(13-01-2016, 04:21 PM)Mark Hennessy Wrote: Something to check for - early sets can break out into motor-boating, and the cure is to add an earth wire from the ground of the tone control PCB to the earth point on the volume control. You'll find that the later sets will have that done already, but there's a chance the earlier one might not.
And to keep the tradition alive, the new power amplifier connectors can be as unreliable as the older ones
When I checked to see if there was an earth strap going to the volume control, there was not one there, and so I added one as sugested.
The plug and socket had very bad connections and switch cleaner did not cure it. The symptoms were a VERY loud low frequency howl, I believed caused by a loose connection to the signal screen.
Each connector was carefully removed from the plug and reshaped, then the retaining clip was raised again with a sharp blade before inserting back in the plug body. I agree this is a poor connector, should have been gold plated or even silver plated rather than bare metal may have been better. It was still possible to cause it to howl when wriggled even after cleaning and reshaping the contacts, and adding the earth strap, but it was OK if left alone.
I found a common fault on a RP72 and a RP72MB, The FM was intermittent. It was very difficult to locate the cause, even the force of spraying freezer spray anywhere on the board caused it to go intermittent. But I eventually traced the problem to a broken solder joint on L3 on the FM tuner module.
This may be caused by movement between the circuit board and the tuning cap, so I made sure the 3 screws securing the board to the tuning capacitor were nice and tight.
I also put plenty of solder above and below the wire to the coil to hold it firm.
The LW fault on the first RP72 was caused by a broken / dry joint on the LW aerial trimmer capacitor. It was shy of solder causing a weak joint and the capacitor had somehow lifted from the board.
There were a few other solder joints looking like this so I went round them all and re-flowed with fresh solder.
The last job on the RP72 was to refit the negative snap connector back onto the battery holder where it had been pulled off. I used a large washer on either side of the nylon and bolted the spring contact back into place. It is important to use a countersunk type screw on the outside and cut the surplus thread on the inside of the spring, or else the spring will not compress enough.
Mike
