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SANSUI AU-G90X HUM PROBLEMS

Subchannel

Subchannel

Active Member
Dear forum:
Just got an AU-G90X on my bench with hum on PHONO input. It´s both Ground hum and 50Hz mains hum.
It also bleeds into the other inputs, though much much lower in strenght of course. The amp is untouched from new so all pot´s and contacts needs cleaning. Before I start to dismantel and measure, anyone here familiar with the model and the problem? Thank you in advance.
 
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Firstly - recommend a thorough view / check of the AU-G90X's grounding scheme to ensure all wiring is sound etc
 
Thanks for replying. Everything looks perfect inside, hav not found any loose/ broken wires/ pcb´s. Measured the inputs and all good, but when setting the PHONO SELECTOR to MM the right rca connector for PHONO is shorted to ground. I could not read this from the schematics, but I know many amp´s use shorting plugs for phono input when not in use, is this maybe a part of the design?
 
When measuring the PHONO input, both MM and HIGH MC is correct, at 100Kohm and 100ohm. MC TRANS HIGH is 3.2ohm (should be 40ohm) and MC TRANS LOW is 2.3ohm (should be 5.3ohm).
As stated this is an original amp with no modifications, the only part I can see that may have been replaces is the voltage regulators on the PSU for phono/ preamp PCB
Any suggestions anyone?
 
From your description it sounds like there are one or more wiring errors? - which would mean that someone has been in there and removed some connectors or connections and put them back incorrectly. You will need a multimeter, the schematic, an analytic mind, and a lot of patience to find the issue(s).

Note that on some models of amplifier Sansui used 'shorting' phono connectors, in other words when no cable is inserted into a phono socket it shows as shorted (inner to outer), when you plug a cable into that socket the short is removed. So if you believe there is an unwanted short on a particular phono socket, plug a cable into it and measure at the end of the cable. I don't think the AU-G90X has these, but I thought I would mention it anyway.
 
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Not visible by the naked eye, but several solder joints on the input rca´s was not in contact with the pcb. Phone rca was also broken inside the connector itself. Replaced electrolytics on phono card. Problem solved!
Thanks again for input, most appreciated.
 
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