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Onkyo M-508. A couple of questions for the experts

I must have tweaked the main circuit board too much when reinstalling it as I found a whole bunch of NEW cold solder joints on the front half of the board (I had already reflowed nearly every one on the back half). Hopefully reflowing all these will get it back up and running properly... so tedious, but it must be done.
 
mondialfan said:
Did you test the LED? I wouldn't suspect the LED has suddenly failed especially since you just completed all of this work. Due to the fact you were working inside I would expect that something is mechanically open in the bias circuit somewhere, either a trace or a wire. I've seen this a happen in amps similar to this that have a separate circuit board for the bias transistor. Check all the wires leaving the main board traveling to the small daughterboard for the bias transistor Q443 are they well seated in the connector and still soldered to the daughterboard?

Per the schematic,
Whats your voltage drop across R421 and is it changing when you move the bias adj pot? How does this V drop compare to the same resistor in the working channel?
Whats your voltage drop from the Anode of D417 to the cathode of D419, in both the working and non-working channel?
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I reflowed about 30 joints on the main board then checked all six small transistors on the heat sink (Q443-Q448: 2SC3419 / 2SC3298 / 2SA1306) they all check out fine- no dead ones and there is continuity from the transistor to the wire to the circuit board. All 12 main output transistors check out fine.

I got no voltage at on R421. Voltage drop from D417+ to D419- is .4mV with power off, and -1 with power on.

I think I'm in way over my head on this one, I might just pack it up and send it to BK precision Audio to fix (the guy that 3D printed the capacitor blocks for me)
 
Okay you will not get a voltage drop across R421 until you figure out whats wrong at D417 & D419. You should have much higher voltage from d417 to D419 than that.

Is D409 & D411 illuminated?
 
skinnypuppy said:
If you can’t figure it out there’s a guy out in Edgewood I’d trust to go over it..pm me if you end up needing his info
Click to expand...
will do... I'm trying a few other things (I cleaned all the liquid flux residue off the board and found 12 more suspicious cold joints) but if they don't work then I'll need someone who is not as amateurish as I am !
 
Does anybody know what adjustment the two metal boxes on the main board adjust? the idle current is set with the blue pots, but I don't know what the others are for:
16AAA. completed board- front.jpg
 
The parallel parts R451, C439 & L401 create a band pass filter to the output stage driver. The adj inductors L401/402 were very likely set to a specific inductance value at the factory and then installed into the boards. It's expected that they will never need adjustment.
 
I think you've probably got a cold solder joint or cracked trace in the front end of the circuit before the bias transistor. If you want to take some comparison measurements we can see if we can isolate your issue.

These measurements are all with the amp powered on. You can connect it to a DBT as long the bulb is large enough to let the amp actually turn on.

What is your DCV as measured across R411 and in the opposite channel R412?

Next clip your ground probe to an "E" connection, the point between R425 & R427 which are attached to each other via "E" looks like a good spot for this. Measure the DCV on the + side of C413 it should be around +3.6V. Now measure the - side of C415 and report your DCV, it should be about -3.6v. We know from your earlier measurement of D417+ to D419- that you only had -1V so one or both of these are missing or incorrect. That D417+ to D419- that you measured before should be measuring approx 7.2VDC.
 
BTW its best if you have a grabber type clip on lead for the ground and if you do not have a shielded red probe for your meter then wrap some electrical tape around the end so only the tip is exposed. This is to minimize the possibility of a slipped probe ruining your day. I've had a few of those bad days and they're best avoided at all costs.
 
I can only check measurements from the bottom side of the board - I'm 6'4",300lbs and my hands are the size of a catcher's mitt and don't fit anywhere in the amp to hook up anything. Also, I can only read a very few of the numbers from the top and cannot locate R425/427, C413/415 or D417 & 419) - I can easily find them from the bottom of the board, but I don't know where to hook up the ground lead to an 'E" (My huge mitts are the reason I had to take the thing completely apart to work on it)

I did reflow EVERY single trace on the board with liquid flux & silver solder, took forever but soo many of them were cracked 1/2 around and a few were completely broken, yet the problem persists...
 
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Can you get to R411 & R412 to measure the DC voltage drop directly across them?

For the other measurements R425 is next to C425 and R427 is also next to C427 unfortunately those resistors are at different positions on the board and not near each other. In your picture of the board in post #87, R425 is at the front left edge of the board. You should be able to measure a short between one side of R425 and one side of R427 this will be the "E" bus. Are the components labeled on the bottom side of the board? If not it may be quite difficult to ensure you're on the right pin from the bottom.
 
The Boards are all labeled on the bottom and pretty easy to read because there is nothing to get in the way and there is room for my mitts
20A. output transistors soldered.jpg
 
You can attach your ground probe to the copper capacitor plate for the channel under test if you're working underneath. It'll have to be the ground plate for the channel that your testing because the two ground planes for L/R are isolated by back to back diodes D435/D437 so you'll not measure the right voltage if you're using the L channel copper plate as ground and measuring the right channel voltages.

Anytime I'm testing on a live circuit I always like to have one probe (usually the ground reference) physically attached. Then I can fully concentrate on where/what I'm doing with the positive probe. If I was working with a set of oversized "mitts" I'd have to get a set of extra long maxi-grabbers like the Pomona 5911B set. This would allow you to keep your hands out of the circuit but reach down inside and clamp onto something for ground reference when working on the top side of the amp boards.

When testing on the backside of the board that is as tightly populated as this one I like to use a needle sharp probe. If you're using a standard sized probe tip then one errant slip and you could be shorting across two pads and your problem could suddenly be much worse with additional dead parts.

One of my pet peeves is going back and forth between PDF pages trying to trace out signals so the first thing I always do is to use MS Paint to create a large unified schematic. I tried attaching one here but it doesn't come up in full rez if you're interested in having it PM me.
 
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I'll give it another look on Tuesday- family commitments until then (AKA the wife wants to do stuff and says leave it alone for awhile !)
 
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