Vintage flanger is not flanging

[terjewinther]

It could be that my 512D SAD ICs are defect of course, but I have run into these issues:
- I could not find a 15 - 18 volt trafo, so I used an older (big) one that is giving off 19 volts. Doesn't seem to be a problem, because internally there is 12,1 volts and 6,07 volts as it should, and the LM317 is not getting particularly hot (no problem touching it even after a lot of time). Hope I have not fried something....
- It passes audio, but does not flange in any settings
- The internal clock seem to be running all right, and I could calibrate it with a frequency counter to the desired 40kHz, and I can see that the internal LFO is altering the clock when I turned up the depth and rate pots.
- On the scope I can see the input getting into the SAD IC, although somewhat divergent waveform from the original input (not much, but slightly changed)
- The output of the SAD does not look good, though; it seems to be more or less a copy of the clock with some significant offset.
- I removed the SAD IC, and noticed that in the "on" position with the LED on, the sound is noticeably louder, and with more high-frequency content, so there seem to be some sort of (soft) distortion going on. I have of course triple-checked all components, and even swapped out the TL072 og 4558s in the signal path to eliminate bad parts. Have not tried to change the BC550 (Q3), but as that is not directly in the signal path, it should not matter.

Especially the slight distortion of the unaltered sound getting through at the preamp stage is working me a bit. I am not that good in op-amp maths, so may I ask how much amplification there should be at the preamp stage?

I have an original SAD1024 - but only one - so I am a bit reluctant to try it without getting the SAD512D ICs working (I have several of those).


Terje WInther
WintherStormer

[Synthbuilder]

Hope I have not fried something....

Your transformer should be fine. All the power has to go through U1 so any excess input voltage will be regulated to around 12V. If U1 is not getting too hot then all is well.

On the scope I can see the input getting into the SAD IC, although somewhat divergent waveform from the original input (not much, but slightly changed)

This sounds like it's behaving correctly up to this point. The input amplifier will have gain of around ten but only for mid to higher frequencies. At lower frequencies it has a gain of one. So it will boost treble frequencies and leave all the others as they are. This is the signal at pre-amp out. The signal going to the BBD has some additional processing. U8 (pins 5, 6, 7) acts to reduce the signal a bit and act as a soft clipper to prevent the BBDs being overloaded. Then very high frequencies are attenuated by the low pass filter based around Q3.

The output of the SAD does not look good, though; it seems to be more or less a copy of the clock with some significant offset.

Hopefully the input offset, controlled by the BIAS trimmer, has not been set correctly. If the offset is set so wrong you'll get no output. However, if you don't get any signal when the BIAS trimmer is moved I think this points to the BBD being bad.

I'm assuming that the two round pads on CLK have been linked together.

[terjewinther]

Thanks a lot for the reply regarding this.
It turned out you were right: some of my BBD ICs are indeed faulty (sadly, to my disappointment), and some of them needed careful calibration to work properly. The calibration description says "turn the trimmer slowly" - that is an understatement; they need to be turned *really* slowly to find the right spot! And as always: the various trimmers interact somewhat, so several rounds of calibration are needed to find the "sweet spot" of proper operation.
But the important thing is this: it works!
I noticed while calibrating that the mode switch matters, especially for feedback, and found out that the position of the mode switch is not mentioned in the calibration procedures. It is actually not mentioned at all, so what does that do really?

[Synthbuilder]

... and some of them needed careful calibration to work properly.

Did you have a oscilloscope? If you don't have access to one, it could be quite tricky to find the right trimmer settings.

With the BIAS calibration using a 2V input does make it tricky to find the ideal position. In practice a 2V input signal is larger than you would normally use, so precise setting of BIAS is not always needed. Indeed, some old Flanger and Chorus pedals don't even have a trimmer for this setting and just use a fixed voltage. On a 'scope it's pretty straightforward to see what's happening as the waveform gets obviously clipped at either the top or the bottom depending on the BIAS setting.

I think as a BBD ages it gets harder to select the correct bias required for clean operation. If you have only a very narrow range of usable positions it could be because the BBD is on its way out. It might be worth checking you definitely have a 5K trimmer in there. A 50K trimmer would make it very tricky to set BIAS correctly.

the various trimmers interact somewhat, so several rounds of calibration are needed to find the "sweet spot" of proper operation.

Yes, there is some interaction between the trimmers but if you do the calibration in the order given in the Builder's Guide you shouldn't need to go back and change any of the previous calibrations. The RNG setting slightly affects the ideal BIAS setting, and as the BBD ages this will be more pronounced. The LVL and FBK interact as increasing BBD output level (LVL) will also increase the overall feedback level, so LVL must be set before the FEEDBACK trimmer.

I noticed while calibrating that the mode switch matters, especially for feedback, and found out that the position of the mode switch is not mentioned in the calibration procedures. It is actually not mentioned at all, so what does that do really?

That's interesting. The mode switch should only affect the feedback (FBK) trimmer calibration and probably only then if you want it to self-oscillate (which I personally wouldn't recommend). If it affected the LVL calibration you should probably check the resistor values of R48 and R49 - both should be 33K. However, if you flip the switch between finding the null point using the Manual pot on the front panel and adjusting LVL that will give you the wrong setting of LVL. The switch must be left in either setting when carrying out the LVL calibration. That is, it shouldn't be moved before the LVL trimmer is adjusted once a null point has been found. I should probably make a note of that in the Builder's Guide.

The mode switch controls the phase of the feedback signal. One way is inverted, the other way is not. The original BF-1 had no inversion. You get a different tone to your flanged sound. The switch only has an effect when the Feedback pot is not set to zero.

[terjewinther]

Did you have a oscilloscope?

Yes. I first did it just by listening to the sound. That was quite hard. As soon as I got the oscilloscope up, it was much easier.

The mode switch should only affect the feedback (FBK) trimmer calibration and probably only then if you want it to self-oscillate (which I personally wouldn't recommend). If it affected the LVL calibration you should probably check the resistor values of R48 and R49 - both should be 33K. However, if you flip the switch between finding the null point using the Manual pot on the front panel and adjusting LVL that will give you the wrong setting of LVL. The switch must be left in either setting when carrying out the LVL calibration. That is, it shouldn't be moved before the LVL trimmer is adjusted once a null point has been found. I should probably make a note of that in the Builder's Guide.

The mode switch controls the phase of the feedback signal. One way is inverted, the other way is not. The original BF-1 had no inversion. You get a different tone to your flanged sound. The switch only has an effect when the Feedback pot is not set to zero.

Thanks: Yes, now that you explained that, I understand. The mode switch does give the pedal more range for effects, so that is welcome.
And indeed: the differences are really exposed when you are just at the edge of self-occilation.

The various resistors and trimmers are correct, so any strange behavior I think is due to the BBD chip. As some of the other BBDs in the batch I have are failing, I suspect that the ones working could start to fail any day. I better have fun with the flanger while I can.