Pioneer SX525 LOUD audio Crunching/distortion noise in left channel...

[Unclematt]

HI Everyone! I am new to this site. I have had the pleasure of owning a Pioneer SX-525 for the last several years now and has been quite a reliable low powered solid state amp.

Unfortunately the left channel scared the heck out of me one day by making a horribly loud sustained crunchy distorted audio signal through the left channel only. I am a complete novice when it comes to electronics of this stature, but I would really like to learn how to repair and maintain my amp moving forward. Anyone here have an idea as to what may be causing this?

I opened up the cover and it appears that several of the caps on the main amp are leaking... There is some orange residue near and around the bottom of four separate caps on the main amp board (AWH-006).

Thank you for everyones help in advance!

 

[nickdinatale]

I had a very similar problem a while back with a Marantz 2270. Turned out to be a bad transistor on the tone board. Sounds like a recap is definitely in order, but I'm sure some helpful Pioneer folks will be along shortly to point you in the right direction as to which other components/boards you should be looking at. Welcome to AK and good luck!

 

[spark1]

I think you are describing glue, used to secure larger capacitors. While some such glues become corrosive or even conductive with age, I don't recall this being a common problem with Pioneer units.

I would guess a problematic transistor, not knowing anything else about the problem.

Does it do it on all sources? Does it occur even with no input? Is the noise volume sensitive?

 

[Unclematt]

Hi Nick, thank you for the welcome! I have been frequenting this site for years and I am glad I finally signed up. I may still recap anyways just to add longevity to my unit after we pinpoint the problem.

Hi Spark as well. To give you a quick run down, yes it makes the sustained audio crunch/distortion on both A and B speaker settings, radio (FM,AM, Noise Filter), Phono, and I have yet to test the headphone jack. I did notice however on the wire block diagram that the headphone jack line is in-line with the speaker output. Not sure if that is worth testing as I am without a 1/4" to 1/8" jack at the moment.

It does occur without input as well, yes. The noise volume is kinda sensitive it seems as well, yes.

What transistor should I be looking for and on what board?

 

[century tek]

It's most likely a noisy transistor in the left channel. Your receiver could have that problematic differential pair (single 5 pin transistor) 2SA798.

 

[Unclematt]

It's most likely a noisy transistor in the left channel. Your receiver could have that problematic differential pair (single 5 pin transistor) 2SA798.

Thank you Century for the reply as well. What part of the block diagram can I find this transistor? (AWH-006? Or?) I mentioned above I am a novice and am learning in the process of repairing this unit. We all have to start somewhere! Thanks for your help.

EDIT:
I just read through the Block diagram and did not see a 2SA798 transistor in the circuit or part lists.

 

[century tek]

Look at main amp board AWH-006.

Probably look at the 2SC1318's and the 2SA726's. Pretty simple. A lot of those pre drivers are the same.

I would prolly start with getting some freeze spray and hit each one. The noise will usually quiet down so that's your bad one.

 

[blhagstrom]

Have you ever cleaned the controls (deoxit)?
Lose of contact within the controls can cause loud noise, especially the volume control which is a variable resistor tied to ground. No ground connection path leads to full audio (loud) noise.

Noisy transistors typically make small static like noise, not loud noise.

Do the lights flicker when it makes noise? If that has the rotary power switch with the speaker selector, those can get wonky. But there again, it’s a static crackling noise typically not loud.

The goo around the caps is assembly glue. Forget that.

The only visible things that can be seen in problem electronics is bad solder joints or char.
A bad solder joint is possible here, but it takes a good eye to spot those and some can be really tough to see. Char is a dead give away but that is more common in dead units.

My vote is dirty contacts. Work all the controls a bunch of times, perhaps even when it acts up. If there is an effect/improvement, it’s dirty controls.

 

[century tek]

Noisy transistors can be real loud. I had the 2SA798 on my 8500II start out as a background crackle but the more I used it, the louder it became and then it would eventually throw it into protection and not come out until I repaired it.

Unclematt, do you get this without any music playing?

 

[blhagstrom]

Noisy transistors can be real loud. I had the 2SA798 on my 8500II start out as a background crackle but the more I used it, the louder it became and then it would eventually throw it into protection and not come out until I repaired it.

Unclematt, do you get this without any music playing?

Hmmm, I would call that a failing transistor.
But, technically, the ones we usually refer to as noisy are failing as well.

 

[Unclematt]

Strom! Thank you so much for your reply. I ended up cleaning the unit and the noise stopped.

Thank you everyone for your detailed help and input into the possible problems. I am so glad I decided to sign up for this forum finally. Happy holidays everyone!

 

[Unclematt]

Well, I’m back with some terrible news. The issue was only temporarily fixed... I returned back home, turned on my amp, and the sustained crackling noise in the left channel across all outputs has returned. I checked transistors on the main amp board and all are showing less than .8v in both the pnp and npn style transistors on the left channel. I re-cleaned the switches and pots... the issue remains. I have hit a wall. Any other thoughts and suggestions?

Thanks!