OTA problems. No signal after pin 5

[ohmelter]

Hello. I don't think my last post stated my problem accurately, so here goes. I built a Ross Phasor clone and I'm not getting any signal after pin 5 of the first OTA, I'm using two ota's and a dual opamp as input and output buffer. Please help

[frequencycentral]

You should probably read the debugging thread:

http://www.diystompboxes.com/smfforum/index.php?topic=29816.0

.......then provide a little more information for us to go on. 

I'm sure a few people round here have built the Ross Phaser.

[ohmelter]

Here's the link to the schematic
http://www.tonepad.com/getFile.asp?id=99
there isn't any signal coming through the end of the circuit. I cut the wire to the second stage of the dual op-amp, at the output, and traced it back to see where the signal starts and stops. I get signal coming in and signal up to pin 5 of the first 1600. Nothing else.

[Paul Marossy]

Maybe your first 600 got fried. Sometimes that can happen to IC chips due to static discharge.

[ohmelter]

Replaced first 600 with another...same thing..no signal after pin 5

[ohmelter]

Here are the voltage measurments for the IC's. I used "Ropez" schematic. Measurments on OTA's were virtually identical..I mean if one pin was  2.121 the other the same pin might be 2.16 or identical. So I'll only list one OTA for brevity.
Ic1                  IC2                   
1  2.32                1  .004             9    .867
2  2.32                2   0.00            10   0.00
3  2.22                3   2.19            11   9.07
4  0.00                4   2.121           12   0.00
5  2.19                5   0.00             13   2.03
6  2.19                6   0.00             14   2.188
7  2.82                7   0.00             15    0.00
8  9.00                8   .884             16   .004

[01370022]

I don't know if it'll fix the problem, but looking at your voltages, I'd say there's a problem with your Vbias network. From looking at the schem anything connected to Vb should be in the vicinity of 4.5V.

[ohmelter]

I just lowered resistors to get about 4 volts out of the bias network. Still no change.

[Paul Marossy]

I don't know if it'll fix the problem, but looking at your voltages, I'd say there's a problem with your Vbias network. From looking at the schem anything connected to Vb should be in the vicinity of 4.5V.

Hmm... good point. For example, you should be measuring 4.5V at Pins 3 & 14 of IC-2 & IC-3. You're measuring only half of that.
And at IC-1, you should be measuring 4.5V at Pin 5, but you only roughly have half of that.

I'd say there is a strong possibility that this is part of your problem.

[ohmelter]

Before 10k resistors I have 9.06 volts from the battery. Between the two 10k resistors, in series with the 9v supply, one going to ground I got 2.1 or so volts. I then lower the resistors to get 4.0 volts at all IC's, still no difference. Gotta' check it again.

[01370022]

You lowered resistors?   Both of the Vb resistors should be the same. 10k on the schem.

Sorry if I've misunderstood this, I have trouble visualising a problem sometimes.

Is the build on PCB or vero?

It's about now that I'd start going through the circuit and the build carefully, checking that all the components and the wiring nodes are correct, and all going off to the right places.

Also check that the LFO is working correctly. I can't remember the exact workings of the circuit since it's been a while since I built one, but if this isn't working properly, it may monkey with the output.

Good luck with it anyway, it's a really nice phasor

[01370022]

Ah, didn't read your last post before posting.

Change your Vb resistors back to 10k, then go check the Vb trace and make sure that EVERY connection that's been made to it is made on the schem. By the sounds of it, somethings connected to the Vb trace that's not meant to and that's pulling the voltage down. It could be something simple like a solder bridge somewhere.

If there's no wrong connections on the Vb line, check/replace the Vb supply cap (10uF). It could be the wrong polarity or could've gone open with excessive heat or something weird

[ohmelter]

Ok, will do. I'll try it out, double check, and get back.

Anthony

[ohmelter]

Alright....Found out that by cutting the 27k and 1.8k resistors (before each OTA) out of the circuit, it brings my V bias back up to 4.5v. With them in circuit it reads about 3.4v. What gives?

[01370022]

That sounds a bit odd. Try to isolate the problem a bit further. Reconnect the two resistors one stage at a time, taking Vb measurements each time. One of those 4 stages could be wired incorrectly. If you find one stage that does cause Vb to drop, make sure to test it independently of the other stages to isolate it further. Then go over it with a fine tooth comb to see if you can spot the trouble.

Also if the first stage is correctly wired (doesn't pull Vb down) then if you connect up the 27k and 1k8 resistors then you should also see signal at the output of that stage. If it doesn't then there's something a bit more insidious going on and I'll have to have to open up my phaser and have a squiz

[ohmelter]

Every stage drags down the voltage as they are connected. I start at 4.5v and as each stage is added their sum brought it down to 2.1 volts. Everything is OK with just the 072. I've checked my tracing and schematic about 10 times. Guess I'll check again.

[Paul Marossy]

Every stage drags down the voltage as they are connected. I start at 4.5v and as each stage is added their sum brought it down to 2.1 volts.

Strange. I'm trying to think of why that might be happening. Maybe that 100uF cap to ground off of Vref has something to do with it? What if it were faulty?

[01370022]

Every stage drags down the voltage as they are connected. I start at 4.5v and as each stage is added their sum brought it down to 2.1 volts.

Strange. I'm trying to think of why that might be happening. Maybe that 100uF cap to ground off of Vref has something to do with it? What if it were faulty?

Thats just plain freaking weird. I thought the same thing about the caps too, but discarded them cos it worked fine with just the '072. I'll dig out the datasheet later when I get back home and open up my unit and see what readings I get off that.

[01370022]

Sorry man I didn't get to look inside my unit tonight, but just having a look at the data sheet and the schem, and a thought occured to me, check if the LFO is working correctly and oscillating properly. My reasoning (tired and not thought out) is that if the control voltage from the LFO section is near ground, then it'll pull the OTA output to ground which means that it'll be sinking the bias current through the feedback loop, pulling Vb down.

If someone who has a better understanding of how the circuit works can see I'm wrong, feel free to shoot me down and replace my theory with one that works.

I hope that provides some illumination, if not, I'll try and get my unit open soon and try and see what's going on inside.

I'm gonna hit the hay now as I've had to retype too much of this to correct typos

Good luck dude

Jon

BTW what PCB layout are you using? or is it perf?

[ohmelter]

thanks for hanging in there. It's perf board.

Anthony