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]

Guys, Thanks for all  your help.  The schematic Kent posted is correct for my set. 

I tested the power transformer again: the dim bulb test passed, ran it up directly to 120 VAC with no tubes in it and it pulled a little over .5 amp, put the 2A3's in and ran it up to 120 VAC and the filaments glowed just fine.  Transformer is fine (thank you Norman).  I suspected the 5Z3 had a short but it didn't appear to be shorted.  The unit had been recapped (filters only) so I assumed they were fine but just for grins I put an ohmmeter across C202 and it was a dead short. The caps looked relatively new but this one is definitely my problem. 

Who would have thought that a recapped cap could be bad!  Another lesson learned.  Kent, I won't be needing that power transformer after all.  Thanks again and Merry Christmas.

Joe

[Kent King]

Glad you've got it going - almost a Christmas miracle!!

Kent

[Joe Miller]

I finished recapping the Amp/PS and powered it up just fine.  I connected the speaker (I thought I had an open FC) and found all good including the FC.  Just for grins I connected the tuner (it has been "partially recapped") and that baby plays like a million bucks.  Now here is my question: Someone has recapped some of the caps  in the tuner but none of the "bathtub" types.  Should I go through and recap them or are they good to go (I know there may be differing opinions on that).  The bathtubs look like they might be the oil filled type but not sure.

Recommendations please.

Thanks, Joe

[Norman S Braithwaite]

They are wax-paper.  I would replace them.  My preference is to melt the old guts out and install new capacitors in the case.  Beware that some of these are actually a combination of a choke between terminals and a capacitor from one terminal to the body of the capacitor.

Norman

[Joe Miller]

Do the guts come out okay with a heat gun, like the Philco tarry ones do?

[mike hadley]

Hi Joe,

The Bathtubs mounted on the rear wall backs are filters they consist of a choke and cap,

the backs are soldered on, once removed the filter networkcan be seen

The choke is on the right and the paper cap is on the left in the photo, be careful as the wire used in the choke is quite fine, a polypropylene axial cap will fit in the can quite nicely.

[Joe Miller]

Those son of a guns are riveted in. Looks like a big job.

[Norman S Braithwaite]

The filters are in low impedance cathode circuits hence modest leakage through the capacitors will not impact circuit performance to a significant degree.  If a filter capacitor in the unit is shorted, the set will still operate but bias will be lost in the local the stage causing some distortion and a warm tube.  There is no significant risk to vital components as a result of a shorted capacitor in the bathtub filter units.  Some collectors leave the originals in place.

Norman 

[mike hadley]

Joe;

Unfortunately all of the bathtubs are riveted in, as my aw15 was in such a mess when it came into my ownership I decided to restuff the bathtubs.

They all have soldered on backs and they are all potted in tar, there are two caps that I didnt remove as the audio interstage transformer is mounted on the other side of the chassis and would need to be removed to get at the rivets.

I used a small blob of silicone caulk to glue the new cap on to the bathtub and tried to make a neat job,

here is a photo of the underside of my chassis, the caps in question are to the right of the wave switch

Not ideal but tidy.

Mike

[Kent King]

I light of Norman's comment above (low-impedance cathode circuit filters) - I do not replace those 3 bathtub units in the AW15 sets unless the set is not operating well. I do replace other paper caps and the other bathtub units throughout the set, the leave the 3 on the back apron unless it can't be brought to playing spec. 

Kent

[Joe Miller]

Thanks guys, I'm going to do a deep dive on what has been done to mine before I get carried way. Will probably go Kent's way since the set is playing so good as is.

[David C. Poland]

I agree - leave those 3 apron mounted cap/coil units alone unless there is a good reason. Ditto for the previous 12 tube DELUXE. I have never had a reason for the several AW-12's and AW-15's over the years.