westinghou: Help with AM bleed through when switched to phono

I recently restored this Westinghouse H-458T6.  I was able to bring it up on my variac when I first put it on the bench.  It played on AM and on the phono input with my phone.  I then disassembled and found that the main can cap had been replaced at some point.  I thought oh, maybe I will not need to replace that.  But after checking found it to be bad.  So I proceeded to replace it and all the paper/wax caps. Checked all the voltages and everything appears to be within tolerance. I restored the cabinet and fixed some problem areas in the speaker cone.  Bench tested it with the phono input and it sounds pretty good!

So time to put it back together.  Unfortunately when I hooked up the antenna, I noticed that the AM was still on when switched to phono input.  Can anyone provide some suggestions for what might cause this?  If this is normal, is there anything that can be done to eliminate it?  I was considering adding a switch to the antenna, but I was hoping to get some advise from someone here before I make any modifications like that..

Thanks! James

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A common reason for "playthrough" is problem with the holder for the detector/AF tube (6AQ6 in this radio). For example creepage currents between neighbor pins as can happen with overheated or dirty paper bakelite holders, or bad contact at the cathode pin. Check what happens if you wiggle the 6AQ6 while listening. Usually, playthrough of those reasons will be evident even at minimum volume.

Another possibility could be some wiring error, creepage or other problem around the phono/radio switch. Check that the wiring corresponds to the schematic.

A third possibility could be coupling through the B+ rail, i see from the schematic that it has no RF decoupling at all, just an electrolytic capacitor C14C.  Try if the situation improves by adding decoupling with for example 100 nF plastic foil capacitor from B+ to chassis, located near the positive end of R3 or R7.

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Hi Torbjörn,

 

Thank you for the reply and suggestions.  

 

• I checked the 6AQ6 for any change when I wiggled it and did not observe any change.  I removed it and cleaned the pins and socket again, still no change.

 

• I am confident that I was careful to replace the capacitors exactly as they were originally.  I realize that its possible someone made a change at some point, but everything looked original other than the can capacitor which had been changed.  Of course its unlikely, but possible that it had a wiring error all a long from the factory. I have attempted to check what I see in the radio against what is shown on the schematic.  I am still learning, and this is an area I lack confidence in that I have it right. I have attached a photo of the radio, with the 3 capacitors in this area marked.  I also attached the schematic with my mark up of what I think I see in the radio.  It looks like it is correct, but would appreciate if you could check this and give me your feedback.

• I also marked on the schematic the R3 and R7 resistors you mentioned I might need to add a 100nF to the chassis.  Would you suggest I go from R3 to chassis ground has I have marked on the schematic?   Again, appreciate your feedback on this, as I want to be sure I understand it correctly..

Best regards,

James

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Hi James and Torbjörn,

sorry for jumping in, but there's another possibility. As I understand the operation of the phono switch, its two right prongs connect the front end (lower screened lead) to the volume pot (upper screened lead) in radio position. A 45 degrees turn will connect phono at the upper prong to the pot, while the radio signal is shortened to ground via the lower prong. If that contact is bad this might be the cause for crosstalk.

Good luck,

Steffen

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Thanks for the input.  I was suspect of the switch as a possible cause of the issue.  I did do a thorough  cleaning of the switch, but still have the issue. Maybe the switch is bad? I hope that is not the issue, as I see no easy fix for that. 

I will try Torbjorn suggestion to add the 100nF cap.  I just want to be sure about where I should connect it.  

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Hi Torbjörn,

I finally got back to this project after some business travel. I tried the 100nF capacitor on both R7 and R3. No difference in the amount of bleed through from the station when switched to phono.

 

Is there anything else you suggest I might check?  I have not tested the 6AQ6 tube, could a bad tube cause this?          

Here is a link to the photo with the new capacitors around the switch marked.

My last resort will be to put a switch on the antenna.  When the antenna is disconnected, I have no noise when switched to phono.

Thanks,

James

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