Pioneer SX-9000 now on the bench...

[Guest]

A very early solid-state stereo receiver from Pioneer. The SX-9000. It was TOP of the line at that time which was 1972.

62 watts into 8ohms, reverberation circuit built-in (hope you like echo), can power 6 speakers simultaneously, 2 mic inputs, 2 phono inputs, tape, aux, AM/FM, tone color and more. It had every possible option that the early 70s could offer and Pioneer packed it into the cabinet. It weighs 38 pounds. It was also common for these to come with factory schematics. Pretty cool.

This model is FILLED with noisy transistors. Early silicon semiconductors were still being figured out and it didn't help that these were also being run at their absolute limits. This will be a ton of work.

[Mackey]

Interesting Stuff! I wish I could watch you work on something like that. I like the look of the unit. Clean, retro vib.

[Performa]

Now that is a pretty piece of Hi-Fi gear, rock on sir!

[Guest]

Well, it turns out the right channel has a shorted output driver transistor from a smoked pre-driver transistor that let DC hit the driver (output). 2SC1115 is the model from Sanken (still in business!!). No longer made for the last 40 or so years so the closest replacement (and WAY overkill) is the ON Semiconductor MJ21194G. Left side is fine, but I will replace all 4 so the Hfe (gain) is all matched on each side.



The little semi that cooked:

[flipoclock]

wow, that is one fine looking unit, it will be even cooler when it is done

[Guest]

Yes, this particular circuit topology is very warm-sounding, much like a vacuum tube amplifier. The output is coupled by capacitors so the sound takes on a warmer tone.

[Guest]

New outputs. Only one was cooked, but I will replace all 4. The old ones were 10A and the new can handle 16A. BEEFY.

The factory used a weird black material for thermal bridging between the outputs and the heatsink. It is removed, but the stains stay. It stains the aluminum, but doesn't interfere with the new thermal paste or heat transfer.



New mica insulator.


Also gets new heavy duty dual-diodes in the power supply. Oldies were 1.5A and new ones are 16A.

[flipoclock]

Nice work! What is the reasoning behind 'upgrading' the outputs and diodes to 16A? Bear in mind that I'm a noob with regard to audio stuff

[Guest]

Very good question! Sometimes a modern replacement that has the same physical layout and meets, or exceeds, the OEM output can sometimes be more robust since it was probably designed to also work for other applications that required a higher amperage. With this old gear we are sometimes limited as to what will fit and also operate the same if something needs to be totally replaced.

It is like replacing the transmission in your car, but the new one has heavy duty gears. Works the same, makes the car move at the right speed, same amount of gears, same engine power going to it. It just has the ability to take more power.

Or, think of a capacitor rated at 220uf and 6.3 volts. Not terribly common to have in stock. You can replace it with one rated at 220uf and 16 volts and the circuit will still operate the same.

[Guest]

They simply do not make switches like these anymore....exquisitely designed and built. The SX-9000 can power 3 sets of speakers. 6 speaker pairs! These are the switches that enable/disable each pair. A, B, C, and the fourth button cancels all of them.

They are quite gummed up with dielectric (non-conducting) grease that has gotten goopy and hard. Believe it or not the company that made them is still in business in Japan and was established in 1948! Alps Alpine – Electronic Components and Automotive Infotainment

[fromnowon]

That is very nice! Wow, stamped steel. They even left openings to be able to clean and re-lube the mechanism.

[Guest]

All done and ready to make music. Such a brilliant amp design and it sounds warm, but bright. Love these early solid-state units. Strange thing about this one as I finished it up. Someone had apparently tinkered with it long ago and changed out a ceramic disc capacitor on the amplifier board. A tiny one.

They used the wrong value and it caused it to filter some of the highs on the right channel. They added a 1000nf, or 1uf and it needed a 56pf. It was minimal, but I could just hear it...something was a little bit off and it really was hard to notice when the balance was evenly split. The component they used must have been used, as well as it looked old just like the one in the left channel from a physical size and shape view.

Also has a real walnut veneer cabinet.

[flipoclock]

That is one beautiful result of which you can be proud! Nice work!