Mike express build

[Miket]

Looks like I was wrong. *facepalm*
My last diagnosis was declared while playing with c9 disconnected, from the previous test. I'm surprised it actually worked with the circuit broken like that... All be it, less powerful. I reconnected c9, and it was back to itself.



I replaced c10 and c12, in case they were leaky, also tried increasing C12 to 500pf, still nothing.

One thing I'm wondering about it, should there be masses of gain at 9 o'clock on the volume knob? Mine is almost completely clipping the signal at that point, I've got the feeling that the output stage is just getting hit way too hard, and it's causing all this mess.

Any ideas?

[geetarpicker]

The volume knob setting where the amp will begin to clip is much dependent on volume pot taper. On my original '85 it's clean up to about 11am, but many pots these days (even audio taper) come on so quickly some amps will get pretty gainy well before that.

[Tillydog]

My last diagnosis was declared while playing with c9 disconnected, from the previous test. I'm surprised it actually worked with the circuit broken like that... All be it, less powerful. I reconnected c9, and it was back to itself.

C9 is the coupling cap before the PI, isn't it?

If so, you shouldn't get any signal through with it disconnected. The fact that you did suggests that there is something funky about your wiring - I would guess a poor ground somewhere between the main smoothing cap and the PI. Check the filter caps in the PSU, too (may have dried up if they are old).

Do you still have the NFB disconnected? If so, the amp will clip much sooner than with it connected.

[Tillydog]

A couple of joints for you to double-check (particularly the grounds from the smaller smoothing caps)

[Miket]

Thanks tilly. Will do