The Pioneer SX 1250 is one serious, high quality electronic monster. Sold from '77 to '79, it was Pioneer's top of the line receiver and boasted 160 watts per channel in a sixty pound chassis. The build quality is very high and properly restored they sound great. Not all of the late '70s monster amps and receivers sound great, but the SX 1250 truly does. I restored one about 6 months ago and neglected to write it up at the time, but it recently wandered back into my life due to a problem.
This unit's original problem was two blown main filter caps. My original thought was it was just time for the caps to go, now 30 years old, but the restoration revealed problems on the regulator card. Someone had previously done a completely inadequate repair and that card required replacement of pretty much every component, active and passive. Poor voltage control could have been a contributing factor in the death of the main filters. A 1250 deserves quality parts and someone didn't give this receiver the respect it deserved.
I replaced all 4 main filters with lovely Hitachis and recapped all 4 vertically mounted cards, the AC relay board underneath, the phono section, and the preamp cards behind the face. The two relays were burnished and all the pots and switches were squirted. The post restoration listening was a total pleasure.
Time passed and the unit ended up back at Mike's for resale. Mike set it up for a listen and proceeded to punch all its buttons. I confess I never use most of the tone controls on my receivers and I must have missed testing the two high cut filter switches towards the left side of the front panel, just above the bass and treble pots. When Mike clicked them there was a significant pop in the left channel, so it was time to peel the face again and make it right.
These two filter switches sit on their own board above the tone control board which needs to be removed for access. It had been pulled to spritz the switches with blue foam and deoxit, but no components had been replaced in the original restoration since it only had two small transistors, 4 tantalum caps and a few film and ceramic caps. These components are not typically points of failure, but 'typically' is not exactly 'ever'. Mike's estimated order of likely offenders was transistors, then tantalums and then the ceramics. Mike checked them all out in circuit and all seemed normal.
Mike swapped the transistors and the offending pop was still there.
Next up were the 4 relatively low voltage blue egg tantalums, at 4.7mfd and 10mfd, and that solved the problem.
Here is the modified board with two new transistors and 4 electrolytics replacing the tantalums:
The SX 1250 was reassembled and Patricia Barber spun on the highly tweaked Samsung DVD player with very impressive results. The SX 1250 is a head turner, one of those amps that sound good enough in the next room to drag you in for a listen. It even sounds great on Mike's beloved Infinity Quantum 2's that can get very agressive with many amplifiers. It takes an amp with considerable low end control and weight to balance out the 3 emit tweeters on the front.
It's a great looking piece of gear for those with a strong back, a very heavy duty shelf and a taste for the top of the line. It's a great representative of the very best Japanese solid state engineering in the '70s.