Problem with a Rocktron Big Crush

[jwbink1500]

Hi everyone,

I hope posting a troubleshooting repair question here is OK. I know most seem to post Diy and build problems.

I have a Rocktron Big Crush that has problems. I had modded this pedal to get rid of the volume loss that is in the stock pedal. Pedal worked great for about a year, so I don't think the mods caused this but who knows. I got help at Indyguitarist.com originally, but everyone seems to have abandoned that site. My original post was almost 2 months old before this site admin offered some help. I noticed some of the members there are also members here, too, so I thought I'd ask.

The Sustain knob has kind of turned into a volume knob now. It starts out low volume and increases like a volume knob would until about 3:00, then the volume shoots up abruptly and then drops off completely by 4:00 (just short of being max sustain.) The attack knob seems to have no effect on this effect. Level knob still works as it should.

Here's the schematic: http://i395.photobucket.com/albums/pp37/jwbink1500/BigCrush-2.jpg Might have to right click and save.

I believe it may be the IC but I'm still learning and might be way off. The IC is a LM3080. All transistors are C1815. Recently replaced P2 with a new alpha pot and that didn't work.

The IC measurements in relation to the ground are:
Knob at 12:00          at Max
1    0                           0
2    4.06                      2.83
3    4.03                      2.73
4    0                           0
5    .64                        .72
6    4.04                      .66
7    8.30                      8.29
8    0                          0

Brian Wampler advised the following Mods to remove volume loss:
c40: 1uf
r4: jumper
r25 (connected to r4): 1m
c25 (connected to r38): 1uf
c1: 1uf

I worked backwards through the circuit with and audio probe and came to  the IC but I'm not sure how it's "supposed" to act or sound. With the probe I get sound at the base of Q4 and barely anything off of the collector or emitter.

I'm sure someone can shed some light on the problem for me. Let me know what you think or if I need to post somewhere else.

Thanks
Joel

[R.G.]

Can you take transistor pins voltage measurements for us? There's clearly something wrong, but it appears to be in the transistors, not the IC.
When you say "knob" I'm guessing you mean the sustain control.

[jwbink1500]

Thanks a million for the response R.G.   

    Before i begin I did mean sustain control.

Here's the measurements from all transistors:
Q1
C 9.59
B 8.99
E 8.32

Q2
C 8.32
B 3.13
E 2.62

Q3
C 8.32
B 0.66
E 0.12

Q4
C 7.31
B 3.63
E 3.05

Q5
C 8.05
B 0
E 0

Q6
C 8.05
B 0
E 0

Q7
C 8.05
B 0
E 0

Q8
C 8.05
B 0
E 0

Q9
C 8.32
B 8.05
E 7.49

Q10
C 8.32
B 3.13
E 2.64

Let me know what you think.

Thanks again for the reply!!!

Joel

[PRR]

Annotated plan:  http://imgur.com/qCuvv

As bias current increases, LM3080 out goes to ground, but also the inputs pull non-negligible current to ground.

> I get sound at the base of Q4 and barely anything off of the collector or emitter.

Q4? That appears to be stable and operating correctly. The E and C signals will distort but at very high level, over half a volt.

[jwbink1500]

PRR,
Nice pic, btw. Where do you think the problem is? I actually thought the problem might be Q4 because Q5,6,7,8, had no voltage readings. But I'm also VERY new to circuits and still learning. Let me know if I should give additional measurements.

Thanks

Joel

[PRR]

> Where do you think the problem is?

R.G. knows the circuit; I'm bumping mainly to catch his sharp eye.

I annotated because that many 3-leg parts as a list is just too much for the mind; penciling the numbers ON the plan works better for me and many debuggers. However none of the transistor readings look way-wrong or unworkable.

The '80 voltages don't seem right to me, but I've forgotten a lot about that chip. I hope R.G. spots this.

[R.G.]

Q1
C 9.59
B 8.99
E 8.32

Q2
C 8.32
B 3.13
E 2.62

Q4
C 7.31
B 3.63
E 3.05

Q5
C 8.05
B 0
E 0

Q6
C 8.05
B 0
E 0

Q7
C 8.05
B 0
E 0

Q8
C 8.05
B 0
E 0

Q9
C 8.32
B 8.05
E 7.49

Q10
C 8.32
B 3.13
E 2.64

These are OK.

Q3
C 8.32
B 0.66
E 0.12

This one is badly wrong.

At this point, you need to separate Q3 problems from IC problems. Temporarily open the wire between pin 6 of the IC and the base of Q3, then remeasure Q3 pins. If it comes up to the base at 3-4V and the emitter about 0.5-0.6V lower, then the IC is pulling the transistor base down and there are problems with the IC. I kind of think the IC has issues because pins 2 and 3 are falling with the change in the sustain knob setting, and that's not normally how the 3080 works. The 3080 does have a sudden-death mode. Ever putting more than 1ma into pin 5 when the chip is powered damages or kills it. If you've ever had R22 shorted and turned the sustain knob to zero ohms, it has been damaged. Can't tell from here without more tests, but it sometimes happens.

I generally try never to say "damaged IC" because many beginners automatically assume that a part is bad and not the soldering and wiring problems that are 90+ % of the issues. But it does happen, and the 3080 is prone to it. So separate the IC from Q3, see what Q3 does, and we'll go from there.

[analogguru]

Funny to see the schematic of the Aria ACP-1 (and similar units) in a Rocktron pedal.
But more interesting is the emitter resistor of the phase-splitter (R 25/R 19).  In the original Aria pedal it is 3k3 (instead of the 10k in the Rocktron).  This makes more sense.  First I thought a mistake in the schematic, but the voltage readings let me believe that it is a mistake happened during cloning.

analogguru

[PRR]

> badly wrong. .... then remeasure Q3 pins. If it comes up to the base at 3-4V

Looking back:

> The IC measurements in relation to the ground are:
> Knob at 12:00          at Max
> pin6    4.04               .66

So Q3 base does go to 4V (normal) when the '3080 is starved, drops when '3080 is fat.

> I kind of think the IC has issues... ... I generally try never to say "damaged IC" because many beginners automatically assume that a part is bad and not the soldering and wiring problems that are 90+ % of the issues.

About what I was thinking, but didn't want to commit.

On third thouht: _I_ would look-close at pin 7. Is power really getting to it? (Not just the pad.) But be REAL careful probing: a probe-slip off this power pin to other LM3080 pins is risky, perhaps fatal to the chip.

> the emitter resistor of the phase-splitter

We could speculate that it gives an interesting asymmetric action.

I agree it is more likely someone got carried-away with "10K".

[jwbink1500]

Wow, thanks for the response guys!

So I disconnected the base of Q3. Here are my measurements:

Q3
E  0v
C  8.32v
B  nc

Measurement on pin 6 of IC: .65v
Pin 7 measures 8.31v on the leg so it appears it is getting power.

I never jumped R22 but I did remove it at one point to check the resistor out of circuit but that was after the problem had started.

If the IC is bad, a CA3080 will work as a replacement, correct?

Thanks,
Joel

[jwbink1500]

Thought I might bump to catch someone's eye. Anyone know if the above Transistor is messed up or working correctly?

Also, I noticed R. Keeley uses 2SC1849 Transistors in his comp instead of the 1815. Is this a compatible Transistor for the Big Crush? Is it an "upgrade" over the 2SC1815? I thought if I have to order some anyway, then I might try those but wanted someone elses input.

Thanks
Joel

[PRR]

There's three things on this node. '3080 pin 6, Q3 Base, and the 150K resistor. (And a cap but that's unlikely to be at fault.) I was not sure what R.G. wanted to test. I suspect he wanted the 150K still biasing Q3 Base. Your open-base measure is perhaps not useful.

However: what is '3080 pin 6 doing? Is it solid at +5V? Or lower?

Put Q3 Base back.

NOTE which way the '3080 is oriented..

Is the '3080 in a socket? Just take it out (wrap it in tinfoil). Re-measure Q3 Base.

If not in socket.... well, I suspect you are going to replace it anyway. So get brutal. Use small sharp wire-snips. Cut pin 6 so the chip is not connected to the PCB. Re-measure Q3 Base.

I read nominal as +5V at the "+5V" supply near R42 R43, maybe 4.5V at Q3 Base, Q3 emitter 0.6V lower. This should be stable with signal (though with the snipped '3080, there won't be signal at most points).

You might just go ahead and keep snipping the legs from the '3080. Without a DIP-head iron, that's the only way to get a bad DIP out. Snip each pin, toss the body, then use small needle-nose and a hot iron to remove each pin from the PCB. The holes will fill with solder; quickly re-heat then poke a sewing-needle through (solder won't stick to cold chrome). Buy a '3080 and a 8-pin DIP socket. Solder the socket. Insert the '3080 Right Way Round.


> uses 2SC1849 Transistors

There's no magic or special-specs in how these transistors are used. Any small Silicon NPN is fine.

[jwbink1500]

PRR,

Thanks for the reply!

OK, so I disconnected pin 6 of the IC without clipping and measured with base of Q3 reattached. Voltage at base of Q3 varied depending on sustain knob position.

With sustain (P2 on Schematic) turned up all the way, voltage was 3.62v and with it all the way down it was 4.13v. The other knobs had no effect on the measurements taken. As I said, pin 6 was disconnected from this test. Does this sound right?

The reason I asked about the 2SC1849's was because of this http://www.robertkeeley.com/Audiokt88/kross.html I didn't know if there might be slight tone improvement or not.

Let me know what you think.

Thanks,
Joel

[PRR]

> Voltage at base of Q3 varied depending on sustain knob position.

That't not right. Q3 has no direct connection to Sustain.

Over near Q1 is a "+9V" and a "+5V" point. Are these stable and the appropriate voltage? Mostly "stable"; whether 10V-6V or 8V-5V isn't important but it should not vary even a tenth volt. 

[jwbink1500]

Thanks PRR,

Pin 6 of the IC is still disconnected at time of these tests

I tested C28 and got no voltage. I tested where R42 & R43 meet and got 3.84 with sustain at max an 4.38.

The 4.38 begins to slowly decrease as I increase the sustain, and when I get to that point where normally the volume drops off, as I described in my original post, the voltage suddenly drops to about 3.84. The 9V supply off the emitter stays steady at about 8.3V regardless of sustain position.

It does sound like the cap has failed correct? Should I replace it with another electrolytic or different type (poly film for ex.), or does it matter? Could this be causing all of the other problems?

Thanks
Joel

[PRR]

Replace the cap just because cheap-n-easy.

But I'm inclined to think the LM3080 has blown its PNP side. Pins 2 3 and 5 will still pull current, but without the PNPs then 3080 T1 T2 work as diodes instead of transistors, pulling 100X more base current than normal. Increased pin 5 current causes pins 2 and 3 to pull large current in R5 R7, enough to load-down the R42 R43 junction.

[R.G.]

Replace the cap just because cheap-n-easy.
But I'm inclined to think the LM3080 has blown its PNP side.

That's what I make of it too.

[jwbink1500]

Thanks for the reply fellers,

I'll replace both the IC and the cap and go from there. I'll have to order the parts from SmallBear. I'll post the results after I receive and swap.

Thanks a billion guys!!!   
Joel

[jwbink1500]

Hi again guys!

I finally got around to changing some parts on the comp. Started with cap. Replacing it made no noticeable change. Next I removed the IC and installed a socket and the CA3080. Viola!! Good as new!

Thank you RG and PRR for all your help! Could not have done it without you! 

Joel