samson s6 distorted output

[pontiacpete]

The pre amp sounds like it's working properly, I took the pre amp out to another amp and it sounds fine except the reverb feeds back and starts to oscillated. This could be another issue.
Anyway Going out of the "amp in" jack the signal is distorted.
I'm confused about a few things here.
When I probe with the oscilloscope I find the signal is distorted down the middle rail, for instance, on either side of L701.
ON the other side of the R741, the end of the resistor that is connected directly to IC701 pin2, there's no signal.
The +/- voltages are 55 with no ripple.
So my first question is which pin is the input of IC701?

Am i wrong to think that the signal path comes down the middle whrere the ballast resistors meet ?


Help.!!

[Jazz P Bass]

IC 701 is a Protect IC.
The signal comes in, from the left, into the Long Tail pair.
Q701 & 702.
From there it drives Q705 & 708.
Take a few basic Vdc measurements & you may get lucky & find the culprit.
The base of Q710 & 711.(the Drivers)
The base of Q714 & 718 (the Outputs)
Scope the output without a load & then with a dummy load (8 ohms).
Compare the two.

[Enzo]

Yes ^^^^

The output of the amp runs from the inductor, up and over IC701 to the output relay and speaker terminals. R741 samples that output and feeds it to the IC. The IC only controls the speaker relay and is not the source of your distortion.

One thing I like to determine right away is whether the load matters. RUn the amp with no speaker load. Still distorted or now clean? This tells us where to look.

Wwhat is the nature of your distortion? Missing top or bottom half? CLipping? Both sides clipped or just one?

[pontiacpete]

Thanks JP and Enzo for responding.
Here's the voltages on the bases of :
Q710 = 7.1vdc
Q711 = 4.8vdc
Q714 = -.16vdc
Q718 = -.018vdc

Scopiing the output with an 8ohm load the wave is looks ok on the bottom half but the top half is blurred and distorted
Scoping the output without a load it looks similiar, with the blurry top half but the bottom downward part of the wave is dashed.
Hope my description means something or should I take a picture.

Should the voltages be the same on the bases of the transistors?

[pontiacpete]

Thanks JP and Enzo for responding.
Here's the voltages on the bases of :
Q710 = 7.1vdc
Q711 = 4.8vdc
Q714 = -.16vdc
Q718 = -.018vdc

Scopiing the output with an 8ohm load the wave is looks ok on the bottom half but the top half is blurred and distorted
Scoping the output without a load it looks similiar, with the blurry top half but the bottom downward part of the wave is dashed.
Hope my description means something or should I take a picture.

Should the voltages be the same on the bases of the transistors?

[pontiacpete]

Now when I use 2 meters on either side of R721 there's a -.72 equally on Q714 and 718,
I turned svr702 a little.
Maybe my readings were wrong above. Not sure
Are the voltages too low?

[Jazz P Bass]

0.72 Vdc sounds about right.
Maybe a tad high.
Class AB, the devices are barely on.
Q710 & 711 voltages do not make any sense.
If the bases really are that high, then wouldn't Q714 & 718 be higher at the base.

[MarkusBass]

Have you tried checking transistors for shorts/open before probing the amp with the oscilloscope? It looks like your measurements in post #5 were wrong. Can you again check voltages on Q710 and Q711 (including collector and emitter)?
The voltages that you posted are either wrong, or show that there is a problem with the transistors.

Mark

[pontiacpete]

Sorry, for the confusion I don't know what happen with the previous readings so I made some more.
Q710 - B = 1.5vdc, E = .7vdc, C = 57vdc
Q711 - B = -.75vdc, E = .7vdc, C = -57vdc

Do these make any sense?

[pontiacpete]

Sorry, for the confusion I don't know what happen with the previous readings so I made some more.
Q710 - B = 1.5vdc, E = .7vdc, C = 57vdc
Q711 - B = -.75vdc, E = .7vdc, C = -57vdc

Do these make any sense?

[MarkusBass]

No, the voltages on Q711 do not make sense (unless there is a problem with this transistor). The amp is symmetrical so may expect voltages to be symmetrical too. Are you sure that voltage on emitter of Q711 is the same as on emitter of Q710 (and it's positive)? So there is no current flowing through R721. Maybe it's open or Q711 failed. BTW, have you checked any components in the amp yet? Especially R721, Q710, Q709.

Mark

[pontiacpete]

The emitters are both -.7vdc on Q711/710. I found that without a load the voltages are equal but are at .16vdc, both positive.
R721 is not open, it measures 270 ohms in circuit.
I did a diode check on Q711 while circuit and there are no shorts.
I made some resistance measurements
Measuring across CR703 -- 1.1m ohms
Measuring across cr706 -- 390K
Resistance betw C and E
Q710 - 430K
Q711 - 289K

Betw B and E
Q710 - 850K
Q711 - 440K

Q709 - diode check shows no short

Does this help at all?

[pontiacpete]

So you are saying that it's OK that they are both negative?
And both positive w/o a load?

[pontiacpete]

I still don 't know why these readings are not going so well for me.
Maybe I'm grounding my probe well enough,, I clipped to the heatsink which appears to be ground.
pPerhaps there's a bad clip lead in my batch.
Anyway, now I'm reading 14.4vdc on the base of Q710
and 12vdc at Q711's

Sorry for the confusion
So they are conducting?

[MarkusBass]

So you are saying that it's OK that they are both negative?
And both positive w/o a load?

No, I said exactly the oposite. Maybe this is a language problem - English is not my native language. I said that "the amp is symmetrical so you may expect voltages to be symmetrical too" meaning that if you get +1.5V on one side, you should expect -1.5V on the other side. It is hard to help you because you provided some voltages three times and every time they were different. It looks like you have problems with simple measurements. Maybe you shouldn't use heatsink as a ground reference. Can't you find any component that is soldered to the ground? What about the power supply?

Mark