tblake10 Posted February 13, 2014 Share Posted February 13, 2014 Trying to bias my dynaco st-70. One side reads fine. The other side show a very low reading and it won't move off that even when I turn the bias screw. Any ideas? I tried different power tubes on that side, same result. Tried different rectifier tube too. Quote Link to comment Share on other sites More sharing options...
Don Richard Posted February 13, 2014 Share Posted February 13, 2014 According to the schematic found online, the two channels are fed from the same bias power supply through separate 10K bias adjustment pots. The one giving trouble may have a bad connection, is dirty, or it has failed. With the power unplugged from the wall, after 10 minutes or so open the amplifier up. If the wiring connected to the pot is solidly connected, spray the affected pot with something like Caig's Fader Lube (follow directions on the can) and work the pot back and forth several times. Blow the excess out of the pot, let it sit for 1/2 hour, then plug the amp in and turn it on. If the bias still won't set properly the pot is probably bad. Quote Link to comment Share on other sites More sharing options...
mustang guy Posted February 13, 2014 Share Posted February 13, 2014 ...and keep one of your arms behind your back. An electrical jolt in one arm and out the other passes right through your chest where your heart is. Can kill ya! Quote Link to comment Share on other sites More sharing options...
tblake10 Posted February 13, 2014 Author Share Posted February 13, 2014 Thanks for the suggestions. I'll give it a look. Quote Link to comment Share on other sites More sharing options...
tblake10 Posted February 24, 2014 Author Share Posted February 24, 2014 Finally worked up the nerve to open it up and just looked closely and felt to see if the connections around the bias adjustments were good. Everything looked good. Hooked it back up and now the I am getting a bias reading on both sides. Guess I got lucky and jiggled something right? Thanks Don Richard and Mustang Guy for the responses! Quote Link to comment Share on other sites More sharing options...
babadono Posted February 24, 2014 Share Posted February 24, 2014 One night I got lucky and jiggled something right, now I'm married But seriously I'd say you dis lodged some dirt in the pot or rattled a cold solder joint. Are you sure all the solder joints look good? I would clean the pots and re solder the connections. Just my $.02 Quote Link to comment Share on other sites More sharing options...
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