Pioneer A400 power problem

[redart]

Hi,
I'm trying to troubleshoot my venerable Pioneer A400 audio amplifier. It started making odd sounds then the power on protection relay started taking longer than usual to click in, then finally refused to click in at all, and fuse resistor R308 (circled on schematic) starting to smolder. When I replaced the fuse resistor with a normal resistor it blew straight away (I didn't realise it was a fuse resistor initially). Presumably something has shorted - one of the output transistors maybe (Q1 to Q4), or Q5 or Q6 - driver transistors ?.
I'm hoping someone can help me troubleshoot from the attached schematic - where best to look and how to test ?.
Thanks in advance.

 

[Nigel Goodwin]

Check the output voltages (on L401 and L402 as handy points) - these should be VERY close to zero volts, if not the amp on that channel is duff.

I'm a little mystified as to why R308 is burning up, which is part of the protection circuit - but check those voltages first.

 

[redart]

Hi Nigel and thanks for the reply. I guess R308 is doing it's job as clearly there's a bunch of current going through there that shouldn't be. The output voltages tell the story - one side is sitting at -40V, the other is at -13V. These voltages correspond to marked DC voltages (no input signal) at various other points on the schematic, but my troubleshooting skills don't have an answer as to why they would be showing up on the outputs.

 

[Nigel Goodwin]

Bit strange that BOTH outputs are duff - check the outputs of the two voltage regulators Q237 and Q238, notice they are also fed via fusible resistors.

 

[redart]

Hi Nigel,

emitter, collector and base of Q237 are all sitting at just under +40V, Q238 all just under -40V. Not sure about you but this makes me even more confused !

 

[Nigel Goodwin]

Hi Nigel,

emitter, collector and base of Q237 are all sitting at just under +40V, Q238 all just under -40V. Not sure about you but this makes me even more confused !

No that's OK, they are only for smoothing - effectively amplifying the value of the capacitor on the base.

 

[leal]

Hellow guys

If you don't have any problem with the current amp (bias) just replace the differential transistors and this should solve your problem. We call this (popcorn) efect, remember the stage that is affected is the differential voltage amp and will cause the voltage to swing out of ctrl. makeing the resistor fry up. always take precaution useing a 100w bulb in the main pwr fuse. adj bias to 500mv.
good luck.

 

[redart]

Thanks Leal. My theory isn't that strong so I can't pinpoint the differential transistors. Can you help me out ?. Am I looking for a common emitter configuration ?.

 

[redart]

Have done some more testing....lifted the legs of the output transistors Q1 - Q4.

Q1 and Q3 appear to be good - 10 ohms between base collector and base emitter one way, open the opposite way, and open both ways collector emitter.

Q2 and Q4 both appear bad however. Q2 reads 10 ohms base collector and base emitter one way, and 38 ohms going in the other direction. Collector emitter is less than 1 ohm both ways. Q4 is less than 1 ohm on all combinations !.

Also tested Q5 and Q6 and these appear to be good, however Q6 appeared to have a dry solder join at it's base. Both Q5 and Q6 are C4137, however I haven't been able to find a pin out diagram for this. Only datasheet I've found says it's used for audio temperature compensation.

Is it worth replacing Q2 and Q4 ? ($22 cost). Do these readings help explain the initial symptoms and the blowing of R308 ?. Would a dry solder on Q6 have any significance?. If Q1 and Q3 are good why did that channel have voltage on it's output ?.

Any help / suggestions would be much appreciated.

 

[Nigel Goodwin]

Yes, the output transistors been short explain everything - and is really what we all expected.

However, just replacing the oujtput transistors may very well cause them to blow again - it all depends if anything else caused them to blow.

As a professional engineer I always prefer to change ALL transistors in the DC chain (which is a LOT in this amp), but for cost reasons you might just change the outputs and see what happens.

R202 sets the bias current, and rather bizarrely seems to be configured so if the slider goes O/C then the bias increases to maximum?. So you need to carefully check the bias when you power on.

 

[leal]

Ok here’s the deal

If you have one Ch. Out removes the bad transistors and replaced main Pwr. Fuse with a bulb as mentioned earlier. If you have a variable X former adj. Voltage from ¼ to a proper workable operating V. you could work with. This method has worked for me for many years that I’ve been in the field. Before you fire up the unit, use contact cleaner and pre lubricate the bias Ctrl gently, don’t forget to mark its positioned.
Now that you have the bulb in place Crank the Pwr. On. And look for a dim glow at this point. Measure the bias voltage of the remaining transistors from B to E of the PNP. That voltage should be around 500mv. If not, make proper adj. until unit is within range.
If the glow of the bulb is too be bright orange. Then you might have additional problem in the remaining good Ch.
Now replace the bad transistors with new ones and do the same procedure that I mentioned earlier.
As you mentioned before, you said that “I have rail voltage at the output” If you do have voltage at the output. I assume at this point that you checked everything in the circuit and all bad components were replaced. Then proceed to replace differential transistor Q281 and Q282. This should solve in most cases the fault. If this won’t solve the problem then there must be a bad component to be found.
I hope this could be useful to you. Good luck.

 

[redart]

Thanks again guys...your help is much appreciated. Rather than buying 2 new transistors (they're $12 ea here in Oz!) I was thinking of connecting up one channel at a time using the two good transistors I have, but was unsure if there was any reason not to do this. Then if that channel works, swap the transistors over to the other channel and check that side. I need to limit the bias current (using the bulb), then check the bias voltage of the transistors and adjust if necessary using VR201 and VR202. Does this sound about right ?

 

[Nigel Goodwin]

The obviously reason for NOT doing it, is that you're VERY likely to kill those transistors as well.

 

[redart]

Thanks for the feedback Nigel. In your previous post you suggested I might change the outputs and see what happens, so is it the act of connecting one channel at a time that is the bad idea, and if so, why?. Can the left and right channels not work independently, or are they so co-dependant that having one open would cause the other to blow ?

 

[Nigel Goodwin]

They should work independently - but it's likely that the duff channel could well blow the output transistors again if you replace only those. You might be lucky, but based on my experience of repairing hundreds of power amplifers over many decades it's rarely only the output transistors.