Aion Quadratron (Lovetone Doppelganger) LFO Depth

[benny_profane]

My quadratron build has a slight issue with the LFOs when dual mode is activated. In single LFO mode, the entire range of knob is useful. In dual LFO mode, the upper third or so locks up the LED at full brightness with no oscillation.

The build has the suggested LEDs (Tayda diffused green). The span pots have been increased to B2K from the original B1K. I've tried tacking a 2k resistor in parallel with the pots to simulate the stock value, although I don't think that this is related to this problem. The problem is exhibited when the pot is completely turned up and there is no resistance between the output of the op amp and the series resistor / LED array.

I've tried swapping the IC6 with another TL022 with no change in behavior.

I pulled one of the LEDs for testing. I'm getting consistent Fv measurements with two different meters for that LED and samples from two different batches of green diffuse LEDs from Tayda.

I'm stuck (much like the LEDs).

[LaceSensor]

Change the span controls to B1k, problem should go away.
If it doesnt, you have an issue somewhere else. Check all parts, with specific reference to the series resistors on the span controls (R27-32).

By now, this circuit is quite well characterised. The schematic hasnt changed in a meaningful way from the reverse engineer I did many years ago, and subsequently did a few runs of PCBs (at least a couple hundred, and I can count on one hand the number of replacements - all were user error, too).

Its absolutely normal for the indicated oscillation to be different in dual than single LFO mode as demonstrated by my vintage unit.


Nice looking enclosure by the way

[benny_profane]

Thanks for taking a look (and the compliment!). I'm a little confused as to why the SPAN control would have an impact here since the lack of cycling is exhibited when the pot is essentially shorted? Do you think that there'd be a difference between the 1KB pot and a 2k resistor in parallel?

R27 – R32 are all 330R.

[Govmnt_Lacky]

Thanks for taking a look (and the compliment!). I'm a little confused as to why the SPAN control would have an impact here since the lack of cycling is exhibited when the pot is essentially shorted? Do you think that there'd be a difference between the 1KB pot and a 2k resistor in parallel?

R27 – R32 are all 330R.

This is just a shot in the dark here but.....

Did you verify that the Span pots Lugs 1 and 2 are shorted? NOT Lugs 2 & 3? Just wondering if by chance the boards are incorrect and causing issues.

[benny_profane]

This is just a shot in the dark here but.....

Did you verify that the Span pots Lugs 1 and 2 are shorted? NOT Lugs 2 & 3? Just wondering if by chance the boards are incorrect and causing issues.

I didn't. I can check on that too. Although I very much doubt that that's the case since it would mean that the controls would operate in reverse.

[LaceSensor]

you should take some voltage readings on the LFO IC
It might be that its still "swinging" the voltage but you arent seeing that on the LED

[benny_profane]

you should take some voltage readings on the LFO IC
It might be that its still "swinging" the voltage but you arent seeing that on the LED

For an LFO exhibiting the 'locked up' behavior, I'm getting an voltage swing at the output of the LFO, but it is steady at 2.29V at the output of the buffer before the LEDs. Danfrank had mentioned that the Tayda LEDs may have an mcd value that are too high. The green LEDs from Tayda are rated at 2-3k mcd. Would a lower mcd value help? I don't really want to change the series resistor here because, since you mentioned, this is a well-documented circuit that has proven to work. That being the case, a variable must've been introduced. I think that must be some characteristic of the LEDs.