Pioneer SX-525 DC voltage at speakers, can't find it need help

rocknroller

Active Member
Have roughly 19-23vdc at both speaker left and right outputs. See block diagram and amp schematic below. Pulled and tested Q11/Q13 tested just fine. Thought maybe C17 at pin 11 was leaking though it tested fine. Replaced (both channels) no change. Maybe the C6/C7 2200uf caps off pins 9/10 though tested fine too. Replaced (both channels) no change. Oddly saw 5 volts across those BTW. 0.5 ohm emitter resistors all good. I'm not seeing how 23v is getting out to the speakers. Anyone?

SX-525 Amp.jpg
SX-525 Block Diagram.jpg
 
23v is the half supply volts, which goes to the o\p capacitor, unless the cap is shorted there wont be any dc on the speaker side ?, connect a 10ohm resistor on the o\p and see if it if it gets rid of it - or kills the resistor, is there 40v from the regulator-diode

It gets rid of it. I just found a similar thread/problem on another forum where the answer to them was "Do you load the output with a resistor or a loudspeaker or only with the scope? The time constant of 2200 uF with a 1 megohm or 10 megohm oscilloscope probe is 2200 or 22000 seconds, respectively. That is, without extra load you have to wait for hours for the capacitors to get charged - and if their leakage is as bad as electrolytic capacitor manufacturers specify, they'll never get fully charged at all."

So I guess it's not actually a problem, but by design. I just always thought (as the other user did) the DC voltage on the terminals was a bad thing. It appears it's only there until a speaker is actually connected I guess, but only in a design where the output cap is coupled the way it is like on this amp. Learn something new everyday.
 
It gets rid of it. I just found a similar thread/problem on another forum where the answer to them was "Do you load the output with a resistor or a loudspeaker or only with the scope? The time constant of 2200 uF with a 1 megohm or 10 megohm oscilloscope probe is 2200 or 22000 seconds, respectively. That is, without extra load you have to wait for hours for the capacitors to get charged - and if their leakage is as bad as electrolytic capacitor manufacturers specify, they'll never get fully charged at all."

So I guess it's not actually a problem, but by design. I just always thought (as the other user did) the DC voltage on the terminals was a bad thing. It appears it's only there until a speaker is actually connected I guess, but only in a design where the output cap is coupled the way it is like on this amp. Learn something new everyday.

In the OLD days, they didn't have reliable or matched pnp / npn output transistor pairs, so they were "feeling their way around in the dark" as far as amplifier circuitry design.

Once they "got em", then we could have amplifiers that didn't need "output capacitors" and DC output could be adjusted to zero volts to ground.

So WE have to compensate for the type of circuitry in our troubleshooting advice.

The big division in older amps is whether or not it has a DC blocking capacitor in the OUTPUT circuit to the speakers. And yes, in that type of amp a speaker load is needed to check for unwanted DC "getting out".
That type of amp does have a voltage adjust, to set it at Vcc / 2 for symmetrical clipping (and most clean output power).

In this 525 design, they had pnp and npn output transistors - they just hadn't switched over to a +/- power for the amps and outputs yet.
 
That type of amp does have a voltage adjust, to set it at Vcc / 2 for symmetrical clipping (and most clean output power).

Can you explain the vcc/2 comment? Are you saying take the input rail voltage (50v on the schematic, but seeing slightly less) and dived that by two and measure (where? Emitter of Q13 with respect to ground? Q11? ) to adjust to half that? Is this a no-load, volume zero adjust? Or some dummy load? And am I adjusting VR1/Vr2 in that circuit or from someplace else from the power supply source? I saw a you tube video of someone adjusting what looked like VR1/VR2 to set a made up idle current. He choose 30ma seemingly arbitrarily, as there is nothing int he service manual about it. He looked at a later model sx-535 that wanted a 15ma setting, but he felt that was too low at the end of the range. Without any second verifiable source, I didn't want to touch it.
 
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