Pin 1 - +40.1VDC (checked against ground @ pin 7)

Pin 4 - -40.1VDC (checked against ground @ pin 7)

Looks like just a bad 3886. I'll drop an order in and get one on the way. I'll report back when I get it to make sure that was it. Thanks for the help guys!

Baja

Quote from: Roly on July 29, 2013, 12:36:11 PM

Disconnect the speaker and measure the DC output voltage with the Gain and Master volume at minimum.

I measure 40.1VDC at the speaker terminals.

Is the hum influenced by the volume, treble, or bass controls at all?

No, volume and tone controls have no effect on the hum.

I've been out on the road for the past couple of months, and this last weekend I ran into a guy that gave me a really nice looking Behringer bass amp that needed repair. I ended up in a hotel with WiFi for a few days, and thought I'd try to find some information on this amp. I have looked for the last couple of days for a schematic, but it's proving to be a tough on to find. I'm on the road, but I have the trusty old fluke multimeter and a bit of time so I thought I'd take a crack at this.

I got her opened up and found that it's powered by a LM3886T. I pulled the input from the power Trans off and checked it. It shows as a 28-0-28, and measures at 31-0-31VAC. There's no discernible burn marks or broken components on any of the boards, but when you fire the amp up it sends a medium volume 60Hz tone through the speaker. There's a couple of sets of wires that run from the amplifier/power supply board to the preamp boards, but not having a schematic is a bit tough on figuring this out.

Any chance someone has a schematic for this board or can give me a clue where to look next? I verified that the rectifier diodes are good, and none of the big caps are shorted, but I'm kind of lost from there.

Baja

That's way cool Roly. I volunteer for alot of the youth hunting events around here, no matter if it's something I do well or not. I always remeber my Grandad taking me out when I was little, and it was never about what you killed, but about the act of going. Some of my fondest memories were packing through the woods with a gun and my Dad (even though he secretly hated hunting), and the comraderie that came with it. It always feels great when you see a kid light up telling someone a story about thier first hunt.

Baja

I'm hoping to get at it tonight and break it down. We have been on forced 4 day weeks for the last month due to our wonderful economy and sometimes the 3 days off falls just when I need to get in the shop and work on something :grr

I'll report back when I get it broke down and know more!

Baja

I have the MPF102's, but I was waiting on the J201's I ordered. I think I have enough parts to breadboard up your new shematic so I'll get back whern I have it up and running.

Baja

Well, I got some time this afternoon to drop in the fix. Although there was a reduction in the volume, there is still a pop from both relays. I wonder if changing the .1uf caps to a larger value might be necessary?

Baja

I got some time yesterday to redraw the schematic, but the repair shelf was full so I ran out of time before I could try the fix. I should have it done this afternoon so I'll post up the results after.

Here's a copy of the schematic with the fix added:




Baja

Just a quick update:

I've been building a few different preamps to play with, but I ordered some J201's from overseas and I'm still waiting.....

I'll be sure to throw an update when I get all my parts and get topluggin the modules together.

Baja

OK, sounds good! One question, how big should my 100 ohm be? I figured everything for current @ 9V, but if there's a large spike coming through, should I up my resistor size to 1/2W?

Baja

That makes sense. I didn't think about the diodes not catching that spike. I'll drop in a couple of resistors on Tuesday when I get back to the shop.

Thanks!

Baja

My MXR clone already has a 1M to ground just before the cap. It's the only mod I did to this one, and it solved the pop issue when I had it in the stomp box.

I'm going to throw a .1uf cap across the switch contacts tomorrow and see if that doesn't tame the beast a bit. I had a delay pedal a few years ago that I had to solder in a 1M between the input pad on the PCB and a spot I cleared off on the ground plane to get it to stop. That one had a different pop than this one, not quite as solid but alot of that is probably the different speaker I was using then.

Baja

Ok, here's a couple of schematics that should help. The first is a schematic of the clean boost circuit I used, and the second is schematic of my relay board and how the circuits are wired up.

The only other schematic is the MXR distortion +, but I'm not sure if I can post it without getting in trouble (it's got another website's schpeel on it).



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Baja

I'd love to, but photobucket seems to be having a melt down with it's uploader :grr

I'll try to find another photohosting place and get it up asap.As for photobucket... :trouble

Baja

I've been screwing around with a few different preamp ideas over the last couple of months, and I decided to build a dedicated "test mule" tda2003 amplifier board to try out the different bread board circuits on. I bought a amp kit through one of the online places, and it works fantastic. I'm very pleased with the sound of the amp, but I've had other problems that are sending me into mental overdrive :duh

One of the problems that I have been unsucessuful in fixing is a switch pop issue. I have a protoboard that has a modified MXR distortion plus and a mosfet clean boost on it. I've played it as a pedal, and it works great in front of everything I've plugged it into so far. I got this idea fron one of Juan's posts and tried to add it in as the preamp for this little amp, and the switch pop is driving me nuts. I originally designed the circuit to use 2 different 3PDT switched to turn each effect on and off separately, so when I decided to add it on to the front of my test mule (I call it Pepe), I used a couple of 9V relays I got from one of the online places to do the switching so I could use mini toggles or a foot switch to switch the effects on and off. The footswitch works great (and is quiet), and the switches work, but create a nice fat pop every stinking time. I added a 1uf cap across the relay coils and a diode to keep from blowing it up, but I'm still getting that pop from the front panel switches.

I'm kind of lost here, but I figured I'd throw it out and see if the amplifier gods had another tidbit of info that might keep me from pulling my hair out :grr

Thanks ahead for any help,

Baja