A77 Speed Issue
 

Morning all. I have a problem that has developed on one of my A77's. It's a MkIV standard 3.75/7.5ips model. All is fine from startup but after a while, when it's thoroughly warmed up, on 3.75ips it reverts to 7.5ips. If I turn it off for a while and go back later it starts ok then reverts so it's obviously(?) temperature related. The audio quality in and out (when running at the correct speed) is very good. Before I start diving in can anyone point me in the direction to start looking. The machine has been completely recapped on all boards. This appears to be a recent problem (I don't use it all that much now).
Any advice would be most welcome.
Mike

Re: A77 Speed Issue
 

I'd say around Q203 is where I'd look first - if it's breaking down it will pull the 555 to the slow speed tach frequency.

Re: A77 Speed Issue
 

Try measuring the collector voltage of Q203 at at 3.75 it should be low, see what it is when the fault occurs.

Also measure the voltage at the red wire AE1, this should be 21V at 3.75IPS,

Let us know your findings.

Re: A77 Speed Issue
 

Thanks guys, I will do that but not until after Christmas. Will have a bit more time then. It is an odd one though. I’ve had the machine on this afternoon and it plays perfectly at 3.75. I would have thought that it would be well up to temperature by then. It’s not as if it’s dusty internally either, it’s spotless. It’s also not the case that I record a lot at 3.75 but it is handy to record the R3 Throught the Night broadcast direct from the tuner.