Thursday, 24 January 2019

Pioneer SA-6200 - low and distorted

John called.

"Got this Pioneer amp I've bought. Not sounding too good. Can you take a look?"

Yeah, why not?

It's Pioneer SA-6200 , dating from around 1974. It's described as a medium power versatile amplifier.

Powering up and it's very distorted on the right channel.

It's obviously never been apart before. It's full of dust inside. A good sign, means it hasn't been got at!
I clean up the pots and switches as a matter of course, but the fault is certainly in the power amp.

It's the usual 70's push pull single rail arrangement. AC coupled to the speakers.

Checking some voltages, and something's very amiss on the input stage. The voltages on the input current mirror, Q1 & Q5 are very wrong.

Sure enough C1 has failed, and is almost short circuit.

So it's duly evicted. Looks like an electrolytic, doesn't it? Well, it isn't. It's made by Sanyo, and is, in fact, a tantalum capacitor. When they're of this age, they're more evil than the electrolytic!

It's rated at 0.47uF at 25V. I haven't got any, so I've substituted a 1u 63V poly. It'll out-live me. I've changed it's best mate on the other channel too. 

Talking of capacitors... most of the electrolytics on the amp are in poor shape too.. many physically leaking and crusty. Let's change them while we're in there ...

The re-capped board is fitted back in, and tested. Sounds good :) Making about 14.5 watts RMS into 8 ohms. It's supposed to be 15. Near enough after 44 years ;)

The amp is buttoned back up, and given a soak testing with a bit of Emerson, Lake and Palmer.

One interesting thing to note, is the amp has two phono inputs. One doubles up as a mic input. Both the inputs feed the RIAA equaliser, so your mic also has the RIAA EQ curve applied! Ah well...

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