GR-300 repair help

Started by larsove, November 07, 2022, 10:50:54 AM

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larsove

Hi!
I'm a new forum user - But I have been using Roland GR300s as an active touring musician for years. Happy to have found a forum where one might get help and tips of keeping these precious machines alive. I am handy with a soldering iron, and have an oscilloscope & Mmeter. but I am really bad at reading schematics, but trying to learn every day!
So - to my question:

I have a GR300 that has no synthsound output. I get clean guitar signal, and I also get LED response on the "String select" section. But I get no synth signal out.
I know the cable and my GR303 is working because I have another working GR300.
I have not recapped the entire board but have all the caps available to do so. But I was wondering if anyone have ideas or tips on where to look with a oscilloscope or multimeter. Of if you have had similar experience with your GR300.

Best,
Lars Ove
Norway

expelec

Always check the power supply voltages first.  And if they haven't been replaced, replace the blue tantalum caps in the power supply.

Ok, you see signal on the String Select LEDs, so input signals are there.  It's unlikely that all the VCOs would be dead. The signals then get mixed and go to the VCF and the output Syn VCA.  You can check pin 15 to see if anything is coming out of the VCF, but my first suspect would be the Syn VCA section.  There's a bunch of stuff involved here but I'd probably look in this order:

1. Check IC14. pin3 to see if you have some voltage when playing.  If not:
     Are you getting Syn Vol signal from guitar?
     Possibly problem with gating based on Total Envelope signal
2. If you have input and output (pin 1) of IC14, could be dead LED/LDR combo in the Syn VCA.  If the LED dies, then you'd have no synth sound.
3. If you have output at IC15 pin 1 (post VCA synth signal), then could be the muting circuit (Q27 etc.) which follows.

HTH

larsove

Thank you!
I have checked the power supply and I've also replaced both blue tantalum caps and both 1000uF35v caps.
Will look into the rest , thank you very much!

larsove

Quote from: expelec on November 10, 2022, 07:28:24 AMAlways check the power supply voltages first.  And if they haven't been replaced, replace the blue tantalum caps in the power supply.

Ok, you see signal on the String Select LEDs, so input signals are there.  It's unlikely that all the VCOs would be dead. The signals then get mixed and go to the VCF and the output Syn VCA.  You can check pin 15 to see if anything is coming out of the VCF, but my first suspect would be the Syn VCA section.  There's a bunch of stuff involved here but I'd probably look in this order:

1. Check IC14. pin3 to see if you have some voltage when playing.  If not:
     Are you getting Syn Vol signal from guitar?
     Possibly problem with gating based on Total Envelope signal
2. If you have input and output (pin 1) of IC14, could be dead LED/LDR combo in the Syn VCA.  If the LED dies, then you'd have no synth sound.
3. If you have output at IC15 pin 1 (post VCA synth signal), then could be the muting circuit (Q27 etc.) which follows.

HTH

I get a constant reading of -5.5 - -6 v out of pin 1 on IC14 - sometimes -8.5(after unit has been powered on for a while). And around 11v on pin 3 while playing .and a reading of 0.09 when not playing.

No output at pin 1 IC15.
(Multimeter set to DC - measured at pins and black lead at "A GND")

expelec

Pin 3 seems to be correct for playing/not playing, but Pin 1 values do not seem correct.  If you have positive voltage on Pin 3 the opamp will try to make Pin 2 equal to Pin 3 and it can only do that by turning transistor Q24 on with positive Pin 1.  Pin 1 should be around voltage of Pin2 (same as Pin 3) + .7V.  If Q24 were bad, open circuit, Pin 2 would stay at ground (via R176) and IC14:1 would saturate near the positive rail, but instead you have negative voltage at Pin 1.  Makes me suspect IC14.

larsove

Quote from: expelec on November 15, 2022, 06:40:54 AMPin 3 seems to be correct for playing/not playing, but Pin 1 values do not seem correct.  If you have positive voltage on Pin 3 the opamp will try to make Pin 2 equal to Pin 3 and it can only do that by turning transistor Q24 on with positive Pin 1.  Pin 1 should be around voltage of Pin2 (same as Pin 3) + .7V.  If Q24 were bad, open circuit, Pin 2 would stay at ground (via R176) and IC14:1 would saturate near the positive rail, but instead you have negative voltage at Pin 1.  Makes me suspect IC14.
Yes, I'll try to find a replacement for IC14 (uPC4558c if I read the schem correctly) and go on from there.
Thank you so much for your guidance - really helpful!

expelec

Yeah, same as RC4558, MC4558, NJM4558 etc.  Might be convenient to put an 8 pin socket in there and try any dual op amp you have: TL072, 082 etc.

larsove

Quote from: expelec on November 17, 2022, 05:30:45 AMYeah, same as RC4558, MC4558, NJM4558 etc.  Might be convenient to put an 8 pin socket in there and try any dual op amp you have: TL072, 082 etc.

Hi again - I swapped out the faulty NEC 4558 - and changed all the caps on the main board and ............ drumroll ............

It's alive again!!!

Thank you for all your help!

Chrismiami

Quote from: larsove on December 02, 2022, 06:10:14 AMHi again - I swapped out the faulty NEC 4558 - and changed all the caps on the main board and ............ drumroll ............

It's alive again!!!

Thank you for all your help!

How many were axial, how many radial?  8)

larsove

Quote from: Chrismiami on April 27, 2023, 11:36:57 AMHow many were axial, how many radial?  8)

All of the caps on the main board are radial. Only the two 1000uF/35v caps in the power circuit are axial