ACC 261 Tremolo Reverb PCB Hum Hunting

[onedamnote]

Working on finding the extra hum getting picked up in the reverb circuit on a 261 head. Making some color troubleshooting aids to help the probing process. This is a blend of the top and bottom Reverb PCB (without markings at the moment).





Touched up all the solder joints, replaced the electrolytic caps, set the tremolo bias, (power-amp bias is a bit off) overall quiet and plenty of power which makes some low level hum really shine. Maybe ripple or some leaky tantalums or leaky diode next to 100uf cap. (old cap in picture).

• Wires Left to Right:
• Yellow/White: is Signal In from Q3E off of Preamp board.
• Orange: 25+vDC current from regulator
• PCV-White To reverb tank
• White Blue: to Reverb foot pedal switch
• Solid Yellow: Signal Out to Base of Q4 on Preamp Board
• PVC-White/w Ground: Reverb Tank Return
• Red: +70v DC from Transformer Filter Cap 6800uf 75v (could have excessive ripple causing the noise?) Would like to build a updated filter cap using new caps versus NOS big Blue from early 2000s.
• Twisted Ground: Grounded Reverb Output RCA style.
• Green/White: Tremolo Foot switch

Schematic Drawn in Scheme-it: https://www.digikey.com/schemeit/pro...dd109844d12fcc




From Service Manual:

[onedamnote]

Bottom Waveform: DC Ripple monitoring on the +70v rail.

Top waveform is probe monitoring input at Q4 Base which is the return signal from the reverb. The hum amplifies when adjusting reverb pot from zero to on. (with or without tank plugged in).

[onedamnote]

Located two shorted diodes in series with the Power Board Bias adjustment. This burnt out the 200 ohm VR (R108). I replaced the VR and changed the output transistor for all like kind, RCA 40411s late 70s run. (no way to "match them" but hopefully the balancing resistors are in good shape. Re-biased the amp, which in turn reduced overall noise being amplified, now I have to locate a few noise generating resistors on the channel one pre-amp board in the treble circuit. I have an Eico 147a to fix-up which can do noise test and signal sniffing.