematic I posted, He had a tuning cap in the circuit.. He states in the article that an antenna coil or an oscillator coil will work, with a tuning cap..
THANK YOU Marty
Reply With Quote
Audiokarma
  #26  
Old 08-11-2013, 02:49 PM
jr_tech's Avatar
jr_tech jr_tech is offline
VideoKarma Member
 
Join Date: Sep 2009
Posts: 4,510
For sure... but I used all the 365pf tuning caps that I had when I built a number of tuned loop antennas for myself and several friends... still digging in "junk boxes", might find something suitable.
"Fixed tuned" worked well enough for my purpose of testing the operation of the circuit.

jr

Update: I "biased up" the G2 somewhat by connecting it through a 470k resistor to the B+ supply. G2 now sets at about + 25 volts. The modulated output is no longer missing the negative information from the modulating signal and the transmitter continues to oscillate when the modulating signal is removed, but it is not very symmetrical (sorry 'bout the fuzzy handheld pix). The tube is now drawing more current and the B+ supply has dropped to about 110 VDC. Obviously, the 10k resistor in the power supply will have to be reduced, and likely more filtering will be needed to handle the increased current load. More work is needed, but out of time for today.

Last edited by jr_tech; 05-06-2017 at 02:20 PM.
Reply With Quote
  #27  
Old 09-15-2013, 02:22 PM
cbenham's Avatar
cbenham cbenham is offline
VideoKarma Member
 
Join Date: May 2006
Posts: 471
Quote:
Originally Posted by bob91343 View Post
Yes R2 is the critical component missing from your unit. It's from the screen to B+.
Referring to the posted original article above, I think R2, 3.5 meg, from B+ to the screen grid must be large enough to allow modulation
with a very high impedance source. With a lower impedance source, [headphone output from a CD player, etc.], it could be much smaller.
Hope this helps.

Last edited by cbenham; 09-15-2013 at 02:41 PM.
Reply With Quote
  #28  
Old 05-01-2014, 12:40 AM
Winky Dink's Avatar
Winky Dink Winky Dink is offline
VideoKarma Member
 
Join Date: Apr 2010
Location: Meridian, Idaho.
Posts: 583
The Wayback Machine

(for some reason I can't embed my photos, so I have to use old-fashioned links)

After several frustrating attempts to get a Li'l 7 AM transmitter to work, I decided to try something else. After reading Jay Allen's (radiojayallen.com) description of the product, I bought the SStran AMT3000. I'd never worked with a circuit board, and it was rough going at first...

https://picasaweb.google.com/lh/phot...eat=directlink

...but I managed to finish the build in two days...

https://picasaweb.google.com/lh/phot...eat=directlink

...and, son-of-a-gun, it works beautifully. Good reception throughout the house, and now I know there's always something good to listen to on 1530 AM radio. And now, when the grandchildren visit this summer, we can enjoy an authentic experience listening to old-time radio shows.

The name "Wayback Machine" refers to Sherman and Mr. Peabody on Rocky and Bullwinkle, a favorite TV show from my youth.

https://picasaweb.google.com/lh/phot...eat=directlink

With the experience of building this transmitter and with all the suggestions above in this thread, I'm going back to the Li'l 7 once again.
__________________
Winky Dink
Damn the patina,
Full speed ahead!
Reply With Quote
  #29  
Old 05-02-2014, 05:24 PM
bandersen's Avatar
bandersen bandersen is offline
RCA 741PCS
 
Join Date: Mar 2009
Location: Chicago, IL
Posts: 8,085
I'm glad you're happy with it. I've had my 3000 running 24/7 for months with no issues
__________________
Here are my Vintage Radio & TV YouTube Channel and Photo Gallery
Reply With Quote
Reply


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -5. The time now is 05:02 AM.



Powered by vBulletin® Version 3.8.4
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
©Copyright 2012 VideoKarma.org, All rights reserved.