Scott AW 15 Finicky Oscillator

[Michael J Boessen]

Hi, All:

I've been working on a Scott AW 15, with a lot of help from folks here, and I really appreciate it!!

I have the cabinet looking awesome, most of the chrome polished up, and everything working right up to the High band, and there I am stuck. 

Based on a suggestion here, I ordered 3 NOS 56 tubes of 3 different brands: a Tung Sol, an RCA, and a NU.   I also have an Arcturus that tests about 70%.  I'm not concerned about the blue band.  The RCA seems to do the best across all of the other three bands.  Good sensitivity on white and red, and works on the green band  from 22 MHZ down to 13 MHZ.  I've been 3 days, on and off trying to get this band to work.  I have tinkered with the values of the resistor and capacitor on the grid of the oscillator.  I put on the 2 .05 caps on the filaments as someone here suggested and cut the cathode ground wire as short as I can make it, about 1 1/2 inch.  I have even tried tinkering with the lead dress, but have not had any success with that and have everything back to as original.  I have even tried 3 different 2A7 mixer tubes.

I am stumped.  Other than fooling with the windings on the oscillator coil of the band switch wheel, I don't see what else I can do.  If I were missing the high end of the band I wouldn't worry about it.  Nothing above 18, usually anyhow, but 10 to 13 mhz is pretty busy.

I can't imagine what anyone can suggest to fix this, but would sure appreciate any suggestions.  

Best

Mike

[Michael J Boessen]

Hi, David:

OMG.  That tube cover!  I've never seen anything like that.  Are you kidding?  NO air holes?  Sure seems like that would shorten tube life quite a bit. 

Yes, with the separate power and receiver chassis, you would either have to run your power leads in with the receiver cable, which would have to induce a little hum that the discriminating Scott engineers would not have liked, or you would have to have a separate power cable, which would be little different than the separate cable going to the current design switch.  I suspect, given what you have told me about how finicky the Scott guys were about having a "quiet" chassis, that this was the driving factor in the choice of switch location.

It is, without a doubt, an amazing radio.  Some of the design aspects, however, leave me a little at a loss.

Best

Mike

[David C. Poland]

The weighty tube cover does have a hole in the top sized to allow for a grid cap,... and ventilation.

But, it does not merely slip over the tube, depending on the tube shape and height. 

Rather, remove the tube, slip the lead weight up from the tube base, then lower the copper cove and screw the cover onto the lead weight. then insert the tube with cover back into the radio socket.

A taller tube leaves the weighted cover hanging in the air with the cover resting on the glass top, restricting ventilation.

The early tubes had low amplification factors, so microphonics were perhaps a greater problem.

Just an interesting artifact from early radio history.

[Lars Edwards]

Hello Mike,

Your comments are very interesting.  I've been working with an AW-12 Deluxe for some time now and it also has mediocre BCB reception.  This appears to be a later version and some of the circuitry is close to that of the AW-15.

Some of my latest attempts to resolve what seems to be oscillator performance are similar to yours. 

I obtained 4 new 56 tubes - so a total of 5 including the original that was in the set when I got it. 

Out of the 5 tubes 2 perform good on all 4 bands.

the 3rd tube performs on the BCB band but on the SW bands it cuts out out at about mid range going from low to high frequency.

The last 2 tubes perform on the BCB but on the SW bands especially the 2 higher frequency bands I get a lot of squealing.

So for now the radio is working on the BCB and the dial alignment is pretty much spot on across the band. However, performance on the BCB is not what I remember( the radio was owned by my aunt and uncle and gifted to me many years ago).  I have done all the checks for the antenna switch and it is ok.

One other observation is that during the IF alignment I had relatively sharp peaks except on the 1st IF.  That one is very broad and because of that I suspect there may still be a problem there.

On the SW bands the sensitivity seems to be good.

Any thoughts from you or additional ones from David or any other members of the group are certainly welcome !

Thanks

Lars

Michael J Boessen said:

Hi, All:

Got a new, matched pair coil wheel and tuning condenser from Kent King and installed them yesterday.  My worst fears were realized, as it made almost no difference.  Perhaps broadcast sensitivity is a trifle better and maybe gained a small amount at the low end of the red and green bands.  Now the oscillator dies on the red band when I tune down to about 5mhz.  The green band almost makes it down to 12 mhz.  Needless to say, this was depressing.

Since I have an extra coil wheel, I figured I had nothing to lose, so I decided to put the old one back on there and start fooling with the windings.  The old one worked almost exactly the same with the new tuning condenser as it did with the old one, so I left the new tuning condenser in there.  I will not fool with the trim tabs on it.  

 After poking around very gingerly at the 2 ends of the oscillator windings and making a small difference, I again decided I didn't have anything to lose so as carefully as I could, I broke about 3 primary and 3 secondary windings loose from the glue and just packed them together. I now am able to tune the entire band from 10.5 to 21 mhz. It's a little iffy trying to get 22, and sensitivity is extremely poor at the low end.  Also. dial linearity is out the window. I can put it on at the high end, but 11 tunes at about 11.5 and the padder doesn't change it much at all.

So I went to the 4.5 to 10 mhz band and pushed them around a little as well. Not near as much as on the higher band. I now have that band working end to end, and linearity isn't too bad. There is no padder for that band, but I can live with what I have there. Also, sensitivity is passable on that band.

I still agree this cannot be the correct solution, but it must indicate something.  

On the other problem of mediocre Broadcast sensitivity, I am also still mystified that I can open up the antenna switch on broadcast and it makes almost no difference in receiver sensitivity.  I had John Goller try it on his AW15, and he said the difference was a lot on his.  I replaced the Broadcast antenna coil with a new one from the same set as the coil wheel and tuning condenser, and it made no difference.  I have chased every wire and replaced every component from the antenna to the second IF stage.  With the slight increase in Broadcast performance I get with the new parts, I guess I am going to let it out the door with that problem.  It is passable, but certainly not what I think an AW15 should be capable of.  

Still need inspiration on the shortwave oscillator problem.  I don't expect you guys to have any better ideas.  I realize this is a very strange problem, and I'll just have to keep fooling with stuff.

Appreciate the help from everyone.  Sorry this has been such a protracted thread.   I've done hundreds of radios.  This one is the worst.

Best

Mike

[Michael J Boessen]

Hi, Lars:

There are smarter guys on here than me.  I presume you have replaced the caps in the IF coil cans.  The resistors in those coil cans on my set were pretty far out of spec as well.  Also, I'm guessing you know you need a tuning can to put in place of the chrome one while tuning.  You have to have that smaller bottom cover in place for all but maybe one of the bottom trimmers.  I believe I recall that the IF tuning was a little soft on one of my IF trimmers as well, but don't recall which one it was.  Concerning the birdies, I had a few of those as well, but don't remember what the solution to that was.  I never found a 56 that would work clear across all the bands.  I finally made an adapter for a 6AB4.  I had to change the grid biasing components quite a bit to make it work due to the much higher gain and wider bandwidth of the 6AB4.

Concerning the broadcast sensitivity, they warn the user in the operating instructions not to put more than 70 feet of antenna on the set, which I thought was strange.  Usually, longer is better, up to about 140 feet, I think.  Anyhow, I went outside my shop and strung up 30 feet to a nearby tree, and the darn thing actually worked better.  Maybe you have too much antenna?  That said, I never did get it to perform as well as I thought it should on broadcast.  I have done lots of 5 and 6 tubers that worked better on my 70 foot antenna.

That's all I have, but, again, there are smarter guys on here than me.  Maybe they will have some ideas.

Best

Mike

[Lars Edwards]

Mike

Thank you for the suggestions and information.

I have replaced the resistors in the IF cans due to values that were up to 50% off. Paper capacitors were also replaced.

I recapped the entire unit except for the mica capacitors.

In order to make the IF alignment easier I used a small chassis punch and put 1/2" holes in the tops of the IF cans and will put metal "plugs" in the holes after finishing alignment.

On another forum I ran across some info about checking all the solder joints.  I did find one that was bad in the antenna coil assembly and that helped significantly.  I also resoldered all the connections around the 1st IF and that also helped.  As long as I was working on it I reflowed all the connections on all the tube sockets.  Again, a small improvement.

As of late last night the performance on the BCB is very good. I'm located in north Alabama and was listening to stations in Chicago, Atlanta, St. Louis, Cleveland, Pittsburgh and Nashville to mention a few. Today I am receiving all the local stations plus Nashville (WSM) which I could not get previously.

The antenna that I am using right now is my ham radio multiband vertical ( height is about 24 feet) with a ground plane of about 24 radials. It has coax feedline about 100 ft long.  This is probably not ideal but seems to be working very good.

Now that the radio is working much better I'll work on getting an antenna in the range of 70' total as well as having a good ground.

I've seen references to doing the IF alignment with the oscillator tube removed and I may give that a try to see if makes much if any difference.  The Scott instructions do not mention that.

One more thing that I have discovered now that the radio is performing much better is that the sensitivity switch is a bit flaky on the most sensitive position. It take a bit of fiddling to get it set.  All other positions are good.

I'd like to get your opinion on whether you think that adding the capacitors to the oscillator filaments will help out.  It is hard for me to imaging that out of 5 oscillator tubes only 2 are satisfactory.

Again, thanks for your advice and help.

Lars

[Michael J Boessen]

Hi, Lars:

I'm not sure what the caps on the oscillator filament are supposed to be for.  One of the things the Scott guys were very focused on was to have a very quiet receiver, IE, crank up the volume with no signal, and hear nothing in the speaker.  I have to say, my AW15 project was indeed the quietest receiver I've ever encountered.  By way of speculation, the caps may have been to shunt any RF that might be picked up on the filament leads and superimposed on the oscillator waveform.  I did put them on.  They didn't change the problem of the oscillator dying at the low end of the upper 2 bands at all.  Caps are cheap.  Why not put them on there?

Glad you got your receiver working better on BC.  If  your experience is the same as mine, the 70 foot long wire will underwhelm.  Mine did better with 35 feet than 70 feet.

Best

Mike

Lars Edwards said:

Mike

Thank you for the suggestions and information.

I have replaced the resistors in the IF cans due to values that were up to 50% off. Paper capacitors were also replaced.

I recapped the entire unit except for the mica capacitors.

In order to make the IF alignment easier I used a small chassis punch and put 1/2" holes in the tops of the IF cans and will put metal "plugs" in the holes after finishing alignment.

On another forum I ran across some info about checking all the solder joints.  I did find one that was bad in the antenna coil assembly and that helped significantly.  I also resoldered all the connections around the 1st IF and that also helped.  As long as I was working on it I reflowed all the connections on all the tube sockets.  Again, a small improvement.

As of late last night the performance on the BCB is very good. I'm located in north Alabama and was listening to stations in Chicago, Atlanta, St. Louis, Cleveland, Pittsburgh and Nashville to mention a few. Today I am receiving all the local stations plus Nashville (WSM) which I could not get previously.

The antenna that I am using right now is my ham radio multiband vertical ( height is about 24 feet) with a ground plane of about 24 radials. It has coax feedline about 100 ft long.  This is probably not ideal but seems to be working very good.

Now that the radio is working much better I'll work on getting an antenna in the range of 70' total as well as having a good ground.

I've seen references to doing the IF alignment with the oscillator tube removed and I may give that a try to see if makes much if any difference.  The Scott instructions do not mention that.

One more thing that I have discovered now that the radio is performing much better is that the sensitivity switch is a bit flaky on the most sensitive position. It take a bit of fiddling to get it set.  All other positions are good.

I'd like to get your opinion on whether you think that adding the capacitors to the oscillator filaments will help out.  It is hard for me to imaging that out of 5 oscillator tubes only 2 are satisfactory.

Again, thanks for your advice and help.

Lars

[David C. Poland]

FYI - check out the Scott Super antenna - first version first offered for the AW-15 and also the AW-23.

See   Info Archive, Set Folders, Accessories, Super Antenna System

- a double di-pole with 110 foot twisted lead in - to a chrome plated outboard antenna switch hung on the middle rear tube shield.  The 2 position switch 1) for BC and low SW and 2) for the other higher SW bands. The switch had a coil inside to alter the antenna performance. IMG_0867.jpeg  Has to be in correct position for the band selected.

I made a repo of this antenna years ago, and used it for several years. an original: Super%20Antenna%20-%20early%20double%20doublet%201935.jpg

Later versions super antennas had a transformer at the antenna wire conjunction, for use in later Scott  models having two antenna posts beginnig with the late 7 knob AW-23 and forward. 

[Lars Edwards]

David,

Thank you very much for the information !

[Lars Edwards]

Mike,

Thanks for the input.

In regard to how quiet the receiver is I do have a high level of  noise external to the receiver here.  Based on my checking this RF noise is due in large part to the new power meters that the local utility installed.

There are also a couple of noisy transformers within about 1/4 mile.

I'm guessing that there is also significant line noise due to these items.

I believe that is why the best antenna here right now is the vertical with the coax and very good ground plane.

I am going to go through the receiver and clean all the tube sockets and power supply/amplifier and speaker sockets just to make sure that this is not overlooked.

Best Regards

Lars

[Michael J Boessen]

Hi, Lars:

One of the worst things for antique radios is LED lighting.  The biggest offender I have seen are the LED replacements for fluorescent tubes, but all LED lights are possible sources.  Anything that has a power supply with a switching regulator (IE about any electronic device) is a likely problem as well.  I have had my cell phone charger interfere with some radios.

Cleaning the sockets and pins is never a bad idea, but I'm not optimistic that it will help much, if at all. 

Good luck!

Best

Mike 

Lars Edwards said:

Mike,

Thanks for the input.

In regard to how quiet the receiver is I do have a high level of  noise external to the receiver here.  Based on my checking this RF noise is due in large part to the new power meters that the local utility installed.

There are also a couple of noisy transformers within about 1/4 mile.

I'm guessing that there is also significant line noise due to these items.

I believe that is why the best antenna here right now is the vertical with the coax and very good ground plane.

I am going to go through the receiver and clean all the tube sockets and power supply/amplifier and speaker sockets just to make sure that this is not overlooked.

Best Regards

Lars

[Lars Edwards]

Mike and Dave,

Now that the receiver sensitivity is quite good I'd like to be using it for more extended listening times.  During the testing and alignment the listening times were relatively short after the set was warmed up good.

As received this radio had one 45 and one 2A3 in the amplifier.  There was another 45 tube in a box but it does not work right and has tested gassy. They must have had a 45 go bad and they replaced it with a 2A3.

These tubes are relatively expensive so I'd like to get your take on whether to get another 45 to replace the 2A3 or go ahead and get a couple of 1619's and make the converters to use these in place of the 45's.

Again, thanks for your advice.

Best Regards

Lars

[Michael J Boessen]

Hi, Lars:

Well, this might be better addressed by someone smarter. On the upside, the 1619 has lower filament current and higher plate voltage rating.  It has higher plate current, so might tax your power transformer and audio transformers.  Plate load of the 2A3 is 2500, vs 1500 for the 1619, so your audio transformer won't be a good match.  Concerning gain, harmonic distortion, etc, these are over my head.  If that is what you are after, I guess there might be some upside.  The AW15 I did was PLENTY loud.  I never turned it up past 1/4.  Not sure about the 45 business.  Mine has 2A3 outputs.

That's all I got

Best

Mike