[Eric Adams]

Hi Guys,

It's time to pick everyone's brain again.

I have this  Zenith H725 with a problem. On AM, with full volume, barely able to hear any stations. FM nothing. I have replaced all caps (not micas). All resistors have been tested and replaced if out of specs. All tubes tested. I've also swapped them with known good ones. All connections double checked for accurate component replacement. On AM, I'm not concerned about FM for the moment, with my signal tracer I go as far as pin 6 on the 12BA6-the 2nd IF where it connects to the 3rd IF can. The signal stops there. I replaced the 3rd IF can-no change. I did find out that if I jump from where the signal stopped (pin 6 of 12BA6, 2nd IF) to pin 6 of the 12AU6 Limiter, I get great volume and all AM stations up and down the dial.

Any ideas where to look?

Thanks,

[Radioroslyn]

Your limiter is really working!!!! Limits everything!!!
Have to run to work but will have a look this evening.
Can you post a diagram?
Terry

[Joe Rossi]

Hi Eric,  Ay-yi-yi, too many pin 6's, too many 12BA6's!  I'm presuming that you are referring to pin 6 on the 1st IF 12BA6 which connects to the 2nd IF can at the .002 cap, because pin 6 of the 2nd IF 12BA6 connects to the 3rd IF can.  No matter, re-check your ground connections between the 1st IF and the 3rd IF, and the both 12BA6 tube sockets and surrounding wires for continuity.  Keep us informed!  Joe

[Eliot Ness]

....... I replaced the 2nd IF can-no change. I did find out that if I jump from where the signal stopped (pin 6 of 12BA6, 2nd IF) to pin 6 of the 12AU6 Limiter, I get great volume and all AM stations up and down the dial ....

Looks like you should be looking at T3, the third IF transformer.  You'd probably get similar results jumping from pin 6 of the second IF to pin 1 of the Limiter.

[Eric Adams]

OK, John. You're right. The signal stopped at the 12BA6, 2nd IF to the 3rd IF can, not the 2nd. I corrected my OP.

[Ron Ramirez]

Is T3 connected correctly?

AM signal is detected at pin 6 of the 19T8. The detected AM audio leaves T3 via the junction of R10 and R11, goes to the band switch, and from there to the volume control.

I am surprised you are getting AM audio at the screen grid (pin 6) of the 12AU6 limiter - unless I am missing something, I don't think you should be.

Or do you mean you are connecting a jumper between pin 6 of the 12AU6 and pin 6 (also screen grid) of the 12BA6 2nd IF? If so...then you have lost screen grid voltage to the 12BA6 2nd IF. You are in effect restoring that SG voltage to the 12BA6 when you connect the jumper.

The R1 in series with the SG of the 2nd IF now becomes suspect, as does C13, .002 uF. Make sure that R1 is not open, and that C13 is not shorted. Your SG should be connected to terminal 1 of T3, also.

Measure voltages, especially at the plate and SG of the 2nd IF tube.

[Eric Adams]

Thanks Ron. I will dig into it and let you know.

[Eric Adams]

Hey Ron

I don't know how I missed it! I thought I checked EVERY resistor. Ooops.  You called it. You said " The R1 in series with the SG of the 2nd IF now becomes suspect. That was it. It was open!

Thank you!  

[Ron Ramirez]

Great!

[Eric Adams]

Yup! Now all I have to do is get the FM to work. Just static. Aligned it, but nothing.