On the CR11/CR12 side. I'm wondering if I should just jumper a wire from R50 to the inside att/c3 junction on the meter board, as it seems to die en route somewhere. The previous owner attempted a few repairs, perhaps a solder trace got too hot.dmp said:You have a negative control voltage on R50 but not on the attack pot???
Which side of R50: CR5 or CR12/CR11?
Right. I'm talking about the output pot itself. Regardless of the ceiling control position, I still get signal with output off. I think it might be time to double check the calibration now. Thinking of making sure R7 isn't way off.dmp said:Nice!
The ceiling also attenuates the signal
dmp said:I think I found the proverbial smoking gun with these 525s.
The ones I built both had hum and a non-musical distortion (same as discussed on pg 14 by Mich and Peter). The distortion was not caused by the FET - it was caused by the meter circuit. After turning R55 all the way off (so the meter would not light at all), the distortion disappears. There's a ground problem when the meter circuit shares the same ground as the in/out pot grounds. I think the current from the meter circuit was causing a ground loop between in/out pots and A3. Easy fix - I cut the trace leading from the neg side of CM1 to the GND on the meter board, and ran a wire from Neg CM1 to audio ground at the hole by gold finger 5. With this change, both of my 525s are quiet & distortion free. Even at heavy compression they sound smooth - a huge improvement. And the hum is gone
A second mod I'd recommend - after tightening down the mounting nuts a few times (or a zillion in my case), the silkscreen over the ground plane developed shorts, which connects audio ground to the chassis, not desirable. Easy mod was to cut the traces by gold fingers 5 & 13 to disconnect the pcb ground plane from audio ground. Then the short to the chassis through the mounting bolts makes the pcb ground plane a chassis ground, which is better for noise rejection.
With these mods the 525s are fantastic. I just played through them with guitar and singing and they are smooth and clean.
Crazy it’s been that long ago!dmp said:I went back to try to figure out what you were asking and thinking it is this mod, 10 years ago so I don't remember details. But the 5&13 had to do with the ground plane - should be pretty easy to see what is connecting to the ground plane.
From the 500 series specs, pin 5 is audio ground and pin 13 is power ground.
Regardless of the ceiling control position, I still get signal with output off.