I hope the is the correct forum to ask this question.

I have a vintage solid-state guitar amplifier that I need to diagnose and fix. It has a stereo power amp and one side of the amp is not producing any output, so that's the first place to look. Luckily, I have full schematics, and the schematics even include expected voltages and signal shape at various points in the circuit given a specified test input signal (400 Hz sine wave).

The front of a guitar amp is expecting an "instrument" level signal. Typically, with these types of signal chains for guitars, guitar amps, and guitar effect, one deals with "instrument" and "line" level. These mainly refer to the "strength" of the signal. Line level is stronger and more resistant to load. While instrument level is weak, like it would come directly from a microphone or guitar pickup without any amplification. The amp's input is expecting "instrument" level.

Can anyone recommend how to set a typical function generator to output an "instrument" level signal? I believe it would be something like adjusting the output impedance. I need to generate this so that my testing of the circuit is as close as possible to the test conditions referenced in the schematic. To strong of a signal will likely throw off the reference specs, so I'm trying to make my function generator match the test conditions referenced on the schematic as closely as possible.

Thanks!

 

Welcome to AAC!

a specified test input signal (400 Hz sine wave).

Your schematic should specify the test signal amplitude too. What is it?
A 10kΩ resistor in series with the function generator output should give a reasonable simulation of a guitar pickup.
This may help.

 

Your schematic should specify the test signal amplitude too. What is it?
A 10kΩ resistor in series with the function generator output should give a reasonable simulation of a guitar pickup.

The schematic does not specify that, so since it is a vintage guitar amp, I'm going under the assumption that would be typical guitar pickup output levels. Pickup outputs vary quite a bit, with 125mV being fairly typical. Ranges from 90mV to 400mV are not uncommon.

I've attached a snippet of the schematic that specifies the reference signal. A few test point measures are on this snippet as well.

EDIT: schematic does *not* specify ...

 

BTW ... thanks for the 10k resistor in series tip! And also the "pickup equivalent circuit" link. Thanks!