Channel switching schematic problem

[paul525715]

I've recently built the "Aiken" Soldano X88R channel switching schematic for an amp that I'm in the process of building. I also added relay based circuits to switch fx loop and relay circuits on and off. These are fed from the same 5V DC supply (from a voltage regulated rectified 6.3V filament tap).
There is a schematic below.

Thing is this... when I'm using the channel switches, everything works fine (the LEDs change etc etc). The second I switch the fx loop or reverb switches on or off the channel LEDs dance about too. I've checked the wiring and it's all correct, so does anyone have an idea what's causing the problem and how I can fix it?

The only thing I could think of was the fx / reverb relays causing Voltage / current dips when they were switched on or off.

Is there anything I can do to stop the problem? It's driving me slowly mad.

Thanks,

Paul

[raiken]

Wow, where did you dig up that old schematic? I took that off my website about ten years ago!

Make sure you have diodes across the relay coils like you have on the LED/HF11 relays.

Make sure you have the digital circuitry power and ground isolated and decoupled from the relay power. Put bypass caps (0.1uF in parallel with 10uF or so) in the digital switching section.

Make sure you have good grounding (preferably a ground plane) for the digital section.

Make sure your power supply is capable of sourcing the current necessary to run all those relays. If the supply voltage dips, the digital circuit may malfunction.

Also, it looks like you are using a transistor to drive a relay that then drives an LED and opto-fet switch. While it will work, you really don't need a relay in that case, you can use the 2N7000 transistor to directly turn the LEDs and optofets off. The only time you need relays is if you need an isolated switch, or you don't want optofets in your signal path.

Randall Aiken

[paul525715]

The diodes did the trick. Thank you so much. I found the schematic on schematicheaven.com. You have just bought my next build one step closer to completion.

All the best,

Paul

[raiken]

The diodes did the trick. Thank you so much. I found the schematic on schematicheaven.com. You have just bought my next build one step closer to completion.

All the best,

Paul

Those diodes are there because the relay coil is basically a powerful inductor that you are switching on and off. When the current flow is cut off in an inductor, the magnetic field collapses, and produces a large voltage spike. The diodes give that spike somewhere to go.

Having said that, in taking a look at your schematic again, I see something a bit disturbing - your LED/H11F1 relays are switching +5V on and off to those devices (5V on pin 4 and 13), which is fine, but your reverb and effects loop devices have the switched line coming from the top side of the relay coil, essentially switching both the relay coil and the supply to those parts on and off. That is not good. You should have pins 4 and 13 going to the +5V supply, and you should move your switch to the ground side of the relay coil, leaving the 5V supply uninterrupted, and giving the coil diode a constant path to divert the spikes back.

Also, if you care to post a schematic of your LED/H11F1 scheme, I'll draw you up a better way to switch that stuff. Actually, if you have the original schematic I posted from way back then, it had several suggestions on the schematic for ways to drive LDRs and photofets without relays.

Randall Aiken