Ampeg SVTpro

[pontiacpete]

This came to me blowing fuses. The issue was cracked solder joints on the IEC. Now it seems to have a slight distortion that trails off after certain notes. I’m kind of hoping this might be a bias issue. Does anyone know how to adjust AP1 10K trim pot?
42601h15.pdf

[nevetslab]

This came to me blowing fuses. The issue was cracked solder joints on the IEC. Now it seems to have a slight distortion that trails off after certain notes. I’m kind of hoping this might be a bias issue. Does anyone know how to adjust AP1 10K trim pot?
[ATTACH]n977620[/ATTACH]

The Bias Pot AP1 is adjusted to yield an average of 25mVDC across the eight Source Resistors R32 thru R39 (0.47 ohms/5W) at nominal line voltage and no signal applied. Assuming the MosFET's are still the original matched parts, the current thru each part should be reasonably close. If they had been replaced by non-matched parts, you'll have to make sure there aren't one of more MosFET's that are showing much higher current thru the resistors.

[pontiacpete]

They were running less than a volt across these resistors. I raised it up to 25mvdc and it sounds much better. There are 2 mosfets that are at about 18mvdc. Should there be any concern about this?

[nevetslab]

They were running less than a volt across these resistors. I raised it up to 25mvdc and it sounds much better. There are 2 mosfets that are at about 18mvdc. Should there be any concern about this?

Not surprising to find a couple MosFET's less than 25mV after readjustment. 25mV thru 0.47 ohms is 53mA & 18mV is 38mA, so you're in the ballpark. No need for concern.

[pontiacpete]

Thanks for the help!