AW 15 Amp Problem

[Joe Miller]

I just started on an AW15 restoration and found that my Amp is not wired per the schematic that I am using from EH Scott Google Docs: https://drive.google.com/drive/u/0/folders/0B0K3TBSz_27rNjMzYzk3Zjc...  

On this schematic item 22 is noted as a "No. 2496 Audio Choke" but is shown as an interstage transformer with split windings on the primary and secondary.  The secondary measures 266 ohms and 268 ohms measured separately to ground.  I only see 4 leads coming out of #22, two of which are tied to ground, the other 2 connected to the to the 500 ohm resistors (#5, #8).

When attempting to measure the primary from pins 5 and 6 on the Jones connector I measure 57K ohms.  When I try to measure each section of the primary from pin 4 to 5 and pin 4 to 6, I show an open.  Further investigation reveals that pins 5 and 6 are connected to the 28k resistors rather than connected to the interstage transformer.  That is why I am measuring 57K for the primary (pins 5 and 6 are not really connected to the primary). So I am guessing that my schematic is wrong.  

Is there another schematic for this amp or am I missing something?  Also, not sure what the 28K ohm resistors are doing.

Any assistance would be appreciated.

Thanks,

Joe Miller

PS: I may have a bad power transformer too as it measures less than 5 ohms

[Joe Miller]

New development....Hooked everything up and enjoyed listening to it for about an hour.  Took it apart and put in the cabinet and turned it back on and it played great.  Was just about ready to slide it up to the wall and it blew the fuse.  I took the amp out and sure enough the fuse was open.  Last time it was doing this I found C202 shorted even though it had been recapped. Well I found it had happened again to the new cap I had just installed, but it did take some continuous playing to get it to short.  What voltage should this set run at the first cap off the rectifier?  I think the voltage is a bit too high and over time the filter cap breaks down.  I also noticed that I had left the 110/120 volt switch in the worst position (i.e. fewest primary turns resulting in the higher secondary voltage.)  So, I will make sure it stays in the right position.  Will probably put 2 20uf caps @ 450V in series to gain some higher rating for that cap position.  Thoughts?

Joe

[Kent King]

Merry Christmas!

Well, the setting of the hi/lo switch is very important these days as line voltages are higher. Many times the hi/lo switch is bad and I just wire the set to always be in "hi" and leave it at that. Since it blew on startup, it is probably the initial voltage surge that is blowing the cap. I know some folks here advocate for and install varistors on the line, which eliminates the inrush surge voltages. Your approach, or maybe buying a 500v or higher filer for C20 would also be reasonable. Today's line voltages are notably higher than the 1930s, so we do have to take this into consideration.

Kent

[Alex R. Whitaker]

I would put the two 20 mfd @ 450V caps in series, with equalizing resistors across each, to keep this from happening in the future.  Also, what Kent said about wiring the hi/low switch permanently in "high" is a great idea.  I think about every town/city has over 120 VAC at the outlet.  Not much need for the high/low switch anymore. 

Joe Miller said:

New development....Hooked everything up and enjoyed listening to it for about an hour.  Took it apart and put in the cabinet and turned it back on and it played great.  Was just about ready to slide it up to the wall and it blew the fuse.  I took the amp out and sure enough the fuse was open.  Last time it was doing this I found C202 shorted even though it had been recapped. Well I found it had happened again to the new cap I had just installed, but it did take some continuous playing to get it to short.  What voltage should this set run at the first cap off the rectifier?  I think the voltage is a bit too high and over time the filter cap breaks down.  I also noticed that I had left the 110/120 volt switch in the worst position (i.e. fewest primary turns resulting in the higher secondary voltage.)  So, I will make sure it stays in the right position.  Will probably put 2 20uf caps @ 450V in series to gain some higher rating for that cap position.  Thoughts?

Joe

[Alex R. Whitaker]

BTW, Merry Christmas, all.  Enjoy your Christmas lights twinkling off of the chrome of your Scotts!

[Joe Miller]

Hope everyone had a good Christmas.  

I am still messing around with my AW15 and have 2 remaining questions:

1. My tuning meter was working perfectly, but is now pegged to the right. I may have overdriven it when the voltage was high.  I replaced the 56 tube and it is still pegged.  I see that there is an adjustment on the 56 tube cathode.  I will see if I can adjust it to fix the meter swing, but first I have to find the adjustment.  Anyone know where it is located?  I hope I don't need to take the chassis out of the cabinet to get to it.

2. My right hand side control is a three-position switch.  The escutcheon says sensitivity and shows 5 positions.

Schematic says it is a Static Switch.  It acts like a sensitivity switch though.  Position 3 has most gain, when moving to position 2 it goes nearly silent and then gradually increases in volume but at lower gain.  When moved to position 1 I lose audio.  So what do I have here, Static or Sensitivity switch?  Why no audio in position one?

Thanks for your help,

Joe    

[David C. Poland]

Joe - Consider - Scott radios were built in Chicago, home of several 50,000 watt AM stations, not to mention WLW Cincinnati broadcasting "experimental" at 500,000 watts at night. Stories in Ohio of nighttime WLW playing on local fence wire and folks tooth fillings and coast to coast reception. But many stations were much lower power like 5000 down to 250 watts. While here in Columbus, our stations were 5000 watts or less. Scott radios were very sensitive and designed to pick up weak, distant stations "ordinary'"radios would miss. That is a tremendous range of station strength/weakness to contend with.

And -  Scott was selling his optional Super Antenna, a double di-pole with twin lead in to that antenna switch often seen on hung on the rear middle tube shield - to further improve reception.

My opinion is that the single AVC circuit on the AW-15 alone couldn't deal with strong local stations, given the overall sensitivity of your Scott when using a good outside antenna. Hence the Static (Sensitivity) control to reduce over all sensitivity of the radio for strong stations - to maintain clear enjoyable audio.

The 1937 Philharmonic had AGC at both the IF stage and the RF stages for that reason. The AW-15's static control position was indeed a sensitivity control, with several positions to even out reception. Needed to throttle back the gain to a level to avoid over driving the 4 stage iF and 3 stage audio amps into distortion on strong stations. 

So if you lack a near-by clear channel 50,000 watt station, then position 1 silence seems proper for you. Here in central Ohio I run my AW-15 in static 3 position, and can only get a few stations in position 2 at night unless I use my tunable loop for AM. A really good outside antenna would probably provide good reception in position 2.

As for pegging the meter, is it stuck, or just non functional. Does it swing at all when powering up the radio?

[Joe Miller]

Hi David,

Thanks for the info.  I think my switch is working fine then.

I think the tuning meter is maybe stuck.  It was moving just fine earlier on startup and was sensitive to signal strength.  I think I switched the static switch from 3 to 2 when on a strong station and it may have pegged it.  Is there an easy way to "unstick" it?  Also is there a tuning meter adjustment pot somewhere on the chassis.  One shows on the schematic.

Joe

[David C. Poland]

Is a really lightweight movement.

Suggest you dismount the meter so you can carefully inspect it, and make a continuity check as well.

[Joe Miller]

will do, thanks.

[David C. Poland]

My experience, AW-15 meters action is not particularly sensitive, and need a decent signal to register any movement. In contrast, the AW-12 movement seemed more sensitive weaker signals. 

I have had several examples of both models. Meter circuits vary with these models.