DOD 680.. BBD busted?

[birt]

hi,

i've got a DOD 680 from a friend on the bench and it doesn't do anything. the rocker switch was a shot so i temporarily bridged that one to get power to the circuit. without a guitar/amp connected i don't even get the LED to light up so there is definitely something wrong.

the 7815 gets pretty warm and i measure only 7,5V on it's output. so i take the SAD4096 out of it's socket and the voltage at the regulator shoots up to 15V. and the LED goes on with the footswitch.

it seems to me that the SAD4096 if dead but i would like to know for sure that it isn't some other component somewhere... as those things are unobtainium and i don't want to bring the bad news to the owner without being absolutely certain that this is the problem.

so does anyone have a schematic with voltages i can check? or other things i should check?

[Scruffie]

You provide the voltages, we'll check 'em.

But that isn't a particularly healthy sign for the SAD4096, no. Good news is, unlike the 5101, 1024 or 512D, it should be reasonably easy to replace with a MN3005 on a daughterboard.

[birt]

ok so what points in the circuit do i need to check?

[Scruffie]

We need voltages of every pin of every IC (without the SAD4096 in).

Here's a schematic

[birt]

LF351N
1- 7,60mV
2- 7,54V
3- 7,40V
4- 0V
5- 5,6mV
6- 7,54V
7- 14,92V
8- 3,2mV

4013
1- 14,9V
2- 7,48V
3- 7,45V
4- 0V
5- 7,48V
6- 0V
7- 0V
8- 14,92V
9- 14,90V
10- 7,48V
11- 7,54V
12- 0V
13- 7,48V
14- 0V

4558
1- 9,46V
2- 7,47V
3- 7,47V
4- 0V
5- 7,27V
6- 7,61V
7- 7,61V
8- 14,92V
the 3 other 4558 opamps are in the same ballpark so nothing special there.

NE571N
1- 830mV
2- 1,81V
3- 1,81V
4- 0V
5- 1,81V
6- 6,88V
7- 6,88V
8- 1,81V
9- 1,81V
10- 5,38V
11- 1,80V
12- 1,81V
13- 14,78V
14- 1,81V
15- 1,81V
16- 971mV

[Scruffie]

They all seem okay except for pin 1 of the 4558, that shouldn't be higher than the other pins (especially on all 3 chips) reading error?

If that's just a mistake then yeah, unfortunately it definitely seems to be a dead SAD.

[birt]

i checked again; it's only on one out of four 4558's that pin 1 is at a higher voltage. in all others 1 and 2 are at the same voltage.
this opamp is the one where the output goes to the SAD and the trimpot. but in the actual pedal there is a 100K connected between pin 1 and 2 that i don't see in the schematic. (i don't think i will trace out the whole thing though, it's not worth it without that SAD)

[Scruffie]

Check that the 47nF from pin 1 hasn't turned short with the continuity setting on your meter, if it hasn't, replace that 4558, the voltage should then (hopefully) be in line with pins 2&3 and then it might be worth giving the SAD4096 another shot.

[birt]

the cap is fine and i get the same results with a new 4558. (that opamp stage has pin 2 connected to the 15K, two 100K resistor network that ties it to the output of the previous gainstage)

[Scruffie]

I'm working off the assumption the + & - are just flipped in the drawing, is that not correct? Either way, the output shouldn't be higher than the inputs, there's no offset I see... Is the 50nF cap from the repeat pot okay too?