Pioneer SPEC-2 right amp board makes soft hissing noise

S

snailman

Member
I am repairing a SPEC-2 for a friend, and since it exhibited symptoms similar to those in this thread (http://www.audiokarma.org/forums/showthread.php?t=140255), I replaced RL1, Q4, Q5, Q6, Q7, Q12, Q13 on both amp boards, along with quite a few blackened resistors. I also regreased the power transistors.

My friend said that he was listening to it, and it suddenly went into protection, along with a small puff of smoke.

The amp now works fine, but the meters still float about halfway upscale, and there is a pop in the right channel on powerup.

After bench testing (about an hour, sounded great), I turned the volume down to zero, and noticed a soft hissing coming from the right amp board (not in the audio). There are no visible problems and it doesn't smell warm.

Should I be worried about these symptoms?

Thank you in advance for any help!
 
What does offset measure?

Hissing? That's a new one, sort of. I've heard electrolytic caps hiss as they were in the process of exploding, either from being installed backwards or being overvolted.
 
I have not checked the DC offset yet. Will test, and post results sometime tomorrow.

The sound seems to be coming from the lower middle/right of the board.
It is quiet enough that I didn't notice it before, just when powering down.

Other than the pop at powerup (only after it's been unplugged, it seems), it sounds great, even on the cheap speakers in my bench system. (wood glued rat shack cones, anyone? :D)
 
Check for reversed capacitors, a reversed electrolytic acts like a short circuit, but the power dissipated could be translated to electrolyte pressure.

THEN look for cracked or unsoldered connections - you MIGHT be hearing arcing, ESPECIALLY around the relay coil circuit - the inductive kickback may be bridging the gap.
 
I heard a bit of a hiss when one of the STV broke.
But it got hot quick and blew an output.

Perhaps the other STV let go.
 
Left Channel: full scale (250mv) until protection turns off, then stable at 0mv
Right Channel: full scale (-250mv) until protection turns off, then stable at 0mv

As far as I can tell there is no visible arcing, or reversed caps.
 
check the dc voltage across each relay coil, AT the relay coil terminals, not the pad, the terminal sticking through the pad.
 
Well I found the problem. The replacement RL1 for the right board is defective.

I switched the relays between left and right, and the sound was then coming from the left board.

The switched contacts on are constantly closed, which would explain the pop on powerup.

I will obtain a replacement, and measure the voltage then. (coil on defective relay measures about 400 ohms when removed, other relay is 0. I am assuming that this will affect the reading.)
 
Relays arrived today, and have been installed.

On powerup, there is now soft static on the left channel, which comes and goes.

The boards themselves are quiet, other than the click of the relays at powerup.

Other than the static, the unit sounds good.
 
snailman said:
Well I found the problem. The replacement RL1 for the right board is defective.

I switched the relays between left and right, and the sound was then coming from the left board.

The switched contacts on are constantly closed, which would explain the pop on powerup.

I will obtain a replacement, and measure the voltage then. (coil on defective relay measures about 400 ohms when removed, other relay is 0. I am assuming that this will affect the reading.)
Click to expand...

Power up pop gone? Protection relay operating normally?

On powerup, there is now soft static on the left channel, which comes and goes.
Click to expand...


All original transistors? We routinely replace q4, q5, q6, q7 because they take a beating.

You could swap q1, q2, q3 between the channels and see if the soft static moves.

That would narrow the culprits considerably.
 
markthefixer said:
Power up pop gone? Protection relay operating normally?
Click to expand...
The pop is gone, and the relays are operating normally.

markthefixer said:
All original transistors? We routinely replace q4, q5, q6, q7 because they take a beating.
Click to expand...
I have replaced q4, 5, 6, 7, 12, and 13 as well as rl1 on both boards.
As recommended by EchoWars in the thread linked in the first post, I replaced the 2SD610's with MJE15032G's and the 2SB630's with MJE15033G's

markthefixer said:
You could swap q1, q2, q3 between the channels and see if the soft static moves.

That would narrow the culprits considerably.
Click to expand...
I will try this, and post the results.

I have also noticed a few questionable solder joints that I didn't catch before, so I will reflow them.
 
Update:

Sorry for the delay, but things have been quite hectic for me the past few months, leaving no time for listening to equipment, much less working on it.

Now that I finally have free time again, I found the cause of the static:
My bench speakers have RCA connectors, and the plug was quite loose on the left one.
Pinching the ground shell tighter got rid of the static!

The SPEC-2 sounds great again, and my friend is quite happy with the results.

Thank you for your help!
 
C4 (not the explosive kind! :D) is installed correctly on both boards.

3 hours of bench testing later, and everything seems OK.
Definitely one of the best sounding amps I've ever heard!

Again, thank you for all the help!
 
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