SVT 2 PRO stuck on mute

[Markorock37]

I have an Ampeg SVT 2 PRO on the bench stuck in mute. Mute light is on and won't switch off with the switch. Amp is biased up correctly with green leds and the standby/fault switch lights up green as well. Power stage works fine when plugged into the power amp in jack. I have read this is an issue that others have ran into, but I see no fix anywhere yet. Curious as where to start before I just pull the preamp board out. All solder joints look good but I haven't gotten out the magnifying glass yet.

[The Dude]

Here's the schematic. ampeg_svt-2_pro.pdf

The mute circuit is just below the input. First check the mute switch. Then, check Q3, Q4, Q5, Q6.

[Markorock37]

I tested all components in the mute circuit. Two of the caps didn't test right to me so I replaced those, both .022uf. Switch tests good, cycles on and off on my meter. One of the npns (Q5), and the jfet (Q3) did not test good in circuit but did out of circuit in my cheap component checker. The search goes on.

[Jazz P Bass]

What is the base voltage at Q5? (F input)

[Markorock37]

.66V at base of Q5.

[Markorock37]

One odd thing I have 16.4V at R101, but the other side is at .1V, so either that resistor is bad or something is pulling the voltage down.

[Jazz P Bass]

Find out why Q5 base is high.
If it's high, then Q5 is On.
That is Mute.

[g1]

'F' to Q5 comes from the footswitch jack. I think oxidized contacts or bad solder could cause this issue.

[Markorock37]

Well wow. That was it.

[g1]

Now that is a feature I will call a design oversight.