short?

[ptrx]

i'm very new to electronics, and i have a problem with a new mxr dist+, i'm determined to fix it though...
a wrongly polarised 9v adapter was attached to it, and stuff got burned. A 1M resistor had obviously suffered (it was black) and i replaced it. still nothing. JD Sleep told me the problem could also be the IC or the diodes (btw thanks for yr fast reply!), and i followed the online instructions to test the whole circuit. i.e. with a multimeter checking for volts from ground to various points on the board. I get zero readings all over! Also, when testing the circuit using a battery, the led lit up briefly and then died completely.  is this indicating a short circuit??? Either that or i'm missing out on smthng obvious here. It's a factory unit though, and the paths are pretty hard to make out.  i feel like i'm on a bit of a dead end, and a tip would help me out a lot on my quest..... :shock:

[AL]

Is there a diode near the battery/power supply input? If so this is for reverse voltage protection. Change the diode.

AL

[ptrx]

unfortunately, no. there are only the two diodes for distortion.

[ptrx]

i'll put one in once this thing gets fixed though.... :lol:

[AL]

Ok, well it was worth a shot. So, without anymore info it's going to be hard to figure out the problem. I'm not sure what the new MXR+ schematic looks like. The old one is available in the schematics section of this site. It uses a 741 opamp - datasheet here http://www.national.com/pf/LM/LM741.html

My suggestion would be to build an audio probe from the GEO site (link above) or, I think there is info for the probe at General Guitar Gadgets. Build this !!! It's invaluable and has saved me hours of debugging time. It's a quick, inexpesive build.

AL

[ptrx]

thanks for your advise, AL!!!
i did however manage to get some readings on the multimeter (like i said i'm a rookie), and as advised on the beginners thread, here's a list of voltages :
the meter was set at 20
R1 10k  0.82/0.82
R2 1M   0.90/0.82
R3 75k  0.90/1.65
R4 1M   0.90/0
R5 1M   1.07/1.18
R6 4.7k 0/0
R7 10k  0/0
R8 34k  1.66/1.68
R9 1M   1.68/9.43

C1 (F39, 103j) 0.82/0
C2 (F37, 102j  0/0
C3 (1u0)         0/0.90
C4  (F30, 479j)  1.07/0
C5 (1u0)         1.17/0
C6  (F37,102j)  0/0
C7 (100uF 16v)  1.66/0

D1      0
D2      0
D3  (led)  1.65/0

IC: i got 4 readings, clockwise: 1.67/1.17/1.07/0.83

i hope this is of any relevance, and i hope i got the readings right... i'll certainly check out that audio probe! greetings from belgium!!!

[AL]

Ok, which schematic are you using? Without that I can't be sure what your number designations are for. I'm assuming those are voltage readings on each side of the component? (ie R1 10K 0.82/0.82) That's 8.2 Volts on both sides of the resistor? If that's what your reading you've got a problem. It's a resistor - you should have a voltage drop. You may want to set your multimeter to Ohms and check your resistors to see if they are reading correctly.

Ohms Law V=I*R  Voltage = Current * Resistance. Remember that one.

Also, check the data sheet for the 741. Pins 4 and 7 are Voltage supplies (positive and negative). I'm not sure which way your "clockwise" reading is. There should be a small indent on the chip. Looking at the chip you should read (with the tab on your left), from the tab - left to right - pin 1 etc. Check the datasheet. Pin 7 is your positive supply so check that reading first - if you've got no voltage you've got a problem.

And build the Audio Probe - you won't regret it.


AL

[ptrx]

ok cool that's allready a lot of usefull stuff. All i've got is the old diagram, which i think isn't of much relevance.  it's hard to make out the paths on the high tech print job of the new mxr... there kind of layered. Anyway time for some serious studying methinks. i'll be back as soon as i see the light.