The Spectrum +3 (Z70830 PCB) was the first fully Amstrad designed Spectrum (the grey +2 was a hastily ‘cloned’ version of the 128 Toastrack given the production deadlines in place after the Sinclair takeover).
Unfortunately, when designing the audio circuitry, critical errors were made in the amplification circuit that heavily distorts the audio produced.
This design error was fixed on the +2B machines (Z70833 PCB), and the redesigned circuit is relatively easy to apply to the +3.
Firstly, replace the following resistors on the +3 PCB with the given values below (all 0.25W types):
R72: 820 ohm
The redesigned circuit on the +2B added some extra resistors for which there is no location on the +3 PCB. These need to be added as follows.
R204: 10kOhm – install between the base of TR5 and ground (or between adjacent legs of R50 and R67).
R206: 390 ohm between the emitter of TR5 and 0v. This is best done by removing TR5, bend up the emitter leg and install R206 in the through hole formerly occupied by TR5’s emitter. Then replace TR5 and solder the emitter leg to the resistor as shown below.
Finally for best results, remove ceramic capacitor C37, next to the modulator.
These modifications when completed should result in audio quality that supersedes that of the +2B.
Original article by ZXGuesser with assistance from Ian Gledhill can be found here.