Vintage MXR 117 Flanger Not working

[ugly_guitar_guy]

A friend gave me a non-working MXR 117 Flanger and it just doesn't pass any signal when engaged. I popped it open and looked at the connections and everything "seems" to still be in place connection-wise.

Now, I've heard a few things about this model not working. First is that they need to be biased. I can see the trim pots, but how do it know which one to turn and which direction?

Second, it could be the SAD1024 that went out. Fortunately, I work for a place that just happened to have a brand new old stock sitting around waiting to be snatched up. Problem is, I want to make sure that something else in the circuit didn't kill the SAD1024 and have the same thing happen if I put the new one in (if that is indeed the issue).

Third, that some of the caps may need to be changed. How do I determine this?

I'd really like to think that if I put the new SAD1024 in that its gonna fire right up and be happy, but with how rare they are I want to be absolutely sure first. Any help?

The pinout readings on the SAD1024 are:

P1    0
P2    5.4
P3    5.17-6.45
P4    0
P5    9.35
P6    9.33
P7    13.9
P8    5.17-6.4
P9    15.27
P10  6.22-6.87
P11  8.82
P12  9.75
P13  0
P14  5.68-6.8
P15  4.86
P16  0

Thanks!

[theehman]

You should audio probe the inputs/outputs of the SAD1024A to see if signal's going in and coming out. 

[Mark Hammer]

Ron's right.  There's a whole lotta points/places/ways where the signal can get held up without implying anything amiss about the BBD chip.  For example, the 4013 CMOS flip-flop that provides the complementary clock pulses to the BBD could be fried (as happens to CMOS chips).  Cost of a replacement 4013?  Much less than a buck.  Maybe the LM340 that regulates the supply voltage from the transformer is fried.  Cost of a new LM340-15?  Less than a buck.

certainly, for safety sake, take the SAD1024 out and put it in some conductive foam, while messing around.  Then measure all the voltages on all the V+ pins of the chips, just to make sure that the other devices are working properly.

[Fender3D]

...Third, that some of the caps may need to be changed. How do I determine this?...

You know the proverb "Beat your wife regularly; if you don't know why, she will." don't you?
with old caps it's the same...
change them you don't know why, they will 

[Thomeeque]

factory schematic
redrawn schematic

P3    5.17-6.45
P8    5.17-6.4
P10  6.22-6.87
P14  5.68-6.8

Looks like clocks (4013) are ticking - even I would expect 7.5V (half of VCC) here, but it is probably messed by your voltmeter (3+14 and 8+10 are connected, readings should be same) - what voltmeter did you use exactly?

P7    13.9
P9    15.27

VCC regulation (LM340) seems to be alright. P7 value is weird, should be same as P9, pins 7+9 should connected, measure again.

P2    5.4
P15  4.86

P15 (input of second BBD line) seems to be misbiased, should be similar to P2 (and I believe 5.4V is closer to right input bias voltage). Maybe C6 is leaky and pulls bias voltage delivered by R2 down (or BBD is ill).

Definitely try audio probe and trace where signal gets lost.

Good luck, T.

[ugly_guitar_guy]

Hey guys, well I rewired the power cable because it seemed a bit frayed where it went into the chassis, and powered it on and it made a humming noise with a slight flanger sound underneath it (when before it made none). After a minute of messing with it, I bumped the switch and voila! Clean flanger sound. Now, it wasn't as thick of a flanger sound as I expected (not quite like what I've heard on youtube), but the speed worked, and the depth worked but it just wasn't as thick sounding. Obviously this means the switch needs to be changed, so I'll do that, and I'll take a minute to redo my pin readings to see if things match better than my initial test.

Looks like clocks (4013) are ticking - even I would expect 7.5V (half of VCC) here, but it is probably messed by your voltmeter (3+14 and 8+10 are connected, readings should be same) - what voltmeter did you use exactly?

I have a craftsman 82015 multimeter. I'll put in a fresh battery and try again.

[theehman]

Might check the tantalum cap on the output.  I've seen them go bad and cause a faint flanging.

[ugly_guitar_guy]

Sorry for the dead thread reply, but I finally got the flanger working. The switch was done. Replaced it with a new 3PDT (I'm going to add a floating LED plate to it since it's a '77 an I don't want to drill into the case) and it fired right up!

Time to drop this bad boy into the pedal board! And now I have a backup SAD1024 to boot. Good times