Step 9

[Darin]

Should the value of R 31 ( 27K ) change after is it on the board ? I measured R 8 ( 27K ) on the board and it still measures 27K but R 31 measured 8 K. Thinking i made a mistake i removed it and it measures 27K off the board just as it did before i installed it. But I am only getting just over 2 volts at TP9 where as i should be going from -5 volts to + 5 volts acording to what i read in my instruction. Could i have a bad resistor ? I dont have another 27K to try.

[Qiaozhi]

Should the value of R 31 ( 27K ) change after is it on the board ? I measured R 8 ( 27K ) on the board and it still measures 27K but R 31 measured 8 K. Thinking i made a mistake i removed it and it measures 27K off the board just as it did before i installed it. But I am only getting just over 2 volts at TP9 where as i should be going from -5 volts to + 5 volts acording to what i read in my instruction. Could i have a bad resistor ? I dont have another 27K to try.

Measuring component values in-circuit is not reliable, as other components can affect the results. If you want to check the value of R31, then disconnect the Audio pot first. Also, it would be worth checking (with the pot disconnected) that one of the outer pins of PL6 measures as +5V, and the other as -5V.

[Darin]

Measuring component values in-circuit is not reliable, as other components can affect the results. If you want to check the value of R31, then disconnect the Audio pot first. Also, it would be worth checking (with the pot disconnected) that one of the outer pins of PL6 measures as +5V, and the other as -5V.

PL6 on outer pins has -4.98V and +4.97V ( with the pot disconnected ). With pot connected R31 has a range of -4.75V to +4.75V on the side coming from PL6. The other side of R31 is ranging from -.16V to +.16V. TP9 has the same .16V range.

[Qiaozhi]

PL6 on outer pins has -4.98V and +4.97V ( with the pot disconnected ). With pot connected R31 has a range of -4.75V to +4.75V on the side coming from PL6. The other side of R31 is ranging from -.16V to +.16V. TP9 has the same .16V range.

I don't have an MPP open on the bench to double-check at this time, but I'm certain that the voltage swing at TP9 should be greater than 160mV.
When the audio pot is turned to the +5V end, the voltage at TP9 should approach -5V, and vice-versa. I would suggest checking the component values around U9a, and possibly replace U9a itself. From your measurements it seems like U9a is operating as a unity-gain amplifier, instead of amplifying the voltage at the inverting input by 47x.

[Darin]

Thanks Qiaozhi,
I will try that.

Thanks for the help.

Just relized I should have labled this post TP9 not step 9 sorry havent got there yet.

[Darin]

Thanks Qiaozhi, I checked all the components again then ordered a couple 27K resistors, TL071 's as well as some other parts to try and get my SM PI PRO 3 working from Mouser.

I got some parts in from Mouser today, replaced R31 ( 27K ) voltage is now good on both sides of R31 and at U9 but TP9 was still at 160mv.
So I replaced TL071 and now everything is working as it should.
I also got a 5K audio Taper for the external volume control, maybe I can finish it up tomorrow night.

Thanks again for you help

Darin