Trying to understand a Tremolo circuit...

[Gil]

Hello everybody !

I'm trying to figure out which VR's I should tweak to get more Depth
from my piano's tremolo. The problem is that even when the Depth pot
is at max. position, still, the tremolo depth sounds very shallow...

Can somebody help ?

Thanks !

[Nasse]

I think your pic only shows tremolo oscillator but not the audio path

[Gil]

ok, fixed.

[gez]

VR9 and VR10 would be my guess (gently tweak them and see).  If your piano spontaneously combusts, then I reserve the right to be wrong on this matter...

[Nasse]

Is taht piano old anyway and you can have new for little money. Don´t know for sure but I believe those ldr s make voltage dividers with that transformer looking thingy and those 22k resistor perhaps (R135 and R136?). Perhaps those could be tweaked. Looks messy my brain starts hurting.

Or Gez is right... What if you could easily try other leds, put there some new slightly more bright ones. Dont know what J1 and J2 are, can you put some new tremolo circuit there and take the lfo for it from the old circuit

[Gil]

You must be kidding. This is circuit is of the most amazing electric piano ever made probably. A Yamaha CP80.

Being a light bulb based tremolo explains the smoothness and silkiness of its behavior, and how amazing it actually sounds !
(reminds me of the tremolo in blackface TwinReverbs and Champs...)

So basically I assume that either the LED do not put enough light, or the light-sensitive resistors are too old ?...

[Gil]

OK, this is really interesting. I check the datasheet for the LED type part - mcd527,
and realize this is a pair of a LED and a light sensitive resistor in a single package ??

http://www.datasheetarchive.com/pdf-datasheets/Databooks-2/Book277-644.pdf

[gez]

Have you tried adjusting the trimpots yet?  They control the amount of current the drivers feed to the LEDs (by setting bias of said drivers). 

[Gil]

Not yet.

Two question -

1. Isn't it a wrong approach, to change the calibration settings rather than replacing the old components
which presumably effect the way the over circuit operates ? I thought about recapping the circuit first,
maybe replace the opamps with fresh ones, assuming that would bring the circuit back to original "specs" ?

2. Should each of the two trimpot affect the depth for its dedicated channel (i.e separate Depth control for the
left and right channels ?)

Thanks

[gez]

1. Isn't it a wrong approach, to change the calibration settings rather than replacing the old components
which presumably effect the way the over circuit operates ?

No, it's not the wrong approach.  The tremolo is working, therefore the components are working.  If the piano has been gigged with, then the trimmers might have be knocked out over the years.  Or perhaps it was meant to be subtle in the first place, and that's how they set these things up in the factory.  Either way, my approach would be to try the trimmers first.  Why replace stuff if it's unnecessary...and you won't know that until you try.

Don't go overboard.  Tiny amounts of adjustment.  If you don't hear much, just leave it (time to tweak other components).

I thought about recapping the circuit first, maybe replace the op-amps with fresh ones, assuming that would bring the circuit back to original "specs" ?

In this case (from what I can make out  - there are a few 'mystery components'), it doesn't look as though recapping is going to give more depth.  However, if your keyboard is old and you're working on the thing it makes total sense to replace any electrolytics.  As for op-amps it's only worth replacing them in the audio chain, and only then if the old ones are ancient, noisy types.

2. Should each of the two trimpot affect the depth for its dedicated channel (i.e separate Depth control for the
left and right channels ?)

Yes (from what I gather).

[gez]

...having said all that, there are one or two electrolytics in the oscillator circuit.  Would be wise to replace them.

[Gil]

OK, thanks.

I'll try that and report the results ! 

[mth5044]

That is a really nice piano!

[Gil]

Ok, so I tweaked VR11 and VR9 and found that they do not change the depth of the effect,
rather than used to make the depth appear or not. That is to say, when set to the "sweet spot",
there will be some depth heard, but when tweaked either clockwise or counterclockwise,
no depth at all....

[gez]

Ok, so I tweaked VR11 and VR9 and found that they do not change the depth of the effect,
rather than used to make the depth appear or not. That is to say, when set to the "sweet spot",
there will be some depth heard, but when tweaked either clockwise or counterclockwise,
no depth at all....

OK, makes sense as the trem with many keyboards is subtle (think 'shimmer' rather than 'chop').  The way the LDRs are setup is weird.  They seem to load the previous stage to a greater/lesser extent.  If that's the case, then it's going to be difficult trying to get any increase in depth. 

One thing you could try is to increase the amplitude of the LFO's waveforms.  Try reducing the resistance on the VR8 and VR10 trimmers (should increase amplitude of signal going to the drivers).

I have to blow up the schemo to read the markings, which is difficult as they come up blurred.  So, I've just noticed that I misread the trimmers for the drivers.  VR9 and VR11 are the trimmers you should have been tweaking.  Try these again.  If no joy, then give VR8 & VR10 a go.

[gez]

The way the LDRs are setup is weird.  They seem to load the previous stage to a greater/lesser extent. 

OK, I've got it now.  It was difficult to make them out, but there are a couple of series resistors that form dividers with the LDRs.  I went back to the schematic to search for them as the LDRs work anti-phase...so they had to be there to prevent one half loading the other.  I think (difficult to read) they're R135 & R136.  Basically, they're 22K (that much I can make out).  Try the trimmers first, but if still no joy these would be the components to tweak.

Increasing the value of the resistors (within reason) will give you a little more depth (don't expect miracles).  However, it will also reduce the amplitude of the signal at the output (you're dividing down the signal), when the tremolo effect is engaged.  If you make the resistors too large, the output will be quiet and noisier.

Edit: you'll probably have to recalibrate the trimmers if you change the value of those resistors.

[Nasse]

I was thinkin- what- if- those electrolytic caps particularly in series with the lfo ouput before those transistors drivin leds

If some of the uf s are dead and few of those who are alive are limpin could this make the amplitude smaller

Has anybody tried gsi soundware plugin?