MXR Analog Chorus back from the dead

randella

I've had a good bit of help from this place already, so I thought I'd share this on the off-chance it helps someone.

My MXR Chorus fried itself a while ago - I wasn't about to return it to the shop as I knew full-well it was my cheap-ass power supply that did the nasty when it shorted one day.  So if you've got an analogue time-based effect (chorus, flange, delay, phase) that's no longer effecting, chances are it's the BBD (bucket-brigade delay) chip that's goosed.  In my case, it was the part numbered 3207 in the bottom-left of the picture.  £3 bought me a new one off Fleabay.  Some pedals use obsolete parts that are now unavailable, in which case you might be out of luck.

I checked the pinouts as the new part was a different brand, and they were the same.  So an hour with a solder sucker, I've dropped the new chip in and it's working a treat.  I do have a background in electronics, but if I'm honest 98% of it was forgotten as soon as I was sat down with a Jalfrezi and a pint of Cobra on graduation day .  If I can do it with my moderate soldering skills and some careful reading, I'm sure it's not beyond the wit of any other electronics hobbyist with a steady hand, half a brain, a soldering iron and the internet.

Anyway, there you are.  I might have a cold one to celebrate.

randella

Also, buy a decent power supply, not some shee-yte from Maplins.

lasermonkey

Have a wis! There's a certain feeling of satisfaction to be had from fixing stuff yourself. In my experience, swirly pedals that have stopped swirling have usually been down to a knackered capacitor in the LFO section, but a dodgy psu can cause havoc!

randella

Ah cheers @lasermonkey. Yeah, that PSU ran through my pedalboard like a toddler having a tantrum - aside from anything else, it also took out my Vox Joe Satriani overdrive which is all SMD stuff, so no chance of resuscitating that one.

Also glad you've spotted my deliberate mistake . A phaser's going to use an LFO, of course it is! I told you it'd all gone to pot since I had that curry!

ICBM

This is something I'm beginning to really dislike about modern very-high-current power supplies - if you use the wrong one, or set the polarity or voltage wrong on the ones with options, they can easily fry a whole board full of pedals because they have enough current available to simply burn out any protection diodes without shutting down, and then cook the circuitry too.

Well done for fixing that!

randella

Gah, @lasermonkey - have been doing more reading and it looks like I've talked myself into a hole with this LFO business, I need to go back to school. They all use an LFO; in a delay-based circuit, the oscillator varies the BBD clock, right? Anyway, I've learned summat tonight, so I owe you

@ICBM Thanks! I wasn't sure I could get the old chip out without cooking something with the iron, but a bit of patience paid off. The new chip in that picture isn't as neatly soldered as the rest of the circuit but I'm guessing these through-hole boards are wave soldered? If so I don't feel too bad.

As for the PSU, in this case it was entirely my fault for cheaping out and using a crap variable voltage switched-mode wall wart effort with one of those daisy chain cables. It was inevitable that it wasn't going to end well.

ICBM

randella said:

@ICBM Thanks! I wasn't sure I could get the old chip out without cooking something with the iron, but a bit of patience paid off.

The best way is usually to carefully destroy the old part, if you can't get in close enough to just cut the legs - ideally you want to leave just individual pins in the holes in the board, which are then fairly easy to remove one at a time without having to put too much heat in, otherwise you can end up lifting traces and pulling out the through-plating.

randella

@ICBM Ah, that's good advice, I feel slightly less Neanderthal now! What I ended up doing was removing as much solder as possible from the board, then very gently twisting the old chip back and forth with some pliers until it broke free leaving eight leg bits on the PCB which I cleaned up with the iron, tweezers (sorry Mrs. Randella!) and the solder sucker.

Mental note made to buy some much more delicate snips which would have enabled me to simply cut the legs, like you suggested.

thermionic

I would have soldered in a socket myself, just in case it happens again

randella

Lol, if it happens again I'm taking up flower arranging

ICBM

A massive Zener diode as a reverse and over-voltage protection across the power supply input would probably be better - it would ensure that it doesn't happen again.