Reparing an old Roland GS-6 Digital Guitar Sound System - Page 1
Forum section: electronics / beginners

author: obiwanjacobi
author_rank: Super Contributor
timestamp: December 22, 2012, 02:47:40 pm
content: The following schema is the part of the unit that doesn't work.The VR board (left) contains 7 pots that are read by the processor. IC18 and IC22 (4066) switch (multiplex) these signals to 4 CPU pins (after going thru a low pass). The program monitors these levels and shows them on the display of the unit (3x7-seg) when they change. So when you turn one of the pots it displays the number (pots 1-7) and its value (0-99).Currently the program thinks that pots 4 and 5 are constantly changing and flashes their values on the display (very annoying).- I have checked the +5V power supply. Its fed by a 7805 and seems fine. There is less than 50mV noise on the +5V.- On the low pass filter I can monitor the pot signals fine. ALL pots work and I can see no spikes (AC coupled various time/div).- The flipflop sends out pretty stable switching signals - if it didn't I should also notice it on other pots.- On the CPU pins I can see the pot levels for the two pots (6 and 7, 2 and 3 and 1) but on pin 42 I see only some small spike. The voltage collapse (or can't source the current for my 1 MOhm probe?). At that point (when pin 42 is collapsed) the annoying flashing stops (the program doesn't see the value changing) and the other pots work just fine.- Then I lifted pin 11 from IC18 and IC20 and tested again. The problem was still there. So I though that at least the passives of the low pass were good.- Then I lifted pin 11 from IC18 and IC20 and soldered a wire to pin 42 of the CPU. The other side of the wire I soldered directly to the input (C42) of pot 5. At that time I "thought" I had it working and that the 4066's were busted.So I ordered new 4066's and replaced them (my first SMD replacement) but when I tried the problem was still there. Luckily I didn't make things worse either.So now I am not sure how to continue. It could be pin 42 of the CPU that is faulty but those old 8098's are hard to come by...Thoughts?

author: PA0PBZ
author_rank: Super Contributor
timestamp: December 22, 2012, 03:03:32 pm
content: Sounds like a broken PCB trace between the pins 10 of IC18 and IC22 and pin 42 of the processor.It seems the only logical conclusion looking at what you describe.

author: obiwanjacobi
author_rank: Super Contributor
timestamp: December 22, 2012, 03:12:06 pm
content: That does not explain why I can't put my probe on CPU pin 42 without it collapsing - and the program stopping the flashing of the values...

author: PA0PBZ
author_rank: Super Contributor
timestamp: December 22, 2012, 03:14:47 pm
content: It does, when you touch it, it is no longer floating....

author: SeanB
author_rank: Super Contributor
timestamp: December 22, 2012, 03:28:49 pm
content: Connect 42 to 41 or 43 and see if the inputs are the same on both channels. If so then broken track, if not sad news you need a new ( or good used) 8098.

author: PA0PBZ
author_rank: Super Contributor
timestamp: December 22, 2012, 03:33:41 pm
content: QuoteDoesn't sound like a broken processor...

author: obiwanjacobi
author_rank: Super Contributor
timestamp: December 22, 2012, 05:21:46 pm
content: Quote from: PA0PBZ on December 22, 2012, 03:14:47 pmBut then it should start to work, not collapse..?Quote from: SeanB on December 22, 2012, 03:28:49 pmI am a bit weary to connect these pins together. Basically you are shorting out any difference in pot-voltage. Can the 4066's take that?Quote from: PA0PBZ on December 22, 2012, 03:33:41 pmThat analysis (when i thought I had it working) is obviously flawed. I am not sure what I looked at in retrospect. So we should probably skip that remark altogether.I will see if I can find any broken print tracks or bad solder joints in the vicinity.

author: PA0PBZ
author_rank: Super Contributor
timestamp: December 22, 2012, 05:30:16 pm
content: Just put a wire where the track should be and see if it works, it will not do any harm.

author: obiwanjacobi
author_rank: Super Contributor
timestamp: December 22, 2012, 06:01:03 pm
content: Yep, broken track alright. The pin 10's of the 4066 were attached fine but didn't connect to the CPU pin 42. So a little wire fixed that.Thank you very much, I learned something new today.

author: PA0PBZ
author_rank: Super Contributor
timestamp: December 22, 2012, 06:16:45 pm
content: Pfew, it was more trouble to convince you than it was to find the problem(maar mooi dat het gefixed is)

author: SeanB
author_rank: Super Contributor
timestamp: December 22, 2012, 06:23:11 pm
content: Dir waar ja, maar hy is nou reg.

author: obiwanjacobi
author_rank: Super Contributor
timestamp: December 22, 2012, 06:44:37 pm
content: Quote from: PA0PBZ on December 22, 2012, 06:16:45 pmAnd that is what I have learned.I can follow anybody's directions, but I want to understand it too, that was why I was discussing it.