A head-scratcher

[Mark Hammer]

I have a Behringer clone of the old Boss DC-2 Dimension C.  Terrific little pedal, especially for the money.  Actually does more than the DC-2, but that's another story.   I put it on one of my pedalboards.  Here's the weird thing.   When battery-operated, it works just fine.  When I plug in the external PS, it switches fine and I get the clean signal, so clearly all the op-amps are getting power.  But I don't get chorus.  Put the power plug in, and the battery is switched out.  Take the power-plug out and it switches back to the battery just fine, wth chorus intact.

If the unit used MN3007s, I might understand that the bias voltages might be thrown off by a PS  that was somehow less than 9v.  But it uses MN3207s and powers them with a regulated-down 5V  The PS would have to be well below (v to not power the regulator in the pedal.  I might add that I have Rebote 2. on the same board, and its 5V regulator works just fine.

I thought it might be the home-made male-male adaptors I made for snuggling up the pedals closer together, since the output jack is also used for switching and a bad insert angle might mis-switch, but nope, it's not them.

The supply is a 500ma wallwart, used to power 7 pedals (the biggest current draw likely coming from the Rebote), daisy-chained from a little plastic distribution box.  So I don't know what's going on....not unless there is something amiss with the wallwart itself.  I probably need to try another one.  Lord knows I have a big box of them to choose from.

[R.G.]

I would get out the voltmeter and measure the DC going to the board with battery and then with the DC wall wart.

[alparent]

I have a couple of months old BOSS PSA-120 if you want to try with that?

[Mark Hammer]

Thanks Alain.  I'll start with my big box of adaptors, and if none of them improve the situation, I may ask to borrow one of yours.

[Mark Hammer]

Reached into the big bin of wallwarts and pulled out a Zoom adaptor rated at 9VDC/300ma; more thsn enough to power all 7 pedals.  I measured the first adaptor I was using, which was also supposedly 9VDC/500ma.  I'm used to wallwarts reading a bit more than they state on the case, but this adaptor was such a bargain that they seem to have thrown in a few extra volts at no extra charge: it read 15.6vDC, both with no load and with a load.

I've gone over the schematic (which I assume is near-identical to the Boss schematic) and I can't seem to spot anywhere that switching might be "confused" with a V+ of 15.6VDC rather than +9vdc.  The bias voltages for the two BBDs are taken from the output of the onboard regulator so even if the starting supply voltage is high, the regulator output should be within reason.

But, the long and the short of it is that the PS was the problem.  All works just fine now.

[antonis]

it read 15.6vDC, both with no load and with a load.

So you've got a super extra "blue" regulated & stabilized wall wart PS...

[samhay]

The LM78L05 regulators have thermal overload protection.
Perhaps with the extra 6V it has to drop from your overzealous power supply, it is getting warm - I don't expect Behringer have used a heat sink - and it is going into protection mode. The datasheet doesn't say what happens at this point, but I expect the result is that you get <<5V from the regulator.

[Mark Hammer]

As good an explanation as any.  While I'm not itching to pull the pedal apart again and probe (when I'd rather be fixing and building things), you have me curious about whether the bias voltage for the BBDs is messed up as a result.  I guess the simple test would be to use the initial adaptor and see if monkeying with the bias trimpot brings chorus back.