I have been having an ongoing saga with my Beveridge 2sw-1&2 ( the "2" having the tone switch on the input). I have had much conversation with a good member here @cvanc and he has been a good mentor.
I have had these for just over a year and they sit for weeks or months as I get inspired or not. When initially rebuilt (see my other threads) the dc voltages were all within reason. However, I soon discovered that both amps suffered from what was to be oscillation manifested by white type hash noise on the program material and sometimes wildly going off and driving operating current up and blowing fuses.
Over that time , lots of hair pulling, ferrite beads wire routing and the like proved to have no effect. I am pretty competent around tube electronics but I have a healthy respect for these as trials take longer as safety is paramount for me.
In the end I have zeroed in on the power transformers in both these units as being faulty - perhaps modulating and beating through the different windings and causing the oscillation. Both units came to me in operable condition with blown apart tubes in one and popped caps and rectifiers in the other. These ran away at some point.
I had new transformers wound with a bit of experimentation to arrive at the final design. One for both heater supplies and preamp board 60v and one for the high voltage. I am going to do further experimentation but right now the new transformers totally clean up the oscillation.
The yellow trace is input and purple and pink traces are at the +3200 and mid point --- see before and after.
Other news is that I picked up a packet of old Beveridge correspondence files that would of been to a dealer from the factory in the 1976 - 1979 timeframe. Lots of interesting back and forth in the letters contained in this file along with griping and thoughts on a review from
J Gordon Holt of Stereophile magazine. In that packet was what looked like an original blue print schematic and the 2x positive for the preamp board PCB.
In my quest to isolate the source of oscillations I managed to track down a quantity of original spec Toshiba PNP S1376 driver transistors directly from Japan that were part of the original boards back in the day. I can confirm that the substitute Central Semiconductor CEN-U57 works as well - its available from Mouser and Digikey for the moment and has original pin out ECB as well.
A few more experiments to follow in this maddening journey.
I have had these for just over a year and they sit for weeks or months as I get inspired or not. When initially rebuilt (see my other threads) the dc voltages were all within reason. However, I soon discovered that both amps suffered from what was to be oscillation manifested by white type hash noise on the program material and sometimes wildly going off and driving operating current up and blowing fuses.
Over that time , lots of hair pulling, ferrite beads wire routing and the like proved to have no effect. I am pretty competent around tube electronics but I have a healthy respect for these as trials take longer as safety is paramount for me.
In the end I have zeroed in on the power transformers in both these units as being faulty - perhaps modulating and beating through the different windings and causing the oscillation. Both units came to me in operable condition with blown apart tubes in one and popped caps and rectifiers in the other. These ran away at some point.
I had new transformers wound with a bit of experimentation to arrive at the final design. One for both heater supplies and preamp board 60v and one for the high voltage. I am going to do further experimentation but right now the new transformers totally clean up the oscillation.
The yellow trace is input and purple and pink traces are at the +3200 and mid point --- see before and after.
Other news is that I picked up a packet of old Beveridge correspondence files that would of been to a dealer from the factory in the 1976 - 1979 timeframe. Lots of interesting back and forth in the letters contained in this file along with griping and thoughts on a review from
J Gordon Holt of Stereophile magazine. In that packet was what looked like an original blue print schematic and the 2x positive for the preamp board PCB.
In my quest to isolate the source of oscillations I managed to track down a quantity of original spec Toshiba PNP S1376 driver transistors directly from Japan that were part of the original boards back in the day. I can confirm that the substitute Central Semiconductor CEN-U57 works as well - its available from Mouser and Digikey for the moment and has original pin out ECB as well.
A few more experiments to follow in this maddening journey.
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Hi ,
It has been a battle with these speakers!
A lot of trickery with the figuring out where this oscillation was coming from.
I built one transformer with 36v@ 750mA, 36v@ 750mA,60v,tap@ 36V 1.25A 130VA
I built a a second separate high voltage transformer.
The high voltage transformer should theoretically be at 1130v but my original does 1200v. I did design a transformer at 1130v in one experiment -but then I moved to the final design at 1200v @ 300mA 360VA
both are Class H isolation.
A very painful process with these units......
It has been a battle with these speakers!
A lot of trickery with the figuring out where this oscillation was coming from.
I built one transformer with 36v@ 750mA, 36v@ 750mA,60v,tap@ 36V 1.25A 130VA
I built a a second separate high voltage transformer.
The high voltage transformer should theoretically be at 1130v but my original does 1200v. I did design a transformer at 1130v in one experiment -but then I moved to the final design at 1200v @ 300mA 360VA
both are Class H isolation.
A very painful process with these units......
I struggled with exact the same problems some years ago.. And I also looked in to every aspect except the power transformer... I mean I had all voltages as i should when not playing... and whe i started to play it was buzz sounds from BOTH channels?? Sa what happends is that when you get corona discharge in the power tranformer you pollute the whole room with EMC disturbances that modulates the sound.
kcin... I know how much you have struggled! Been there done that. But i am happy for you now when you finally solved the issue!! GOOD WORK!! And hopefully there is people around the world reading this and do not have to do the same journey as us!
kcin... I know how much you have struggled! Been there done that. But i am happy for you now when you finally solved the issue!! GOOD WORK!! And hopefully there is people around the world reading this and do not have to do the same journey as us!