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Hello folks,
This posting in cases anyone else runs into this issue.
I just got finished battling with this pa that would motorboat with bass turned up on the channels & mains. You would think filter/decoupling caps somewhere on each channel or possibly the main filters...NOT!
I replaced all the 100uf decoupling caps on the preamp stages as well as the main preamp board as well as the few small electrolytics on the power amp board. All to no avail.
It turns out the 24Vdc to power the front preamp board is weirdly derived from the same divided down supply that feeds the Diff-amp and bias stage in the power amp. I subbed in the 24V and motorboating all gone. Best I can figure that is where the low frequency feedback is getting through. No filter cap at that point diminished the motorboating.
So, I actually never did figure out what the real issues given the time I had already spent getting to this point. Note: I subbed in new caps for the main filters just to cover all bases even though I was pretty sure they were ok. I also tested the them on the Sencore LC103, and they were definitely good.
So you ask 'What ever did you do'. I finally just decided to build a series pass circuit with a LM7812 with 12V zener in it's center (ground) lead given I didn't have a LM7824. This pic doesn't show it, but I tapped the Main B+ off of a disc cap to the left of the pic. Since the leads on all of the components are folded over & soldered heaftily on these boards, there was little change it would unsolder the component from the board with quick soldering & all.
It worked like a charm and got the thing out of my face
. Hope this helps someone along the line.
This posting in cases anyone else runs into this issue.
I just got finished battling with this pa that would motorboat with bass turned up on the channels & mains. You would think filter/decoupling caps somewhere on each channel or possibly the main filters...NOT!
I replaced all the 100uf decoupling caps on the preamp stages as well as the main preamp board as well as the few small electrolytics on the power amp board. All to no avail.
It turns out the 24Vdc to power the front preamp board is weirdly derived from the same divided down supply that feeds the Diff-amp and bias stage in the power amp. I subbed in the 24V and motorboating all gone. Best I can figure that is where the low frequency feedback is getting through. No filter cap at that point diminished the motorboating.
So, I actually never did figure out what the real issues given the time I had already spent getting to this point. Note: I subbed in new caps for the main filters just to cover all bases even though I was pretty sure they were ok. I also tested the them on the Sencore LC103, and they were definitely good.
So you ask 'What ever did you do'. I finally just decided to build a series pass circuit with a LM7812 with 12V zener in it's center (ground) lead given I didn't have a LM7824. This pic doesn't show it, but I tapped the Main B+ off of a disc cap to the left of the pic. Since the leads on all of the components are folded over & soldered heaftily on these boards, there was little change it would unsolder the component from the board with quick soldering & all.
It worked like a charm and got the thing out of my face
. Hope this helps someone along the line.
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