SR-150 SSB Output Mod
Don Power, K0TNP

I have found a problem with SR-150's that is very easy to cure.

The symptoms are low SSB output. C.W. output normal. That is 100 watts out on C.W. and only about 50 on voice.

On my rig (the only one I have had experience with) the problem turned out to be un-bypassed B+ bus. No bypass shown on the diagram. So maybe at a later date they corrected the problem.

Referring to the schematic-- The triode section of V-3B is a cathode follower in the balanced modulator. If you follow the plate supply back to it's source, there is no audio bypass. This makes V-3B an audio amplifier. And following this audio output, it goes through R-113, supply for the aalc amp. V-5B. and then C-154 and voltage doubler CR-7 and CR-8 and appears as aalc voltage. Reducing the gain of the driver stages. A 1 or 2 MFD cap will bypass the plate of V-3B and make it a cathode follower as it was meant. This will allow the aalc circuit to operate normally. The output on voice will be up where it should be.

Don Power, K0TNP

This page last updated 17 Jul 2001.