Hitachi HA-7700 butchered by previous owner

[matski]

Hi all, I've just picked up a late 70s Hitachi HA-7700 mosfet integrated, that I knew had issues before I bought it, but didn't expect the balance pot to have been removed completely and the speaker selector wires to have all been cut. Before I dive in and wrestle with the schematics any ideas why someone would do this? I am thinking for a shorter cleaner signal path and sacrificing the above functions. Nothing is burnt out inside, no obvious reconnection point for the speaker selector without further disassembly. I'm just after quick thoughts

Thanks

Mat

 

[ilimzn]

It was done because... audiophools.
The way it was done shows obviously that the person who did it was following a mantra but without real understanding. For instance, if you knew what 'removing the balance pot' means, which is taking it out of circuit, you would not PHYSICALLY remove it. Neither would you cut speaker selector wires and leave them there cut, rather bypassed them with wires directly from the speaker relay. The idea being 'I will remove the functions I do not use expecting to shorten the audio path' which might well have been pure wishful thinking because one has to know about as much as the people who made the amplifier to make those decisions.
Unfortunately, you are faced with at least a minor overhaul. Depends how far you want to restore it.

 

[matski]

Hi ilimzn,

Thanks for your reply. My thoughts exactly. There was no need to remove it entirely as you say, now I have to find a 250k pot with a long enough shaft to reach the front panel. I can do the restoration work I am sure. I can follow a schematic well enough and solder well. There do seem to be some non original looking additional wires in there too so definitely a case of blind surgery. I'd like to get it as originally intended, as physically it is in superb condition.

Thanks

Mat

 

[ilimzn]

I used to own one, it's a great amp, well worth restoring.
Unfortunately I must warn you that the 250k balance pot is not an ordinary pot. Each element only has half of it's track made as a resistor, the other half is a conductor. In the mid position, the left section connects the left wiper to the left-wise pin, the right section connects the right wiper to the right-wise pin. When you turn the pot towards the left, the left channel remains un-attenuated (permanent connection to left pin), just the right one attenuates. Reverse for the right=wise direction. Unfortunately the only source for such a pot would be another amplifier with the same type pot. The resistance itself might not be too critical, though.