Pioneer SX-1250 Receiver -- Some schematics

[jackinnj]

Here are 2 schematics of the power section of the Pioneer SX-1250:





Any suggestions?

[unclejed613]

the top one is the amplifier section, the bottom one is the power supply..... looking closer at the labels above the dotted boxes, it says so...

[jackinnj]

I just bypassed the preamp in the SX-1250 with a Linn LK-1. The very beefy amplifier sounds very good, but the preamp seems to leave a lot to be desired.

[unclejed613]

"back in the day" it was a very good sounding receiver, maybe the P/S for the preamp and the preamp are due for a recap?

[jackinnj]

I was very pleased that the potentiometers and balance controls operated smoothly. I know from my Pioneer SX-838 that these are often problematic. I've got to replace a couple of the lamps, and the input selector switch needs a bit of TLC.

[tiefbassuebertr]

check out the voltage regulator PCB of your second schematic of burned aeras. Replace all electrolytic caps arround this and also the zeners and transistors.
A second very importand step:
all silicon devices with more than 25-30 degrees temperature above ambient temperatur must be mounted on heatsinks. Because the space therefore isn't enough, use as heatsink chassis devices close to the formerly PCB place. Keep in mind that isolated mounting is absolutely necessary.
After do this you get a strongly enhancing of the realibitity and exceeding the lifetime of the caps on the supply-PCB. Five or six years ago I have this perform by various SX models of my brother (1250, 1280, 950 and 980 I think).

[KBK]

The entire SX and integrated amplifier range were designed so that the regulated pre and pre driver PS rails would FAIL in about 10-15 years of normal use. They put the rail buffer caps above the regulating transistors, thus ensuring that heat would overcome them some time in the future. When they finally 'pop', they generally take out the entire amplifier output section at the same time.

Like some sort of time limit use fuse that was purposely designed into all of the pioneer stuff...they blow to kingdom come --if not taken care of ahead of time.

The point is that if you have a unit of that type (Pioneer receiver or integrated amplifier from about 1976-1977 to about 1985-86) and design, you must go in and re-cap the entire pre-driver and preamplifier power supply regulator rails. It is just a matter of time until catastrophic failure.

Gorgeous stuff, tho', Jack. I love it. Audio porn from Japan. Practically brings a tear to my eye when I see mint ones. I want them so bad.... Like a warm hug, it makes me feel comfortable.

[jackinnj]

Originally Posted by KBK

Gorgeous stuff, tho', Jack. I love it. Audio porn from Japan. Practically brings a tear to my eye when I see mint ones. I want them so bad.... Like a warm hug, it makes me feel comfortable.

I guess that I was pretty taken aback by how dreadful the RIAA/pre sounds. I've hooked the 1250 up to a couple of preamps by now and the power section sounds better and better.

I used an SX-838 daily, for 10+ years and it didn't blow up. It did succumb to the knife with new power supply caps, LM4702 driver circuitry, lateral MOSFET output stage and ALPS pots. The extant pre is single-ended with cap coupling and needs a bit of work.

[KBK]

Yeah...when you find yourself pouring over schematics for a 30watt Luxman receiver and putting in relay'd 100kuf power supplies, vishay resistors on a shunt attenuator, black gates, and then working out all the chassis damping, etc...you know you've got it real real bad.

Building them hot rod sleepers with old style good looks.


One thing you don't see a lot of people doing is putting t-03 or 220 sized (whatever the given case requires) ceramic insulators in their gear. It is necessary in RF gear so the eddy current don't fry things up, and it REALLY clears up the transient response under power loading. Big time. Especially when you've taken care of everything else so it shows through as one of the last major points/sources of transient and HF distortion. It's like the last light going out so you finally have total darkness. It's a big change in the last bits that most forget is in there - to be taken out.