Help needed

[douglas wright]

Working on all wave 15, no sound or reception. Speaker rebuilt, everything good on speaker.
Looking at schematic I see primary field coil only has 5 volts going to it, is this correct? all tubes are good.
I am used to radios with higher b plus going to the field coil.
What am I missing?

[David C. Poland]

Even with the higher line voltage elevating the B+, 470 volts is a bit much. Initial surge likely even higher hitting the 1st Filter cap ahead of the 30 Henry choke the hardest.

You figure why no hi-lo switch? do you have power transformer with tap for 110 and 120 volts? if so, use the higher one to reduce B+ voltage ratio step up (meaning: the tube filaments cools down a bit and dial lamp dims a bit). The Hi position was intended for line voltages from 115 to 130 back in the day when line voltages were more variable.

Or do you have a replacement power transformer that is not quite right and needs some mod to tame the high voltage winding?

Scott Tech Service specs say for test bench AC supply set to 115 Volts and hi-lo at HI. Should produce 330 plate volts at the 2A3 plate. and about 190 volts on the plates of the IF amp tubes.

A 5Z3 is a robust tube - (a 5U4 is basically a 5Z3 on an octal base.) Should not be failing. 

If you have a Variac, use it to attain the proper B+ voltages. Your new 1st filter cap failing? using new 10 MFD filters caps? 450 volt rated filter caps should be sufficient with proper power transformer.

 

[David C. Poland]

How about a photo of you amp.

[douglas wright]

My transformer is tapped, 115 or 120.
Not sure how to post photo on reply.
My transformer looks original

[douglas wright]

All electrolytic caps are new

[douglas wright]

Kent, 

The tube will start arcing, then the filaments will go out.

Test the tube and tube is dead.

I think I measured 5.2 volts at 5z3 filaments.

[douglas wright]

17113359616581633919601195299621.jpg

[douglas wright]

douglas wright said:

My transformer is tapped, 115 or 120.
Not sure how to post photo on reply.
My transformer looks original.

Photo of amp in link further down in the replies.

[David C. Poland]

If you have not tried a new 5Z3, do so. An old tube may be suspect. Did it test ok earlier.

You have a proper size fuse? fuse did not blow?

Also, was the amp right side up when you powered it?

If on its side, orient the amp so pins 1 & 4 are horizontal per the tube manual, lest the hot filament droop and short to plate.

For a photo, click on the paper clip above this text box. select the photo from your computer to upload to this thread.

[David C. Poland]

At 11:38pm on March 25, 2024, David C. Poland said…

Again, if the hi=lo switch is missing, use a meter to determine which primary post is for the higher mains voltage. you should have 3 lugs for the power transformer primary.  Probably can't tell with resistance measurement of the primary.

photo   MG_1056.jpeg   of my AW-15 transformer primary lugs for reference. Notice the partly obscured 110 and 120 labeling.

So REMOVE the rectifier tube. Then move the power cord connection  between the two primary lugs and see which give you the LOWER voltage measured between the high voltage center tap and one of the tube socket rectifier plates. use the 500 volt AC scale.  Maybe put one hand in your pocket for safety when you power on. You are seeking the one that shows a LOWER high voltage.

While you are at it, double the measured voltage to see about what your first filter cap is seeing as a full wave output.  CAUTION: With the rectifier tube inserted, the speaker and receiver absolutely need to be plugged into the amp to draw down the working high voltage somewhat. 

If you are inadvertently feeding 125 volts AC into the "lo' side, the B+ voltage will be way high.