Pioneer SA-6500 (MKI) Double bump sound at power on...

JK-47

JK-47

Member
I've had a friend bring me a Pioneer SA6500 to repair that was recently recapped with two dead output transistors, blown protection fuses. This time around It had been used with 4ohm speakers at high volume until the one channel said "no more". Both protection fuses blown, and probably a lot more.

Opening it up, I replaced a few resistors and all 4 output transistors with TIP4CG and TIP42C per AK sub reconditions.Left side was tested dead after pulling and testing all four.

Powered the thing on to test with speakers, sounded great, except for a non functioning Loudness switch. Note that I had the speakers off and volume low when I powered it up. Discovered the failed Loudness switch was a transistor. Easy enough.

Powered it on again with the speakers on and volume kind of low, I got my Loudness function back, but also heard what I can only describe as a buzzy double thump in my test speakers. You could see it pump left, then right then left then right again in the woofers. Bulb limiter lit up initially, dimmed as its supposed to, then lit up two pulses. Sound quality afterward was great, not distorted as I can hear.. I know these old guys are prone to the "on button pop". So I replaced the snubbers on the power switch and at the accessory plug. Now it's just a dull double thud. It's only noticeable when the speakers are turned on, volume on of off. Nothing seems to be hot, Every transistor with the exception of the one SM2B41 on the protection board. All caps were new, from good sources, Nichicon... 10000uf100v filter caps. All resistors looked good. I replaced two 680ohm resistors that looked dark, but tested near dead on. All transistors are subbed through AK threads. With the exception of the TIP41-TIP42, I have used them all many times before.

2SA640 subbed with KSA992FBU
2SC1222 subbed with KSC1845EBY
2SC1438 subbed with KSC1845EBY
2SA850 subbed with KSA1013YBU
2SC1735 used OEM
2SA777 subbed with KSA1013YBU
2SC1318 subbed with KSC2383
2SA733 used OEM
2SC945 used OEM
TIP41-TIP42 used for output.

DC offset is L 60mv R 50mv, a little high, but it seems these pioneers can be in this range normally. I am not hearing distortion at generous volumes.

Any advice on where to test next would be appreciated. Is this double thud normal? I don't see this in any other amps that I can remember.

Thanks in advance
 
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I'm back at trying to sort this out again today. Any advice would be appreciated. I haven't seen this kind of power surge at power on before. I'm going to start by pulling and testing the TIP41-TIP42 transistors, and make sure they're ok. Then go backwards through the transistors that I installed. Has anyone had issues with the subs listed above in this model Pioneer?
 
JK-47 said:
I'm back at trying to sort this out again today. Any advice would be appreciated. I haven't seen this kind of power surge at power on before. I'm going to start by pulling and testing the TIP41-TIP42 transistors, and make sure they're ok. Then go backwards through the transistors that I installed. Has anyone had issues with the subs listed above in this model Pioneer?
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Where di you get the transistors?
 
test speaker drivers push out then in, left and right. I've checked all solder connections, double checked pinout. I made good notes for each transistor as I replaced it.
 
If you are still on the DBT that may be your problem. Once you have confirmed there are no shorts the DBT needs to be removed. You can't check it out properly or adjust anything while on the DBT.

Craig
 
It does the same thing straight into the Variac at 120v. I am concerned that the transformer may have damage due to pushing 4ohm speakers until the protection blew the fuses.
 
JK-47 said:
I am concerned that the transformer may have damage due to pushing 4ohm speakers until the protection blew the fuses.
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I wouldn't worry to much about the transformer. More than likely outputs short out . I have never seen a damaged transformer from overheating and amplifier.
 
avionic said:
I wouldn't worry to much about the transformer. More than likely outputs short out . I have never seen a damaged transformer from overheating and amplifier.
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I'll pull and test the TIP41-42 pairs. They were replaced before I noticed a burned resistor wrapped in insulation.
 
Check out section 6.4 in the Service Manual, the muting circuit. Maybe it's not working correctly and passing on turn-on thumps.

Craig
 
OK, I think what I'm looking at is that KSC2383 may not be a good sub for 2SC1735 in this application. that being Q15 2SC1735. I need to check my notes on where I got that sub from. Q14 is KSA1013YBU subbing in for 2SA850. I'll swap these out and test the caps and resistors in the morning and go from there. Thanks again.
 
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