Guys:

Was working on a Zenith 5D811 table radio - an All American Five.

I tuned the RF front end trimmer on the variable cap for max RF received at the high end of the dial (high frequency).

When I put the antenna back into the radio case, it appears that the metal of the chassis actually DETUNED the antenna a little due to proximity effect - actually enough to notice and cause me to retweak the RF trimmer with the antenna close to chassis.

Is this normal? I have never seen this happen before.

Lou

Yes, this is normal. Any good-sized piece of steel near the antenna loop will change its inductance, and therefore its resonance point.

:Yes, this is normal. Any good-sized piece of steel near the antenna loop will change its inductance, and therefore its resonance point.
:So the vendor should have made provision for tuning the antenna in the cabinet?? Or do the service techs just say " its good enough". It's not a HUGE detuning effect but it is noticeable.

Usually the loop antenna was attached to the fiberboard back of the radio, not the cabinet. So for most small sets it would have been possible to remove both the chassis and antenna from the cabinet and then place the antenna in its "normal" position with respect to the chassis before the final dial calibration steps were done.

:Usually the loop antenna was attached to the fiberboard back of the radio, not the cabinet. So for most small sets it would have been possible to remove both the chassis and antenna from the cabinet and then place the antenna in its "normal" position with respect to the chassis before the final dial calibration steps were done.

:Thanks - not the case here . The antenna and chassis are separate except for the interconnecting wires. So I have to use the spacing between the chassis and the antenna as a guide for the final RF tweakup.

Lou

Right- but once the chassis and antenna are removed from the case, you have full access to the tuning padder(s). It's just necessary to remember to position the antenna near the chassis (as it is when buttoned up in the case) when doing the dial alignment.

Just a long shot---Make sure you didn't reverse your antenna wires when you put it back on. It makes a difference. I had an untouched original set that wasn't performing well, no matter how I tried to tune the alignment...turns out it was reversed antenna leads from the factory.

Interesting....

Well when I did the initial; alignment, I did not pay attention to the position of the antenna.

Also since there are only two connections to the antenna I wonder why polarity makes a difference? Since the wires were resoldered by someone else I have no idea if they are correct - but perhaps the schematic can tell me.

Lou

:Just a long shot---Make sure you didn't reverse your antenna wires when you put it back on. It makes a difference. I had an untouched original set that wasn't performing well, no matter how I tried to tune the alignment...turns out it was reversed antenna leads from the factory.
:

It's the inside winding that goes to the tuner. The outside winding may go to AVC line.

:It's the inside winding that goes to the tuner. The outside winding may go to AVC line.
:
:
Thanks Warren

I just checked the set wiring and the trimmer on the top of the RF tuning cap is wired to the inside of the antenna - and the outside is wired to the stator I believe.

Does it really make a difference? I am curious.

Lou

::It's the inside winding that goes to the tuner. The outside winding may go to AVC line.
::
::
:Thanks Warren
:
:I just checked the set wiring and the trimmer on the top of the RF tuning cap is wired to the inside of the antenna - and the outside is wired to the stator I believe.
:
:Does it really make a difference? I am curious.
:
:Lou
:
Hi Lou,
C4 on the schematic is in the AVC circuit. It should get the outside of the loop.

Best regards,

Bill Grimm