TR707 bizarre power problem

[Circuitbenders]

Heres a weird one i'm hoping someone can help me with. I've got a TR707 with a very strange power problem. I plugged in the normal Roland 12V power supply and nothing happened so, assuming that it had a broken power jack as all 707's seem to have, i connected my bench power supply to the power socket solder pads on the board and it worked fine. Thinking that this meant the socket was broken i replaced it and plugged in the Roland supply again and still nothing. So i connect the Roland supply to the solder pads and it still doesn't work, but actually connecting my bench power supply to the power connector via a power socket works fine. 

Basically this 707 will work fine when powered from my bench power supply, where it draws about 200ma, but it won't work at all from any other plug top power supply, and i've tried about 10 different ones with different polarities and all capable of supplying well over 200ma. Is there some kind of difference between a bench supply and a plug top supply because as far as i can see they are doing exactly the same thing. I've measured the voltage further into the circuit with both a plug top supply and the bench supply and it seems to be identical, it just won't run on any plug top supply.

This one has totally baffled me

UPDATE:
Using the bench supply the voltage regular pins on the 707 board can be measured at 5V-0V-11V. Using any other supply its 0V-0V-11V. So i'm guessing the problem might be something to do with that. The question is, why?

[Gordonjcp]

I haven't really got a solution for you, but I must say you've got a really really interesting problem...


Do you know if the power supplies work correctly with other equipment?

[Circuitbenders]

the same power supplies are perfecly happy to run either of my other two 707's, just not this one.

On another note, the crash and the rides seem to be missing from the main mix on this 707. They sound fine from their individual outputs where their levels can be controlled using their sliders, but they aren't present at the main mix outputs regardless of the level sliders.

Something tells me someones been messing around with this thing.

[Circuitbenders]

The power problem turned out to be a damaged voltage regulator and a rather battered looking power diode, which may or may not have been causing a problem but i replaced it anyway. I'm still a bit baffled as to why it would work with a bench power supply and not a plug top one but i'm not going to argue with it.

I've also solved the problem with the crash & ride sounds missing from the main mix if anyone comes across the same thing. Basically on the 707 the sounds go through the individual volume sliders and then onto the mix bus via the  individual outputs, which are switched sockets. When you don't have a jack in the individual output the volume sliders are connected directly to the mix bus via the switching on the individual socket, but when you plug in a jack the sound is removed from the mix bus by the socket switch. If the switches on the sockets are a bit tired, or as in this case some idiot has replaced the individual outs with non switching sockets (doh!) the sounds effected will not be present in the main mix.

[Gordonjcp]

Maybe the semi-faulty regulator was drawing too much current and pulling down the plugtop PSU, but the (presumably regulated) bench PSU was content to keep going?

[Circuitbenders]

well, i tried two regulated power supplies but i can only assume that you are right and the more 'hardcore' build of the bench supply kept it running under pressure. I did notice the plug top power supplies were making faint buzzing noises and the heatsink on the voltage regulator was getting hotter than usual, which would indicate a high current draw, and which is what lead me suspect something was wrong with it.