Newbie, help requested, Boss CE2 clone

[R2e]

Hi, I've built a Boss CE2 chorus chlone using a Tonepad PCB (aka Chorral Chorus). It is exhibiting the classic gating syndrome, a distorted noise coming through only when I hit the strings hard. The only difference to the Tonepad spec is a TL072 instead of TL022 (which I couldn't obtain and was advised the 072 would be fine). I'm very confused by the results I'm getting (Base lower than emitter in most cases), which are as follows -

                                                  E                                  B                                C
Q1                                          5.11                               3.98                            8.31
Q2                                          5.47                               5.35                            8.30
Q3                                          5.22                               4.33                            8.29
Q4                                          3.34                               3.40                            8.17
Q5                                          0.00                               0.46                            6.40


IC1 (1 to                1.29   0.08    6.09    0.00    6.15    6.17    4.64    8.27
IC2                           Pin 1 fluctuates 7.62 to 1.25
                                Pin 2 fluctuates 4.30 to 3.99
                                Pin 3 fluctuates 6.24 to 2.00
                                Pin 4 0.00
                                Pin 5 fluctuates 4.3 to 3.98
                                Pin 6 3.79
                                Pin 7 7.59
                                Pin 8 8.22
IC3 (1 to                8.12    4.23    5.41    0.55    0.00    4.16    5.48    5.48
IC4  (1 to               8.12    4.16    0.00    4.23    0.37    6.87    2.50    0.55

D1                            (A) 1.51      (B) 2.49
D2                            (A) 2.50      (B) 6.30
D7                            (A) 0.00      (B) 8.20

Any suggestions as to what might be wrong gratefully received. My electronics knowledge is rudimentary and not up to the task of debugging but I am trying to learn as I go along.  Incidentally I've built another half dozen effects from Tube Screamer to Tremelo without any problems, or at least without ones I couldn't solve.   

[lowstar]

hi r2e,
did you check different settings on the bbd trimpot ?
cheers,
lowstar

[R2e]

Hi Lowstar,

Yes, fully to one side I get the gating, move off the stop -  nothing

[Mark Hammer]

It is exhibiting the classic gating syndrome, a distorted noise coming through only when I hit the strings hard.

Classic symptom of an electrolytic capacitor turned around the wrong way.

[R2e]

Hi Mark,

I've checked that at least a couple of times! No pain in doing it again though.............................

[R2e]

Nope, 6 electros, all round the correct way. Any other ideas, anyone?

[RaceDriver205]

If youve checked everything and you can't see what could possibly be wrong, heres what you do:
Check the soldering.
Still won't work?
Check the copper tracks.
not working yet?
Check the soldering.
G#D DAMN IT!, WHY WONT THIS PIECE OF JUNK WORK!
Check the soldering.

Ive gone over a circuit board like a hawk over and over again trying to work out what in gods name is wrong with it. Ive analysed it down to the precise area of the circuit that isn't working and still not made it work. Then one day id be cursing at it and telling it all the horrible things im going to do to it, when I see a tiny, microscopic, barely visible fleck of copper bridging two closely spaced tracks. ARGH!
Hope this helps 

[R2e]

Believe me I've been going over it with a magnifying glass, have remade all the solder joints, desoldered any doubtful solder links. I'm still no closer to identifying where the problem may lie. After a week of this, I thought I would try the forum before trying to achieve perfect pitch by landing it in the middle of the rubbish bin (garbage can) on my way to the lunatic asylum!!

[alteredsounds]

Could you have overheated a component when soldering?  Have you tried audio-probing?

Cheers,
Nick,

[R2e]

I don't think so, the IC's are all socket mounted and the Transistors have been removed, sockets fitted and new transistors tried without any success. Do the figures I got tell anyone anything about where to look next?

[Mark Hammer]

Sometimes the DC voltages at all of the various IC and transistor pins can be diagnostic and pinpoint the problem, and sometimes not.  It's always a good idea to measure them, and it's certainly easy enough to do.  Other times, though, you have to use an instrument signal and measure AC, following the signal through and noting where it diverges from the expected level. 

To do this, set your meter to whatever range will let you read things between 50mv and 500mv AC (making sure to reconnect any probes that are AC or DC-specific), and with the guitar plugged in strum it (all 6 strings) and measure the AC voltage reading at the emitter of Q1 and at the output of IC1 (pin 1).  If you have passive pickups and nothing else on ahead of the chorus, you should probably have a reading of 50mv or so at Q1, and somewhere arond 5x that at the output of IC1.  Measure the output at IC1b (pin 7).  That should be close to the same reading as pin 1.  Now measure the output at the junction of C17 and R26.  If it is close to the same reading as pins 1 and 7, then congratulations you have established that the clean signal is fine and makes it all the way through.  If it chokes at one of those points, then you have identified what MIGHT be wrong.

Yoiu can also verify the expected levels at the emitter of Q2 and Q3.  If the level seems fine at Q2 but not at Q3, then the problem is somewhere between them.  If the level is fine at the output of IC1a but not at the emitter of Q2, then the problem is somewhere between *those* points, and so on.

The obstinacy of the circuit to NOT pass signal unless under duress (i.e., hard strum) certainly behaves like a reversed electrolytic, though since you indicate having double and triple checked for such matters, we obviously need to take you at your word and rule that out.  What it doesn't sound like, though, is a misbiased BBD.  There, you would expect continuous clean sound and ugly but constant delay sound.

Just to make sure we're covering all possible angles, I've had instances in the past where circuits behaved like caps were ass backwards, but the problem was due to an intermittent in one of the connections at the input.  Sometimes a fracture in the patch cord, and sometimes an intermittent on the stompswitch.  We've discussed stompswitches gone bad before.  It can happen that the goop used to momentarily hold the rocker contact in place during assembly of the stompswitch starts to flow when too much continuous heat is applied during soldering.  The goop coats the surface of the contact and you end up getting very poor conduction between contacts inside the switch.  These can occasionally mimic backwards caps because they can also end up conducting only whern the signal level is high.  The solution to this is to disassemble the switch, swab the goop off with a Q-tip, and reassemble.

[RaceDriver205]

Ah! I know. Tonepad has a habit of putting transistors the wrong way round in its layouts. Try checking all the transistors are the right way round by checking their leads (collector, base, emitter) match the schematic.

[R2e]

Thanks Mark and others, I'll give all this a try and hopefully work my way through the problem. Think I need an 'Electronics for Dummies' book..............................