Fender Steel King pre-amp clip

[soundinvst]

Fender Steel King amp has a pre-amp clip LED lit. The first problem of note are at TP11 which is made with the footswitch connected. Test at TP11 should read +13.4VDC with the footswitch reverb LED on and -13.0VDC with the footswitch reverb LED off. I'm getting -12.7VDC with the footswitch reverb LED on AND off.

Thanks in advance for your help!
fender_steel_king.pdf

[soundinvst]

Sorry for the broken link

[g1]

Do you have +15V at TP3 and at pin 8 of U8 ?

[The Dude]

^^^^^^^ That. And, is it clean/well filtered?

[soundinvst]

Yes, all op amps, U1-U15, have -15v on pin 4 and +15v on pin 8

[J M Fahey]

Ok, check that F3 at the low voltage supply is fine.

Some 15-20 VAC pass through it, to feed low voltage regulators but raw AC also feeds footswitch jack.

Classic Fender trick is to feed AC to footswitch and different footswitchable diodes clip that and generate different DC voltages at the Fsw circuit, which gets decoded by comparators and switch this and that on and off.

There is a complex table showing varying DC voltages at different test points depending on which individual foot switch is presses, check voltages match it or not.

Post results.

[soundinvst]

Thank you for your response!

P3 voltages are -15.65 and +15.65
TP10 FS tuner/mute LED on: -13.8, off: +14.7
TP11 FS reverb LED on: -12.7, off: -12.7
TP12A FS FX loop LED on: -12.4, off: -12.4
TP12B FS FX loop LED on: +0.15, off: +0.15

Not switching @ TP11, TP12A and TP12B