On Tue, 2 Jun 2020 18:28:17 -0400, M Philbrook
wrote:

In article ,
says...

The ac line relay cycles for 500mS, without display indications.
This with or without main TX connected.
Housekeeper is live and rises to 13V during the start-up
attempt. Power detect line ok.

Anyone familiar with start-up sequence of this thing?

Looks like you've got to disassemble the whole thing to get
at the processor/control area.

Is it safe to bypass relay, to run up the power gradually?

RL

I have no idea basically what you are talking about except maybe a power
management circuit that isn't getting a watch dog trigger from the CPU ?

Normally those kinds of circuits requie some sort of feed back
information to keep it alive...

I Suppose the proper thing to do is watch that feed back information to
see if this is the case.

In some cases it could be a rather high speed pulse coming from a
Digital timer or CPU. when that stops the circuit will time out and shut
down..

I assume this is what you are referring to ?

Even with a full schematic, the requirements for the main processor
to keep the line relay closed and running is not obvious.

Two of these cycles from O/I switch latches the unit off until
line input power itself is cycled.(There is a dual flipflop in the
housekeeping circuitry).

All supply lines are monitored to develop 'power good', but there's
no obvious power circuit failure evident during the period of relay
closure - except that the front panel doesn't even blink.

All service troubleshooting involves interaction with the front panel
display. . . .with specialized harness extenders and board
substitutions.

The harnessing is nuts, with signals routed through 4 or 5 cards and
connectors before arriving at their destination. I expect the
organization of power circuits and control logic will be similarly
convoluted.

I've told the owner - no repair / no charge - take it to Yamaha
service - but it hasn't been picked up since.

RL