SX-2500 Strange Bias Behavior

[bayernfan]

Got an SX-2500 on the bench and it's exhibiting some strange behavior.

Cold (amp has been off several hours) it starts fine, both channel slowly creep up to 10mV (measured across emitter resistors). It holds that value well, but once I begin amplifying a 1 kHz sine signal for longer than a few minutes, it completely loses bias in right channel (begins shooting back and forth rapidly between 0 and 20 to 30mV), and the left channel starts moving around quite a bit though not as erratically.

Once this occurs, if I disconnect the load (switch to speaker off), the right bias drops to 0mV and the left bias steadies itself again. If I switch back to speaker on (A or B), it begins the wild bias current behavior again.

I've done cursory (in-circuit) tests of all the transistors and diodes on the main amp board, including the 2SC984 transistors that strap to the heat sink. Both channels match in all tests. Checked resistors in the bias circuit as well. Both output capacitors test good (don't think that's the issue). Inspected the soldering and traces, everything appears fine.

Obviously something is heating up and becoming problematic. In the back of my mind I'm wondering if it's a trim pot on the board.

Any advice would be appreciated. It's rare that I work on a Pioneer of this type (cap-coupled).

 

[chazix]

once I begin amplifying a 1 kHz sine signal

You know it's only appropriate to check bias with no signal, right?

(And with no load. I know some manufacturers call for making the measurement with a load. I just think they are wrong . My reasoning is that, for amps with DC-coupled output, any DC offset at the output will distort the bias measurement. I think the SX-2500 is AC-coupled, so it doesn't matter for that case.)

 

[bayernfan]

You know it's only appropriate to check bias with no signal, right?

(And with no load. I know some manufacturers call for making the measurement with a load. I just think they are wrong . My reasoning is that, for amps with DC-coupled output, any DC offset at the output will distort the bias measurement. I think the SX-2500 is AC-coupled, so it doesn't matter for that case.)

Yes, bias is set and checked with no input signal.

The described behavior is when there is no input signal. I only mention the input signal to describe driving a load for a duration to get the amp warmed up.

 

[chazix]

Yes, bias is set and checked with no input signal.

Yikes, then this sounds like a strange syndrome, indeed!

The only thing I can think of that could affect bias measurements in these circumstances, depending on whether the speaker switch is on or off, is DC leakage in the output coupling capacitors.

I should confess that I'm no SX-2500 expert. (Have worked on at least one, but so long ago that different sorts of problems may be cropping up now.) Hopefully someone with more recent/relevant experience will come along.

 

[bayernfan]

Yikes, then this sounds like a strange syndrome, indeed!

The only thing I can think of that could affect bias measurements in these circumstances, depending on whether the speaker switch is on or off, is DC leakage in the output coupling capacitors.

I should confess that I'm no SX-2500 expert. (Have worked on at least one, but so long ago that different sorts of problems may be cropping up now.) Hopefully someone with more recent/relevant experience will come along.

I actually considered this as well. The output coupling caps test well with my Peak ESR, but of course that is not testing DC leakage. The fact that switching load into and out of circuit has such a dramatic effect on things makes me think they could be the culprit.

 

[Bruno Primas]

If you haven't already, the bias trimmers are garbage on this era of Pioneer and must be replaced.

I have a 2500.

 

[bayernfan]

Pulled the four trimmers. Put my DMM across each one.

The two bias trimmers (220 ohm) were fine in the middle but at the ends would suddenly spike to around 2k ohm.

The two center voltage trimmers (100k) were fine at the ends and absolutely crazy in the middle. As I rotated smoothly in one direction it could bounce around by as much as 15k ohm up AND down.

If one of these old timer Pioneers comes across the bench again, these are instant replace items for sure.

Hopefully between the new trimmers and output coupling capacitors, the issue will be mended. Going to use Bourns 3386F (250ohm and 100k ohm). Parts arrive tmw.

 

[bayernfan]

Well the new trimmers and output capacitors did nothing to resolve the issue, but I do think I may have traced the origin.

Against my better judgement, I replaced the 2SC458 transistors on the control amp and phono amp boards with KSC1845 (instead of KSC1815). I measured the IN pins on the control amp, no AC. Then measured the OUT pins, sure enough some significant AC swing.

Let's hope replacing all these 1845s with 1815s resolves the issue.

 

[Bruno Primas]

Using 1845s shouldn't make it not work. Double-check that the pinout is correct.

 

[bayernfan]

Doing some research on AK right now regarding KSC1845 oscillation. It seems some older Marantz and Kenwoods units are sensitive to high gain transistors in the preamp stages. Switching from 1845 to 1815 solved the 'motorboating' issue several members had experienced.

 

[Bruno Primas]

There are only two 458's on the main amp. There are however, 2SA484, 2SC484, and 2SC984s that can get very confusing when replacing if you're not careful.

 

[bayernfan]

I'm referring to the Control Amp board:



I replaced the 2SC458LG transistors with KSC1845 instead of the correct KSC1815. I triple check pin-out, so that's never an issue.

On the Main Amp board, the only two transistors that have been replaced are the two 2SC458, which are now KSC1815.

 

[bayernfan]

Replacing the 1845's on the Control Amp board with 1815's resolved the issue. I recommend using KSC1815 for all 2SC458 in the SX-2500.