pioneer_sx-1250_09

Pioneer SX-1250 Monster Receiver Repair & Restoration

A lovely customer of mine named Frank rang me to tell me that his beloved Pioneer SX-1250 monster receiver had died in a cloud of acrid smoke. Naturally he was devastated, and he asked me if I could restore the old beast for him.

The Pioneer SX-1250 is a monster receiver in the truest sense of the term. There are several large receivers like this, created at a time when the Japanese manufacturers were competing for bragging rights. Another similar model is the Sansui G-8000 Pure Power DC Stereo Receiver. I repaired one of those monsters, here.

The Pioneer SX-1250 had died, literally in a cloud of smoke, so in terms of diagnostics, I was looking for a smoke generator. I found one – well two actually. The first was on the bottom panel, where one of the large electrolytic main filter capacitors had spewed its electrolyte out of the pressure release vent and directly onto the metal bottom panel. This most definitely would have made a noise and created some smoke.

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What a beast – this really doesnt convey the size and weight of this monster receiver!
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This is what spewed out of the vent of the main filter capacitor that failed, due no doubt to age and resultant high ESR.
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This is how the capacitors sit in the chassis, two per channel, for the +ve and -ve supply rails.
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They are big buggers…
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This is where the electrolyte was ejected from – the pressure release vent on the bottom of the big filter capacitor.

The other possible culprit was one of my favourites – a Rifa epoxy film capacitor, X-rated for connecting across active and neutral, that was blackened and burnt. These things fail with monotonous regularity. This particular cap was 0.033uF and I have lost count of how many of these I have seen fail. What happens is that the epoxy cracks, allowing moisture and other contaminants into the case, which then causes a partial short, which then allows a small amount of current to flow and then more until finally – bang! They blow like a firework, often when the equipment is turned off.

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Here we have another very common occurrence – an exploded Rifa mains capacitor, sitting across active and neutral.
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New high-quality replacement, to left of center, this one  a modern plastic film cap not like the Rifa which was made of made metalised paper and epoxy resin.

This repair required partial dis-assembly of the power supply. Specifically, I removed the two pairs of main filter caps as i had decided to replace them all with brand new, high-spec and slightly larger capacitance Nippon Chemi-Con parts, imported from Germany. The standard capacitors were 22,000uF, I chose 27,000uF replacements.

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These are the replacements from Nippon Chemi-Con — beautiful capacitors, slightly larger capacitance than the originals, which will help the bass performance if anything.
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The new capacitors next to the old ones. Note the smaller size, despite the increased C.

There are several considerations when replacing main filter capacitors and they apply to any amplifier or power supply. First is the mechanical fit. The original caps were secured by a system of fixings that required the cap diameter to be precisely 63.5mm. This is a standard size, but it just means that you have to choose the same diameter caps to replace the originals, unless you want to have to mess around with adapters and kludges, and I never do.

Then there are the electrical properties of the capacitors to consider. These needed to be 80V rated and the originals were 22,000uF, so ideally the replacements would be that size, or slightly larger. I say slightly because this really must be calculated. When the equipment is turned on, the main filter capacitors charge up and as they do, they effectively present a short-circuit across the mains, until they have charges. This charging or in-rush current as it is known will be designed for and the fuses specified to allow for this current and not too much more.

If you randomly use larger capacitors as people seem to like to do, you risk increasing the in-rush current to the point where fuses could blow or worse still, other parts of the power supply circuit could be overloaded and damaged. In this case however, 27,000uF capacitors were within what the design could tolerate and worked very well. They were not as tall as the originals, but were the same diameter and even the same brand – Nippon Chemi-Con – a very high quality choice, used in industrial and laboratory environments.

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New capacitors in place and don’t they look great! They fit perfectly into the original capacitor clamps. This is where time spent researching the correct parts pays off. You don’t want to be messing around with parts that don’t fit and require kludges – this is not cool with vintage gear.
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A close-up view of the of the new capacitors
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Pretty looking things aren’t they!

The new caps had different fastener requirements to enable the bus bars and wire connectors to be held in place, so I used a new set of M5 Allen bolts and lock washers, which looked great once installed.

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New Allen-headed fasteners look great and really clamp down on the capacitor terminals and buss-bars. This helps to create the lowest power supply impedance, which then helps the amplifier sound its best.
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Close-up of the new fasteners, they are all torqued down nice and tight, with shake-proof washers, no way these are coming loose.

After replacing the dead X-rated mains capacitor and all the mail filters, it was time to power up the beast. Apart from my room lighting dimming as the filter capacitors charged up, she turned on without a problem and it was extremely satisfying to hear the “click” of the protection relays after the circuits stabilised.

Next part of the job involved carefully adjusting the power amplifier blocks. Amplifiers generally require two standard adjustments – DC offset nulling and idle current or “bias” adjustment. Adjusting the DC offset involves connecting an accurate multimeter across the speaker outputs and measuring any DC present. Ideally there should be none at all, because DC voltages flow continually through the voice-coils of the speakers and are dissipated as heat. Too much DC will literally cook the voice-coils. There will usually be a servo circuit which continuously adjusts the DC to a zero point, but this zero point usually needs to be correctly set and I did that here on the SX-1250.

The second adjustment to make is to what we call standing current or bias. This represents the amount of current flowing through the output devices at the “idle” or no musical signal condition. This adjustment is critical because it sets the point at which the output transistors will switch from class A to class B operation and plays a crucial role in how much heat is dissipated by the output devices when no signal is present. These standing currents must be accurately set after the amplifier has had a chance to warm up and stabilise.

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Setting bias by adjusting the voltage drop across an emitter resistor, to 100mV

I also did a lot of cosmetic work to this beautiful amplifier. I cleaned the entire chassis, removed all the knobs and cleaned them in warm, soapy water, cleaned the front panel with Ambersil foaming cleaner, cleaned the genuine wood veneer and nourished it with a little oil, lubricated the tuner dial mechanism and a couple of other things.

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Original and un-treated wood finish, note the improvement in the wood after treatment, in the images below.
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Wood looks much better now, after treatment with a special nourishing oil rub.
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You can really see the improvement in the wood finish here – the depth now is amazing.
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She looks beautiful now with nourished wood finish and cleaned fascia and knobs.

Lastly I tested her, though headphones as I usually do. Testing showed that the switches and controls were noisy, a result of dirt buildup. To clean them, I used isopropyl alcohol. I got into every nook and cranny of the front panel controls with the isoprop. Once everything was doused, and being extremely careful not to allow the isoprop to migrate to the tuner display for example, I exercised all the switches and controls for several minutes to work the alcohol through. Quite a bit of fluxey, yellow residue dripped out of the end of the from panel sub assembly after being sprayed with isoprop, and the controls worked perfectly afterwards.

Needless to say, when Frank arrived to pick up his pride and joy he was thrilled. He couldn’t hide his excitement at seeing his amp so clean, with beautiful new main filter capacitors and working better than she had in years. After all this work, the amp would have sounded perhaps better that she ever had, even when new, because of the improvement I made to the power supply which flow through (literally) into all the signal that pass through it.

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Power-up, what a gorgeous piece of electronics – they REALLY don’t make them like this anymore.
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Such a great looking piece of equipment
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Beautiful…
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More to come with other restorations, soon!

9 thoughts on “Pioneer SX-1250 Monster Receiver Repair & Restoration”

  1. Looks Awesome!! Very nice job, indeed!
    My old gal, SX-1250, runs and looks great but would like to get the caps replaced and have her tweaked up.
    I live in Hudson, WI….are you near by or know of anyone that would be willing to do such work?
    Thank You!
    Best Wishes!
    Mike

    1. Hi Mike, I am located in Perth, Western Australia, so a bit far to assist in this instance I think! Best of luck with the restoration though. Regards, Mike.

      1. Thank You, Mike!!
        Yes indeed you are a few miles away!!
        Very nice work you did on that unit…I think it looks nice!!
        Look forward to getting my old gal tweaked and peaked! Found a fellow I’ll be shipping her off to for the maintenance and tweaks she may require. He has done many of them as well as other units….many happy customers!!
        Peace N Smiles My Friend!!

        Mike

  2. I am going to show this Pioneer SX 1250 work you have done to my techs here in upstate NY, USA hopefully it can help my 40 year old capacitors in the Pioneer SX-1250 move out. Everything else has been completed silver solder everywhere, new caps everywhere it will be stunning. Thank you for leading the way!

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