In the text of the thread I linked to it says;-
Quote:
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They both use the same mechanism and the schematic and manual covers both models and a 550 model that I have never yet seen.
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OK, it's not quite clear that the 550 model is covered, but surely it's worth a try? In any case when working on vintage equipment it's often necessary to use a service sheet for a similar model, taking into account minor differences.
Why not compare the circuit with what you have in front of you? Is the transformer and transistor count the same? Are the transistor types the same or equivalent?
The reason I suggested cleaning the contacts of the slide switch is that in record mode it supplies 9V to the bias oscillator circuit (the two transistors at bottom left of the diagram) and the erase head. Easily checked with a meter even if you know nothing at all about how the circuit operates.
If 9V is present then check the voltages on the transistors in the bias oscillator and report back. If you have a scope you could check whether a bias waveform is present.