Shruthi SMR4 mkII,: no sound at pin 3

Discontinued products Shruthi
#1

Hi,
I build my second shruthi (the first one is the SMR4 version 1) yesterday and I have a problem. When I finished the filter board, I test it and it was ok. When I finished the controller board everthing was ok. But later: no sound.
I tried to follow the signal, and I have no sound at pin 3 (just a crapy noise when I touch the pin with the audio cable). I verify all the soldering points that seems ok (I verify that all the socket connectors are ok and it seems ok). I changed all the IC (with the one of my first shruthi): no more success. I also checked all the voltage power supply: without IC: I always have +5V or -5V at the power points.

Do you think I burn one item on the board from the first tests and some time later?

In which area I need to investigate? I don’t have the same filter board working so I cannot check the voltage by comparison. Do you have some points that I can check to find the failure?

Thank you for your help.

Best Regards.

A. Gillièron

#2

Most of the time it’s a bad solder- or connection point somewhere. Even if it looks good sometimes the contact is not good.
I suspect the CV for the filter isn’t coming thru check the connectors between the boards, the trimmer and IC4 and connections around it.
And it is always nice to see a hi-res photo of both sides of the boards.

#3

Hi,

thank you for your help. this is the pictures, I hope it can help.

https://drive.google.com/folderview?id=0B6Ky59dNxs3qNjd6aXFLM1h3NHc&usp=sharing

I’m investigating. I check all the solder connections. The trimmer is ok, because I can mesurate the ohm value when touching the solers connection and the small point on the PCB (put especially for testing?).

I check the output of the 4 AMPS of IC4. The values change when I turn the boutons or play a note. The value of IC4C is constant. So for me IC4 is ok and the resistors and capacitors around that also?

I suspect the transistors Q2 and Q3. is it normal that the Q3 values are very low (about 20mV)?

Thank you very much for your support

A. Gillièron

#4

IC4 pin 7 should vary between -50mV and 100mV (very approximately) when you change cutoff from min to max. Which voltage do you read there?

#5

When cutoff is MIN, I have 44mV
and Max: -38mV

Thank you

#6

sounds OK… with adjustment of the V/OCT trimmer this will get closer to the right range…

Next step: remove IC7. Current between pin 1 and 6 when cutoff = 0 and cutoff = 127?

#7

Hi,
I try to mesurate the current between 1 and 6, but I think I have nothing mesurable. I think about a multimeter problem, but it seems not. What are the I expectables?
The tension between 1 and ground is 1mV.

thank you for your help

#8

Did you use the Ampere setting on your meter? You should measure mA and not mV.

#9

Yes I do… I have nothing (I try also with a second Multimeter, the minimum range of mine is 20uA).
I try to remove also IC4. Without IC4, I can mesurate beween pin 1 and pin 6 of IC7:
0.5mA when cutoff is 0
0.2mA when cutoff is 127

I hope this help

#10

Post a photo of your board? Maybe some transistors mounted backwards?

#11

Hi,
I posted a picture in the drive adress just in the third post. I don’t think there is a transistor wrong position.

#12

Hi,
I finally took the time to trouble shoot. I found the problem. Q2 or Q3 were out of order: I removed both and replace them. Now everything work perfectly.

Thank you for your help

Alain