Local oscillator

2/27/2002 8:44:22 PMSteve

One of my many hobbies (like you) is restoring antique radios & things. I've several radios in my collection but there are 2 I am having difficulty with.

One is a 1938 Truetone by Western Auto.
I've had it for perhaps 15 years, the 1st of my collection.
It is 3 bands: AM, Middle and SW.
AM works great and is quite sensitive.
SW is very sensitive in the middle of the dial.
Middle is weak in a tiny center portion.

I have recapped the chassis, that fixed the supply hum, avc and other things.
I have the schematic and checked voltages but the SW and middle wave sensitivity is still a problem.

I've aligned the set using the manual.

I have discovered the local oscillator output amplitude drops on both ends of the tuning dial.
AM slightly,
SW output is zero at both ends,
Middle sharply peaks in the middle then falls off.
Osc output would look like a bell on a chart.

I'm using my scope to check these.
I've also checked osc coils for opens/shorts.

I've replaced the caps again in the osc circuit but the problem remains and I am at a loss.

My 2nd question is about my 1927 Stewart Warner TRF AM receiver.
The 3 tuning caps & drive were a chore to rebuld.
Now the front end is aligned and works geat, too bad the output transformer secondary is open, thus no output without an external amp.
Any ideas how to repair-replace this transformer and keep the set looking original?
The original is riveted onto the breadboard with specialized large hollow rivets (To make it difficult)

For me, original appearance is everything, as most of my other sets have new caps inside old ones, etc

Thanks for your time and any ideas!
Steve

2/27/2002 10:56:46 PMNorm Leal

Hi Steve

I'll get started...

Was your radio aligned at the high end of the band? Padder adjusted at the low end? Sensitivity shouldn't be peaked in the middle. You should have separate trimmers for shortwave.

Did you align the IF Transformers with a calibrated signal generator? If the IF frequency is off you will not be able to align the dial.

The oscillator signal strength will vary as you tune but this shouldn't have an effect on sensitivity.

What Model is your Stewart Warner? Most had audio interstage transformers but not output. Small audio interstage transformers are available as well as replacement coils for 1/2" x 1/2" metal cores.

Any chance of mounting a modern output transformer under the chassis? Most early radios had high impedance output for a speaker. Can you take output direct from the tube plate?

Norm

: One of my many hobbies (like you) is restoring antique radios & things. I've several radios in my collection but there are 2 I am having difficulty with.

: One is a 1938 Truetone by Western Auto.
: I've had it for perhaps 15 years, the 1st of my collection.
: It is 3 bands: AM, Middle and SW.
: AM works great and is quite sensitive.
: SW is very sensitive in the middle of the dial.
: Middle is weak in a tiny center portion.

: I have recapped the chassis, that fixed the supply hum, avc and other things.
: I have the schematic and checked voltages but the SW and middle wave sensitivity is still a problem.

: I've aligned the set using the manual.

: I have discovered the local oscillator output amplitude drops on both ends of the tuning dial.
: AM slightly,
: SW output is zero at both ends,
: Middle sharply peaks in the middle then falls off.
: Osc output would look like a bell on a chart.

: I'm using my scope to check these.
: I've also checked osc coils for opens/shorts.

: I've replaced the caps again in the osc circuit but the problem remains and I am at a loss.

:
: My 2nd question is about my 1927 Stewart Warner TRF AM receiver.
: The 3 tuning caps & drive were a chore to rebuld.
: Now the front end is aligned and works geat, too bad the output transformer secondary is open, thus no output without an external amp.
: Any ideas how to repair-replace this transformer and keep the set looking original?
: The original is riveted onto the breadboard with specialized large hollow rivets (To make it difficult)

: For me, original appearance is everything, as most of my other sets have new caps inside old ones, etc

: Thanks for your time and any ideas!
: Steve

3/25/2002 8:42:28 PMSteve

Norm:
Sorry it took so long for me to return to this board.
The chassis model number is D 699.
Each band antenna impedence adjustment has its own trimmer, as does each local oscillator.

The last time I worked on this chassis was perhaps a year ago.
I use the set once in a while to show friends.

These are some of the alignment procedures included on the schematic:

IF alignment:
ext osc @ 465Khz in series with dummy ant, applied to the ctl gred of the 6K7 and adjust the output xfrmr.
Move ext osc to grid of 6ABG & adj input xfrmr.

AM was 1st adjusted to 1720Khz, then
1550Khz, then600KC.

SW band is adj to 17Mhz, then 6Mc

Middle is adj to 5000Khz, then 1800Khz.

The ckt is arranged with 6A8 as the mixer, 6K7 as the IF amp and 6Q7 as the detector.

AM works fine, only a little less sensitive on the ends.

SW is dead at the very ends and peaks in the middle.

Middle wave is dead all across the dial except for a small spot in the center.

The output of the local osc taken at the grid of 6A8 while in the Middle wave is zero except for the center.

SW is not so bad, but is zero at the ends.
I suspect my testing equipment is out of alignment, so I
plan to do the equipment alignment and then try to
repair this radio again.
It is a console radio I love, if you want you can see it here:

http://personal.atl.bellsouth.net/atl/s/t/stevedez/antiques/true.html

Thanks again for the help
Steve

3/25/2002 8:44:28 PMSteve