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Onkyo TX-4500: distortion driving me nuts

gadget73 said:
ok, so no bias and audible distortion sounds like crossover distortion issues. Have to figure out why it won't bias, and I'd wager that will sort it out. Check Q504 and the stuff around it, if something is funny there it will likely affect the bias and since it is connected to Q506 it may be causing both issues

wild guess but possibly the offset adjustment is adjusting things far enough that its providing some bias for at least one side of the output pair. If so that should cut down on the distortion a bit at low levels at least. Use that to zero the offset and see what you get from that point.
Click to expand...

Voltages on Q504 and 604 with a load connected are completely different.
Q604 shows high negative voltage (>3v) whereas Q504 does not.
But what the hell does it mean.
I've traced that part of the circuit and nothing was found.
Either i'm blind or I am missing something.

But I agree, this is the first thing I checked when this problem arose.
Both 2SC945 transistors are new (the old ones were destroyed)

BB
 
Bert Beukema said:
Hrmmmm. There are only two ground wires and they are soldered on their terminals.
They lead to a common chassis ground.
Can't see anything wrong there :-(
Click to expand...
If you swapped the internal speaker connections and the issue still reside on the right channel, that's the weird issue, since there's no components between the selector and the speaker jacks besides wire. Mind ohming out both ground wires from terminal(s) to common ground point? That selector that R/L solder to is one component (labeled S814A/B), I assume 2 sided. Both + from left/right should connect to point 2 (if I remember correctly) on both sides, even if one connection is crap the issue should still follow when rerouting the internal speaker outputs.
 
Mico said:
Are you referring to these two mysterious pots? I used to know what they did, will look into my notes.
Click to expand...

Yessir, exactly those smack in the middle of the board and not a peep in the manual on them.
Weird. But then again, the terminals to set idle etc were also missing ;-)
 
Bert Beukema said:
Both 2SC945 transistors are new (the old ones were destroyed)
Click to expand...

no chance the pinouts on the new are different from the old ? I've done that before and spent a lot of time chasing my tail because of it.

those extra pots are odd. Most of the emitter current would flow through the 0.47 ohm but those pots are part of a chain of stuff across that. Output looks to go through a diode to whatever "20" goes to. Whats is terminal 20 connected to? Output power meter or something maybe ?
 
Hajidub said:
If you swapped the internal speaker connections and the issue still reside on the right channel, that's the weird issue, since there's no components between the selector and the speaker jacks besides wire. Mind ohming out both ground wires from terminal(s) to common ground point? That selector that R/L solder to is one component (labeled S814A/B), I assume 2 sided. Both + from left/right should connect to point 2 (if I remember correctly) on both sides, even if one connection is crap the issue should still follow when rerouting the internal speaker outputs.
Click to expand...

No problem of course, I will do this first thing when i'm back in the lab.
For now my wife is threatening me with heavy sailor swearing.
Maybe a good thing to quit for the night (it's rather late over here).

BB
 
gadget73 said:
no chance the pinouts on the new are different from the old ? I've done that before and spent a lot of time chasing my tail because of it.

those extra pots are odd. Most of the emitter current would flow through the 0.47 ohm but those pots are part of a chain of stuff across that. Output looks to go through a diode to whatever "20" goes to. Whats is terminal 20 connected to? Output power meter or something maybe ?
Click to expand...

Actually that did happen when I accidentally used KSC945 transistors instead of 2SC.
But I corrected that mistake, these are most certainly ECB :-)
 
Bert Beukema said:
Oh yeah, there are also the mystery trimpots in the middle of the PCB.
These are labelled R537 and R537.
Unclear what these are doing nor whether or not they could impact the circuit.

BB
Click to expand...
R537 & R637 are part of the protective circuit and used to adjust for low ohm load that the relay will open. 4ohm load relay closed and 2.5ohm load relay open.
 
Bert Beukema said:
for the record, these are the changes made in semiconductors:

2SC1681 --> 2SC2240
2SA706 --> 2SA1220 (tried the old A706 but no change anywhere)
2SC1625 --> MJE15031
2SA815 --> MJE15030

These should all work normally, i've used them often.

BB
Click to expand...
Q501/Q502&Q601/Q602 = 2SC1681 --> 2SC2240
(E-C-B)
Q503/Q603 = 2SA706 --> 2SA1220 (E-C-B)
Q504/Q604 = ????
Q505/Q605 = 2SC1625 --> MJE15031 (B-C-E)
Q506/Q606 = 2SA815 --> MJE15030 (B-C-E)

What was used for the Q504/Q604 ?
 
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I hate to ask this, but did you check all the right channel transistors with a tester just to be sure?
 
Bert Beukema said:
These are labelled R537 and R537.
Unclear what these are doing nor whether or not they could impact the circuit.
Click to expand...
They are part of the current sensing protect circuit. If itt triggers, the relay will click open and power needs to be removed before they will reset. So don't think that is this issue...
Wonderuing if bad base or emitter resistor to output ttransistor. Too bad you are scopeless right now.
 
Hmmm. If Q504 is getting hot, it's passing too much current. Only way that can happen is if it's being switched on constantly (higher or lower than expected base current) or some closely associated component is bad.

I will download the schematic and see if I see anything (no promises, I'm an amateur) and get back. The incorrect voltage should be a clue, look for where it could be coming from, especially a bad resistor since you have not changed those. If you have an open resistor in the bias control circuit, you could be railing the amp after all!

OK, looked at the schematic.

R504 is the center voltage (actually zero balance) VR. Make sure it's good. Should have very near zero volts at the emitters of the output transistors.

R516 is the bias adjustment VR, verify that all voltages around it are good and that you don't have an open resistor or VR. Don't know if you replaced those, I think you did. Make sure you don't have a cracked trace!

This is from the service manual for the base model, I'll look at the Mk II one as well.

Mk II has an integrated differential pair, so if you have discrete transistors for Q501 and Q502 you have the Mk I. Only one VR for bias on the Mk II, center voltage (offset) being off would require replacing the twin transistor.

If you had an output shorted, I'd be quite suspicious of resistors R517 through R522, the resistors in those positions on amps I've had with shorted outputs were fried.
 
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HPMguy said:
Hmmm. If Q504 is getting hot, it's passing too much current. Only way that can happen is if it's being switched on constantly (higher or lower than expected base current) or some closely associated component is bad.

I will download the schematic and see if I see anything (no promises, I'm an amateur) and get back. The incorrect voltage should be a clue, look for where it could be coming from, especially a bad resistor since you have not changed those. If you have an open resistor in the bias control circuit, you could be railing the amp after all!

OK, looked at the schematic.

R504 is the center voltage (actually zero balance) VR. Make sure it's good. Should have very near zero volts at the emitters of the output transistors.

R516 is the bias adjustment VR, verify that all voltages around it are good and that you don't have an open resistor or VR. Don't know if you replaced those, I think you did. Make sure you don't have a cracked trace!

This is from the service manual for the base model, I'll look at the Mk II one as well.

Mk II has an integrated differential pair, so if you have discrete transistors for Q501 and Q502 you have the Mk I. Only one VR for bias on the Mk II, center voltage (offset) being off would require replacing the twin transistor.

If you had an output shorted, I'd be quite suspicious of resistors R517 through R522, the resistors in those positions on amps I've had with shorted outputs were fried.
Click to expand...

Exactly my thoughts. Something failed in that part of the circuit due to cascading apocalypse.
And lo and behold, even though I checked them three times over, a resistor failed.
R622 (10 ohm, 1W) had gone high (23Mo).
I was getting differential readings because of surrounding components.
Desoldering all resistors showed the failure, shows you something about testing in circuit :-(
Changing the resistor = no more distortion = no more overheating of the transistors.

Now let's attach the output transistors again and see if we can set bias.

Many thanks for your insights, it is invaluable to spar in these situations.

BB
 
sregor said:
They are part of the current sensing protect circuit. If itt triggers, the relay will click open and power needs to be removed before they will reset. So don't think that is this issue...
Wonderuing if bad base or emitter resistor to output ttransistor. Too bad you are scopeless right now.
Click to expand...

yeah, i'm flying blind these days but for a few weeks can manage.
I had some DIY DSO scope somewhere but I don't know if I can find the time to get to constructing it.
And learn how the damn thing works. I just bought it as a fun project because it is so tiny.
And my scope is a gentle giant next to it.

So those mystery pots are a protection circuit, does the manual show how to set them up?
I mean, it is near impossible to get to them when the pcb is in the machine.
If there is any setup involved i'd rather do that immediately.

BB
 
hamrules said:
I hate to ask this, but did you check all the right channel transistors with a tester just to be sure?
Click to expand...

Sure, changed them three times and even put back some of the original transistors.
But they all check fine. It was a resistor (a bit hidden) that had gone high in the mega-ohms.

BB
 
hamrules said:
Q501/Q502&Q601/Q602 = 2SC1681 --> 2SC2240
(E-C-B)
Q503/Q603 = 2SA706 --> 2SA1220 (E-C-B)
Q504/Q604 = ????
Q505/Q605 = 2SC1625 --> MJE15031 (B-C-E)
Q506/Q606 = 2SA815 --> MJE15030 (B-C-E)

What was used for the Q504/Q604 ?
Click to expand...

Q504/Q604 = 2SC945
No change on the original transistors used.
Onkyo used a commonly available transistor.
Just remember the KSC version is NOT ECB
 
Sorry, have had an extremely shitty day with a work backlog that just won't give up.
I'll try to do the last steps tomorrow morning.

BB
 
Bert Beukema said:
So those mystery pots are a protection circuit, does the manual show how to set them up?
I mean, it is near impossible to get to them when the pcb is in the machine.
If there is any setup involved i'd rather do that immediately.

BB
Click to expand...
Yes, but requires a signal gen. and O'scope, I think.
 
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