Modding an SMD pedal

O

Orson Anton Welleson

New member
Hi all,

I had bought a couple of Rat clones in recent weeks. One is a Little Bear R.Attack. It appears there is some variance from the original Rat circuit (see this thread on FSB, relevant schematic is about 1/2 way down - https://www.freestompboxes.org/viewtopic.php?t=24444)

I had intended to mod this little bugger, due to it not quite sounding like the other Rat clone I bought, and the Muroidea I built. It lacks the bite and snarl the Rat circuit is known for. Upon opening it up, I see the PCB is somewhat hybrid SMD/through hole, so I thought I'd just remove and replace the appropriate components. The few changed components from the original Rat circuit do not have the holes for the through hole components that the other components do. I had various ideas about trying to tack resistors on to the SMD, assuming that parallel resistors (for example) would result in the needed value.

After studying the board at length today, I have concluded this is a lost cause, and will have to accept this rather mellow Rat clone as it is. I thought I'd ask those who might know, before re-boxing the circuit board, etc, and setting it aside.

As I said, I believe it is a lost cause, and am going to build another Rat on vero, and use this enclosure for it. If I'm giving up too soon, I'd love to hear it, but if otherwise, I'm cool with that.
 
Can you post a pic of the board?

From what I can see on the schematic, assuming the markings in red are where the R.Attack differs from the RAT, the most important ones are the different filter values in the op amp feedback loop, the compensation cap being larger, and the clipping diodes being asymmetrical.

Using a naive RC filter calculator it seems that the low pass values were 1540Hz and 60Hz originally, and in the R.Attack are 1450Hz and 78Hz. Frankly that doesn't sound like a hugely impactful change, to me, but it is there.

Depending on where those resistors are, you could try just switching out the SMD resistors? Depending on the size, it shouldn't be too bad, plus you could get away with just switching out the two resistors. Switching the 110 to a 103 would make it pretty much exactly the same, and with a 100ohm resistor you would get to 1590Hz, which is pretty close. I suspect that small of a difference won't be very noticeable though.

Similarly you could just switch R5 to a 560 ohm resistor. I suspect it won't make a huge difference either, to be honest, but I can't say for sure.

No idea how the comp cap affects things tbh. Is it still a LM308 in yours? The one posted in that thread was through hole, so it's also possible yours has a different op amp - and different values all over, for that matter.

The asymmetrical clipping might make the biggest impact out of these if you ask me, are the diodes through hole? Shouldn't be too bad to desolder (or just snip) one of the two out that are connected and just jumper it instead, if you so wish.
 
In spite of having been building and modding pedals, my electronic technical knowledge is beginner level. Tracing a board is doable, but this one stumped me. Having said that, there were what *appeared* to be changed component values, but was unable to confirm where they corresponded to the original Rat schematic. As you can see, some components don't have through holes, or some do have holes, but they are too small to have been intended for a component.

I had similar thoughts about the diodes as well, but D1/D2 have holes in the board that do not look like they were intended for components/other diodes. D4/D5 on the other hand, look like they could be. I also thought about swapping out the K30A transistor (I've never seen that specific designation before) for a 2N5458. I have some I bought almost twenty years ago, and never did anything with. I would think changing the transistor would *not* have any noticable effect, so chose not to.

I have a board for the PPCB Informant. I figure my time is better invested in that, and later I can built a V1 Rat on vero and use the Little Bear enclosure for that.

Thanks!
 
I was about to write "Disregard this thread" but it seems that's already happened! Ha! So just for grins, I replaced the op amp with a genuine, Asian LM308 I bought from Synthrotek. Normally, most generally agree that op amps don't make much of a difference in drive pedals, although in my experience, the ZenDrive is an exception. After changing out the op amp in the Little Bear, it came to life! It has all the bite and snarl a Rat usually has, although there's one difference. I notice the 1.5K resistor has been changed/increased. I came across a thread on Freestompboxes.org, where Mark Hammer discusses that 1.5K on the tone control, and that raising 1.5K to either 4.7 or better, 10K will tame the Rat *without* neutering it. That appears to be what is going on here.

The op amp I replaced has markings on it of "LM308" that are so faint as to barely be legible. While both op amps are imitations, the new one is either a better quality, or the original op amp wasn't working as it should. It made all the difference in the world. I'm going to try the Mark Hammer mod on my Muroidea.

Just an FYI.
 
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