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Hello folks,
I'm posting this for the benefit of others that might encounter this issue.
This Beta Lead had a nasty buzzing and square type pulse on the output under load that was a bit intermittent. The amp did pass signal until the pulsing started.
Now I know from other models of the original Sunn solid state amps that Sunn added mods to prevent them from oscillating but that was primarily to prevent parasitic oscillations that were usually in the super sonic range and generally a result of the newer epitaxial transistors having higher frequency response compared to the originals.
No mods seem to be available for the Beta Lead other than in the service manual which only states what has been changed after certain serial numbers but no reason for the changes.
I was able to isolate the pulse to the output/pwr supply board. I changed all the smaller electrolytic capacitors just to eliminate any possible issues there. Interestingly, none of those smaller caps checked bad on any of the tests my Sencore LC103 ran on them giving me reason to possibly not worry too much about the ton of them on the preamp board unless there arise some issues there.
Anyway, after troubleshooting which as you know is pretty difficult on a direct coupled power stage with more than burned resistor or shorted xistors, I decided to measure the bias. A really big DUH hit me square in the face!
The bias was super high.
Turns out the 25K bias pot was open. Even the most seasoned techs overlook the obvious sometimes.
For a source, the 25K CTS type bias pots in the Fender HR Deluxe or Deville will fit perfectly in the pcb. It is available from Fender or AmpRepairParts.com. Fender #0027873000 AmpRepairParts.com #FBP-AP25KL.
Hope this helps others.
Peace, love, dope (Cheech and Chong reference for those 'Seasoned' guys out there)!....glen
I'm posting this for the benefit of others that might encounter this issue.
This Beta Lead had a nasty buzzing and square type pulse on the output under load that was a bit intermittent. The amp did pass signal until the pulsing started.
Now I know from other models of the original Sunn solid state amps that Sunn added mods to prevent them from oscillating but that was primarily to prevent parasitic oscillations that were usually in the super sonic range and generally a result of the newer epitaxial transistors having higher frequency response compared to the originals.
No mods seem to be available for the Beta Lead other than in the service manual which only states what has been changed after certain serial numbers but no reason for the changes.
I was able to isolate the pulse to the output/pwr supply board. I changed all the smaller electrolytic capacitors just to eliminate any possible issues there. Interestingly, none of those smaller caps checked bad on any of the tests my Sencore LC103 ran on them giving me reason to possibly not worry too much about the ton of them on the preamp board unless there arise some issues there.
Anyway, after troubleshooting which as you know is pretty difficult on a direct coupled power stage with more than burned resistor or shorted xistors, I decided to measure the bias. A really big DUH hit me square in the face!
The bias was super high.
Turns out the 25K bias pot was open. Even the most seasoned techs overlook the obvious sometimes.
For a source, the 25K CTS type bias pots in the Fender HR Deluxe or Deville will fit perfectly in the pcb. It is available from Fender or AmpRepairParts.com. Fender #0027873000 AmpRepairParts.com #FBP-AP25KL.
Hope this helps others.
Peace, love, dope (Cheech and Chong reference for those 'Seasoned' guys out there)!....glen
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