low bias on swan 500cx

[WT4T]

Went per the manual to set bias current on the 500cx. zero'ed the meter, turned the carrier balance for lowest reading with the mode switch in trans, tried to adj the bias pot, the most I can get is about 20MA. I checked the supply no problem there,rebuilt with the new board, and does run my 250 ,with the proper bias just fine. I flicked the mode switch back and forth one time seemed to go to about 40ma ...max.. but would not show that again. I will clean the bias pot, the pins on the power connector and look at the relay contacts,after that ,will need some guidance.
thanks for any help
dale wt4t

[AD4U]

Since the PS works OK with another Swan rig, the problem is in the rig, not the PS.

The first thing I would do is clean the bias pot(s) - some rigs have a bias pot on the PS and one on the rig - and the rear panel power connector and the final tube sockets and tube pins and try again.

If this does not solve the problem I would find a set of known good final tubes (6KD6 I think) and substitute them for the ones in the final and try adjusting the idling current again with the good tubes.  "Soft" tubes will DEFINITELY cause you not to be able to adjust the bias (idling current) properly.  I think idling current on a Swan is around 60mA.  Soft tubes will result in very low idling current no matter how low you set the bias voltage.

If that fails I would carefully check in and around the final compartment in the bias circuit for a leaky electrolytic cap or a resistor(s) that have gone way high in value due to heat and / or age.

If that fails, in a 40+ year old rig, never assume that somebody did not make some well intentioned modifications that did not work.

Dick  AD4U

[WT4T]

thanks for the info, will put back on bench and see what turns up.
thanks
dale wt4t