MXR M-80 Bass DI No Output

[Archie Speed]

Hello, folks. First post for me. I recently inherited a MXR M80 Bass DI that has no output signal. All of the led indicators function properly, the in & out jacks check out as no shorts or opens. 9v battery is good & appears to be putting pwr out to board. Voltage appears to be getting to all of the op amps. I say 'appears' as I'm not sure what the correct value should be. Same with the test point values. There are 15 tp's that I've found. TP VOM readings range mainly around 3.76 with a couple of zero readings.

I'm having a lot of issues as the only schematic I have to work off of is this:
MXR M80 Schematic

Not the clearest print for old eyes to work thru. I inadvertently draped my finger across the area of R94/C97 & did get output, albeit noisy. So I'm still conductive.
I also notice what I believe to be indications of a short between the top of the TLC2264 op amp & the back of the pot above it (divot in the chip/small flash mark on the pot).

Any suggestions on how to better isolate the issue than my simply ordering/replacing oa's? As you can tell, I do not play much at the component level. Any guidance will be greatly appreciated.

Archie

[J M Fahey]

Sorry but that schematic is unreadable.
But it´s just the .gif or .jpg preview of a .pdf file, hosted in Scribd.

If somehow you can download the .pdf (by registering/paying/begging/whatever), I bet it will be much more readable.

Otherwise, only guessing

[Archie Speed]

Thanks, JM. I've contributed to Scribed coffers & have the .pdf. Is it permissible to post such a downloaded file on this forum or is that going to anger the copyright demons?

Archie

[J M Fahey]

If you made a page with a full project, PCB designs, full build-it-yourself layouts, etc, (even sold kits or the fully built product) *that* would clearly damage the copyright owner for sure.
But here you, as a Tech in this field, are asking fellow Techs for technical opinion to solve a problem, and I bet that's covered under the "Fair Use" provisions.
Similar to Doctors showing colleagues X-Rays or blood analysis from patients, to get a "second opinion".
So I bet your posting it here or, if you wish, uploading it to Dropbox or similar and posting just the link would be fine.

[g1]

Here's that schematic in pdf form:

[g1]

The TP's will be of no use unless you can say where they are connected to (ex.U4A pin8 etc.).
There should be 2 supply levels, around 9V (VCC) and around half that (VCC/2).
All the IC's should have at least one pin with approx. 9V.

[Archie Speed]

Thanks for the replies. Checking all of the ic's on the board, each brings in an 8.42vdc reading. 14 pin op amps reading across 4+11, 8 pin across 4+8 & 2 mux/sw across 8+16. That's using a 9v batt that metered in as 9.59 at the start of the checking. Going thru the schematic & checking the vcc & vcc/2 points yielded 8.23 at all vcc points indicated on schematic. vcc/2 points clocked in at 4.11 with the exception of the location at R44 (gate sw, upper right of schem) which read full 8.23.

Archie

[g1]

You have 8.2V at the side of R44 that meets the gate of Q4 ? Is the gate light turning on and off properly?

[Archie Speed]

You have 8.2V at the side of R44 that meets the gate of Q4 ? Is the gate light turning on and off properly?

Correct. Gate LED/sw on/off all good visual indications.

[Archie Speed]

Solved. Replaced op amp TLC2264C(D) with new & works fine, lasts a long time.

Inside the box, this chip lives directly beneath the 'gain' pot. And judging by the previously noted small flash mark, the pot got 'stomped' on & driven into the chip causing the arc & spark that killed it. So I insulated the back of the pot with tape to keep things on the up & up.

Archie

[Stoffie1974]

Solved. Replaced op amp TLC2264C(D) with new & works fine, lasts a long time.

Inside the box, this chip lives directly beneath the 'gain' pot. And judging by the previously noted small flash mark, the pot got 'stomped' on & driven into the chip causing the arc & spark that killed it. So I insulated the back of the pot with tape to keep things on the up & up.

Archie

Hi all... concerning this topic i found some pics about this problem.
The casings of the pots are "floating" only 1mm above the electronic components, and i can imagine that when you're "in the heat oh the battle" and you need to push a switch, but your foot is a little out of position, you don't think about those knobs that are only a few inches away... you press it with enough force, and... silence .
Or if for example a bolt has loosened.
I don't understand why EH didn't saw this coming !
For a pedal that costs 179€ in Germany, you'd expect better.
I own this great pedal too, but after seeing this i immediately built a simple modification to prevent it from happening.
I just added a mylar between the pots and the components. (see pics)

[salecello]

mxr m80

Hi all... concerning this topic i found some pics about this problem.
The casings of the pots are "floating" only 1mm above the electronic components, and i can imagine that when you're "in the heat oh the battle" and you need to push a switch, but your foot is a little out of position, you don't think about those knobs that are only a few inches away... you press it with enough force, and... silence .
Or if for example a bolt has loosened.
I don't understand why EH didn't saw this coming !
For a pedal that costs 179€ in Germany, you'd expect better.
I own this great pedal too, but after seeing this i immediately built a simple modification to prevent it from happening.
I just added a mylar between the pots and the components. (see pics)

Stoffie, please help me if you can. I'm proud owner of mxr m80. I bought it used in good condition, but preowner removed cable for battery. I want to put it back. I bought the cable with battery connector, but I dont know wher to connect it in the pedal, and which connector to use. I think it is Nylon connector with 3 pins, but I'm not sure. If you can make the photo inside the pedal where the battery cable is connected I will be very grateful. Thanks in advance, Aleksandar. Sorry for my English, but I hope you can undersud my problem

[Archie Speed]

Does this help? Inside/back of MXR Bass d.i.

[salecello]

Does this help? Inside/back of MXR Bass d.i.

Archie Speed, that's it. Many thanks. It's nylon connector.

[salecello]

Archie, it was very helpful for me, but I compared iside of my M80 with your photo and I noticed the alien into my pedal. Today I'll make the photo of my pedal and send you back to see this strange thing and if you can explane me what kind of modification preowner done. I tried pedal on the stage and it work fine, but I want to know what is it. Thanks one more for your time to make the photo and for your quick answer.