MXR M87 Bass Compressor Repair - May Be Of Use To Someone?

[bassace97]

So it seems to me there are two separate common problems with these compressors:

 

1.  Failed output TLC2262 Op amp.  Symptoms are the pedal looks to be working correctly with the gain reduction LEDs working, but with no output when the pedal is engaged. fixed just by swapping the output op amp normally, tricky without SMD soldering equipment but doable.

2. Failed ratio switch.  All gain LEDs stay on permanently instead of going off after a few seconds of turning on.  Fixed by replacing the ratio switch as mentioned by @fretbass.

 

 

[jimbobothy]

… one thing stopping me going for one of these is all the above unfortunately. 

[bassace97]

4 hours ago, jimbobothy said:

… one thing stopping me going for one of these is all the above unfortunately. 

Despite all the reports of the failures it's a very small number that actually fail - there are plenty of these in touring use that take a lot of abuse that are over 10 years old now and absolutely fine - If you got one new now I think you'd be fine.

[dchang0]

Thanks to everyone for this discussion--I am repairing an M87 now.

 

FYI, the exact part number of the Alpha rotary switch in my MXR M87 is:

 

SR1712F-0204-15K0A-N9-N

 

It is not exactly the same as the one fretbass mentioned that Mouser carries. Here are the differences (marked in bold above):

 

K0A = knurled knob, not flatted knob as indicated by F0A

15 in front of the K0A = 15mm tall, measured from the base of the bushing, not 20mm

 

Unfortunately, nobody sells this exact part, so what I am going to do is take the knurled plastic actuator from the 2011 Rev. D PCB I have and transfer it into the body of the brand new rotary switch. The other option is to simply glue together the broken pieces inside my original switch, but this requires a more careful desoldering without damaging the switch.

 

Also, I can confirm that it is U8 (not U7) that will get replaced by an AD822ARZ (8SOIC form factor) or the newer replacement suggested by Analog Devices, the ADA4622-2ARZ. The ADA4622-2 has a higher slew rate and higher bandwidth and happens to cost less than the older AD822ARZ that it replaces.

 

Removing the old TLC2262 is much easier if you use Chip Quik removal alloy, a special low-melting-point solder designed specifically to make it easy to remove surface-mount ICs with many pins. I'll probably use Chip Quik to remove the rotary switch too, though those are through-hole pins.

[dchang0]

Follow up on my repair:

 

1) Turns out I didn't need to replace the TLC2262 at all. The only problem I have is the broken Ratio rotary switch. By virtue of its being broken, there is no output when the footswitch is engaged. (The other telltale symptom is that the LED graph at the top lights up completely right at power-on and stays lit up the whole time, never dropping down.)

 

2) I desoldered the Ratio rotary footswitch with only moderate effort using this trick. Using a Hakko 808 power solder sucker, I completely removed the solder from the ten small pins so that they were standing naked in their holes. This means only the two larger housing mounting clips needed to be desoldered to remove the entire switch. I couldn't suck all the solder out of the mounting clip holes, so what I did was fill them in with leaded solder and heated the solder up by holding the iron there for a while, then I tilted the switch housing to pull the mounting clip out of the heated solder. Repeat for the other mounting clip on the other side, and the whole switch lifts out pretty easily.

 

I didn't bother with the Chip Quik removal alloy, even though I have some.

 

3) Then I took apart the Alpha rotary switch, p/n SR1712F-0204-15K0A-N9-N.

 

Not good. The black plastic actuator has two metal brushes on the bottom that are simply melted into the plastic. One of them had broken off. This means I cannot simply put the metal brush back into a slot or hole. I will have to find a glue that could grip enough on the black plastic in spite of the grease OR re-melt the black plastic perfectly to hold the metal brush. Neither approach will be easy or durable. It also means that I cannot transfer the 15mm knurled black plastic actuator to a new switch body, as it is the part that is actually broken, not the base.

 

I asked Mouser if they would order the correct part number (15mm knurled shaft) and they recommended the in-stock 20mm flat shaft instead. I guess they don't want to order the correct part for me.

 

So, right now, I am asking MXR tech support if they will sell me just the rotary switch for a reasonable price. If that doesn't work, I'll ask Alpha Taiwan if they'll sell me just one sample unit. If that doesn't work, I'll order the wrong part from Mouser, cut the shaft from 20mm down to 15mm, then change the knob from one for knurled shafts to one for flat shafts or solid shafts (with set screw).

 

What a pain! All this due to a tiny piece of melted plastic breaking off.... (see photo).

 

Edited February 24 by dchang0

[tauzero]

Banzai Music have the exact switch: https://www.banzaimusic.com/alpha-2p4t-rotary-switch.html

[dchang0]

@tauzero

 

WOW, you are amazing! I wasn't able to find this even while searching for the exact part number in my search engine--yet there it is, the exact part number on their page!

 

THANKS!

[dchang0]

Sheesh, this is crazy. I just tried Google instead of my usual search engine (Ecosia), and other sources are popping up.

 

SynthCube carries it at the lowest prices I've seen so far (USD $2.95)

 

https://synthcube.com/cart/index.php?route=product/product&product_id=8606

 

SmallBear Electronics carries it too at the same low price:

 

https://smallbear-electronics.mybigcommerce.com/rotary-switch-2p4t/

 

I'll buy from SmallBear since I've bought from them before and need some other rare parts from them too.

 

There's a supplier in Japan who carries a slightly different part (the bushing is 5mm instead of 7mm, but since the MXR doesn't use the bushing at all and the knurled shaft is still 15mm measured from the base of the bushing, this switch will work):

 

https://akizukidenshi.com/catalog/g/g116035/

 

I hope this helps somebody--thanks again to @tauzero!

 

 

UPDATE: Jim Dunlop TechRoom emailed me back:

 

"The part number for the rotary switch on the M87 Bass Compressor is ECB557. You can orders this by reaching out to our Orders team ([email protected]) and provide them with this part number and your shipping address for an accurate quote with taxes and shipping."

 

 

 

 

Edited February 26 by dchang0