To save graunching the film ribbon, with sub-boards flopping around on
the bench. This amp will give the analogue output,vol and tone change,
without the display pcb connected, but with "computer hash"
superimposed, but the SD/USB pcb needs to be connected via its sloppy
connectors.
At the input TL072 there are 2 SMD diodes D5 and D8 marked JV something,
measuring forward .25V in circuit.
With some low level heating , in that area from hot air, part of the
analogue signal drops out. Resoldering those 2D and 4 nearby R, plus the
traditional usual suspects, including the speaker (initially showed
suspect VC tail/pigtail internal PbF problem perhaps) hopefully has
trapped the problem