Zenith T825 FM Alignment

2/24/2008 1:49:39 PMMark Johansson

Hello again! So I got my T825 working on FM, thanks to Bill G., And now I'm trying to get it aligned. I'm using a Heathkit FMO-1 FM test oscillator, Oscilloscope & homemade phase shift circuit for line sweep (from the Heath manual). Here's a pic of my setup:

Aligning the IF stages went well:

My problem seems to be with the Discriminator, I can't seem to get a nice symmetrical waveform. This is the best I can get with the 10.7 marker pip right where the waveform crosses the baseline:

Since I replaced the caps in the discriminator can, I'm wondering if I did something wrong in there, though I did double check my wiring & it all looks OK. Any ideas?

Thanks, Mark.

2/26/2008 4:38:18 PMBill G.

Hi Mark,
That is a nifty set up. I have been trying to do that, but my WWII veteran FM signal generator doesn't have an audio output. By the way I do not recommend using vintage test equipment. I could have restored three radios with the effort I put into getting that piece of junk going.

I have aligned several radios and recorded the reading when an unmodulated carrier was tuned from 9.7 MHz to 11.1 MHz. The negative peak was -0.808, the positive 0.547. I had some asymmetry, but not like your scope is showing.
Have you injected the signal at the grid of the limiter? If not, try that.

Best Regards,

Bill Grimm

3/10/2008 3:30:56 PMEdd

Different alignment hook ups and equipment give different effects, so I can’t come up with a given.

So what info I am needing to know is in the referencing of the the S curve that you have shown and since I have no markers pips for referencing, tell me if the left small + node at the top is lower than 10.7 or higher than 10.7 and I will then clue you in as to what steps to take next.

73's de Edd

:Hi Mark,
: That is a nifty set up. I have been trying to do that, but my WWII veteran FM signal generator doesn't have an audio output. By the way I do not recommend using vintage test equipment. I could have restored three radios with the effort I put into getting that piece of junk going.
:
: I have aligned several radios and recorded the reading when an unmodulated carrier was tuned from 9.7 MHz to 11.1 MHz. The negative peak was -0.808, the positive 0.547. I had some asymmetry, but not like your scope is showing.
: Have you injected the signal at the grid of the limiter? If not, try that.
:
:Best Regards,
:
:Bill Grimm