Dyna Comp debug help pls!

[Al Heeley]

I have built the Dynacomp circuit as per the gallery pages here, and constructed myself an audio probe to try and find out why it's not working.
http://www.aronnelson.com/gallery/main.php/v/MarkMs-Gallery/album15/album76/DynaComp_revision_1_LAYOUT.gif.html
I trace the signal down from the input through C1 down to R6 and over to the middle leg of Q1. Here it disappears, nothing on either of the outer legs.
Sorry I'm quite new to all this. Does this mean a faulty transistor? The solder joints look ok. where do I go next? what would I expect from either the upper or lower leg of Q1? (I've plugged an ipod in as a signal source, and yes i did check that the song track hadn't ended!)

I also have an issue with C10, the 1uF electrostatic cap - am I correct the +ve leg goes back to the lower leg of Q1 and the -ve goes to the IC leg 2 or is it the other way round?

Many thanks in advance,
Al

[Al Heeley]

just checked on the schematic - all the electolytic caps are oriented correctly, (I am assuming the square holes on the pcb are for + and the round ones are for -ve or normal) can anyone advise on the transister issue pls and next step in tracing the signal?

[R.G.]

See "Debugging: What to do when it doesn't work".

[Al Heeley]

OK I have some voltages.
Note: std dyna comp circuit, no mods, no parts substitutions.
CA3080 IC
pin 1=0
pin 2=6v
pin 3=6v
pin 4=0     <-- is this odd? this should be the pin to ground?
pin 5=0.7v
pin 6=4.73v
pin 7=9v
pin 8=0

Q1 c=9    b=2.6 e=2.5
Q2 c=9    b=0    e=0
Q3 c=8.8 b=0    e=0
Q4 c=8.8 b=0    e=0
Q5 c=9    b=8.8 e=8.3

[Al Heeley]

OK, I have traced down and mended 2 bad solder joints. Here are the new pin voltages. I seem to be stuck now so if anyone is able to help me out I'd be most grateful!

IC: CA3080E                         Q1        Q2        Q3        Q4        Q5
pin 1=0   pin 8=0                    c=1.5   c=6.9    c=0      c=0      c=8.4
pin 2=6   pin 7=9                    b=2      b=2.7   b=0.6   b=0      b=8.9
pin 3=6   pin 6=3                    e=9      e=2.1   e=8.8   e=8.8   e=9
pin 4=0   pin 5=0.6

[R.G.]

OK, I have traced down and mended 2 bad solder joints. Here are the new pin voltages. I seem to be stuck now so if anyone is able to help me out I'd be most grateful!

IC: CA3080E                         Q1        Q2        Q3        Q4        Q5
pin 1=0   pin 8=0                    c=1.5   c=6.9    c=0      c=0      c=8.4
pin 2=6   pin 7=9                    b=2      b=2.7   b=0.6   b=0      b=8.9
pin 3=6   pin 6=3                    e=9      e=2.1   e=8.8   e=8.8   e=9
pin 4=0   pin 5=0.6

Did you by some chance get Q1's collector and base mixed up in the readings or typing? If those voltages are read correctly, yes, the audio would not get past Q1. But they're about OK if you just mistook one for the other in typing.
Q2 base should have been the same voltage as pin 6 of the 3080, since they're connected by a wire. Is that difference real? If it is, you have more soldering problems on Q2 base.

On Q3 and Q4, again, are the emitter and collector reversed in typing, or did you get those actual voltages? The emitters of both of these should be 0V, as they are soldered to a ground buss (or should be!).

Same quibble on Q5. The voltage is kinda OK if the emitter and collector are reversed.

Did you check the pinouts of the devices you used and compare them against the layout?

[Al Heeley]

thx for the reply, i think it may be my confusion wrongly labelling the mxr schematic and linking it to the rear view of the pcb, i'll go over and double-check them again. The Q's are soldered on correctly according to the pcb layout with the flat face of the 2n3904 facing right the back pin is C, middle is B and nearest is E, but I see on the schematic I've scribbled C & E onto the diagram the wrong way round. oops.

[Al Heeley]

Here's the correct version:

IC: CA3080
pin 1=0      pin 8=0
pin 2=6v    pin 7=9v
pin 3=6v    pin 6=2.7v
pin 4=0      pin 5=0.6

Q1 c=9, b=2, e=1.5
Q2 c=2.1, b=2.7, e=6.9
Q3 c=8.8, b=0.6, e=0
Q4 c=8.8, b=0, e=0
Q5 c=9.05, b=8.9, e=8.4

[R.G.]

OK, that makes more sense.

All but Q2 look like they have reasonable voltages to work. It's possible that Q2 was originally correct and got reversed with all the others in the swap.

Here's why I'm being fussy about this. An NPN transistor can only work properly if the emitter is the lowest DC voltage, the base is about 0.5 to 0.6V higher, and the collector is higher than the base. If those are not true, then the transistor does not act as an amplifier, at least not as you expect it to.

So give a check on Q2 particularly. The corrected voltages show the emitter higher than the collector. It's probably a typo. I do those all the time.

Did you in fact use the 2N3904 recommended in the parts list? If you did, the pinout is as follows: with the flat fact toward you, pins down, the pins are emitter, base, collector from left to right.

[MikeH]

And in the future it would be in your best interest to use sockets for you transistors.  That way if you put them in wrong, it's a really simple fix to orient them correctly.  And I'll admit, there are many transistors that I use commonly that I know are CBE, ECB, DSG, DGS, etc, but I don't actually know which way they should be oriented, and in this case I usually socket them and make my best guess, and if it doesn't work, I flip them around.  It's a horrible hackneyed method, and I really should be banned from here for doing it, but I'm just saying... it works for me 

[Al Heeley]

thanks for the sanity check! Retested the Q2 legs - looks like a typo on my side, c=6.9, b=4.7, e=2.1  - thats more like it.

[R.G.]

OK, now that we have what pin's what down, we can see a problem more clearly.

Is the emitter base lead 2.7V as previously, or 4.7V?

This makes a big difference, because if it's 2.7 and the emitter's 2.1, that's about OK. If it's 4.7 and the emitter's 2.1, something is dramatically wrong. They can only be 0.5V to 0.6V apart and have the transistor working.

Also, is Q3 base really 0.6V with no signal in, or is it really 0V?

[Al Heeley]

RETEST: Q2 c=6.9v,  b=2.7v,  e=2.1v  - sorry must have shorted out somewhere on the multimeter probe.
Q3 c=8.8v, b=0, e=0, must have acidentally shorted the probe again
 

[R.G.]

OK. That helps clear things up. And is a little frustrating because the voltages now appear to all be within expected limits.

So you may be back to the audio probe - however you have found two bad solder joints earlier and it may now be working better.

[Al Heeley]

just starting to realise what a great tool the audio probe is - found a couple of tiny breaks on the copper tracks into the ic where the signal stops, will carry on checking and resoldering till we get a result.

[Al Heeley]

Thanks for the help guys, that last track repair seems to have sorted it. Last month I would have wasted hours on this and then thrown it in the bin in frustration.
Here's the enclosure for which this is destined - a double pedal of classic mxr analog effects - tubescreamer and dyna comp.

[R.G.]

N.B. - the tubescreamer is by Ibanez, not MXR.

[Al Heeley]

N.B. - the tubescreamer is by Ibanez, not MXR.

D'oh! Where does Maxon fit in then? Did they launch their own version, also later copied by Boss, once they had picked apart the circuit?

[R O Tiree]

MXR (aka MXR Innovations) was formed in the early 70s and was later bought out by Jim Dunlop. Nuttin'at all to do with Maxon or Ibanez, AFAIK.

[R.G.]

D'oh! Where does Maxon fit in then? Did they launch their own version, also later copied by Boss, once they had picked apart the circuit?

The Tube Screamer was originally made by Maxon. After several iterations, the same thing was marketed as an Ibanez. Others copied it later. It is not clear to me whether "Ibanez" was a brand name for the company who was, or owned Maxon, or was a separate company. Maxon continued to make the insides of the tube screamer even after it was marketed as an Ibanez. I did run into a history buff at one guitar show who had one (or more) of every tube screamer ever made on a display, from Maxon through all the incarnations from Ibanez.

As far as I know, Boss never made a pedal called the Tube Screamer, although they have had very similar circuitry.