Acoustic 164 Guitar Amp Relay Switch Problem
We recently restored an original Acoustic Amp 164 Model. This page is to provide information on a problem that the early Acoustic 164 had with clicks between channels 1 and 2. and a solution to the problem. Later versions of the amp may use opto couplers to switch.
When the amp has no signal through it the channel switch button / footswitch is not a problem but when there is guitar signal there is a loud click when going from Vol 1 to Vol 2 and vise versa.
The problem stems from the way the relay circuit was designed. The schematic left shows the preamp stage switching. The master vol 1 and 2 footswitch operates a relay that shorts out the gain ( in the case of vol 1 ) by shorting the output of V2a to ground. When the relay contact is in this position the POT Master 2 is disconnected. When the switch / footswitch is pressed to Vol2 . The contacts ‘unshort’ the gain stage v2a and connect in the master vol 2 pot so as to attenuate the output further.
The problem lies in that when the relay is between contacts you have a state of no output pot vol connected to attenuate the output and also a high gain stage. With guitar signal through it creates a bang or loud click.
The solution to this problem is to use a second Double Pole relay to control the Vol1 / Vol2 relay and also parallel the contacts that the original relay connected. This means that the original realy only operates after the vol 2 pot is connected and only releases after the gain is un-shorted.
The channel switch and footswitch operate by shorting a ground signal to the relay coil. To modify remove the ground signal wire ( the one that goes to the rear footswitch jack socket connector tip) Use this wire to short the ground to a new DP relay coil RL2. Take power for the other side of the coil from -24V marked on the front board. Ensure to use a reverse biased diode across the coil of the new relay to limit the back EMF voltage.
Connect the ground signal to the poles of the new DP relay contacts. Then use one relay contact to energise the original relay and the other relay contact to short either the vol2 connection or the gain V2a.
With the new relay installed the circuit operates as follows : The Footswitch is pressed shorting a ground point A to the new relay RL2. The relay energises and connects both a ground to the original relay RL1 and Vol2 POT connection. Thus the Vol2 pot is in place before the RL1 operates to un-shorted the gain V2a. In the reverse direction The new relay RL2 de energies first connecting a ground to Vol1 point to reduce the gain on V2a before the original relay RL1 releases the VOL2 POT attenuation. This is due to relay release time lagging the removal of power to the coil.
Thanks to Michael Carroll for his Acoustic 164 guitar amp and Pirk for schematics.