开云体育

Whentuning up my TR-4C, slowing increasing XMTR GAIN and peaking RF TUNE, there is a strange BLOP sound? from the speaker. Please see the attached video clip.

I suspected selfoscillation or a leak somewhere. First I checked the bandswitch and groundings, then I did the Mixer and RF Alignment before proceeding to Carrier Balance Alignment. Tubes were swopped one at a time, screen caps in the PA were replaced, and so were C61 between driver and PA and C129 on the wiper of carrier balance control (R85). Diodes in T14 were recently replaced.

Any idea where to look?

Thanks.

Peter OZ8CTH

A pity my video clip didn't work here because it would have been a perfect illustration of the problem.

During tuning up there is a hum in the speaker, but only on 40, 15 and 10 meters, i.e. only on the bands where the band oscillator is used. When increasing XMTR GAIN in X-CW there is midway a strange BLOP sound, which disappears when XMTR GAIN is increased even more, but the hum is still behind.

I did alignment of crystal oscillator injection and injection coupler alignment, replaced V1 and V3, checked groundings and voltages. Everything seemed fine. I pulled out the driver tube (V6) and measured drive on pin 2 with a scope and a 1:10 probe. Nothing unsual happened on any band when XMTR GAIN was turned slowly CW.

?

Peter OZ8CTH