MXR Help........

[strangeways]

I need a bit of help, please. 

I got a MXR Microamp board from General Guitar Gadgets.  http://generalguitargadgets.com/index.php?option=content&task=view&id=92&Itemid=120

JD is great, and I have already built the AD3208 and Tonebender (without any problems). Needless to say, I am a little perplexed why this build, a beginner build, is the one that leaves me scratching my head.   

I have checked all connections several times, swapped out jacks/IC's/pots/Batteries with no luck.  I followed all the helpful layout info on the site, and even went through the schematic, which I am not so great at, but it all made sense.  This thing works when it is bypassed, but will not for the life of me pass any signal when the LED shines.  I haven't made any changes to the circuit, and even went so far as to build a perfboard Microamp, figuring if I followed the layout and schematic, and got the same results, I may cancel out something I overlooked. The IC is a TL071 and the diode is a 1N4001, both from Small Bear.

Anyway, here is some info that may help you help me.  Thanks in advance.

Daryl

Perf Board

Battery- 7.65

Pin
1- 0      8- .12
2- 7.05   7- 3.51
3- 3.91   6- 3.51
4- .12          5- .00

GGG Board

Battery 7.75

Pin
1- 0      8- .12
2- 7.13   7- 3.56
3- 3.56   6- 3.54
4- .12      5- .00

d1-  K-7.13    
        A-7.93

[George Giblet]

You have done a nice job posting your information and link to the pages - I wish everyone would do that.

I think you are interpreting the IC pins wrong.

If you interchange the left and right sides of the chip ie.

1 <-> 8
2 <-> 7
3 <-> 6
4 <-> 5

The voltages look half decent.

When you look at the top of an IC with the legs pointing away from you and the cut-out/dimple/dot at the top, the numbering is a follows

1   v  8
2       7
3       6
4       5

(When you solder the IC in it goes into the PCB with the legs first passing through the non-copper side and then out the other side through the copper - so you end-up with spikely legs on the copper side of the PCB)

With this orientation can you rewrite the pin-outs and voltages?

If the unit isn't working your problem may be elsewhere.

[strangeways]

Thanks George. 

You are correct about the pinouts.  Those readings were from the bottom view.  Not the top.  My mistake. 

So where else could the problem be?  Any other tests to perform?  Not sure what else to do, just short of lighting it on fire.

Please keep the ideas coming.

Thanks,
Daryl

[strangeways]

Here's a little update.......

I built an audio probe from the Debugging page.  Pretty awesome for learning what the components actually do.

Circuit passes signal pretty much everywhere except for at the input (or at least I think it's the input) of the board (from middle pin of 3pdt to board, which goes to  the 22ohm/.1uf cap).  Check the layout, mine is exactly like this.... http://generalguitargadgets.com/diagrams/microamp_lo.gif .  I suspected the input jack, or the connection to the switch, but they are not the culprit. 

It does pass signal on the other lead of the .1 uf cap, before the 1k/10 resistors, and pretty much all the way through the rest of the board, aside from ground.  I tried replacing the .1 uf cap, but got the same results.  Pot boosts output, and I can hear what this thing will do, so I am really itching to figure this out so I can blow out my tele/800, as well as my hearing.

Is this helping anyone pinpoint where my problem might be?

Please, if anyone has built this or has a microamp they are willing to open up, it would be much appreciated.

Right now I feel like I am having a conversation with myself, and it's pretty boring.

Thanks,
Daryl
 

[strangeways]

Bump!!!!

[George Giblet]

Since you are on line.  A quick thing to try is desolder the wire that goes to R9/R10 on the pcb; (ie the blue wire that goes to the bottom right hand terminal of the foot switch).  Desolder the purple wire going to the output jack.  Connect the desoldered blue wire to the output jack.

What this does is bypasses the footswitch.  If it works then at at least you know the circuit is working.

If that's the case you have the footswitch wired incorrectly - perhaps you have it rotated by 90deg or something and just by fluke it half works.  Alternatively you may have a damaged switch.

Anyway it's something quick to try.

[strangeways]

Again, Thanks George. 

I will try that and post with a reply.

I tried 2 switches already, so hopefully I just have something miswired on both.

Thanks,
Daryl

[strangeways]

Tried the wire swap on the output, to no avail.

Problem seems to be on input.  I swapped out the input wire to board from 3pdt middle terminal (green wire) to tip.  I am getting a very faint signal with the audio probe connected to the tip (brown wire).  Volume increases as I move probe to board, somewhere around c1.  I get a faint signal at r1, but it seems something is not connecting to complete the circuit. 

I have swapped out most of the wires, can I solder directly to c1?  Or elsewhere on the board?

I will try playing around.

Thanks for the help.
Daryl

[strangeways]

Pulled 22m resistor (r1) off of board.  Sound comes through, but sounds like some treble is rolled off.  Roughly the same volume as bypassed sound.  Tried different resistor values, but once you get up to 22m, sound won't pass.  What exactly is happening. 

Anyone?

Daryl

[George Giblet]

I probably could have given you better information yesterday but I was tired and in a rush and didn't read your post carefully enough.


I think the problem is you have used 22ohms instead of 22 Mega ohms (22M), and also 10ohms instead of 10 Mega ohms.

If you don't have a these values kicking around, then for the purposes of debugging and sanity, try replacing these with values around 100k to 1MEG (basically a resistor with a yellow or green (preferred) multiplier band, which is the 3rd band for four banded resistors).  Use whatever you have on hand, the highest values will be more like the real thing (and will be useable).

I suspect it will fire-up after that change.  After that see if you can hunt down some 22Meg and 10Meg resistors.  The 10Meg shouldn't be a problem but you might have some trouble getting a 22Meg, you best best here is to use two 10MEG resistors in series.  If you aren't plugging piezo pickups into the preamp I doubt you will hear any difference by outright substitution of the 22Meg with a single 10Meg.

[strangeways]

Thanks for the heads up. 

If memory serves me, I am pretty positive I put in 22ohm, thinking that 22ohm was the same as 22m.     I guess this is the way to learn these things.

I will let you know if it works, and thanks for all the help. 

D.

[George Giblet]

Yeah, easy to do when you are starting out.

A couple of tips.

- The Mega ohm values should be written with a capital M not a small m eg. 1M, 10M, 22M.  M means 1000000.

The small m actually means milli (1/1000th of an ohm) - you won't see milliohms in effects pedals but they do come-up in modern power supplies and battery chargers (they are usually special low ohms resistors and look different).  Don't worry about these at all.

- 1M upto but not including 10M will have a green third band (for 4 band resistors).

- 1M upto but not including 10M will have a blue third band.  Most shops only stock 10M.  On odd occasion you will see 22M and 33M in circuits.   When you get into these high values resistors they are often special types which have the value typed on the side.

[strangeways]

Hey George,

Thanks for the help.  I replaced the resistors on the GGG PCB and it's all working great. 

Did the same on the perf layout with no luck.   Guess I have some more work to do.

Thanks again for all the help, and the lesson!!!

Daryl