Vintage Synth Repair

Your previous mail came, too...


I have SK20, do repairs of my equipment, but my instrument is OK so  
there was no need to open it and start to work on it. I also didn't  
study Service Manual more deeply. So I'm afraid I can't help much...


The only thing I can say is that Roland volume pedal has different  
connection, different pot inside, so generally it's not recommended to  
use it for Yamaha instruments (or Yamaha FC7 for Roland instruments -  
when I wanted to use it with my Roland V-piano, I had to change  
internal connection).


I'm not sure, but maybe this could cause some problems you describe.


Daniel Forro




On Feb 26, 2017, at 4:58 AM, Karl Knuttel pinksoir@...  
[vintagesynthrepair] wrote:

>
>
> Hi,
>
> My SK20 is exhibiting strange behaviour.
>
> The first issue is that the 'Strings' section signal is audible in  
> the 'Polysynth' section mixer output.
>
> To explain a little better, there is a mixer section for Organ,  
> Strings, and Polysynth before the Master Volume and main output.  
> When the String section fader is set to its minimum it can still be  
> heard through the Polysynth section. So when I have the Polysynth  
> fader up and play the keyboard, the Strings section can be heard  
> underneath the Polysynth output at a low volume. It also appears to  
> overdrive the output so that it slightly distorts.
>
> The bleed occurs only on the Polysynth mixer slider. If I set the  
> Polysynth so that it outputs nothing (ie. ADSR all set to 0, filter  
> set to lowest, etc.) and set the Polysynth mixer slider up and all  
> the other sliders down, when I depress any keys I can hear the  
> String section coming through the output.
>
> If I do the same with the Organ section, the problem doesn't occur.
>
> I checked the volume pedal input. There's strange behaviour there. I  
> plugged in a Roland EV-5 foot pedal and at first it altered the  
> volume of the respective sections (Strings or Mixed). Then it ceased  
> affecting the volume. There was a smell of solder/component burn  
> coming from the synth. There was a similar smell emanating from the  
> synth last night when I was checking it as well.
>
> So now when the volume pedal is plugged into either of the pedal  
> input jacks, the respective sections are muted. Very odd.
>
> I opened up the synth and took out the JK board but visually it  
> looks fine. No obvious damage to either of the ICs and the caps all  
> seem to be ok.
>
> The second issue is that the transformer now seems to be  
> overheating. I left the synth plugged in but powered off and the  
> transformer began to smoke and there was a smell of melt coming from  
> the area.
>
> Any help would be greatly appreciated. Here is the service manual: https://elektrotanya.com/yamaha_sk20_synthesizer_sm.pdf/download.html
>
> (Apologies if this is a double post - I posted directly to the group  
> but it didn't seem to go through)
>
> Cheers,
> Karl

Does somebody here any experience with the repair of Electone EL organ?


I've got EL500 (1998) with some problem (from a friend who wanted to  
throw it away) and would like to revive it. The problem is -  
instrument sounded about 15 minutes, then it got frozen, and the next  
attempts to get it work did only this:
- display backlight got on but display was blank
- muting relay on the amp board was activated after three seconds
- instrument didn't start, was locked before the boot


Hard reset nor an attempt to enter Test mode didn't work.


I couldn't find Service manual on the net, but supposing that the  
other EL are build in similar way - which was right - I could open the  
machine following the Service manual for EL90. In the hope that  
cleaning it inside and reseating the connectors and ICs in sockets  
would be enough...


So I have cleaned all from the dust, disconnected boards and put them  
out one by one, check carefully for bad caps, cold solders or other  
problems, cleaned all connectors, repaired slightly deformed membranes  
of tweeter speakers (checked if they are not shorted), cleaned FDD,  
reseated three Mask ROMs on the main board...


I didn't disassembled panel switches board, display, keyboards, small  
boards with connectors, knee lever switch, pedal keyboard, foot  
controller, board with fuses near the transformer...  All looks well,  
like new, even after 19 years.


Then I have assembled all, and after switching on the instrument  
sounded around 15 minutes, then again locked... Then I tried Test  
mode, this time it worked, but I could do only RAM and ROM test.  
During Rhythm Tone Check it was again locked... (but all rhythm sounds  
sounded well when I tried them before).


I tried to disconnect power cable to amp, but instrument didn't  
started at all, no display backlight - probably OS can recognize the  
missing amp and doesn't start.


Now I'm almost lost and don't know what can be wrong there. I suppose  
it is known problem, and the technician with the experience with  
digital Electone would know what it is and how to repair. Maybe  
something on amp board, or problem with the main board or OS Mask  
ROMs? But why... that friend is not aware of any reason why it should  
start such behavior suddenly, they got it in recycle shop, and could  
use it for few years without any problem. Inside everything looks  
pretty well, like new.


I couldn't find any info except somebody had no sound at all but  
instrument worked, and it was some resistor fuse between two pins of  
SANYO amp IC... I don't think it is my case because instrument behaved  
quite normally after switching on, and locked itself after 15 minutes...


I can disassemble and check all the other parts, or replace bigger  
electrolytic caps on amp board, and on another board near the power  
transformer and fuse board, which makes some voltage filtering  
probably... but it can be unnecessary and will not help...


I would be grateful for any hint or advice, or link... Of course  
Service manual copy would be great...


Thanks in advance.


Daniel Forro

I'm working now on EL500 from 1998 which I've got from a friend who  
declared it as non working. After 10-15 minutes of work it stays  
frozen, locked, and following switchings on are unsuccessful - output  
muting relay clicks after 3 seconds, and instrument doesn't boot. Next  
day the same...


I could perform Reset, it didn't help. Test mode showed ROM and RAM  
OK, I couldn't try more tests.


I did cleaning inside, everything looks well, like new instrument.  
Pulled out amp board, main board with two sub boards connected (XG  
daughterboard, effect board) and some other small boards, checked  
caps, checked solders, resolder some, repaired little bit damaged  
membranes of tweeter speakers, reseated three big mask ROMs in  
sockets... no help.


I suspect usual problem with aged components - namely electrolytic  
caps... The problem can also be dependent on warming some components,  
maybe... I'm not sure. Somebody's advice was to replace Xtals...


So for start I'm thinking about the replacing all bigger electrolytic  
caps in PSU and other supply circuits (they are distributed on more  
boards). After purchasing schematics I could found pretty hidden small  
switching supply and pull it out. It looks well.


In this context my question is: can I switch on this PSU outside the  
instrument, that means without usual load, and measure output  
voltages? Is it safe? I don't want to damage it...
Output spec is:
+5 V/5 A
+12 V/0.3 A
-12 V/0.4 A


It's not Yamaha, but Sharp product. It's interesting to find whole  
block inside Yamaha product not made by Yamaha itself. Before I have  
found it only in Yamaha TX816/416 FM rack modules, and it was also PSU  
from different manufacturer.


Thanks for any advice or hint what I should check or replace.


Daniel Forro

You should be able to check the power supply voltage both connected and unconnected to the synth circuit boards. You may see higher voltages when disconnected, depending on the amount of expected loading of the circuit board. But if you are seeing voltages which are roughly close to the right voltages, and not zero, you have a working power supply. Those voltages themselves are safe, although obviously exercise due care when probing the power supply to avoid the AC voltage supply.


Leigh
--
Leigh M. Smith
mailto:leigh@leighsmith.com
http://www.leighsmith.com