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Blackbird VIPfx self oscillate/squeal in Boost Mode (Black '65) UPDATED!

Started by Loztboy, May 23, 2013, 08:46:11 PM

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Loztboy

I'm having some problems with self oscillating/squeal in the boost mode.
This happens when volume and gain is at about 12 o'clock and more (only in Boost Mode)
I have matched the transistors and I use 33K for the bias resistors to get about 4.4 Volt at all testpoints.

Have anyone else experienced this?

I tested it outside the box and thought it had to do with all the long wires on the test rig.
I boxed it up and got the same result.

I tested all resistors and caps before I soldered them in. Normal mode works like it should.

EDIT:
I have now desoldered the transistors and bias resistors and soldered in sockets.
I got the j201's from Tayda.

EDIT2:
I got my TweedyBird up and running with J201's from the same Tayda batch and it's working fine.
Can someone with a black '65 clone crank the volme and gain pot in Boost mode and report back ?
I use a Jazzmaster with 1MEG pots btw.

EDIT3: Installed shielded cable from input jack to 3pdt switch and from 3pdt switch to input at board.
Did not help much.

Loztboy

If I add a buffer in front the problem is solved.
If I only tested that before butchering the board and installed sockets and destroyed a good transistor :D

Does not anyone else experience this?
If I dont plug anything in the input jack and turn up gain and volume in boost mode, it self oscillate like CRAZY.

btw. It sounds freaking AMAZING!

Loztboy

I got a production unit so I could compare mine with the real deal.
There are no 10pF's in the production unit, there are 3 100pF caps. (WIMA 100/100) I soldered one out to measure it, and it is 100pF.

When I swapped the three 10pF in my build with 100pF there are no more squeal.


Here is a photo of the production unit: