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SA6500 subs=oscillation?

cwh

cwh

33
Saving an SA6500 from the dumpster. Unit came with mix of replacement output TRs but rest untouched. Channels idle about 18mv/38mvdc and have 30mvdv and 50mvdc at the test points.

Swapping in MJ15030/31s seemed up my power output to near spec (13vac/21w) and got channels >1/2 a volt in balance.

Today I swapped in KSC2383/KSA1013 for the 2SC1735/2SA850 drivers and output dropped from clean 13v to 8v with oscillation on the peaks. Back to original remedied that.

VAS stage not able to keep up with the change? Ideas?

Thanks for any input.
 
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Likely your output transistors and possibly replacement drivers are far too fast for the original design. Its likely your original Output transistors were around 4MHz. MJE15030's are nearer 30MHz.
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The old drivers are (if my reference is correct) around 65MHz and the KSA2383 etc are about 100~120MHz.

If you have some 2SC2073's and KSA940's on hand, I'd try changing the outputs to those and see if that tames the oscillation. It might not, but if not, its possible that a couple of VERY small ceramic caps (in the order of 10-100pF) might tame the transistors and stabilise the amp. I've not seen the schematic so I can't be 100% sure. But it should all help.
 
QSilver said:
Likely your output transistors and possibly replacement drivers are far too fast for the original design. Its likely your original Output transistors were around 4MHz. MJE15030's are nearer 30MHz.
.
The old drivers are (if my reference is correct) around 65MHz and the KSA2383 etc are about 100~120MHz.

If you have some 2SC2073's and KSA940's on hand, I'd try changing the outputs to those and see if that tames the oscillation. It might not, but if not, its possible that a couple of VERY small ceramic caps (in the order of 10-100pF) might tame the transistors and stabilise the amp. I've not seen the schematic so I can't be 100% sure. But it should all help.
Click to expand...
Thanks for reply. Correct about outputs. Orginals are 4-5mhz new are 30. I believe drivers are >100mhz.

Got a ton of ksc2073s and a940s. Thanks for that reminder. Maybe I’ll give them a try and see how they play with the c1013/a2383 drivers.

MJs do fine with original drivers btw.

Input on plan to tackle getting an equal dc offset and bias? Yet to look at power consumption but at idle the heat sink feels a bit warm.

Appreciate the attention.
 
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