[sdiy] Blown LF13741 and suitable replacement?

[Richie Burnett]

Hi all,

An unfortunate tale, a lesson learnt, and a question for the analogue 
experts...

Today I was giving my old Roland SH-09 a quick test with some powered PC 
speakers that I had handy on my work bench plugged into the line output 
socket.  (Creative T10 multimedia speakers powered from their own Class-2 
wall-wart power supply.)  I played it for about an hour through these little 
speakers and all was fine.

Then I decided to plug it into my mixing desk to record some sounds. When I 
pulled the jack cable connecting it to the powered speakers there was a loud 
crackle (despite me having turned down the volume on the powered speakers.) 
The SH-09 then played out of tune when I connected it to the mixing desk! 
It was initially about 8 semi-tones flat, and playing notes an octave apart 
resulted in less than an octave span in pitch.  This behaviour had me 
stumped, so I pulled the audio cable connecting it to the mixing desk, and 
plugged the multi-media speakers back in again.  Now the VCO plays the same 
frequency (about 6Hz!) regardless of what note I play.  Not a good day for 
the SH-09, or me :-(

On investigation it turns out that the LF13741 JFET op-amp in the keyboard 
CV sample-and-hold circuit of the SH-09 has been cooked.  Playing C notes an 
octave apart on the keyboard presents 1V, 2V, 3V to the input of the op-amp, 
but always about -3.5V coming out.  For what it's worth the VCO still 
responds to changes in the synth's VCO octave range switch, LFO mod, and the 
pitch-bend lever, so I don't think anything else has been damaged.  Fingers 
crossed!

Can anyone recommend a modern replacement for this part, before I pay an arm 
and a leg for what might turn out to be a counterfeit LF13741 on ebay?  The 
device in my SH-09 is actually a circular metal can package, but the pinout 
matches a standard single op-amp DIP-8 package, so I'm guessing a plastic 
DIP version would suffice?

I'm just cross that I didn't think of the *potential* for damage 
connecting/disconnecting Class-2 speakers to a piece of Class-1 earthed 
equipment.  My multi-meter shows 102VAC between the shield on the audio 
cable coming from the Creative powered speakers and mains earth!  Modern 
gear is probably more tolerant than 1980's technology, but definitely 
something for others to be aware of to reduce the risk of damage to other 
vintage gear.

-Richie, 


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