Gretsch Expandafuzz

[Piddlepaddle]

Okay so I've got an expandafuzz and I would like to clone it. The thing is that it uses an ml748cp 14 pin ic in all 5 circuits of the pedal, fuzz, boost, treble, bass, midrange. Now I notice that the contro fuzz circuit is not all that different from the expanda fuzz and there is even an 8 pin IC in the controfuzz clones where it appears there was originally a 14 pin possibly. My question is would it be possible to substitute another 14 pin ic for the ml748cp such as a tl074 or possibly even an eight pin such as a lm358? I might also mention I can't find any data sheets on the ml748cp. Any thoughts would be much appreciated.

[nelson]

Are you sure it's not this?

http://www.national.com/mpf/LM/LM748.html

It's most likely a 14 pin quad opamp.

I would trace it with that assumption and when going to clone it use an equivalent quad opamp.

[tommy.genes]

Don't know if you already saw it, but Analog Mike posted the Expandafuzz schematic in THIS POST.

-- T. G. --

[Mark Hammer]

You need to consider the limited choice in op-amps at the time this pedal was produced.  Unless there is some specific purpose to the compensation applied to the 748 (and from the use of 25pf caps throughout, it would appear there wasn't), you can feel free to use plain vanilla 4558s or even an LM324.

[R.G.]

Mark, you are correct again. The uA748 was an uncompensated opamp with higher native performance than the 741, kind of a next-generation follow on. It's loosely equivalent to the LM301. The stock compensation is with a 30pF cap on the compensation terminals, producing a similar compensation to the 741. Apparently it was once available in a 14 pin package. That's odd.

But the 8-pin LM748 and in fact any of the currently available uA748s will exactly replace it; just map pins to function. In fact, if you look at the pins and the placement in a 14 pin footprint, the noted pins fall into the same pattern as the pins on an 8-pin version. You could solder an 8-pin LM748 into the 14pin pattern and have it work, as pins 1, 2, 7, 8, 13 and 14 are no-connects.

I would use a modern opamp; the LM301 is a direct replacement. The TL071 should work. A reworking using TL072s would be a better layout.

Watch the power supply. It's not shown on the schematic, and it's bipolar, both +9V and -9V. So you'll need to either use two batteries, a transformer to make +/-, or a charge pump to make - from +.

[Piddlepaddle]

R.G. thanks a million. I kind of figured you would be able to replace the ml748cp with an eight pin but wasn't sure because I couldn't get my hands on a data sheet for it. I've got the original pedal here so I'll map it and apply what you've told me above. wish me luck!! Thanks again.

[nelson]

Learn something new everyday!



14 pin opamps.

[Mark Hammer]

I think I have a couple of 14-pin 741s in the parts bin.  Needless to say, they don't find a whole lot of use anymore.

[Piddlepaddle]

Mark, you are correct again. The uA748 was an uncompensated opamp with higher native performance than the 741, kind of a next-generation follow on. It's loosely equivalent to the LM301. The stock compensation is with a 30pF cap on the compensation terminals, producing a similar compensation to the 741. Apparently it was once available in a 14 pin package. That's odd.

But the 8-pin LM748 and in fact any of the currently available uA748s will exactly replace it; just map pins to function. In fact, if you look at the pins and the placement in a 14 pin footprint, the noted pins fall into the same pattern as the pins on an 8-pin version. You could solder an 8-pin LM748 into the 14pin pattern and have it work, as pins 1, 2, 7, 8, 13 and 14 are no-connects.

I would use a modern opamp; the LM301 is a direct replacement. The TL071 should work. A reworking using TL072s would be a better layout.

Watch the power supply. It's not shown on the schematic, and it's bipolar, both +9V and -9V. So you'll need to either use two batteries, a transformer to make +/-, or a charge pump to make - from +.

So I'm looking at this original pedal and I'm assuming the pinout on the ml748cp is a standard pinout as far as numbering goes correct? 1 through7 on the left proceeding down and 8 through 14 proceeding up. Sorry if that sounds like a stupid question but I couldn't find anything on this chip. Looks like pin 1-3 are soldered together 4 goes to pin 8 5 to ground it seems 6 jumpers to the other op amps 7 nowhere. 14 13 and 12 are together 11 jumpers a few things together. 10 looks like +9v 9 isn't connected to anything and 8 to 4. So if I drop a 8 pin lm301 in there oriented the same way pin 1 in place of 1-3 ect.. I'm in business?
Thanks again.

[R.G.]

Lemme see if I can do this all mentally.
Starting from the end that  has the notch or dot, the left hand row of pins is:

1 = NC
2 = NC
3 = Pin 1 on normal chip, = compensation pin
4 = pin 2 on normal chip, = Inv Input
5 = pin 3 on normal chip, = + input
6 = pin 4 on normal chip, = V-
7 = NC

Then around the bottom, starting up the right side:
8 = NC
9 = pin 5 on normal chip, = NC
10 = pin 6 on normal chip, = output
11 = pin 7 on normal chip, = +V
12 = pin 8 on normal chip, = compensation pin
13 = NC
14 = NC

pin 1-3 are soldered together

That's the compensation pin. OK so far.

4 goes to pin 8

This is the inverting input. Does it go anywhere else?

5 to ground it seems

+ input to ground for bias. That's OK.

6 jumpers to the other op amps

To all the other opamp -9V perhaps?? That would fit the theory.

7 nowhere.

That's OK.

14 13 and 12 are together

These ought to go to a compensation capacitor if the theory holds.

11 jumpers a few things together.

I don't know. On the 301, this is a +V pin. I'd expect +9V here.

10 looks like +9v

Hmmm... should be the output pin. Where does it go in the circuit?

9 isn't connected to anything and

That's OK by the theory.

8 to 4.

That's OK, too.

So if I drop a 8 pin lm301 in there oriented the same way pin 1 in place of 1-3 ect.. I'm in business?

Maybe. I worry about pin 10 being at +9V. But if the opamp is damaged, could be.

The 8 pin 301's pin 1 goes into the hole for pin 3 on this layout. That should leave pins 1, 2, 7, 8, 13 and 14 with no pins in them at all. It's sure worth a try.

[Piddlepaddle]

Lemme see if I can do this all mentally.
Starting from the end that  has the notch or dot, the left hand row of pins is:

1 = NC
2 = NC
3 = Pin 1 on normal chip, = compensation pin
4 = pin 2 on normal chip, = Inv Input
5 = pin 3 on normal chip, = + input
6 = pin 4 on normal chip, = V-
7 = NC

Then around the bottom, starting up the right side:
8 = NC
9 = pin 5 on normal chip, = NC
10 = pin 6 on normal chip, = output
11 = pin 7 on normal chip, = +V
12 = pin 8 on normal chip, = compensation pin
13 = NC
14 = NC

pin 1-3 are soldered together

That's the compensation pin. OK so far.

4 goes to pin 8

This is the inverting input. Does it go anywhere else?

5 to ground it seems

+ input to ground for bias. That's OK.

6 jumpers to the other op amps

To all the other opamp -9V perhaps?? That would fit the theory.

7 nowhere.

That's OK.

14 13 and 12 are together

These ought to go to a compensation capacitor if the theory holds.

11 jumpers a few things together.

I don't know. On the 301, this is a +V pin. I'd expect +9V here.

10 looks like +9v

Hmmm... should be the output pin. Where does it go in the circuit?

9 isn't connected to anything and

That's OK by the theory.

8 to 4.

That's OK, too.

So if I drop a 8 pin lm301 in there oriented the same way pin 1 in place of 1-3 ect.. I'm in business?

Maybe. I worry about pin 10 being at +9V. But if the opamp is damaged, could be.

The 8 pin 301's pin 1 goes into the hole for pin 3 on this layout. That should leave pins 1, 2, 7, 8, 13 and 14 with no pins in them at all. It's
sure worth a try.

quote]4 goes to pin 8 [/quote]
This is the inverting input. Does it go anywhere else?
Looks like it goes to a few resistors that lead to a few caps

14 13 and 12 are together

These ought to go to a compensation capacitor if the theory holds.
Along with pin 1 this leads to the compensation cap

quote]11 jumpers a few things together.[/quote]
I don't know. On the 301, this is a +V pin. I'd expect +9V here
Sorry yes it appears that +9v is on 11

10 looks like +9v

Hmmm... should be the output pin. Where does it go in the circuit?
To a resistor then to wiper 3 of the pots. I suspect your right about the output

So if I drop a 8 pin lm301 in there oriented the same way pin 1 in place of 1-3 ect.. I'm in business?

Maybe. I worry about pin 10 being at +9V. But if the opamp is damaged, could be.
I think I was mistaken as the back of the board is marked with a + but now i see it's for one of the caps in the circuit.
I think It should be able to drop right in.

Now the only thing left is trying an 8 pin would any form of the lm301 work such as lm301a?

[Mark Hammer]

Seems to me the most obvious thing to do is to clone it using some "normal" op-amps, and A/B the clone against the original.  If the clone does not behave similarly, THEN you go about use of comparable op-amps to the original.  I've built a couple of Controfuzzes using plain vanilla 1458 chips, and they work just fine..

[Piddlepaddle]

Thanks guys I'll let you know how it turns out or if I have any problems. 

[wohali]

Sorry to resurrect an old thread - but this appears to be the only ML748CP thread on the net!

In cleaning out my old chip collection, I found a stash of these devices - in 14 pin format. I have about 40-50 of them. If you think you could use them, make me an offer.