69 fuzz oscillation

[rangermaster]

Hi.

I have a diy fulltone 69 fuzz(on eyelet board) on my desk that has a lot of oscillation. Starts with gain pot at 3 and then it's synths hell. I already replaced all the wires, pots and the transistors(oc71 and tried oc45's). Didn't help.

The bias is set at around -4.5v (positive ground).

Anyone has any idea ?

Thanks

[Fancy Lime]

Hi,

Can we see some pictures? Might help in identifying potential layout issues.

Andy

[antonis]

fuzz(on eyelet board)

That..

[rangermaster]

I used this. (220ohm is replaced for a 470ohms resistor)

Deleted.

Why not on eyelet board ?

[Fancy Lime]

Indeed, why not?

Photos of the actual thing would be more informative, though.

Andy

[antonis]

Why not on eyelet board ?

Indeed, why not?

Are we talking about an inherently prone to oscillation build, or what..?? 

If yes, items vulnerable to crosstalk/interference exposed redundat legs don't grease the skids, do they..??

[rangermaster]

.

[antonis]

Proceed to all time classic remedies, like Miller caps (100pF between Q2 C-B and 220 - 470 pF between Q1 C-B ..) and lowering 100k feedback resistor value (and/or placing 1 - 10 nF in parallel with it..)

[Fancy Lime]

What kind of resistors are those, carbon comp or any kind of film?

There are two potential issues I can see. The input cap runs in parallel and very close to the collector resistor of Q1. And the Q1C to Q2B connection runs parallel and very close to the bias resistor. If there is crosstalk at these points, there might be oscillations. Try increasing the distances between those components by bending them a little. Any change?

Andy

[rangermaster]

Resistors are carbon film. Only the 470R is a carbon comp(don't have that in film)

I will try bending or shield the jumper wires tomorrow.

[Fancy Lime]

Resistors are carbon film. Only the 470R is a carbon comp(don't have that in film)

I will try bending or shield the jumper wires tomorrow.

Ah well. About the only thing that carbon comp resistors have going for them is that they are less susceptible to inductive crosstalk.

[rangermaster]

Ok well i fixed it.

Made a new bigger board and used metal film resistors. Problem solved

[Fancy Lime]

Great! Do you mind posting a picture or two of the working board as well? I am planning to do a Si Fuzz Face Mojo Build and I am not quite sure what technique to go with. I was originally thinking turret board but I must say I really like the look of eyelet boards.

Cheers,
Andy

[iainpunk]

if you want a sexy looking MOJO technique for simple circuits, i'd go for amp-style turret board, 1/2W vintage red .25% resistors, and Bakelite screw terminals as transistor sockets.

cheers

[Fancy Lime]

Yeah, resistors are the most difficult part for a mojo build. I was thinking large 2W or 5W metal film or ceramic cased wire wound resistors. Much more options for ridiculously overpriced and oversized caps in bold colors. Although sadly nobody seems to make tropical fish anymore. I had not even thought about socketing transistors but screw terminals are a really good idea. Thanks!

Andy