Phase 90 debugging

ljudsystem · February 21, 2020, 04:04:54 PM

I've built a Phase 90 clone using the Guitar FX vero layout (minus the script/feedback switch) http://tagboardeffects.blogspot.com/2014/05/mxr-phase-90-with-script-switch.html

I've probed the circuit and the signal stops at the first opamp (pin 3). All connections are solid, it gets 9v, and I've replaced the first tl072 in case it was faulty but still no signal.

Has anybody got any idea what might cause this? I've been over the layout half a dozen times and everything seems correct.

Here's the schem for reference: https://www.electrosmash.com/mxr-phase90

Rob Strand · February 21, 2020, 04:19:59 PM

QuoteI've probed the circuit and the signal stops at the first opamp (pin 3). All connections are solid, it gets 9v, and I've replaced the first tl072 in case it was faulty but still no signal.

Check the voltage across the zener and the zener orientation.

Check for shorts to any tracks on the zener line.

Set your trimpot to the mid position to see if that changes anything, perhaps a short around the trimpot ckt.

Mark Hammer · February 21, 2020, 07:54:22 PM

I don't think D1 is supposed to be there. At least not directly in parallel with the zener. Seems to counteract what the zener does.

Rob Strand · February 21, 2020, 08:19:03 PM

QuoteI don't think D1 is supposed to be there.

It's a bug on the schematic, the direction it is drawn makes it benign. IIRC at least one version of the original had a diode across the +9V power rail for protection. So the schematic has misdrawn that diode.

From what I can see it's not on the layout.

willienillie · February 22, 2020, 12:08:00 AM

The signal only goes through one resistor and one cap before it gets to pin 3 of the first opamp. Can you post pics, both sides of your board?

Rob Strand · February 22, 2020, 01:55:57 AM

Post the DC voltages on all opamps, at least the output and two inputs.

ljudsystem · February 22, 2020, 03:40:40 PM

ok, here are my voltages from top to (bottom of the layout):

Opamp 1:

1 8.5V
2 8.5V
3 190mV
4 0V
5 190mV
6 8.5V
7 8.5V
8 9.1V

Opamp2:

1 8.4V
2 4.9V
3 190mV
4 0V
5 190mV
6 8.5V
7 8.5V
8 9.1V

Opamp 3:

1 1.4V
2 1.4V
3 0.9V
4 0V
5 0.8V
6 1.2V
7 1.4V
8 9.1V

V across the zenner is 190mV

Hope this helps

DIY Bass · February 23, 2020, 02:17:26 AM

The op amp voltages do not look right. According to the schematic, I would be expecting about 5v or so on pins 1 to 3 of op amp 1. Given that it is a buffer, the voltage on all these pins should be the same. pins 1 and 2 are about a diode drop from the supply voltage. I am betting that there is a short between these pins and something that is near supply voltage

ElectricDruid · February 23, 2020, 06:10:11 AM

+1 agree, the op-amp voltages look very wrong.

I'd take the chips out (I'm assuming they're socketed) and make sure that the Vref 5.1V voltage is actually what it should be, because it doesn't look like it at the moment. As DIY Bass said, a short somewhere is a likely cause.

ljudsystem · February 23, 2020, 07:46:35 AM

Ok, I knifed all the tracks again but that did nothing.

Removed the opamps and measured the vref to 177mV... (vref is at the cathode of the zenner, right?)

duck_arse · February 23, 2020, 08:07:27 AM

is there a chance you have fitted a schottky or a germanium diode intead of a zener?

ElectricDruid · February 23, 2020, 08:27:59 AM

Quote from: ljudsystem on February 23, 2020, 07:46:35 AM
(vref is at the cathode of the zenner, right?)

Yep.

Where *is* D2 (the zener) on that layout anyway? I can't find it!

http://tagboardeffects.blogspot.com/2014/05/mxr-phase-90-with-script-switch.html

ljudsystem · February 23, 2020, 09:36:09 AM

No, it's definitely a 5v1 zenner. Maby it's broken?

There is only one diode on that schematic and it's the zenner (bottom right corner)

Digital Larry · February 23, 2020, 09:46:44 AM

is C8 backwards?

ljudsystem · February 23, 2020, 10:08:03 AM

nope

Rob Strand · February 23, 2020, 04:17:00 PM

Quotenope

C8 is the 10uF at the top right edge, yes?

Maybe a short around the trimpot. Could even be an open circuit somewhere.

With the power off, measure the resistance across the zener to check for shorts on the zener/Vref line.
Check in both meter polarity directions.

Using the continuity setting on the meter, make sure the track which goes to the +V side (cathode) of the zener connects to
- the + terminal of the 10uF cap,
- the trimpot and
- the 10k. The 10k is the one going to the source of the right-most JFET.

willienillie · February 23, 2020, 05:24:53 PM

Quote from: ElectricDruid on February 23, 2020, 08:27:59 AM
Where *is* D2 (the zener) on that layout anyway? I can't find it!

It's called D1 in this layout.

ElectricDruid · February 23, 2020, 06:36:28 PM

Quote from: willienillie on February 23, 2020, 05:24:53 PM
Quote from: ElectricDruid on February 23, 2020, 08:27:59 AM
Where *is* D2 (the zener) on that layout anyway? I can't find it!

It's called D1 in this layout.

Thanks. I got there in there end, with a little help. What was confusing me was that the schematic and the layout don't have the same labelling.

Can we have a picture of your layout please, ljudsystem? Perhaps someone can spot something.

ljudsystem · February 25, 2020, 04:07:53 PM

Sorry about the delay guys, here are the pics.

Rob, I checked the continuity. The trimmer, 10U and 10k are all connected.

Rob Strand · February 25, 2020, 04:44:50 PM

There has to be something serious/obvious that's killing the Zener voltage *and* screwing up the opamp voltage.

---------------------------------------
EDIT1:
One thing that doesn't make sense is the zener/reference is low but the outputs of the opamps are high.

At first I thought maybe a wiring error but the low zener voltage is present on your opamp pins.

So, are you sure you have the opamps in the right way around?

News:

Phase 90 debugging