My Quad 306 amp and quad preamp have provided many years faultless service with Proac speakers. Recently when switched on after 2 months out of service due to house maintenance, the left channel was almost in audible at even high volumes. I opened up the 306, got out the circuit and made the measurements shown at the points marked with rectangular and rounded end boxes. The amp looks undisturbedfrom new (Bought from the Canadian distributor) and the output transistors all have the same pink spots. The manual does not give the allowable tolerance values , but most were pretty close using a top of the line B+K meter.. I have an audio signal generator and a 4 channel Tek (100MHz) scope. With test speakers attached ( didn't want to risk Proacs). The signal generator was set to 800cps and output set to a level I could work with for a decent period and well within the max input signal. The sine wave at both inputs was the same and clean ( looked like no distortion), amplitude was 34 mV peak to peak. Sounded fine on right Channel and only just audible on the left. I checked the output of both channels and to my surprise found good clean sine signals at the SKs 3.2V peaked to peak on both outputs Both outputs showed approx 1mV offsets. My electronic is really from op amps on and I am pretty shaky on transistor although I did some EF86, ECC33 and KT 66 stuff as a kid. Help Please!
So both amplifier outputs have the same amplitude?
If so, then disconnect the speaker wires from the amplifier and swap them between the channels.
If the problem is still in the same speaker then the amp is ok, so look elsewhere. Maybe a connector.
If so, then disconnect the speaker wires from the amplifier and swap them between the channels.
If the problem is still in the same speaker then the amp is ok, so look elsewhere. Maybe a connector.
Hi raymar,
I had tried swapping the speakers so it was not the problem. I could not understand how the outputs on both channels could be OK and the speakers OK. It occurred to me that, on your suggestion, that I had scoped the output on the SK3 pin of the output transistor of the left channel for convenience , not poked up the loudspeaker sockets. That was the problem. the amp had been knocked over and fallen 2 years prior, and the red plastic part of the socket damaged, but it worked OK.. Not having been used for a couple of months maybe it went open circuit or some other fault, cracked? Anyway changed it and everything seems OK. Talk about weird faults.
Thanks for your help and the one word " connector".
Thread moved to Multi-Way.