Vintage Synth Repair

Hi All

 

I came across an odd one yesterday and would appreciate some suggestions.

 

The problem involves a Solton MS100.

 

The user complains that two problems occur these are independent of each
other.

 

The first revolves around the display.  It occasionally flickers at the same
time the user can hear clicking in the headphones, similar to the
interference noise heard when things like fridge thermostats operate.  It is
unlikely to be external interference as this unit has a built in mains
filter.

 

The second problem occurs when working into a stage PA system.  After the
system has been running for a while, an hour has been suggested, the volume
drops, enough to be noticed by the user but not enough to be noticed by the
majority of the audience.  Note that the sound in the headphones does NOT
drop at all.

 

I would immediately suspect a power supply fault for the first problem
however looking at the circuit diagram shows that this unit has a very
conventional setup.  Mains into transformer and onto five separate supplies
using three legged regulators, and these don't normally work intermittently.

 

Any suggestions welcomed.

 

Regards

 

Brian