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PG200 switches work, knobs don't work - FIX Keyboard Synthesizers
Old 31st August 2011
  #1
Lives for gear
PG200 switches work, knobs don't work - FIX

I remember a post here sometime back where a guy had a PG200 that wouldn't send data with the knobs, only the switches.

Recently I had the same trouble (after leaving the mks30 pg200 on all night).

The fix was to replace IC8, the LM311 comparator in PG200.

I couldn't find the original post so I started this thread. Hope it helps someone.
Old 11th August 2013
  #2
Here for the gear
just wanted to report that this made my day yesterday!

ordered a 311 voltage comparator from ebay after reading this. soldered in and good as new!

thanks so much.

i think what happened is in my haste, i plugged in the cable incorrectly. i blew a fuse in the jx3p and the pg200 knobs stopped working.

going to be much more careful in the future, but thanks for this. back in business!
Old 10th June 2015
  #3
Is anyone still having trouble with the comparator? I'm just doing a repair on one, and the part has been replaced before - apparently it's been used 3 times since the last fix.

I've checked the tolerance of the surrounding resistors, and they're fine.
Old 17th March 2016
  #4
Lives for gear
 

I am having the same problem with a PG200 here in my shop. The switches work but the potentiometers do not make any changes to the sound. Based on the following, I am starting to suspect the CPU has a problem on pin 1 which receives the analog signal from the potentiometers. The multiplex circuit works, the D/A words, the comparator works, but the CPU does not respond.

In getting to this point, I have replaced the LM311 as well as the op-amp and tested every passive component and every PC board trace around this area, and checked the CPU clock and all supply voltages for glitches on a 150MHz scope.

Scope photo of test point 2 showing the D/A converter output (top) and test point 3 showing the multiplexer analog out (bottom).


Scope photo of LM311 output, with the output disconnected from the CPU and hooked directly to the scope (top) and test point 3 (bottom). I pulled the output pin out of the socket because the signal was basically gone when connected to the microcontroller, less than 30mV and buried in noise.



Anyone else come into this issue?

Tim
Old 2nd May 2016
  #5
Here for the gear
 

Same Problem with the knobs

Hey friends!

got the same issue with the IC8. Would like to fix it myself or give to a friend who could do it but I don´t know really how to open it up properly. the bottom part is no problem, but i don´t know how to handle to top with all the switches and knobs...could sm.one help me out?

Old 14th September 2016
  #6
adc
Gear Head
 

Hi all. I'm having the same problem so am so happy this post is here! I'm planning on replacing the LM311 but is there way for me to test to see that's actually blown? It's getting the +/-15V supply and most pins have some voltage, the IN- and IN+ have the same 0.4V. Thanks!
Old 14th September 2016
  #7
adc
Gear Head
 

Quote:
Originally Posted by KerrierDistrict View Post
Hey friends!

got the same issue with the IC8. Would like to fix it myself or give to a friend who could do it but I don´t know really how to open it up properly. the bottom part is no problem, but i don´t know how to handle to top with all the switches and knobs...could sm.one help me out?

You just have to pop all the rotary knobs off, pull them off or leverage them carefully with a flathead screw driver. Then the board will come free.
Old 14th September 2016
  #8
Here for the gear
 

changed IC8, still no knobs

Quote:
Originally Posted by adc View Post
You just have to pop all the rotary knobs off, pull them off or leverage them carefully with a flathead screw driver. Then the board will come free.
cheers mate



I recently changed the ic8 for the knobs but still the same problem. any new insights on this issue?

Thanks
Old 15th September 2016
  #9
adc
Gear Head
 

Good to know that there' s other points of failure... since it seems like my lm311 is possibly ok... now if we only knew what else could have failed.

I see in the schematic that IC7 is also in that chain, so that could also be the problem. I just discovered that my LM311 is super hot... so pretty sure that's where my problem is. I'll try replacing that first.

Last edited by adc; 15th September 2016 at 02:45 AM..
Old 22nd September 2016
  #10
adc
Gear Head
 

Fixed! It was the LM311... one way to figure out if that's the problem is if it gets very hot.
Old 19th August 2017
  #11
adc
Gear Head
 

Actually, it's not fixed, seems the 311 is still burning out, so I'm currently looking for the source of this, something up stream of the 311 I guess.
Old 19th August 2017
  #12
Lives for gear
Quote:
Originally Posted by adc View Post
Actually, it's not fixed, seems the 311 is still burning out, so I'm currently looking for the source of this, something up stream of the 311 I guess.
Search for shorts
Old 22nd June 2018
  #13
adc
Gear Head
 

Im still having no luck with this. I tried replacing the lm311 (ic8), the tl082 (ic7), most of the electrolytic caps and I even replaced the m5L8048. Still the same problem, the LM311 gets very hot.
When I plug in the LM311 without the gnd pin it doesnt get hot, and without the -15v pin it doesnt get hot. Not plugging in any of the other 6 pins makes it still heat up.
Any help is much appreciated!
Attached Thumbnails
PG200 switches work, knobs don't work - FIX-jx3p_schematics_06_pg-200.jpg  
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