Question about diode type

[night-B]

Hi guys!
I'm working on a layout for a MXR M108 ten band EQ. Found a schem for it but there are 3 unknowned diodes on the layout (D1 D2 and D3). Looking at the guts can't help because they are smt diodes on the original, and looking at the schem they are zener protection diodes.
Can somebody share his knowledge and suggest a model that would suit the build? I forgot to mention that it runs at 18V.
Thanks a lot

[DutchMF]

The symbol looks like a Zener diode, you could take a look at the datasheet from the IC to get an idea of the voltage. This is just my interpretation, better to wait for the more knowledgable folks to chime in.

Paul

[oldhousescott]

Interesting little circuit. IC4B is set up as a relaxation oscillator with hysteresis. It is switched on anytime the voltage at the anode of D3 is greater than the positive hysteresis voltage. Normally that voltage, at the junction of the anodes of D1, D2 and D3 would be slightly higher than Va. Signal peaks push that voltage level toward Vb.

The manual states that the gain and volume LEDs flash on signal peaks, so this makes sense with what's going on in the circuit.

I don't know why you couldn't just use small-signal diodes (1n914, 1n4148) here, or any value of zener. I can't see where the diodes are ever reverse biased.

[oldhousescott]

On further reflection, I think my analysis about the voltage level to switch on the oscillator was backwards. For the LEDs to be on steady, pin 6 of IC4B would have to be held higher than the positive hysteresis voltage. On negative signal peaks, the junction of the anodes of D1, D2 and D3 will go toward ground. This will reverse bias D3, "opening" the switch and allowing the oscillator to function. I think.

[night-B]

Thanks for these answers, I wish I could understand everything...
I know that OP-amp can work to sum or amplify signals, and they are supplyed by a Vcc ref voltage and a Vee, and I haven't been further than that actually. I feel like I need some electronics teaching now