EH Flanger LFO circuit

[Vsat]

Studied the LFO circuit from the original EH Electric Mistress schematic - the one that uses the LM339 comparators along with half of a CD4013. This was probably done to make use of the left-overs in the design, rather than adding a dual op amp. In the Deluxe version, an LM324 was used instead for the LFO.

A rather odd design - sort of like a "build yer own 555 timer from scratch" along with a comparator that is used as an op amp integrator... Nat Semi app note AN-74 fig. 28 applies here. There is a re-drawn schematic on the net that is confusing since the (+) and (-) inputs on the "op amp" are reversed, although the pin numbers are correct. The schematic makes sense when you know this. The 2u2 capacitor in the "integrator" should be a non-polar electrolytic or film type if problems are to be avoided. The re-drawn schematic shows a regular electrolytic. One end of this capacitor sits at +6V "virtual ground" while the other end swings several volts in both directions.
Regards, Mike

[Arno van der Heijden]

Hi Mike,

Here's another version:

http://img.photobucket.com/albums/v317/StephenGiles/EHelectricmistress_UK.jpg

Btw, I'm the guy with the asymmetrical LFO. Is it possible to diagnose it with a DMM?

[Vsat]

Arno,
Still acting asymmetrical? I don't have an actual unit here but should be possible to figure out what is wrong with a meter. As before, the 2.2 uF electrolytic would be a prime suspect. Checking the voltages along the resistor divider chain that is used for biasing different parts of the circuit could provide some clues.... and power supply voltage and solder joints etc shopuld be checked if they haven't been already. Perhaps someone on the list has an original unit to confirm voltages?
Rgards, Mike

[Arno van der Heijden]

Mike,

Power supply voltage is sitting at 13V, as recommended by EH.
Can you tell me which parts of the circuit should sit at virtual ground, so I can check them?