kenwood KR-3090 distortion. Help with signal flow and voltage readings needed

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rocknroller

Active Member
Heavy audible distortion in power section when playing music, right channel and fairly lower output then left output volume. Confirmed with a pure sign wave and oscilloscope as well at a couple red dot locations shown below. Bad voltage readings shown in red.
1) Trying to understand the audio signal flow path (I'm having trouble figuring out how the signal flow from this). Was going to trace through the circuit with a scope with hopes to see where the distortion started. Though this may be an artifact of #2 that follows below.
2) Thoughts on bad voltage readings? I checked surrounding resistors. All within spec. Is this an indication DM2 (a STV-4H some kind of thermal check since mounted to the heatsink?) is bad? The Power stage drivers (Qm14/Qm18) are bad, both or something else? I don't want to pull Qm14/18 to test unless it's a last resort, as it would require major (complete) disassembly of this unit to get to to remove and test. But I could work around DM2 if that needed to be pulled.


Kenwood KR-3090.jpg
 
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for grins, yank ASO on the offending channel.....

ETA: a cable from tape out to another amp yields? good signal both sides? (actually this step should be first)
 
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Greetings; Well you do have a schema with the sig. path (usually red or blue) you could try a can of freeze spray hit the transistors one by one & may hit on the defective tr. faster than a trace. Also inspect the solder side for aged solder joints.. transistors do heat and cool that affects ages the solder esp. small joints.
Hope it helps.
 
rocknroller said:
Heavy audible distortion in power section when playing music, right channel and fairly lower output then left output volume. Confirmed with a pure sign wave and oscilloscope as well at a couple red dot locations shown below. Bad voltage readings shown in red.
1) Trying to understand the audio signal flow path (I'm having trouble figuring out how the signal flow from this). Was going to trace through the circuit with a scope with hopes to see where the distortion started. Though this may be an artifact of #2 that follows below.
Click to expand...

Kenwood shows the left channel signal path in bold trace. for the Right channel, see the base of Qm2 to the base of Qm8, base of Qm16, base of Qm20, Rm40/Rm42 junction, ...

Are you getting -31.5v on the emitters of Qm6/Qm8? 0.6v on the emitters of Q2/Q4?

I'd inject a signal, verify the signal at the base of Qm2 and start going through the circuit with a scope.
 
quaddriver said:
for grins, yank ASO on the offending channel.....

ETA: a cable from tape out to another amp yields? good signal both sides? (actually this step should be first)
Click to expand...
ASO? What is that?
Yes I had confirmed preamp output sounds fin (both a rec out and the two line into power amp board)
 
rickl said:
Kenwood shows the left channel signal path in bold trace. for the Right channel, see the base of Qm2 to the base of Qm8, base of Qm16, base of Qm20, Rm40/Rm42 junction, ...

Are you getting -31.5v on the emitters of Qm6/Qm8? 0.6v on the emitters of Q2/Q4?

I'd inject a signal, verify the signal at the base of Qm2 and start going through the circuit with a scope.
Click to expand...

Thanks I didn't catch those darker lines.
-29v on the emitters of Qm6/Qm8. 0.7v on the emitters of Q2/Q4. Same values on the other channel so those are good.

The tracing starting at Q2 confirmed what I had already posted in the schematic above with the red dots and voltages. Signal gets corrupt at the collector of Q8, which is also where the voltages start to go bad.
 
rocknroller said:
ASO? What is that?
Yes I had confirmed preamp output sounds fin (both a rec out and the two line into power amp board)
Click to expand...
ASO = 'avoid senseless overload' (I kid) its the protection where it turns on a trans if a resistor network shows enuf DC at the outputs. If could be the input trans to left/right ASO has turned into skank and it is activating with the music, thereby killing half the signal, or the transistor it turns on that drains the signal is bouncing with the music, just yank all 4 (labelled ASO on the SCM you posted) and see if normal.

If not, wont be hard to rebuild this...everything has a sub.
 
When using an o'scope to trace a fault in an amplifier with feedback the fault will appear to be everywhere inside the feedback loop. You can defeat the feedback by opening the loop but that can cause more problems. Checking the DCV on the transistors may be the easiest way to find the problem.

Craig
 
llwhtt said:
When using an o'scope to trace a fault in an amplifier with feedback the fault will appear to be everywhere inside the feedback loop. You can defeat the feedback by opening the loop but that can cause more problems. Checking the DCV on the transistors may be the easiest way to find the problem.

Craig
Click to expand...
Hi Craig - I believe I have done that (at least those marked on the schematic) and I noted differences in red.
 
You need to measure the base-emitter, base-collector voltages of the transistors not just one voltage here and there.
 
If you notice on the bias diode that the voltages are supposed to be symmetric,+1.2 and -1.2. Your voltage are offset 0 and -2.2. So it looks like you are missing something on the positive side to balance out the negative side. Is your output offset the same?

Check all of the base-emitter junctions, there aren't that many. Any that aren't close to .6VDC is a problem.
 
llwhtt said:
If you notice on the bias diode that the voltages are supposed to be symmetric,+1.2 and -1.2. Your voltage are offset 0 and -2.2. So it looks like you are missing something on the positive side to balance out the negative side. Is your output offset the same?

Check all of the base-emitter junctions, there aren't that many. Any that aren't close to .6VDC is a problem.
Click to expand...


Sorry - I am still learning. "output offset"? Where is that measured?
 
rocknroller said:
Heavy audible distortion in power section when playing music, right channel and fairly lower output then left output volume.
Click to expand...
For my understanding, what does this sentence mean?
i.e. - Distortion in the right channel coupled with lower overall volume compared the the left channel? And you believe the left channel is working properly?
There are many different ways to troubleshoot.
But since you desire to inject a sine wave (which is fine) and scope trace it, my approach would be to look at the left channel sine waves with the scope and gain some understanding of a properly operating channel then look at the right channel. In other words, compare the left to right channel sine waves.
Are you able to inject the sine wave into both channels at the same time?
Can you see the sine wave distorting on the right channel?

Voltage readings: I would take the same approach. Take readings on the good left channel then compare them to the right channel. By checking the left channel voltages first and comparing those numbers to the schematic, you verify your methodology is correct. I say this because you have to know what those schematic voltages are referenced to, chassis or some other circuit common.
 
1rebmem said:
For my understanding, what does this sentence mean?
i.e. - Distortion in the right channel coupled with lower overall volume compared the the left channel? And you believe the left channel is working properly?
There are many different ways to troubleshoot.
But since you desire to inject a sine wave (which is fine) and scope trace it, my approach would be to look at the left channel sine waves with the scope and gain some understanding of a properly operating channel then look at the right channel. In other words, compare the left to right channel sine waves.
Are you able to inject the sine wave into both channels at the same time?
Can you see the sine wave distorting on the right channel?

Voltage readings: I would take the same approach. Take readings on the good left channel then compare them to the right channel. By checking the left channel voltages first and comparing those numbers to the schematic, you verify your methodology is correct. I say this because you have to know what those schematic voltages are referenced to, chassis or some other circuit common.
Click to expand...

Left channel is working fine. Yes I did compare channels with sine wave Dual channel scope) and voltages. The schematic I posted showed the differences. Perhaps not ALL the differences in the whole circuit, as I just spot checked a few points at that time, but clearly the problem areas with the signal gets corrupt and the voltages are bad. At the points I showed (red dots on the schematic), the sine wave is corrupted on the right channel, good on the left. Earlier in the audio path signals are good.
 
rocknroller said:
At the points I showed (red dots on the schematic), the sine wave is corrupted on the right channel, good on the left. Earlier in the audio path signals are good.
Click to expand...
"Earlier in the audio path signals are good"
So your scope probing has shown YOU where the sine wave is good and then bad?
If so, please help us out......at what point, at what component is the signal good? At what point do you see it bad?
Please forgive me.....I'm confused because your opening post states "Was going to trace through the circuit with a scope with hopes to see where the distortion started".

Looks like to me, the schematic has bolded lines showing the signal path for the left channel. Scope the transistor bases, collector and emitters in the bolded signal paths. Compare right and left channel scope findings. That should get you to the area where the sine wave goes from good to bad. Then in that area, take voltage readings of every single point where the schematic shows voltage values and post them in red on the schematic.
 
I didn't catch the bolded lines until a poster informed me. I had already taken voltage readings at every single point where the schematic listed the values. They were either the same or close (where close, they were the same across both channels, so regardless of the reading found, I deemed them good. Or bad. Were bad, I listed on the schematic in my initial post.
I attempted to catch the sine wave with a scope at the base of Q1/Q2, but the signal was messes up on both channels. I assume my scope is somehow loading down the circuit there. I also attempted the same at the Collector of Q1/Q2 (which is the base of Q7/Q8) same result. The first place in the path I can see the signals is the at the collector of Q7/Q8, where is fine on left, corrupt on right (top 1/4 of wave is clipped off like a square wave. Is is there that the voltages all start getting screwy as well, as noted on the original post.

I think this is all a mute point now. As I was gathering some (unpublished) voltages readings and comparing across channel. I slipped on Q7 (on the good channel of course) and fried it dead took the fuse with it. Second fuse confirmed dead dead. Nuts. As I stated in the beginning, replacement of most parts on this particular unit requires a complete a total disassembly. There is no access to the trace side of the board otherwise. Given that, and I don't have a replacement anyway, and there's no clear cause of the initial mystery problem, I think I'm going to simply plug the plug, pull the knobs to save for some future project, and move on to something else more productive.
 
rocknroller said:
I slipped on Q7
Click to expand...
Can you make a guess what parts you shorted with the probe Q7 b/c/e? and maybe xxx
It may help identify the failed parts. The amp is fully discrete so can be repaired. Might
as well get your moneys worth and learn something along the way, like it's not that difficult
to repair. Also, "never" probe difficult to access points like transistor or IC pins, one slip...
probe a component connected to the same point, eg, not Qm7base but yes to Rm9
 
mbz said:
Can you make a guess what parts you shorted with the probe Q7 b/c/e? and maybe xxx
It may help identify the failed parts. The amp is fully discrete so can be repaired. Might
as well get your moneys worth and learn something along the way, like it's not that difficult
to repair. Also, "never" probe difficult to access points like transistor or IC pins, one slip...
probe a component connected to the same point, eg, not Qm7base but yes to Rm9
Click to expand...
I know it hit the base.collector pair, spark, then fuse blew. If I had easy access to the bottom like most receivers that have a removable bottom plate, I probably would bite the bullet and send off for a $10 replacement and in a couple weeks hopefully be back in business to start debugging again.. But there's no guarantee something else didn't get taken out with it. And with now two paths to go down and the additional hour plus it will probably take me to extract the board out of this unit, I simply don't want to to be honest. But I appreciate all the help to this point.
 
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