Vintage Synth Repair

Hi All,

I have an Alesis QS6.1 that developed a subtle but annoying hum in the audio outs (main and phones) after around 10 years of good service. I'm suspecting of capacitor malfunction in the power supply.

Has anyone ever serviced the power circuit capacitors in the QS6.1? Were you able to eliminate the hum by replacing them?

Thanks,

~ACO

I don't have a QS6, but the power supply caps are the most likely cause of the problem. They are usually not difficult to replace once you get the synth opened up, and the values of the capacitors are not critical. The rule of thumb is to replace with caps of equal or greater capacity. The only way to find out if they are causing the hum is to replace them. Usually they are not expensive to buy compared to some other types of components such transformers, for example.

> caps of equal or greater capacity
>


And equal or greater voltage :-)


Daniel Forro




On Nov 14, 2015, at 1:57 AM, John Karns johnkarns@...  
[vintagesynthrepair] wrote:

> I don't have a QS6, but the power supply caps are the most likely  
> cause of the problem.  They are usually not difficult to replace  
> once you get the synth opened up, and the values of the capacitors  
> are not critical. The rule of thumb is to replace with caps of equal  
> or greater capacity. The only way to find out if they are causing  
> the hum is to replace them. Usually they are not expensive to buy  
> compared to some other types of components such transformers, for  
> example.
>

Hi everybody,


enjoy some Christmas music on my new YouTube channel if you 
celebrate :-)


Daniel Forró - Forrotronics

I've got KORG PE1000 and could repair some problems like not working  
tones and other usual stuff on the instrument which was not used for  
long time.


Now everything works except one vibrato oscillator - LFO.


There are three LFO's, first one controls first three tones on the  
keyboard, second one the next three notes, third one the next three  
notes and this pattern is repeated throughout the keyboard. So I have  
now vibrato on keys 4 to 9, then 13 to 18, etc. Coupler notes one  
octave higher follow the same pattern but shifted, because note one  
octave up use different LFO. Very clever idea, which helps to get more  
animated sound - all LFO's have the same speed and depth (controlled  
by pots) but different phase.


So I suppose one LFO doesn't work. By measuring voltages at LFO  
outputs (points 50, 53 and 54 in schematics on the page 7) it was  
confirmed - while LFO 2 and 3 have changing triangle wave on the  
outputs and voltage changes from approx. 6.5 to 7.5 Volts, LFO 1  
output gives steady 7.11 Volts.


I have attached Service Manual and I'd like to ask more experienced  
electronic gurus here:


- Which component in LFO circuit would you considered faulty with the  
highest probability? I don't want to change all of them... Could be  
only one half of LM3900 working and the other faulty? Or is it  
transistor?


- How I can find which component is faulty? I have digital voltmeter  
and oscilloscope....  Can I find wrong component by measuring and  
comparing voltages in different locations of the LFO 2 or 3?


Thanks for any hint...


Daniel Forro


P.S.: If attachments are not allowed here, I will send Service manual  
by PM, or it can be found here for download:


http://elektrotanya.com/?q=showresult&what=korg%20pe-1000&kategoria=&kat2=schematics

I've got KORG PE1000 and could repair some problems like not working  
tones and other usual stuff on the instrument which was not used for  
long time.


Now everything works except one vibrato oscillator - LFO.


There are three LFO's, first one controls first three tones on the  
keyboard, second one the next three notes, third one the next three  
notes and this pattern is repeated throughout the keyboard. So I have  
now vibrato on keys 4 to 9, then 13 to 18, etc. Coupler notes one  
octave higher follow the same pattern but shifted, because note one  
octave up use different LFO. Very clever idea, which helps to get more  
animated sound - all LFO's have the same speed and depth (controlled  
by pots) but different phase.


So I suppose one LFO doesn't work. By measuring voltages at LFO  
outputs (points 50, 53 and 54 in schematics on the page 7) it was  
confirmed - while LFO 2 and 3 have changing triangle wave on the  
outputs and voltage changes from approx. 6.5 to 7.5 Volts, LFO 1  
output gives steady 7.11 Volts.


I have attached Service Manual and I'd like to ask more experienced  
electronic gurus here:


- Which component in LFO circuit would you considered faulty with the  
highest probability? I don't want to change all of them... Could be  
only one half of LM3900 working and the other faulty? Or is it  
transistor?


- How I can find which component is faulty? I have digital voltmeter  
and oscilloscope....  Can I find wrong component by measuring and  
comparing voltages in different locations of the LFO 2 or 3?


Thanks for any hint...


Daniel Forro


P.S.: If attachments are not allowed here, I will send Service manual  
by PM, or it can be found here for download:


http://elektrotanya.com/?q=showresult&what=korg%20pe-1000&kategoria=&kat2=schematics

Hi, Fran,


you were absolutely right, it was that opamp. Now new one sits in the  
socket and everything works :-)


Your band looks well! I wish you lot of gigs and fans!


Daniel Forro






On Jan 15, 2016, at 10:32 AM, narfman96@... [vintagesynthrepair]  
wrote:

>
>
> Hi Daniel,
>
> I would replace the bad LM3900 if you are certain the output is  
> stuck at 7 volts. Compare the outputs to the other LFO's. Pin 5 is  
> output 1, pin 4 is output 2, pin 9 is output 3, and pin 10 is output  
> 4. If you can find the offending output voltage stuck at 7 volts you  
> have the bad chip. Everything downstream will be 7 volts too. This  
> is what a tech that works on op amps all the time would call stuck  
> at the rails (power supply voltage).
>
> HTH,  Fran
>
> P.S.  I did not send that other email. Sorry I never replied. We've  
> been busy learning new material plus I've been out booking more  
> gigs. Added some 90's tunes for the millennium crowd.....  The Alibi  
> Band
> The Alibi Band
> Home Music Media Gigs Contact
> View on www.thealibiband.net
> Preview by Yahoo