G-K Backline 600 Pre amp low power and distorting

[TwinBeech]

I have some strange scope readings in the overdrive channel around the 2 FETs, Q1 and Q2. I am not familiar with what I should be seeing so I hope someone can help.

I will try to attach the pre amp schematic.

At U3-7 I get a nice clean sine wave 21V P/P. At the other side of R17 I get a distorted sawtooth type of 5V P/P. At the Gate to Q1 I get a square wave of 1 V. At the Gate of Q2 I get a square wave of 2 V.At the source of Q2 I get a square wave of 1.2V. At pin 1 of U3 I get a dist. saw tooth of 1.2 V. I have checked the value of R17 and also the diodes in-circuit and they are good. Same for the FETs.

Is this what I should see for the FETs or are they causing the distortion to feed back to the end of R17?

Thanks in advance for any suggestions.

[nickb]

That all looks about normal to me.

What is the actual problem you are trying to fix?

EDIT So you fly a twin do you?

[TwinBeech]

The overdrive channel has low power and is distorted. The Normal channel has distortion but to a lower level.

I used to fly Twin Beeches. I had an Air Cargo company out of New Orleans. Still one of my favorite airplanes.

[nickb]

The overdrive channel has low power and is distorted. The Normal channel has distortion but to a lower level.

I would start with the clean channel as it's easier to work on and a lot less complex.

I used to fly Twin Beeches. I had an Air Cargo company out of New Orleans. Still one of my favorite airplanes.

Certainly they are a favorite. I never got further than ASEL so I don't have any personal experience of them. Still I had a lot of fun with singles.

[TwinBeech]

I will try to post some pictures. They are U3-7; R17 downstream; Q1 gate; Q2 gate

[TwinBeech]

The first thing I find is after the two channels come together on U7-1 i get a good sine wave but on U7-7 it is distorted. Moving the contour pot has no effect. I am leaning toward replacing U7 as a start. First pic is U7-1. Other -7.

[nickb]

The first thing I find is after the two channels come together on U7-1 i get a good sine wave but on U7-7 it is distorted. Moving the contour pot has no effect. I am leaning toward replacing U7 as a start. First pic is U7-1. Other -7.

What is the vertical scale of your scope shots? They are meaningless without that information.

PS: Also, what is the frequency? AC/DC coupled?

[The Dude]

^^^^^^ That, plus, what are the DC voltages on U7?

[TwinBeech]

5V/Div

[nickb]

5V/Div

Ouch! Waaay too high. Set the input source to about 200mVpp, use the clean channel and adj the volume to get 1Vpp on U7-1 and remeasure.

And like the Dude said, check the DC voltages (should be first step before breaking out the scope).

[TwinBeech]

pin 4= -14.3VDC pin 8 = +14.3VDC

[TwinBeech]

With 1 V P/P on U7-1 I get .5V with same waveform as before on pin 7.

Edit: That is with a 400mv sine wave at the input. That is the lowest I could get it to go. I have some pads but for a much lower signal. I sent a query to G-K to ask about a print showing test setup but haven't heard back yet.

[The Dude]

Ok, ......so we have supply voltages on the IC, but what are the voltages on the other pins? Most of the time , if the IC is bad there will be DC on pins where it shouldn't be. Are the rest of the pins near 0VDC?

[TwinBeech]

All other pins are at Zero VDC.

[nickb]

With 1 V P/P on U7-1 I get .5V

What? rms, DC, Vpp?

..with same waveform as before on pin 7.

The same amplitude or shape or both?