Compressor debugging (can anybody please check my voltages ?)

[Xavier]

I am building a compressor based on this layout  http://www.sabrotone.com/wp-content/uploads/2011/10/DiamondComp.gif . It is actually the 2nd one I have built , but my brain starts going out of resorces...............

i get no sound (almost). I can hear the eq and volume. All pots seem to work but all I get is this really low output.

The problem seems to be around IC1. I get signal in pin 5 of IC1, but pin 7 (the IC output) shows nothing........... I have *same old story I know  * checked all components and values twice, ticking them in the parts list sheet..............

The status LED worked and now it has stopped working..............


These are my voltages , from a 9,1V battery.


   C   B   E
Q1   8,34   0   0,1
Q2   0,15   0,81   0,15
Q3   8,34   3,33   2,86
Q4   2,86   1,09   0,48
Q5   7,3   0   0
Q6   7,3   0   0
Q7   7,1   0   0
Q8   8,3   0,79   0,28
Q9   0,1   0,7   0
         
IC1 (OP275)         
1   4,2      
2   4,2      
3   4,02      
4   0      
5   4,13      
6   4      
7   0,8      
8   8,3      
         
IC2 (LM358N)         
1   4,15      
2   4,15      
3   4      
4   0      
5   3,96      
6   3,8      
7   4,13      
8   8,29      


Any suggestion ?

[brett]

Hi
a schematic would help a lot more than the layout. But...
Q1 isn't "on". Apparently the base is connected via a resistor to EQ SW 4, which I couldn't find elsewhere.
I only got as far as checking Q1. There might be other issues.
This seems a bit complicated to do on vero. If you can make a PCB it would make things a lot easier.
cheers

[Xavier]

Thanks for your help Brett, I appreciate it !!!

here's the schematic. I checked all solder joints without luck...........

http://smg.photobucket.com/albums/v682/Catacrunch/?action=view&current=DIACMP.png

[Xavier]

I had all transistors socketed, so I removed the sockets and soldered the transistors on the board....no luck. Also checked for shorts and attempted to remove with a stanley blade any potential piece of wire in between tracks..........tried other IC's.........

most challenging circuit to debug to date for me............argh argh argh.......

[brett]

Hi
Is R7 the right value and soldered properly?
Because the base of Q2 should be 1.5 V, not 0.15V. The collector voltage is too low, as well.
If you look at Q2 and Q4 on the schematic, they have a very similar set-up, and the voltages for Q4 look good, but the voltages for Q2 look bad.
cheers

[Xavier]

Yes, R7 is the right value and is soldered properly  

I rechecked all components again (and again) and they are all good............ i redid all the solder joints also. I wish i was able to find a wrong value resistor, but they are all correct.

I changed Q1 but I still get 0 volt (even negative voltage !) in the base of Q1.......

Running out of ideas...........frustrated,

EDIT: PROGRESS !!!! cold solder joint between one end of R20 and lug 1 of the comp knob...now the compressor works but when I switch in the eq circuit, it goes silent,,,,,,,

[brett]

Hi again
I'd have a look at R4. It caries a positive DC bias from IC2B to the base of Q1. The equalier on/off switch should be arrying that bias (about 4 or 5 V) to Q1. 
Also, the diodes on the base of Q2 can't really help but have 0.7V across each of them (assuming they are the right way around - ), so it's quite surprising that the base of Q2 isn't at 1.4V.
Keep at it - there's a dead simple solution.
cheers

[Xavier]

Oh yes it was............since there was no signal at EQ input, I started following the circuit with the probe, and this is what I found:

Cold solder joint at lug 4 of the eq switch !!! this is why when I hit the eq in, all I got was silence...............

So all my problems with this build have been cold solder joints !!!. I never had this in the 7 years i have been building pedals, at least not this amount...............time to change my tin brand ??

Many thanks for your help Brett !!!