Zenith H723 FM Issue

[Knifeswitch]

This is my second 7HO4 chassis I’ve done. First one resurrected without too many issues. This one currently has me stumped. Radio tunes strongly until I get down to 94Mhz at which point the radio goes completely deaf. The local NPR is at 89.3 and usually the strongest signal in the area. I can just barely get it in and none of the other stations come in at all. My RF generator doesn’t even come through. Everything above 94Mhz comes in loud and clear. AT7 is brand new. Other tubes checked fine but I’m testing with a MightyMite and I know it’s lied before. Any ideas appreciated!

[jr_tech]

Are you 100% sure of the chassis number? A later chassis (7LO4) used in the same model radio used a 12DT8 rather than a 12AT7.

jr

[Knifeswitch]

Well it's ink stamped on back of the chassis and the Sam's matches up so as close to 100% as I might ever get. The set didn't wake up til I put the new 12AT7 in. Original Sylvania tested very weak.

[Knifeswitch]

Sorry, I should have posted this to the tech forum. Got it sorted out.

[zeno]

Keep in mind next time a shorting tuning cap will do the same thing.
When you tune lower the plates get closer & can short. Its a bigger problem
on cheap JA sets with small tuning caps.
Most Zeniths used PTO ( Permeability Tuned Oscilator ) for FM
and traditional tuning cap for AM. PTO tunes with coils instead, like
an old car radio. PTO is super mechanically stable but expensive... So
its only used on high end radios & where it is needed.
BTW they can be a bitch to align so leave that be.

73 Zeno
LFOD !

[Knifeswitch]

Yeah the PT was the first place I checked. Thought perhaps a broken core. Turned out the 22pF was open on L4. Bye bye bottom of the band. Thanks for the reply.