Hey all, I have a weird issue that's come up twice now, both Sansui solid state receivers, both the same kind of issue, low frequency oscillation.
First - I have a Solid State 3500. The outputs are original - as are all pre-drivers/tx on both amp boards. Bias adjusts fine on both channels, offset is good, and audio sounds perfectly fine, but when I start turning the unit up hooked up to a load I see a violent low frequency oscillation start. Voltage swings from the outputs, in the scope at the outputs, all the way back to the main bridge rectifier.
When this oscillation starts - audio surprisingly still sounds just fine.
I have replaced all the main filter caps, tried modern subs on the amp boards, subbed new output transistors, and replaced the bridge rectifiers. This has had no effect and so the unit has sat on my work in progress shelf for the better part of a year or so (with the original components re-installed of course, as they were just fine).
This brings me to today.
I opened up a Sansui Solid State 4000 and am experiencing almost the exact same issue.
Today I found out that if bring the output adjustment down on both amp boards - the oscillation ALMOST disappears entirely (not visible through scope or on meter unless REALLY pushed) - but doing this must be just masking a larger issue.
What throws me for a loop is on this solid state 4000, the oscillation was much more pronounced on the left channel than it was the right. Because of this I swapped the amp boards to see if the issue followed and it did.
Do you suspect this issue is power supply related?
What should I look for going forward?
I should also note the original issue on the Solid State 4000 was no bias, this was fixed by replacing the tired STV-3 diodes with 3x1n1004's and bias adjusted and held steady just fine in both channels.
As always, thanks.
- Casey
First - I have a Solid State 3500. The outputs are original - as are all pre-drivers/tx on both amp boards. Bias adjusts fine on both channels, offset is good, and audio sounds perfectly fine, but when I start turning the unit up hooked up to a load I see a violent low frequency oscillation start. Voltage swings from the outputs, in the scope at the outputs, all the way back to the main bridge rectifier.
When this oscillation starts - audio surprisingly still sounds just fine.
I have replaced all the main filter caps, tried modern subs on the amp boards, subbed new output transistors, and replaced the bridge rectifiers. This has had no effect and so the unit has sat on my work in progress shelf for the better part of a year or so (with the original components re-installed of course, as they were just fine).
This brings me to today.
I opened up a Sansui Solid State 4000 and am experiencing almost the exact same issue.
Today I found out that if bring the output adjustment down on both amp boards - the oscillation ALMOST disappears entirely (not visible through scope or on meter unless REALLY pushed) - but doing this must be just masking a larger issue.
What throws me for a loop is on this solid state 4000, the oscillation was much more pronounced on the left channel than it was the right. Because of this I swapped the amp boards to see if the issue followed and it did.
Do you suspect this issue is power supply related?
What should I look for going forward?
I should also note the original issue on the Solid State 4000 was no bias, this was fixed by replacing the tired STV-3 diodes with 3x1n1004's and bias adjusted and held steady just fine in both channels.
As always, thanks.
- Casey
