freestompboxes.org

Hi all.
One of my recent aquisitions from a local musicians marketplace on the web was a Fuchs compressor in a defunct state. The seller told me that this was his second unit. The first one had started to distort with no previous warning and he got a new one through warranty. Now this second unit was working for some time (not sure how long) but then the distortion came along again. At this point he went on to purchase a current Dunlod/MXR super comp as a replacement and sold this for pennies. The pedal arrived and sure enough, the sound was distorting in not so good manner. Opened it up. My first impression was blah. Not the strong goop we've seen way too often, but a rubber goop, all over the board. Otherwise decent build quality, if it werent for the goop. And for what it's worth, this particular rubber is quite easily removable. It'll just takes some effort and lots of patience. Neither of which i had. At this point at least.

At a glance, i was pretty certain what was the problem. The circuit is relatively simple. A DC-DC converter, quad opamp and a VTL5C3 vactrol (plus obvious bunch of caps, resistors, two pots, a transistor and a diode). A MAX1044. This was the thing that first caught my eye. I've seen a few of these broken and since there are sockets of for both ICs i swapped it for ICL7660 which i had at hand. Nope. That's what you get for not measuring and assuming. Then, the symptoms pointed at the opamp. Swapped that and yes. Unit was back to life. I already forgot what the original quad was. But i replaced it wit LM324.

I didn't reverse it any further for now. Should be nice and easy to do, even though the board is two sided - once, or if i get around to it. So. Here are couple of photos for now.

I personally wouldn't put black goop over the top of a socketed IC; I'd worry it might degrade the socket contacts. But I'm sure that's just me being picky.

Do you think maybe one of the opamp channels might be driving too small of a load, causing them to burn out repeatedly?

That would explain it. At first i suspected the pump, as those are quite sensitive for peaks and other interference. And i've seen a few of them dead over the years (usually due to bad, or insufficient protection from minor surges). The first clue should have been that the protection diode (possibly a 10-12V zener) was in tact.

Since it was the opamp and the same issue was with two of the pedals, i'm thinking of a design error. If you look a the opamp channel at pins 12, 13 & 14, it seem to me like those are not connected to anything. Which could be a possible cause of death. As described here: http://www.electronicproducts.com/Analo ... p_amp.aspx
I can't be sure without a proper trace. But if this is the case, every single Fuchs Royal Plush unit is a risk. *If* those pins are left afloat, the fix could be as simple as grounding pin 12 and shorting pins 13 & 14. This being said, the traces could be between the socket and the board. Can't tell from the pictures. I think i'll measure this once i'm back at my desk.
+m

mirosol wrote:That would explain it. At first i suspected the pump, as those are quite sensitive for peaks and other interference. And i've seen a few of them dead over the years (usually due to bad, or insufficient protection from minor surges). The first clue should have been that the protection diode (possibly a 10-12V zener) was in tact.

Since it was the opamp and the same issue was with two of the pedals, i'm thinking of a design error. If you look a the opamp channel at pins 12, 13 & 14, it seem to me like those are not connected to anything. Which could be a possible cause of death. As described here: http://www.electronicproducts.com/Analo ... p_amp.aspx
I can't be sure without a proper trace. But if this is the case, every single Fuchs Royal Plush unit is a risk. *If* those pins are left afloat, the fix could be as simple as grounding pin 12 and shorting pins 13 & 14. This being said, the traces could be between the socket and the board. Can't tell from the pictures. I think i'll measure this once i'm back at my desk.
+m

Thanks for the link, Mirosol. I didn't realize the importance of properly terminating unused op amps, though I was aware of the practice of grounding unused inverters in a CD4049UBE circuit.

You want to terminate the unused + input to 1/2 supply voltage and short its output to its - input. Of course there will already BE a virtual "ground" (1/2 supply) available you can just short the + input to; every op-amp circuit has one. This turns the unused amp into a low-impedance buffer for the virtual ground, which you could use elsewhere instead of the resistor-based voltage divider.

Just checked, the pins 13 & 14 are tied together. As for pin 12, i didn't catch it to be connected to anything. In this case 1/2 of the voltage is ground, or 0 volts - VCC+ is connected to supply and VCC- is connected to negative 9 volts from the DC-DC converter. It would be odd if that one pin was floating free and inverting input and output were connected.

Another thing. I'm not sure why i recalled using LM324. The chip i have in there is TL084.

Now i'm really thinking this should be reversed to see what's wrong with the design...
+m