Vintage Synth Repair

--- In vintagesynthrepair@yahoogroups.com, "Scott S." <picoshark@...> wrote:
>
> I'm working on a SH-101 that has tuning problems, and I can't seem to
> get the modulation to stop. I am not very familiar with operation of
> this, so some may be due to operator error. I have the service manual.
> 
> At one time there was a C battery leak, but it apparently was lying
> flat the whole time, the corrosion was contained to the metal bottom
> lid.
> 
> The tuning is way off, the entire keyboard doesn't quite cover one
> octave. I went through the D/A tune/width/linearity adjustments with a
> voltmeter, the D/A width was off a few millivolts, but did not change
> the problem. The next steps of VCO adjustment require a scope. I
> recently acquired an old Tektronix 465, but have no probes (is 100mhz
> passive good for this?). I am a scope newbie, and am reluctant to
> trust my limited knowledge on this.
> 
> The service manual states:
> 
> "Connect the oscilliscope to the SH-101 OUTPUT. Supply the reference F
> note (based on A = 442Hz) to the scope EXT. Input for the Lissajous
> figure."
> 
> Does this mean connect the SH-101 to channel 1 or 2 of the scope, then
> supply a reference F from another keyboard to the EXT input? I assume
> a sine wave would be good, for the least amount of overtones. The
> further adjustments are talking about adjusting until the figure is
> motionless. If the scope is simply being used to make sure the SH-101
> is in unison with an external synth, I have a good enough ear to
> detect beating.
> 
> Can I stumble through parts of this just with an instrument tuner?
> What kind of probes should I get for an antique scope like this? Is
> this a low-tech enough use that I could perhaps hook up straight wires
> from a BNC connector, or will the impedance be too far off? I live in
> a very rural area, and would like to work on this before the amount of
> time it will take to order probes.
> 
> The modulation seems to be always on and affecting the sound. There
> are Mod sliders for VCA and VCF, both seem to work throughout their
> range. I have read that all the sliders are the same, so swapping some
> could be an option, but it looks moderately hard to get to (not really
> hard, just further disassembly). I am always leery of 20 year old
> plastic connectors and wiring harnesses being brittle.
> 
> Still further, the CV input jack seems to have something stuck in it.
> From my limited knowledge of CV, I think the keyboard is disconnected
> if it is being operated by CV (not the case on this one). I don't know
> if that could contribute to some of these symptoms, I will remove it
> to rule that out.
> 
> Thx in advance,
> Scott S.
>



The Tek 465 is one of the best scopes ever made. 100Mhz passive probe would be fine. Make sure you cal the scope against your new probe when you get one. There's a Calibration loop on the front of the scope

There's also a TEKscopes yahoo page for seasoned people and newbies