need to duplicate this side-effect of a DVM in a circuit!

[rjd2]

hi guys, i need some help on how to accomplish something....

short story: Yamaha CS-5 restoration, IC3(a buffer IC) was shot, replaced with a 741 opamp that shares the same pinout. the problem i now have is that with portamento at 0, the keyboard scales properly, and the keyboard voltage output works perfect. but as you turn up portamento to 10, it adds the portamento affect, but IC 3 pulls the keyboard voltage down a considerable amount. it also throws the scaling out. this only happens south of the portamento pot, so im sure that the new IC is the culprit.

here where it gets wierd.

if i measure the V of pin 1 of IC3, with the negative lead of the DVM to ground and positive lead on pin 1, it FIXES THE PROBLEM! the portamento pot has virtually no affect on the overall keyboard voltage. i've never seen a DVM actually affect the behavior of a circuit, but im obviously showing my ass here.


so, what do i do? thanks for the help!

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[richardc64]

[rjd2]

thanks, richardcs64!

yes, you are exactly right; Yamaha had a TA7505M in there originally. unfortunately, i can't find this IC anywhere, or even a datasheet.

any ideas what would be more suitable there, than the 741 i have now? thanks!

[blue hell]

Cant you trim the unwanted effect away with VR1? As pin1 and pin 5 are for offset control I'd think that would be the purpose of VR1.

[rjd2]

bluehell-looking at the schem, thats exactly what i first thought, cause its the null offset on the 741 schem. alas, no. it provides a very slight correction in voltage/pitch, but does not change the "bad" effect(which is quite huge; portamento up to 10 will cause C1 to drop more than an octave!)

FWIW, i have tried the same thing and gotten the same results on a ca3140e, which of course shares the same pinout.

[blue hell]

odd ... 3140 definitely has high impedance inputs, so thats not the issue. Is the capacitor leaky? (the .22 one).

[rjd2]

nope, cap is fine. looks fine, just tried a .22uf film in there, no difference.....

dunno if this helps, but if i gently blow on the IC3, the drop in voltage gets worse. so the hotter the actual IC, the less drastic the problem.

[richardc64]

[rjd2]

my goodness, you are right! sorry, richardc64. crossing fingers....

[rjd2]

thanks for the help, guys. so an NTE7144 worked, but still, didnt solve the problem. swapping the 10k compensating trimmer worked!!! i noticed it would jump from "low to hi" at a point in its throw. all solved, calibrate and im good. thanks!!!!!

[blue hell]

End good all good, glad you solved it.

[rjd2]

well alas, it was NOT a 100% solution, unfortunately. following up here in the off-chance that someone would understand WHY this would be the case(i.e. is the elusive TA7505m an IC with some magical properties?)

the 10k trimmer, V1, can be set to a value that will provide portamento, and NO voltage offset. in other words, activating the portamento circuit will leave the keyboard voltage unchanged. i did this, and thought i had solved the problem. however, upon going to calibrate the synth, i found that V1 is also the only trimmer to set the keyboard SCALING!!! so i was left with a choice-properly working portamento circuit and atonal scaling, or proper 12-tone scaling, and a worthless portamento circuit.

of course, i chose proper scaling, bail on the portamento function, and re-assembled cause i was tired of the project.

but obviously, yamaha designed this to work in the first place. however, i dont understand how, unless the original TA7505m chip had a property that either an nte7144, ca3140, or 741 IC doesnt have. i quadruple checked all my soldering, no bleeding pins or poor connections, and i also checked upstream and downstream of the portamento circuit, and nothing at all. eh, cest la vie!

[madmaverick]

Greetings all,
this is a very old thread, but here am I.. And this might be useful for anyone searching for help. (I was recently fixing CS-15D with failed buffer IC)

µA741 (LM741 etc..) or NE5534 is the correct replacement. The trimmer there is to make sure that S&H/buffer circuit isnt creating any unwanted offset. Actual scaling is already comming from key assigner and should be perfect (if not, you have a bigger problem..) and any additional calibration is done on VCO side of things.
Probe pin 39 of key assigner to check voltages. Reading has to be from 0.25V to 2V (low C - high C), check if you are getting same values after first buffer, then check that it also corresponds after opamp on TP17 (KVO), if not - use VR1 to balance it. Keep in mind that problem might be even in that trimpot (they are very old at this point ).
After that all is just matter of VCO calibration VR2,3,4. (all VR numbers are relative to mentioned CS-5)