Landtone Fuzz Face clone - Transistor issues

[guyshermannz]

Hi All,

I bought this kit: https://www.aliexpress.com/item/32840495281.html and put it together.

It kinda works, in that if I put pink noise through it from my audio interface (I guess it is at line level) it gives about 10db gain. But, when I put a guitar though it, I get nothing. Also, if I put it after a very high gain distortion pedal I get some output from it. So my assumption is that the signal from the guitar just isn't strong enough to get the circuit going (ie not enough voltage on the base of the first transistor), which makes me wonder if this is a transistor issue?

I tested the hFe of the supposed AC128 transistors on my multimeter and it said 678 for one of the transistors, and 680 for the other. I'm assuming this is actually 67.8 and 68? I assume if it was actually 678 I would probably just get a square wave out for pretty much any input signal (given it is powered off 9V)?

The schematic is here: https://www.flickr.com/photos/57725750@N00/49324232502/

All thoughts on how I can trouble-shoot this further are welcome.

Thanks in advance!

Guy.

[mozz]

Your meter is reading the leakage added so it's not a true gain reading.

[tonyharker]

I would suggest that those transistors are fake or very high leakage.  If your transistors are the same as those in the kit illustration, as far as I'm aware Motorola never made any. BTW AC128s were never made in TO5 cans only in TO1.

[iainpunk]

I would suggest that those transistors are fake or very high leakage.  If your transistors are the same as those in the kit illustration, as far as I'm aware Motorola never made any. BTW AC128s were never made in TO5 cans only in TO1.

motorola did make AC128's in a TO5 package, i have a few genuine NOS.
mine are extremely leaky compared to my normal and rectangular ac128's. i'd recommend changing the bias of Q1. you can do that by replacing the 33k resistor with a 100K pot and a 1k resistor like this and turning the pot until you have sound.


cheers, Iain

[MikeA]

For more info on this kit see https://www.diystompboxes.com/smfforum/index.php?topic=119292.0.  Not likely to be vintage AC128's included in a $30 kit from Ali Express....

[PRR]

hFE is a secondary issue. Measure voltages!!

Since they are likely new-style transistors, the key voltages are
Q2e = 0.1 to 0.7V
Q2c = 3V to 7V

For some reason, THE most popular way to mis-build a FuzzFace is to forget to connect the pot under Q2 emitter.

[iainpunk]

For some reason, THE most popular way to mis-build a FuzzFace is to forget to connect the pot under Q2 emitter.

that the funniest thing i read all evening, 

cheers, Iain

[guyshermannz]

Thanks for the suggestions all, I'll have a crack this weekend and keep you posted!

[guyshermannz]

So I put a pot in series with a 1k resistor instead of the 33k resistor and managed to get some signal out of the guitar, but it is still about -30db vs bypass.

Could this be an input impedance thing?

[guyshermannz]

I should clarify that I got signal by reducing the pot to its minimum

[radio]

So  how are your voltages now compared to the fuzz face Paul posted?

[Steben]

Most fuzz faces today come with on both q collectors a trimpot. Just saying. The perfect trannies are scarce.

[Electric Warrior]

The perfect trannies are scarce.

Not at all. The Fuzz Face works fine with a wide range of collector voltages and thus hfes and leakages. Finding transistors that sound good in a stock circuit is easy.

[PRR]

So I put a pot in series .....

Measure voltages!!

[guyshermannz]

So I put a pot in series .....

Measure voltages!!

Ok, so I started trying to measure voltages and realised I'm not really sure what I'm doing.

Should I have something plugged in to the input and output? If so, what should I have on the input, just a guitar doing nothing?

Also, should I be using DC voltage or AC voltage on my multimeter?

In this instance, because of the way the circuit is running between 0 and -9, I assume I should put the "COM" lead of my meter onto the +9 of my power supply right?

Clearly I'm a noob, but I appreciate your help and patience

[willienillie]

Read DC volts, between ground and transistor pins (collector voltage is of most interest).  With a digital meter, it will read negative, so it doesn't really matter which probe goes where, |absolute value| is enough.  If you need an input cable plugged in to power the pedal up (i.e. from battery), then do so.  Otherwise, it makes no difference.

[guyshermannz]

So I read the voltages.

I hooked the COM lead of my multimeter to the sleeve terminal of the power socket (ie the +9V from the power supply, or the "Ground" in this circuit diagram).

I've posted a couple of images. With nothing plugged in, it looks ok, but as soon as I plug something it it goes kinda weird.




I tried plugging into my guitar, and my audio interface (since the audio interface had got signal back from the pedal when I was putting pink noise through), but it was the same.

Any thoughts appreciated.

[duck_arse]

please post photos of this thing you gave built, nice and clear so we can see "things" - solder side as well [some here like looking at those, but not me]. we also want to see any off-board connections.

[willienillie]

With nothing plugged in, it looks ok

It does.

but as soon as I plug something it it goes kinda weird.

That shouldn't happen.  Maybe something is mis-wired, or maybe there's a problem with the input cap?  But the DC voltages shouldn't be affected by plugging a guitar in.

[guyshermannz]

but as soon as I plug something it it goes kinda weird.

That shouldn't happen.  Maybe something is mis-wired, or maybe there's a problem with the input cap?  But the DC voltages shouldn't be affected by plugging a guitar in.

Ok, here's one thing I'm wondering... given the whole circuit has the voltages backwards, should the input cap be the other way round?

The silk-screen actually has it with the negative terminal towards the tip of the input, which I first thought was wrong (and read in a couple of places) so I put it with the positive terminal towards the tip of the input, but thinking about the fact that  the ring of the jack is at a higher potential than the tip, might it make sense for the cap to be "backwards" as well?