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Beat up Marantz 2226b bias help

Found the KSA992FBU's. The originals in the unit are not bonded with shrink tube, although they are facing each other. Should I bond them and is there any benefit, or should I just leave them separated? Thanks, Rich
 
Steven Tate said:
Probably optional, but if they can easily be shrink-wrapped, I would do it.
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Installed the new ones and did shrink wrap them. Bias rises slower but still consistent. Starts at about 22mv and slows way down in the mid 50's. Checked the dc offset and the power supply voltages and both are set to spec. Any other things to check? I did check most of the resistors in the area and they must have been directly connected to Q728 as they all read pretty close to spec while in the circuit (with the split transistor), but now, with the new transistor, none are even close when checked in the circuit. I think I am going to change out the caps I can get to while I wait for some insight form AK. Thanks, Rich
 
Any time there is a question about the measurements on important resistors in circuit, lift one leg and measure. Just takes a minute, and then you know. The values in that area are important to get the proper bias. Same for trimmer resistance.
 
Steven Tate said:
Does the bias voltage change when you move trimmer R740?
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I can adjust it up, but it is bottomed out trying to lower the bias. It starts about 22mv or so and slowly climbs to the mid-fifties before I turn it off. It goes very slowly after about 30mv but is a steady climb. The unit plays and sounds clear, but I know this is something you should get right. This unit is very hard to work on because to remove the power amp/power supply board you have to disassemble quite a bit, so I am looking on the component side, judging where the component is and then hoping I pick the right solder joint on the foil side, ugh. Thanks for your help Steven, since it rises steadily I am going to guess it going to be a resistor near the transistor I replaced. I think it would be off but steady if it was part of the trimmer circuit. Guess I'll knuckle down and start testing resistors. Rich
 
Steven Tate said:
Bias setting correctly on the other side?
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Haven't checked it since I've been playing with this side, but it did before. Checked 6 resistors in the area and one diode and all were good. Replaced Q716, Q718, and Q726, and the problem remains. Sounds fine, let it play for a while and no issues. The bias tops out at 54.5mv on the right channel.
 
I hate amp problems! Looks like the blown up transistor was on the right channel and the bias issue is on the left if I'm reading the previous posts correctly. But you've changed parts on the left channel so I will assume the issue is on the left side. If not, it's not hard to locate the equivalent devices on the right channel.

Here are some suggestions but first, my amp mantra... Check every last transistor and diode along with all power resistors. OK, we're back!

Check for about 1.2Vdc across the bias dual diode Q738 = pins J730 (-) & J728 (+).
Power off - check the emitter 0.47 ohm resistors R760 and R758.
Check the outputs with the PNP (Q736) one being most suspect given the borked A777. The SM says it's a 2SB617. Hopefully you have a tester like a Peak DCA so you can check gain and leakage.

BTW - Noted two schematic errors. First, but on the right channel, is a voltage at Q710-E of 32 when it should be -32. Second, on the left channel, is an incorrect junction dot at Q710-C to the left side of R722. It does not belong.
 
P.S. Also thought that perhaps the A777 might have opened a trace on the board's back side so check for that too.
 
Richs_Trains said:
Randy, It's 1.7v across the bias diode.

All good
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My bad on the 1.2V, the schematic shows a double diode but is listed as STV-3H so is 3 diodes. You can check against the same points on the other channel - just don't slip and short anything out!

Did you check Q736 as suggested in post #29?

Is the bias pot set for 0 ohms or 100 ohms?
 
rBuckner said:
My bad on the 1.2V, the schematic shows a double diode but is listed as STV-3H so is 3 diodes. You can check against the same points on the other channel - just don't slip and short anything out!
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The other channel is 1.7 also. I did not pull the output because the unit plays clear and even on both channels and I'm afraid if I take it that far apart, I'll never be able to put it back together. I'm nowhere near as talented as most of the people on AK. Thanks, Rich
 
Have not been able to solve the bias problem on the right channel, it still goes up to 59mv and holds there. This is the first unit of this type that I have worked on where the amps and power supply are on one board and it is very hard for me to determine which components are on the left channel and which are on the right. I have pulled several resistors and they have all tested okay, but pulling one leg and then trying to re-seat them is hard for my fingers to do. I have noticed that the parts list has many "pairs" of resistors (ones of the same value) I am wondering if I check and compare these "pairs" in-circuit will it help to point me in the right direction to get me close to the problem? I do have a functional left channel that tests in spec and actually the right channel plays and sound fine, but the bias is 59mv as opposed to 18.5mv. Thanks for any help, Rich
 
Steven Tate said:
Have you checked the bias pot to make sure it is in spec?
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I have not, it appears to be R740 listed as a 100ohm trimming resistor. I am assuming I need to remove it to test it, but since it has three legs, how do I go about checking it with my DMM and what reading range should I get? It is currently bottomed out counter-clockwise and I can only get the bias down to 59mv. Thanks, Steven. Rich
 
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