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It sounds like the CPU has open-collector outputs and the pull-up resistor associated with that signal has gone open circuit. Is there a resistor anywhere between that line you now control with the ATTiny and to the 5V rail? Is the resistor ok and if so, is there really 5V on the rail at the point the resistor connects?


I've attached a picture of this part of the circuit.  There is no connection to the 5v rail in the main circuit.  The record mute signal controls 2 sets of transitors.  2 NPN on the input to the record amp and a PNP which then controls 2 NPN on the output of the record amp.
At the top right of the picture this is connected to the +8v rail via a 4.7k resistor.  There is also 15k resistor prior to the 2 input muting transistors which after the 4.7k resisitor in the circuit.  Both these resistor are ok.

At the moment the tiny85 is replacing the high/low signal that would have come from the processor, I've connected the output to the pin on the connector.  When the processor line was connected in the circuit wasn't muting the majority of the time.  When I removed this pin from the connector the circuit was permanently muted so wouldn't record but played ok.  It was working very intermittently with say 9 times out of 10 not muting.

Unless there are dry joints on the 4K7 resistor or a crack in the PCB trace, it really could be internal damage in the CPU.



--- Quote from: Bryce on 20:16, 09 May 20 ---Unless there are dry joints on the 4K7 resistor or a crack in the PCB trace, it really could be internal damage in the CPU.


--- End quote ---
I think it's internal damage on the CPU, I've pretty much reflowed the whole board including that resistor.  I didn't see any cracks but I know they can be tiny so I may have missed them.  I have soaked tested my fix and it seems to work and is stable so I think i'll have to stop there.  I've spent inordinate amount of time on this already!  The deck sounds surprisingly good really, tape was never a great format and considering most people scorn these twin decks it makes good recordings.  I think the control board is a little cheap, it has hard soldered connections with hot glue on them, but the  rest seems well made and being single sided is very easy work work on.


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