Having spent a few years lying around this one presented more than the average repair challenge.
Quite typical for old amps is a cap job which basically refers to replacing all the old electrolytic capacitors as they tend to dry up with time. This ageing is heightened with lack of use.
Once up and running you can really hear where the reputation for “sound sh!tty” comes from. These amps suffer terribly from excessive noise. Partly, this is because the tone section is active and boosts any noise present in the pre amp. However there are a few things I learnt on this one that I am happy to share with all the many sound city fans out there. My process for noise reduction works like this. I make a record of all tone controls and then measure noise on a spectrum analyser (frequency and amplitude) against all changes that I make. By doing this I can see the benefit of each step and filter out false observations arrived at through my subjective ears that seem to become unreliable after periods of exposure to loud noise.
In the case of the amp I tried all the maxscmod mods that are well distributed on the net but didn’t notice significant benefits. In fact the alterations to the tone stack really seemed to compromise the EQ and in particular the bass response.
What did help is replacing all the screened cable, removing the reverb loop and sensitivity switch – these run your signal all around the chassis of the amp and vulnerable to picking up noise.
I tried FRED to replace the noisy switching of the rectifiers but in fact I could not see any improvement as a result of this.
I replaced the ageing omeg pots for the tone stage and this did help. This particular amp seemed to have developed a degree of microphonics from the pots which can happen when the connection from the pot wiper and the track has lost its mojo.
The most significant improvement was made by replacing as much chassis earthing with wired connections and in particular the bias winding.
By working through this I was able to reduce noise from -27dB to -42dB where a perfectly quiet room is -53dB. That is a massive improvement.