Solid State Guitar Amp Forum | DIY Guitar Amplifiers
Solid State Amplifiers => Amplifier Discussion => Topic started by: ilyaa on January 24, 2019, 02:02:30 PM
finally got this amp almost back to life -
the power amp was busted and, because i could not find a schematic that seemed to match the amp i was looking at, i figured id just replace it with a simple class D thing. got a kit online and wired it up - it even fit the heatsink with no modifications necessary! - and the amp is working fine.
last step: the effects. the reverb is a bit weak (i think i can handle that), but the tremolo is not working at all. the good news is i found the right schematic! the bad news is, i really cant understand how the tremolo circuit is supposed to work - which obviously makes it hard to troubleshoot.
the right schematic is attached - can anyone with a bigger brain than mine explain how this tremolo is supposed to work? and whats that symbol by the LDR?
The symbol is the Greek letter Lamda which indicates the resistor is light dependent. The resistor in a circle above R53 must be a light source (48v lamp?). This is coupled optically to R16 which is a Light dependant resistor.
The IC U2B is set up as a self running low frequency oscillator.
Look above it at IC U3B, pin #9/ R20 also has the Lambda symbol.
I take it that R20 is the LDR.
U2B's oscillating output will allow a change in the resistance of R20.
There is your Tremelo.
The circuit with the photo resistors (U3A and B) looks more like a simple phase shifter... The first thing I would check is to make sure the lamp is flashing.. perhaps it's as simple as a burnt out lamp?
thanks, guys!
lamp was on, but no oscillation -
replaced C45 and C46 - the phase shifting caps - and were back in business!
those were weird old black boxy caps - prob dried out just enough to not do their job in this application -
vibrato sounding great!
had to reduce the gain on the reverb a bit - it was feeding back - but amps put together and back in action!
thanks again!