Tempo One Oscillates On 15m

[hamguy]

I recently acquired a Yaesu/Henry Radio Tempo One. It is a very clean example with an almost mint exterior and a very clean interior.

It required an alignment so I went through the procedure outlined in the service manual. (I have another Tempo One, not as nice, so I am already familiar with the procedure.)

Everything came up to snuff ... except the 15m band which likes to self oscillate. I've poked around with it for a while now and cannot find the cause. If I reduce the drive with the drive tuning control, the oscillation may stop, but at the proper tuning setting, it oscillates (at a 15m frequency). All the other bands operate correctly. I don't know the provenance of this radio, but I did check to see if the 15m circuitry was wired correctly.

Has anyone else seen this? Anyone have a suggestion? I'm stumped (which doesn't happen to often...).

[N8YX]

Has anyone else seen this? Anyone have a suggestion? I'm stumped (which doesn't happen to often...).

Neutralization.

Carefully compare the output circuit (and the tubes used) to that of an FT-101 (pre-Z models). I think they used 6JS6s in both PAs.

If someone fitted U.S.-made tubes but didn't change the neutralization feedback capacitor, you'll get that sort of nonsense. I know that if I don't carefully follow the alignment and neutralization procedures with my FL-101 I'll get exactly what you're describing. (The misbehavior was exacerbated by someone using 1ea Toshiba and Sylvania tube...which was an interesting find, considering the transmitter worked properly otherwise.)

[hamguy]

I am certain the final circuit is neutralized properly, as indicated by maximum output power at the center of the dip.
It sort of seems that it is the driver that is oscillating, but I can't tell for sure. No oscillation if driver is removed, or if finals are removed...
-g

[N8YX]

It sort of seems that it is the driver that is oscillating

Look at the driver tube's plate-blocking capacitor. Another source of nonsense in the FT-90x era. If it's a 1KV postage stamp mica I'd suspect the thing.