Boss DD-2: only half of the compander working?

[drummer4gc]

Hi all,

What could cause only half of the NE570 to work in the DD2 circuit? Ive got delayed signal at pins 14 and 15, but no output from the expander on pins 10 or 12. How could half the chip be working and the other half not? Tried a new chip with the same issue, so it's in the supporting circuitry...I'm just not sure where.

Thanks for your thoughts!

[Slowpoke101]

That side of the NE570 may be being muted. Check the voltage on pin 16 and see if it is less than 2 volts. If it is, several things can cause it. C33 may be leaky and dragging pin 16 down. Q12 or Q13 may be shorted, dragging pin 16 down through D5. Measure the voltages on Q12 and Q13 and see if they are sensible.

Q12 and Q13 make up the Power On Mute circuit that stops excessive noise being made by the pedal when you first apply power. If it has gone wrong you will experience what you are describing.

If no luck, measure the voltages of all of IC2's pins, Q12 & Q13 and D5. Post the results here and we will see if we can help fix your pedal.

[drummer4gc]

Thanks slowpoke. Here ya go

D5 (clearly a problem)
Anode: .78
Cathode: 9.09

Q12
C: 9.09
B: 0 (LED turns off when I measure here)
E: 0

Q13
C: 0
B: 8.86
E: 9.09

IC2
1: .78
2: 1.85
3: 9.1
4: 0
5: 1.85
6: 1.85
7: 2.97
8: 1.85
9: 1.85
10: 4.5
11: 1.85
12: 1.85
13: 9.1
14: 1.85
15: 1.85
16: .78

[Slowpoke101]

Most of the voltages look OK. D5's voltages are fine and so are the voltages on Q12 and Q13.
The voltages on IC2 are mostly OK but pin 10 is concerning.
Due to the age of the pedal a lot of the electrolytic capacitors will be starting to fail. C29 (1uF 50V) may be leaky and needs to be replaced. I do recommend that you replace the following parts (even if they test OK);

C18  10uF 16V
C26  1uF 50V
C28  1uF 50V
C29  1uF 50V
C30  47uF 6.3V
C31  10uF 16V
C32  0.68uF 50V
C33  0.68uF 50V
C34  10uF 16V
C39  1uF 50V
C50  4.7uF 50V

Tantalum capacitors can be used if you can't get small enough electrolytic capacitors that will fit.
Replacing these capacitors should help restore the pedal to working order. Do be careful and do not rush.

[drummer4gc]

A new C34 did the trick. Thanks! Replacing the rest now.

I've repaired a few of these before by replacing all the electros, which I understand dry out over time. I'm curious how this specifically affects the circuit though. Why would a dried out C34 kill the NE570 output? Is it about leaking DC into the comp? Is it generally an issue of leaking DC that makes these dry caps kill various parts of the circuit?

Thanks!

[R O Tiree]

There is a ready market for buying up old DD-2s on eBay, etc, and simply replacing the old electro caps with new ones.  There's a pretty complex formula for working out roughly how long an average electro cap will last (they are lifed items) and one of the terms in that equation has to do with rated voltage compared with the voltage the circuit is supplying.  If there's loads of headroom (say, a cap rated at 50V in a 9V application), then the cap will last a very long time.  Boss's bean-counters being just like bean-counters the world over saw no point in, in their minds, "over-specifying" those caps, so they fail after a few years.

A mate of mine did a handy trade for a while, buying up dead DD-2s for a few ££ and then re-selling them for £££.

[Fender3D]

...Why would a dried out C34 kill the NE570 output? Is it about leaking DC into the comp?...

Shorting to ground pin 1 or 16 the internal rectifier has no gain control current to drive the gain cell.
Many delay based effects use this same trick to kill wet signal...