Replacing relay with optocouplers

[breogan]

Hi!

I am very new to posting in this forum, but I've been lurking for some time. I hope you can help me!

I recently acquired a Mesa Boogie Mark IIa, the very first amp with switchable channels, and is in pretty good condition. The tone is really nice but it has one problem when going from lead mode to rhythm: A loud "pop" noise jumps from the speaker. This noise is quite unpleasant and very annoying and it seems that is related with the switching method. This amp used a relay for switching channels triggered by a external footswitch. This footswitch conects a 7V DC to one leg of the relay to make it change the selection.

AFAIK optocouplers work in the same way that relays does. Using a DC voltage to perform a joint. So I am thinking that It could be a good idea to replace the relay with these. I do have some VTL5C1 optocouplers here at home...

Do you think this could work?

[loudthud]

Those pops are probably due to small amounts of DC leakage into the circuits where the switching is taking place. Look for small amounts of DC on any of the relay contacts in either position of the footswitch and find any leaky capacitor on that node. The leakage can also be from flux left on the PCB that has absorbed moisture. Carefully clean the board and mop up all the flux residue. I use isopropal alchohol for that and a hair dryer to get rid of any remaining traces of moisture.

Replacing the relays with opto's will reduce the pops because they switch state slowly. The circuit might require some small changes to make the opto's operate adequately in circuits where they short the signal to ground because the minimum resistance is a couple of 100 ohms.

[stokes]

I kind of remember readfing somewhere that this was a common problem with the IIA,I think Mesa came up with a cure,try contacting Mesa,you can talk to one of the techs directly.

[breogan]

I've been reading about Mesa history and the Mark IIb had the same problem. This was solve in the Mark IIc model introducing LDR (optocouplers) for channel switching. This method has been used in the MKIII and MKIV too, and in the rectifier series (according to schematicheaven).

However, I've discovered that lonestar models use relays for channel switching, estrange... I've played with a lonestar special and no "pop" occurred when switching channels.

I may be able to get the MKIIa relay to be silence.

[chinrest]

Do I recall something about tacking a small value ceramic cap across the DC terminals of the relay to suppress pops? Or was it a diode? Both?