Forums :: NEW! Web Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Dec Mon 06, 2021 7:07 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 6 posts ] 
Author Message
 Post subject: Stromberg Carlson
PostPosted: Sep Thu 16, 2021 2:16 am 
Member

Joined: Apr Thu 09, 2020 7:33 am
Posts: 189
Hello my fellow members. I'm trying to finish a functional repair on a Stromberg Carlson AM/ Shortwave radio. It looks to be in about the mid 1940s. It's an 8 tube transformer type with pushbuttons and manual tuning with a photo input. The tubes are 6A8 converter, 6K7 IF, 6H6 detector, 12SQ7 audio amp, 12SQ7 phase inverter, two 6L6G outputs and a number 80 rectifier. Also has a tuning eye tube. All of the caps have been changed because none of the original papers measured even close and all had leakage as low as 15 volts on a 400 volt cap on my sencore tester. All new electrolytics too. I then gave it a complete RF and if alignment using my RF generator. The sticker reads its a 455kc if. On this radio you can align the RF trimmer and coil slug. I tuned the coil for the best sensitivity at the low end of the band and the trimmer capacitor at the high end until the RF tracked perfectly with the oscillator. The higher frequency stations come in clear but 710 keel does not and it's a strong station. All I get is AM buzz. I am puzzled. If I apply a 710 khz signal from the generator I can tune it in on the radio dial and peak it on the coil but still no keel station. I wonder if the 6A7 tube is low on performance at the low frequency. It measured weak on the tube tester. Now the SW comes loud and clear after alignment but that AM won't get low stations but is loud and clear on upper AM stations. What do you think? Out of all the radios I done this is the first one that won't tune in Keel.


Top
 Profile  
 
 Post subject: Re: Stromberg Carlson
PostPosted: Sep Thu 16, 2021 6:09 am 
Member
User avatar

Joined: Oct Wed 14, 2009 6:36 am
Posts: 6596
Location: New York USA
Double-check the accuracy of the 455 KHz from your signal generator: Just get a modern digitally-tuned radio and tune it to 910KHz, listen for the audio tone from the second harmonic. tune the signal generator if necessary, to hear it. When the IF stages are not at 455, the lower end of the AM dial becomes "deaf"


Top
 Profile  
 
 Post subject: Re: Stromberg Carlson
PostPosted: Sep Thu 16, 2021 5:57 pm 
Member
User avatar

Joined: Jun Sun 03, 2012 12:40 am
Posts: 2185
Location: Cromwell, Connecticut
Out of curiosity, why is there a 12SQ7 tube in a mostly 6V tube transformer set? Is this an issue, or just a typo?

_________________
Tony

People may not remember how fast you did a job, but they will remember how well you did it.


Top
 Profile  
 
 Post subject: Re: Stromberg Carlson
PostPosted: Sep Sat 18, 2021 1:38 am 
Member

Joined: Apr Thu 09, 2020 7:33 am
Posts: 189
Oops I meant 6SQ7. I been using the same eico generator for a lot of other sets and it turns out fine. The plate and screen voltages on all the tubes are present and strong. The IF and Converter tubes measure just below weak and slightly in the red replace on my tube tester. I had better luck building radios from scratch and getting them to work. I know it's gotta be something staring right in the face. It can't be much it's got all new caps. I didn't check resistors maybe it's a bad resistor. There is one thing about this radio that puzzles me and maybe it could be why the sour performance it's when I try to align the second IF, the top trimmer cap is sensitive to being touched by a small metal screwdriver. I know you're gonna tell me you're not supposed to use metal but the first IF won't do that just the second IF top trimmer. The bottom trimmer on the second IF isn't sensitive to touch just that crazy top one. I wonder if the second IF is wired wrong. Maybe the metal screw on the top trimmer is not wired to ground or to B plus but instead wired to output and it whistles when touched because it might be wired backwards underneath heck I don't know. The screws on some trimmer caps are in same contact with one of the terminals speaking in general about trimmers. It doesn't look like its been hacked up on the bottom of the IF. But I think the problem is in the converter section. It doesn't have an RF amp but it has a three gang tuning cap. The oscillator is wired to the back one and the middle and front are RF. There is a coil on the side of the radio that has a tuneable slug and I think its the antenna adjust but it has no effect. It has a mica cap in parallel with it. There are three tuneable coil slug son top between the Converter tube and tuning cap. The two closest to the front aligns the AM RF lower band and the back one aligns the SW. Underneath there are three trimmers that connect to those coils and align the high band of AM and SW. All must track with the oscillator to give good performance. There are two coils in the back of the radio that align the oscillator. One for the AM and one for SW. There are also trimmer caps connected to those and must be aligned to oscillate at the correct frequency for the high frequency and the coils for low frequency back and forth until it tracks. I can take a working hand held radio with a digital readout and set it to 1170 and hold it close to the Stromberg Carlson and tune it to 710 and the handheld picks up the oscillator on the Stromberg Carlson then I take my generator and apply a strong 710 KC and I can hear my generator out of the Stromberg Carlson and peak the RF coils but it won't pick up 710 keel station just lots of noise. Could the Micas be bad? There are some rectangular caps on the bottom that look like micas but are covered with wax in the converter circuit


Top
 Profile  
 
 Post subject: Re: Stromberg Carlson
PostPosted: Sep Sat 18, 2021 3:19 pm 
Member

Joined: Apr Thu 09, 2020 7:33 am
Posts: 189
I worked on an FM tuner one time and I like to clean tube sockets with deoxit. It was working great until the deoxit got in the tuner circuit parts then it wouldn't pick up squat. I had to get my bottle of 91% spray alcohol and wash it off and dry it with a hair dryer then it was ok. The Stromberg Carlson was rusted bad and all I had was WD-40 to spray on the tuner cap and the tuning slugs so they can move again but AM is not that sensitive to cleaning sprays. I used it before on more modern AA5 line operated sets with no problem and they align perfectly. I noticed that the coils on this radio are larger than modern AA5s maybe that's why WD-40 has more effect on it. I will try washing it off with my alcohol and see what happens. Can a wax covered coil have moisture trapped in it and cause this problem? I have a heat gun I can carefully heat those up just until the wax melts a little and try to dry them out. They all have continuity.


Top
 Profile  
 
 Post subject: Re: Stromberg Carlson
PostPosted: Sep Sat 25, 2021 6:31 pm 
Member

Joined: Apr Thu 09, 2020 7:33 am
Posts: 189
I found the problem.
It was a repairman error. After recapping and aligning I was not aware everything was working fine ON THE RADIO after putting a make shift LOOP antenna on BOTH terminals it works great on all stations. The customer just brought me the chassis and I thought just putting ONE wire on the screw terminal would be good enough but it wasn't it has to have a closed loop antenna


Top
 Profile  
 
Post New Topic Post Reply  [ 6 posts ]  Moderators: Norm Leal, Marcc

All times are UTC [ DST ]


Who is online

Users browsing this forum: btcbar and 8 guests



Search for:
Jump to:  




































Privacy Policy :: Powered by phpBB