DOD 460 Mini Chorus Repair

[clarisso11]

Hi all,

I've just come across this great forum, and was wondering if anyone could be of help with the project I'm working on.
I have done a bunch of testing, but would like to get a second opinion on the diagnosis, since I am still quite inexperienced with electronic circuits.

I'm working on a DOD 460, it only gives the clean signal at the moment when the effect is engaged. I tested all electrolytic capacitors with an ESR meter, and unsoldered and checked the transistors - all good. I've traced the signal with the help of a schematic I found online (sorry for bad quality), and highlighted it:




As you can see, part of the circuit is only a clicking noise (the dotted line), very faint, coming from ODD OUT and EVEN OUT (pin 3&4) of the SAD512D chip - the INPUT signal (pin 6) seems fine. I can't find a signal in the rest of the circuit.


I checked the CLOCK signal (pin 1) of the SAD512D with an oscilloscope, and it does look erratic (although I don't have much experience with oscilloscopes):




Would you say the SAD512D chip is the culprit here, or did I miss something?

[Jazz P Bass]

Here is a 'better' schematic.

[clarisso11]

Very cool, thank you!

[clarisso11]

Does anyone have an idea? I should also mention, the voltage at pin 5 and 8 of SAD512D measures as 8.7V, if that helps..

[Jazz P Bass]

If you read the schematic correctly, pins 5 & 8 should be at a high potential.
So you have good input.
What does pin 3 & 4 look like on a scope or a signal probe?
If 3/4 are good but it 'clicks' after the NPN transistor then I would suspect the transistor.
Here is the SAD512D datasheet:

[clarisso11]

If you read the schematic correctly, pins 5 & 8 should be at a high potential.
So you have good input.
What does pin 3 & 4 look like on a scope or a signal probe?
If 3/4 are good but it 'clicks' after the NPN transistor then I would suspect the transistor.
Here is the SAD512D datasheet:

Thanks for your help, really appreciate it!

I just checked pin 3&4 with an audio probe - no signal, only faint clicking. The signal at pin 6 is very clear and loud on the audio probe, so looks like good input, as you said.
I have unsoldered the NPN transistor and checked it with the diode setting, it all seems to check out.

Here is pin 3 on the scope:
https://ibb.co/tJmChKx

And here is pin 4:
https://ibb.co/51qH6PW

Does that look like a "good" signal? This is where I'm a bit too inexperienced to tell..

[Jazz P Bass]

It 'looks' like a good signal.
So why is it not coming through the audio probe?
Probably need s a boost, hense the npn transistor.
If it does appear that the NAD ic is bad, you are out of luck.
They have not been available for years.

[clarisso11]

Thanks, that makes sense.. I will replace the transistor first, and report back with the results

[clarisso11]

No luck with the transistor, but a new (*old) SAD512D chip did the trick - managed to find one online for a decent price.

Thanks for the help!