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Just got my old vintage big box Clone Theory working again but there's a low hum (sounds like a short pulsewidth squarewave). It's got an inbuilt AC transformer. Filter caps do you guys think? What should I check first?

Maybe you put a "signal wire" to close to a power wire or the transformer? That''ll hum...Sounds dumb but I've done it by accident before but I've never seen inside one of those older ones.

Replace the filter caps regardless, they olddd, but yeah could well be the issue.

In fact, replace all the electrolytic and tantalum caps is a very good idea, many of them could cause hum or issues, just be careful, those PCB traces are very weak.

Thanks guys. Pedal was dead but I haven't changed any wiring, the original fault was in the plug - the neutral wire had come out, glad I checked there first!

I'll go ahead with replacing the electros and tantalums and see what that does. Worked on a same era polychorus once and remember the board being made of cheese so I'll be careful, cheers Scruff.

you can also check the AC voltage on the transformer to make sure there is nothing wrong with the transformer before you go chasing other things down. that supply is a full wave rectifier so I think it should be about 30VAC before the diodes. check the regulator output, it should be 15VDC. if you have a scope you can scope before and after the regulator to check the transformer, power cap, diodes all in one go. the inductance crosstalk that was discussed relies on parallel wires. you can be in close proximity without induction if the two wires do not cross each other's lines of flux.

There is no regulator (well unless it's a late or SAD1024 model, which are just noisy by design) it's regulated by a zener.

As long as the voltage is sub 18V you're okay, i'd guess it's probably between 14.5 - 16V.

A transformer in such close proximity will add some hum regardless but the electro and tants are first to go and at 30+ years old, they are past the use by and will add noise.

I hate to pitch in with no actual input, but for a hot second, I thought this was going to be a thread about a DIY build called the "Humming Clone," and got all excited to see what it was.

Alas, it is not.

I'll just show myself out now.

The electros have actually already been done at some point. I'll check the transformer Rob and see how it's doing. Got no scope so I think I'll just replace the electros and tants anyway and see what happens. It has always been a noisy pedal (though the vibrato is beautiful) but this is a definite low hum - the kind you get when you touch the tip of your jack plug.