Pioneer QX-949 left channel noise

[Anonky]

Hi all,

I have a qx-949 that plays music on both channels on all inputs; however, the left channel has persistent static noise. This occurs on all inputs. For what it may be worth, dc offset in right channel is rock solid at 2.5mV. Left channel was like 60mV. Considering this problem and looking at the manual, I honed in on the power amp assembly as the first place to rule out as where my problem. I measured dc at some easily accessible pins and got <10mV at all of them except pins 28 and 31 on one of the amp assemblies (seen on the foil side of the schematic). Each of which I was getting 60-80mV at any one time. Don’t know anything as to what these numbers mean but the discrepancy makes me think this board is where my problem lies. I’m not yet competent at interpreting schematics, but what components are likely culprits given this problem and the measurements at said pins? I’ve attached pictures of the top and foil side of this amp assembly. Any advice is appreciated. I have all day off tomorrow and would like to get some parts ordered. cheers!

 

[larryderouin]

The 949 is chock full of 2sa725, 2sa726, and a few 2sc1451's. These are the top 3 in the Most Wanted top ten list. The 725 & 726 get noisy (think of Rice Krispies) and need replacement. The 1451 is the #1 and can/will take out the amps in a cascading failure. Basically it's a Pre-Emptive 1st strike! Do all the 725/726 on each board.

Q1/Q2 and Q3/Q4 are differential pairs (affect offset). Match for hFe (within 3points) on a tester. Otherwise no matching is needed. But if your OCD demands it, you can match each pair left to right.

Replacement parts:
2sa725/726 = KSA992FTA $0.41ea up to 10, $0.31 for 11 to 100
2sc1451 = KSC3503DS $0.53ea up to 10

 

[Anonky]

The 949 is chock full of 2sa725, 2sa726, and a few 2sc1451's. These 3 are the top 3 in the Most wanted top ten list. The 725 & 726 get noisy (think of Rice Krispies) and need replacement. The 1451 is the #1 and can/will take out the amps in a cascading failure. Basically it's a Pre-Emptive 1st strike! Do all the 725/726 on each board.

Replacement parts:
2sa725/726 = KSA992FTA $0.41ea up to 10, $0.31 for 11 to 100
2sc1451 = KSC3503DS $0.53ea up to 10

Fantastic information. Much appreciated. Ordering 10 of each. Will report back once I get them popped in!

 

[Anonky]

The 949 is chock full of 2sa725, 2sa726, and a few 2sc1451's. These are the top 3 in the Most Wanted top ten list. The 725 & 726 get noisy (think of Rice Krispies) and need replacement. The 1451 is the #1 and can/will take out the amps in a cascading failure. Basically it's a Pre-Emptive 1st strike! Do all the 725/726 on each board.

Q1/Q2 and Q3/Q4 are differential pairs (affect offset). Match for hFe (within 3points) on a tester. Otherwise no matching is needed. But if your OCD demands it, you can match each pair left to right.

Replacement parts:
2sa725/726 = KSA992FTA $0.41ea up to 10, $0.31 for 11 to 100
2sc1451 = KSC3503DS $0.53ea up to 10

As a follow up to your edit, you think 10 of the KSA992FTA is enough to find two matches? Or would you err on the side of caution and just get like 15-20

 

[larryderouin]

Count how many on each of the amplifier(EQ, Tone control, amp) boards(both of them), total and add 25%. They're cheap enough, and if you need 75 of them it's $23.25 & Shipping.

 

[Anonky]

Count how many on each of the amplifier(EQ, Tone control, amp) boards(both of them), total and add 25%. They're cheap enough, and if you need 75 of them it's $23.25 & Shipping.

Awesome. Also I think I just confirmed one of the 2sa725/6 are the problem. Hit them with a spray of cold compressed air and the rice crispies distortion got exponentially worse. Will update you once I get these components replaced!