Getting rid of a hum in an old drum machine

[Snoozr]

Hi everyone,

Having kind of finished a SID (it makes sounds but I still want to add some more buttons etc someday), I have gotten to the point where I can do some basic soldering without burning my fingers.  :)

I have an old Boss DR 110 drum machine.  It's fun but it has a small hum in it.  Kind of like when you touch a guitar cable and you hear that hum.  Here the hum gets louder when I put my hand near the DC In.  So that seemed like a clue to me.

I found the schematics online for it at http://www.theninhotline.net/dr110/110scheme.zip.

The voice board is in there as VBSCHEME.gif while there is also a CPU board schematic in CPUSCHM.gif.

I was wondering how I might get started in tracking down a hum using this schematic.  I assume something might not be grounded (???) which is causing the hum.

I would have never even bothered if I didn't feel emboldened by my Midibox - so thanks to you all for all the info to date.  If this is too off-topic to ask about, just tell me to shut up and please accept my apologies in advance. 

Also, for those of you with interests in old drum machines, there is a description of how the voicing circuit operates.  Doesn't make a lot of sense to me but it might be neat for the MB-808 folks.

Thanks again.

[stryd_one]

The first thing I would do here is to confirm that there are no issues with ground loops in the audio signals, and that the PSU is working as it should... meaning, try a known-good PSU, and run the synth direct into your amp.

[Goblinz]

The first (and tbh the only ) thing I usually check on broken electronics is the condition of the solder joints/tracks/components. If something looks black and fizzled, bulgged out or otherwised damaged then it needs replacing/the joint resoldering.

[nebula]

That's a strange machine to have a hum issue. Does it hum on battery power?

[/tilted/]

Does it hum on battery power?

Bump. Super highly recommended.

[stryd_one]

try a known-good PSU

A battery is definitely a good option there :) Sorry, obviously I don't know the machine well...

[Snoozr]

Wow - you guys were so right on the batteries!  Also, the PSU is one of these switching voltage kinds (from Wal-Mart no less), so probably it is not doing a great job. 

I will try getting a new PSU for it.  There are only two places to get PSUs around here - the above for El Cheapos and Radio Shack at almost triple the price.  Is there a manufacturer that you all like that is available via Mouser or some such internet distributor?  I haven't the faintest idea who is a quality manufacturer.  This little guy here runs on 9V.  Come to think of it, the Wal-Mart PSU was being used on the SID so I probably need to get something from Mouser for that too.

Thanks again - I was having to undo the hum by using "noise subtraction" (can't remember the right word) of my samples. 

PS - if you have one of these little guys, hang on to it.  Seems like the price is going up on the Bay.

[nebula]

I used to have one, but it was stolen .... but what a great sounding machine - much nicer IMHO than a TR-606.

The best option for a box like this is often your own linear regulated power supply.  The schematics you attached  don't actually show what's happening at the power input, so if it has no regulator, consider spending under 10 bucks on some protoboard, a regulator, a rectifier, and a couple of capacitors.  If there's already a decent power supply on board, any ol' adaptor should do the job.  I think the spec calls for a Boss PSA, which is just a 9VDC adapter with centre negative.

[Snoozr]

Thanks Nebula.  I sense a new project in my near future  :)

[nebula]

Me too.  You've got me wanting to reproduce the drum voice circuits from the schematics you provided!  :)