[sdiy] Roland JX-3p vca - noise

[rsdio at sounds.wa.com]

On Feb 28, 2013, at 07:25, Travis Thatcher wrote:
> I ended up working till late last night so I didn't have a chance to
> crack open my synth (which i'll do tonight) but assuming the muting
> transistors have gone south, would it be un-wise to replace the
> 2sc2878 with a regular 2n3904? Any other drop in replacements that'd
> work if not?

Well, the 2SC2878 claims to be designed specifically for Muting and  
Switching Applications, and the first bullet item in the data sheet  
is the high emitter-base voltage of 25 V. The 2N3904 only handles a 6  
V emitter-base voltage. In addition the collector current rating for  
the 2SC2878 is 300 mA, while the 2N3904 only handles 200 mA. The  
2N3904 has an order of magnitude more bandwidth, but it's probably  
not needed. Of course, the JX-3P circuit may not exceed the ratings  
of the 2N3904, but I'd be hesitant to replace a transistor with  
something different if it doesn't at least match the ratings.

I think that you ended up solving this problem, so I won't try to  
find a transistor that seems to match the capabilities of the 2SC2878  
(which is not available at Mouser).


> Also, would it be really harmful to just bypass the muting circuit
> altogether? Assuming I kept the volume on my mixer low on power
> on/off?

I have "fixed" certain home stereo equipment by removing the muting  
circuit. It certainly doesn't harm the normal operations. I  
instructed the owner to turn off his power amplifier before turning  
off the audio device, explaining that I'd removed one muting circuit  
(since it failed in the mute operation mode).

If it were my JX-3P, I'd want to eventually fix the thing back to  
factory original, but I wouldn't feel too bad using it with the mute  
defeated while waiting on parts.

Brian Willoughby
Sound Consulting