Help needed! shruthiXT + 4PoleMission filterboard


#1

hi everybody,
i thought i give this another shot.

i have build the shruthi XT with a 4pole mission filterboard and from the beginning on it seemed to work fine, i was just not sure about the filter section, as that seemed very “dirty” sounding with a lot hof hi-frequency noise when the cutoff or resonance was dialed up even a little bit. first i thought this is how it is supposed to be and was quite disappointed. then i realized the sound has changed over time, so depending on how the enclosure was positioned it sounded different. that’s not right.

now i took the whole thing apart again for furter test and to check for bad connections (cold solder and such). that i couldn’t find.

but i realized that the 7805 voltage regulator is warm, which i guess is ok, but when i connect the filter board to the XT board, it gets superhot. and i wonder if that would be due to a short somewhere. strange thing is that all the controls and display are working. and until yesterday i even had the same sound again.

then i have read up on the volume pot connection variations, and changed my wired one to the “all bridged” on board version, as i thought maybe the noise gets picked up in the wires. so now i don’t have any sound at all anymore.

still the controls and display works, but the 7805 gets superhot.

and another thing which baffles me is something with the “lightshow” leds, which have been previously connected with wires, which i then took off. now i have just one LED directly on the board. and it is on or off depending on if or where i touch the IC8 right next to it. that is very strange, isn’t it?

ANY HINTS OF WHERE TO CHECK FOR WHAT ARE WELCOME. I HAVE NO CLUE ANYMORE. PLEASE HELP ME!!!


#2

Maybe you could post HD pictures of both sides of both boards?


#3

thanks for your quick response.

here are pictures of both sides of both pcbs.
hope you can spot something wrong.
while taking the pictures i have found one huge mistake, which was:
C37, the one right behind the 7805 on the 4-pole board has been the wrong way around. i flipped the cap, but the results are the same. perhaps that wrong polarity has lead to damage somewhere else? where?

oh, i can only post one picture at a time, bummer.

!


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#9

Looks like you have some capacitor problems.

  1. You could measure the resistance between the output of the regulator (7805) and Gnd. Should be in the range of several hundred Ohms or more. And when powered on measure the voltage between the same pins (5V?).
  2. I wouldn‘t trust the polarity-reversed capacitor (C37) anymore. It‘s probably damaged and causing the regulator to get hot.
  3. C31 should be a tantalum cap.
  4. C9, C15, C22 and C26 should be Polystyrene or Styroflex capacitors with 1% or 2% tolerance. Yours have 20% and more and are not suited for this application. You will hear the difference!

#10

thanks for your reply as well.

  1. resistance from out to ground of the 7805 is 494ohms. the output voltage is 4.9V when the input is 11.11V (that’s what my PSU gives me).

  2. i have replaced C37

  3. and 4. those are good points, that i must have skipped from the BOM, when i have ordered the parts. will try to get the right ones soon.

i also have not found the super exact resistors, for the ones that had very specific values, so i used some that are as close as possible to the original values. the difference is behind the comma, though. (where would i get them, anyway? i’ve checked ebay and went to the biggest electronic components supplier in finland, without finding precise matches.)


#12

ok,

now i have replaced the caps with the right ones (tantalum and styroflex). the filter works much better, but still there are some high frequency artefacts in the signal. they sounds like tuning a radio or vinyl scratching sometimes!?

and the first button from the left (OSC/MIX - SEQ/ARP) does not react anymore. both LEDs function in jam mode, when the sequencer runs, means they blink. what could cause that the button, when pressed is not to react? i cannot access the related page in the menu anymore.

any help still very welcome! don’t know how to fix the issues myself.


#13

Regarding the button problem: have you tried resoldering the joints?
In general: I can see a lot of bad solder joints. Use a little bit flux to resolder them. Or if you don‘t have flux reheat the joints until they melt and then feed a litle bit fresh solder.
Another tip: check your power supply (wallwart). Is it delivering enough power?


#14

hey,
thanks for the reply.
i actually have resoldered the joints before, but now i did it again.
still the S14 button does not function.
might it be that somehow the chip that is reading the buttons state, is malfunctioning? i might have shorted something with the cooling parts of the voltage regulator when they where in a straight position while “sandwiching” the control-board and the filter board. which IC is connected to the S14 button?

the power supply seems not to be the problem i tried different ones.

thanks for any hints.

grm.