So I got my hands on another guitar amplifier for cheap and without the knowledge of how to
use an oscilloscope or variac, etc. (I own neither), I replaced all the electrolytic
capacitors since the amp is over 20 years old and also replaced a few of the power amp chips.
While I do have sound now when I play a guitar through it; I also have a loud hiss. Sounds
like an open mic or radio that is not quite tuned in. This happens without having anything
plugged into the amp and no matter the volume turned up or down, along with bass, mid and
treble set at normal levels.
On closer inspection, I see 2 cement resistors (Pair 1) have small cracks in their cement tops.
They also look like the wire inside them also has raised up, they look a bit deformed compared
to Pair 2 in the middle. Maybe this is normal due to heat? They do have me wondering and that
is why I am asking you pros, "could these cause all this noise even when the amp has been
running for just a few seconds?"
Another investigation I tried was plugging in a ¼ jack cord into each input (one at a time)
and a couple of times this caused the noise to decrease a little bit. A couple of other times
I got a loud pop by plugging in. Seems like this is a grounding issue but how can I check?
I also thought it's interesting that pair 2 and 3 are .33 ohm while this pair 1 with the most
cracking and deformity is 330 ohm. It does match what is on the schematic. What are your
thoughts? Thanks in advance.
use an oscilloscope or variac, etc. (I own neither), I replaced all the electrolytic
capacitors since the amp is over 20 years old and also replaced a few of the power amp chips.
While I do have sound now when I play a guitar through it; I also have a loud hiss. Sounds
like an open mic or radio that is not quite tuned in. This happens without having anything
plugged into the amp and no matter the volume turned up or down, along with bass, mid and
treble set at normal levels.
On closer inspection, I see 2 cement resistors (Pair 1) have small cracks in their cement tops.
They also look like the wire inside them also has raised up, they look a bit deformed compared
to Pair 2 in the middle. Maybe this is normal due to heat? They do have me wondering and that
is why I am asking you pros, "could these cause all this noise even when the amp has been
running for just a few seconds?"
Another investigation I tried was plugging in a ¼ jack cord into each input (one at a time)
and a couple of times this caused the noise to decrease a little bit. A couple of other times
I got a loud pop by plugging in. Seems like this is a grounding issue but how can I check?
I also thought it's interesting that pair 2 and 3 are .33 ohm while this pair 1 with the most
cracking and deformity is 330 ohm. It does match what is on the schematic. What are your
thoughts? Thanks in advance.