Author Topic: tutorials and hints for Dirtbag Deluxe biasing with Osciloscope?  (Read 334 times)

cooder

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I just pulled an almost finished Dirtbag Deluxe board from the rubble and dust of my "I'll get back to that when I have the last missing parts" bin and indeed had the last missing parts to complete it, yay. After a quick ballpark bias setting I do get delay signal, even more yay.
Now I wonder if there's a good tutorial or hints how to bias it with an oscilloscope.
I do have an old school analog scope and a cheapie frequency generator. I did a quick google search but am wondering if more experienced guys here can point me in the direction of which method would squeeze the best out of it?

Any hints and pointers appreciated, also because I have a Total Recall board in the works as well which could also be in for this me thinks.

Paging Scruffie I guess....  8)
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alanp

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Re: tutorials and hints for Dirtbag Deluxe biasing with Osciloscope?
« Reply #1 on: March 09, 2020, 01:32:51 AM »
At a quick 'n' lazy guess, I'd say whack a sine wave or something into the input, and stick your o'scope probes on the input signal and the output from the BBD, and bias until you get the biggest, cleanest sine wave possible.
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Bio77

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Re: tutorials and hints for Dirtbag Deluxe biasing with Osciloscope?
« Reply #2 on: March 09, 2020, 01:19:44 PM »
At a quick 'n' lazy guess, I'd say whack a sine wave or something into the input, and stick your o'scope probes on the input signal and the output from the BBD, and bias until you get the biggest, cleanest sine wave possible.

This is actually all there is to it.  You can dig up the calibration docks for some flangers, delays, etc.  to find the best frequency of the sine wave to use.  I usually use a couple frequencies.  You get the best biasing if you can use an input wave that is big enough that you get clipping on both sides of the sine wave.  Then you just have to even the clipping out on top and bottom of the sine wave.  If you can't get that you dial the trimmer in the middle of where you start to get clipping on the bottom and where you start to get clipping on the top.

It's kind of hard to write about it, but once you start adjusting it you'll know what to do.

For signal generator get the Function Generator PRO app, it's 3 bucks and works well.

cooder

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Re: tutorials and hints for Dirtbag Deluxe biasing with Osciloscope?
« Reply #3 on: March 09, 2020, 02:48:19 PM »
At a quick 'n' lazy guess, I'd say whack a sine wave or something into the input, and stick your o'scope probes on the input signal and the output from the BBD, and bias until you get the biggest, cleanest sine wave possible.

This is actually all there is to it.  You can dig up the calibration docks for some flangers, delays, etc.  to find the best frequency of the sine wave to use.  I usually use a couple frequencies.  You get the best biasing if you can use an input wave that is big enough that you get clipping on both sides of the sine wave.  Then you just have to even the clipping out on top and bottom of the sine wave.  If you can't get that you dial the trimmer in the middle of where you start to get clipping on the bottom and where you start to get clipping on the top.

It's kind of hard to write about it, but once you start adjusting it you'll know what to do.

For signal generator get the Function Generator PRO app, it's 3 bucks and works well.
Thanks guys, excellent, I'll give that a whirl. Thanks for the pointer with the Function Generator app too, sounds like a good idea to have as well!
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Scruffie

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Re: tutorials and hints for Dirtbag Deluxe biasing with Osciloscope?
« Reply #4 on: March 10, 2020, 02:38:18 PM »
DELUXE MEMORY MAN WITH CHORUS AND VIBRATO ALIGNMENT PROCEDURE (8/1/78)
Plug unit into 115V 60Hz. and set feedback and chorus/vibrato fully CCW, blend fully CW, delay fully CW and power on. The pilot light should come on but not the overload light. Connect the scope to pin 2,4,6,10,12, or 15 of the CD4049 and observe the square wave period. It should be between 120 and 140 usec. (add c across 240pf if necessary). Set the delay fully CCW; the period should now be between 8 and 10 usec. Set the delay to the middle of its range. With the switch in the chorus position slowly bring up the chorus/vibrato control, watching the square wave. The period should decrease slightly and become modulated at a rate of slightly less than 1Hz. At max. chorus setting the period should swing approx. 10% of its average value. Set the switch to the vibrato position. The modulation rate should go up to approx. 4Hz. Set the chorus/vibrato control fully CCW. Connect a 250Hz signal of 500mv p-p to the input and adjust the level control for the same level at pin 6 of the NE570. The level at pin 7 should be between .95 and 1.4 V p-p. Watching pin 7, increase the level. The overload light should just begin to glow at about 1.4V p-p and should be almost as bright as the pilot light at about 2.3V. No distortion should be visible in the signal - at this point. Look at pin 7 ot the first MN3005 and turn down the level for a signal of 1.5V p-p. Slowly increase the input frequency. The response at pin 7 of the MN3005 should be flat up to about 900Hz, rise to a max. of about 2V p-p at around 2.5KHz at which point the overload light should glow dimly, drop back to 1.5V p-p at about 3.8KHz and roll off sharply above this. Set the frequency back to 250Hz, the 2nd trimpot at about centre, and connect the scope to pin 7 of the second 4558, which is also the wiper of the first gain trimpot. Increasing the level setting as necessary, set the first bias trim for maximum p-p unclipped signal, then at a lower signal level set the gain to unity (output level = input level). The output level before clipping should be 3.8V p-p or greater. Vary the delay over its full range and if clipping becomes very unsymmetrical at either end, trim the bias a bit for a good compromise over the range. It is normal to see a lot of clock noise at the highest frequencies. The second MN3005 is aligned in the saw way as the first , except that the clock balance trim is carefully set for min. clock frequency at its wiper at maximum delay (min clock frequency) setting. This should be done before fine trimming the bias for symmetrical clipping and again after the bias is finally set, with the input signal to the unit disconnected and the scope gain high. Set the 2nd gain trim last looking at pins 14 and 15 of the NE570 and setting this trim so that the signal is equal in level to that at pin 7 of the 570. If this can't be done, go as far as the trimmer permits and re-adjust the first gain trim to achieve it. Set the level so that the overload light is on dimly, looking at the signal at pins 14 and 15 of the NE570. Set delay to max.(CW) and vary the input frequency from 40Hz upward. The signal should be unclipped and clean over the range. The response should show no peaks, but be flat at about 2.5 KHz and -3db,(x.7) at about 3.2KHz, where only a small amount of abiasing ripple of the waveform should be visible. Set the delay to min.(CCW), The response should now have a peak of about +3db (x1.4) around 2.5KHz and roll off sharply above 3.5 KHz with no trace of noise or abiasing.

That's the factory procedure, of course the 4049 was removed so measure pin 10 or 11 of the 4047 instead where it mentions that.
« Last Edit: March 10, 2020, 06:34:27 PM by Scruffie »
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jimilee

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Re: tutorials and hints for Dirtbag Deluxe biasing with Osciloscope?
« Reply #5 on: March 10, 2020, 04:57:06 PM »



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