Phaser Biasing Trimpot

nickbungus · April 23, 2019, 06:17:06 AM

Hi All

I've got a weird one. I've built a fOXX phaser and all seems good. Bias the fets with a multiturn trimpot - all good. Come back to it 5 minutes later and the biasing needs adjusting a couple of turns. Come back to it again in 5 and needs re-adjusting again. I've built enough of these to know when the biasing is close but no phase, and thats the sound I'm getting each time.

Faulty trimmer? Anyone had a similar issue?

I'm going to replace the trimmer and have a look for any bad solder joints tonight but seems odd

antonis · April 23, 2019, 06:21:08 AM

Measure trimmer's resistance and replace with appropriate 1% metal film resistor (obviously with a combination of resistors..) for checking purpose..

In case of loosy pot, sealing it shouldn't work - replace it..

Mark Hammer · April 23, 2019, 10:25:02 AM

The MXR Phase 90 and 45 addressed the issue of bias drift by using a 5.1V zener to fix the voltage that the bias would be derived from. That way, the battery could wear down from full 9V, but the bias setting would still remain valid. The same strategy was used in tandem with the developent of the MN32xx series of delay chips to provide a bias setting that wouldn't need constant tweaking.

I don't know that the method used by Foxx achieves the same degree of reliability. It might. I just don't know enough to be able to say one way or the other.

anotherjim · April 23, 2019, 12:18:34 PM

It's essentially stabilized by 3 diode drops for approx 1.8v. then an opamp amplifies that by approx x3.2. Schematic calls it 6v, but it could be closer to 5.7v.
I would monitor the 6V with the DMM clipped on and see if it drifts about much. If it is drifting, then change to monitoring the 1.8v over the diodes or use x2 DMM's!

nickbungus · April 23, 2019, 04:01:39 PM

So I was playing with it a bit and left it on. I got a full phase and switched it off. I then left it for a few hours. Switched it back on again. No phase. Waited for approx 30 seconds and then I could hear it slowly start to phase until I had full swing.

I'm going to have a play now and check for soldering etc.

paul.creedy · April 23, 2019, 04:17:59 PM

I misread it as "Phaser Biasing Trumpet" earlier, and then felt sad because that isn't a thing.

nickbungus · April 23, 2019, 04:36:53 PM

I wish I had one of those!!

I've changed out the trimmer for a single turner and still the same. Not that. Because its a gradual thing - could it be a capacitor somewhere? C14 or C19? Just guessing really

nickbungus · April 23, 2019, 05:20:28 PM

So going back to Jims suggestion, I left it off for a while and then clipped my DMM to the 6v rail. It was very static at around the 5.8v mark.

The 1.8v rail stayed at around 1.83v.

They both spiked a few millivolts when engaged then settled but nothing alarming.

Mark Hammer · April 23, 2019, 07:05:44 PM

Quote from: nickbungus on April 23, 2019, 04:36:53 PM
I wish I had one of those!!

I've changed out the trimmer for a single turner and still the same. Not that. Because its a gradual thing - could it be a capacitor somewhere? C14 or C19? Just guessing really

That was going to be my next suggestion (C14/C19). Drift in either of those wold be expected to corrupt the bias voltage.

anotherjim · April 23, 2019, 07:34:39 PM

Quote"Phaser Biasing Trumpet"

I could probably come up with a synth patch worthy of that name.

So it looks like the bias circuit is solid. Next, there are opportunities for the LFO to mess things up thru to the next opamp to the common jfet gate connection. Indeed, the initial lack of phasing could be due to an LFO that is slow to start. Remember, you won't really hear a phasing effect unless the sweep is working.

nickbungus · April 24, 2019, 04:28:16 AM

Thanks guys. I want to confirm this properly but I think it is the biasing going out. I believe I can set the trimmer so that I instantly get phase but then it drifts. I want to confirm this tonight.

Just because I could as I socketed them, I changed the jfets, but the issue continues. The opamps are small smd ones so I cant swap them too easily.

I'll report back after a bit of messing tonight

nickbungus · April 24, 2019, 03:39:09 PM

AnotherJim, you are a legend!

QuoteI want to confirm this properly but I think it is the biasing going out. I believe I can set the trimmer so that I instantly get phase but then it drifts. I want to confirm this tonight.

Sort of. I could get it so that after a second it phased then went out, so I quickly threw that idea out of the window.

So I decided to go and look at the lfo. At this point I was sort of resigned to swapping out an opamp which I was dreading but I'd start with the caps.

Stupidly I did the electrolytics first. In hindsight this was stupid as they are switched on and off with the range switch. Anyway, no change.

Then I changed C8 and C10 (220nf in the schematic but these days I use 680nfs as they give an overall slower speed and leave no overlap or gap between the Slow and Fast ranges). Phase straight away. Left it 5 minutes still phase. Put the kids to bed came down and put the whole thing in a 1590b. Still phase!!!

Thank you so much Jim and Mark. Its guys like you who make this forum so great. Theres no need for you to help us mortals but you do and it really, really helps!

anotherjim · April 24, 2019, 04:59:35 PM

No sweat, it could have been many things back down the line to and including the LFO. Really unusual to get bad film caps - unless you got 0.68uF electro's (they do exist). Film caps don't get stressed in 9v work so are either bad in the first place or solder heat damaged.

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Phaser Biasing Trimpot