Pioneer SX-434 repair thread

[amelbye]

Hi, I'm trying to repair an SX-434 for a friend.

As of now all the outputs are blown. Completely my fault. Long story short, they had to be replaced. Original parts were NLA, so I replaced the Sanyo B507D (PNP) and D313D (NPN) with 2N6042 (PNP) and 2N6045 (NPN), by matching up specs. I found parts recommended across the web but they were all expensive, so I decided to match them up myself.

So I soldered them all in, and here's my first question:
All the collectors are connected together at the heat sink. I can't recall if there were isolation pads or not (I had to set the project aside for a few months and just got back to it). I don't see myself throwing them out, but there aren't any. Should there be?

I don't see them all joined together in the schematic, but I'm hoping someone here will be fimiliar with the type of design and able to tell.

Service manual is easily available (schematics at the end):
Here

or here

[R_J]

Re: Pioneer SX-434 repair thread

You are replacing regular transistors with darlingtons, that will not work
Try MJE15033G and MJE15032G

Well the collectors of the pnp's are connected to -24 volts and the npn's are connected to +24 volts, what do you think will happen if you connect them all together?
Also the heatsink is likely at ground so Yes you need insulators
And you need to check all the driver transistors AND all the resistors in that circut.
Then use an incandescent lamp in series with the a/c line as protection when troubleshooting.

[amelbye]

Re: Pioneer SX-434 repair thread

Thanks R_J!

I didn't realize darlingtons wouldn't work as replacements (because I don't know what darlingtons are), I thought I was being clever

And regarding the heat-sink isolators, I didn't realize exactly what voltages would be present, but this is exactly what I was guessing.

I'll look over all transistors and resistors in this part of the circuit.

The reason I started troubleshooting to begin with was that one channel had no bias voltage. I'll do what you advised and post back if that issue remains.

Thanks again!

[amelbye]

Re: Pioneer SX-434 repair thread

I put in the MJE15033G and MJE15032G and the amplifier works!

Now, here's the procedure for aligning the power amplifier:



I'm confused by this. point 3: does it mean that I should cut the jumper if my voltmeter gets a positive readout?

same goes for point 4, cut the jumper for any amount of negative readout?

I get -16mV on one channel, and -8mV on the other between pin 24/27 and ground.

[R_J]

Re: Pioneer SX-434 repair thread

Those voltages look close enough, I would leave the jumpers in place, they will likely change as the amp warms up.

[amelbye]

Re: Pioneer SX-434 repair thread

after warm-up voltages between point 27 and 24 and ground move to -20mV and -10mV.

I guess these voltages are the center point of the amplifier and should be somewhere around 0 if the output gains are reasonably matched?
In the amps I've done in the past I've adjusted center point by running the amp into clipping with a sine wave input, then adjusted for symmetrical clipping. I guess this is just a simplified way to deal with this?

The bias voltage is 25mV on both channels. Should be more than 5mV it says, doesn't say anything about what to do if it's too high, so I guess I can leave that alone as well?

This amplifier will not be seeing a lot of punishment, so a reduction in output is not a problem, as long as it doesn't affect reliability.