[bobby j]

i have a st 70 with an early mod PC 189A . one side went completely out. no filament, no sound . before i start taking thing apart maybe i can get a heads up as to what to look for. thanks for any help.

[avahifi]

No filament?  Output tubes?  Small signal tube?  One tube?  All tubes?  One channel still working?  I need more specific information to help you.

Thanks,

Frank Van Alstine

[bobby j]

thank you for the reply. right side both output and signal are not lighting up i have sound on the left channel.it was sounding really great then a pop. no smoke. it has 4 large electrolytic two 68uf and two 100uf. i hope this helps.

[avahifi]

You are describing our Super 70i board for the original Dyna St-70 amplifier.  Problems with this are very rare.

There are separate heater taps from the power transformer for the left and right channels, with the three tubes per channel in parallel.  I suppose one shorted tube heater element could pull all three tubes on that channel down.

Are you blowing fuses?  Do you have any spare tubes?  If so replace the three tubes on the right channel and see if that fixes things.  If not, try the following.

If you are not blowing fuses, you could try swapping tubes from channel to channel, one at a time, and watching very carefully to see if the problem changes channels.  Be ready to shut off very promptly as we would not want to have a tube problem damage the other channel too.  In swapping output tubes, you will need to readjust bias settings.

If the problem does not follow a tube, the only other element in the filament circuit is the power transformer and the tube socket wiring itself.  If you have the proper voltage and wiring, it most likely is a tube.

Regards,

Frank Van Alstine

[bobby j]

  aa got it bad out put tube so far so good. thanks again.