MXR M117 repair

[ubxf]

Hello,
i'm trying to fix one of those fx and i changed the electrolytic and tantalum capacitors, then the transistors, now i'm thinking of changing U3 the 4013 IC but i'm having difficulty removing this 14 pin IC. For regular components i just use my trusted blue vacuum pump but for this one not moving and i'm afraid if i apply too much heat i might ruin the trace. Is there a trick to removing those (looks like there is some solder on top as well).

 

[JohnRoberts]

The weak link in these old efx is probably the obsolete unobtanium SAD BBD delay chip.

Are you getting any audio output from the BBD chip?

JR

 

[ubxf]

Thank JR. No i'm only getting hum and distorted sound ,In bypass the signal is fine. I have a spare BBD chip but i thought i would change the clock IC first.

 

[beatnik]

just cut all the ic pins once the ic body is removed you can take pins out one by one it's much easier.

 

[JohnRoberts]

Thank JR. No i'm only getting hum and distorted sound ,In bypass the signal is fine. I have a spare BBD chip but i thought i would change the clock IC first.

is the sound trying to respond to flanging controls?

If the audio is coming out of the BBD the obvious thing to check is the input dc bias trim (dc voltage trimpot setting DC of AC input to BBD. )

The CD4013 a dual d type flip flop is popular for use as a clock driver for BBDs.

they are (were) dirt cheap so swaperooo

JR

 

[ubxf]

Great thank you guys, I'll swap the CD4013 and i'll put a socket before the new one goes in

 

[JohnRoberts]

the sad 1024 is the rare part... CD4013 are probably still being made.

Check the input bias trim...

JR

 

[AnalogPackrat]

If you need a source for thru hole CD4013 in CA check Jameco and Anchor up in the Bay Area They'll both ship. Jameco also has useful items like the tantalum grab bag and older transistors and ICs. Good luck.

 

[ubxf]

Thanks good to know, i ended up finding some here in LA . Hopefully i'll get it going today

 

[Whoops]

You should also post in Diystompboxes forum and Freestompboxes forum as both of are dedicated forums to the build and repair of guitar pedals

 

[ubxf]

Thanks Whoops, i will . I'm still struggling with it

 

[Whoops]

Thanks Whoops, i will . I'm still struggling with it

Probably someone over these forums already has the same problem, so search over there also.
Another nice thing is that PRR posts over there and helps a lot of people in the forums

 

[JohnRoberts]

check the input DC bias trim....

 

[ubxf]

Thanks John, actually i've got bigger problems. In a lot of places where i'm supposed to see 7.6 V i'm seeing 14 V

 

[AnalogPackrat]

Is your 15V rail ok? How about R12, R8 (divider to get half V+ rail) and C9 (half V+ filter cap)? Or something is pulling that reference voltage node up. It's only used by the main signal path op-amps U1, U2, and U4.

 

[ubxf]

15 V is ok, voltage at divider is 7.6 V . I changed C9 so it should be ok i'll check it again in case it failed.

 

[AnalogPackrat]

Which places are at 14V instead of ~7.6V?

 

[ubxf]

U4 pin1 U3 pin2 pin5 SAD pin3 pin14
U1 0v pin1/2/5 2v pin6/7 U2 seems ok

 

[JohnRoberts]

U4 pin1

is opamp output

U3 pin2 pin5

I will ASSume that U3 is the 4013 clock driver.

That makes pin 2 one phase of the clock square wave outputs. If sitting at 14V DC that sounds like the clock is not clocking.

SAD pin3

clock phase A

pin14

Clock phase B... if A is high B should be low and vice versa ... if both are high, no worky.

U1 0v pin1/2/5

wrong

2v pin6/7

wrong

U2 seems ok

great...

====

op amps have pretty simple troubleshooting tells.

First thing to check (always) is proper PS rail voltage present.

Next compare voltage between + input and - input. If the + input is higher than the - input the output should go up, unless there is negative feedback telling it to do something different. If the - input is higher than the + input the output should go down (except for negative feedback yadda yadda.)

If the the +/- input voltages don't agree with the output state and the op is getting rail voltage THE OPAMP MAY BE BAD... typically bad connections are found.

Have fun...

with BBDs no clock no audio signal passed.

JR

 

[AnalogPackrat]

Check the clock lines with a scope. Does the width control affect anything? Is the LFO oscillating?