OCD Clone issues

Jaicen_solo · October 19, 2014, 04:58:24 PM

I've built an odd clone from the perfboaed layout here:
http://revolutiondeux.blogspot.co.uk/2008/07/fullclone-vlod-fulltone-ocd-by.html
The tone and volume controls work, as does the lp/hp switch.
However, I don't seem to have any distortion, just a clean boost. At minimum gain I get no output, so I think somethings wrong there.

Has anybody built this layout, and if so Did you have any issue?? Mosfets are 2N700's, opamp 082 all as specified.

duck_arse · October 20, 2014, 10:05:35 AM

it is of interest that the ocd circuit on the linked page shows the 2N7000 clippers with the gate and drain tied, the perf layout shows the same, but the pcb layout shows source and gate tied. I wonder how many people are disappaointed with the pcb sound results.

as for the problem at hand, voltages! and photos! you know the drill.

midwayfair · October 20, 2014, 11:59:20 AM

Quote from: duck_arse on October 20, 2014, 10:05:35 AM
it is of interest that the ocd circuit on the linked page shows the 2N7000 clippers with the gate and drain tied, the perf layout shows the same, but the pcb layout shows source and gate tied. I wonder how many people are disappaointed with the pcb sound results.

It barely matters whether the clippers are even in the circuit the way it's done in the OCD. And only one of the MOSFETs does anything since the one by itself conducts in both directions -- without the germanium diode there on the other, it would also conduct as a body diode, in which case it's still not going to be any different.

I even recorded a sample with the MOSFET(s) on a switch and the difference with them in is so extremely subtle at all gain settings that I'm sure I couldn't pick them out in a blind test.

Jaicen_solo · October 20, 2014, 01:14:56 PM

That is interesting. I suspect the issue lies with the mosfets, odd that it doesn't distort though.
Of course voltages will be forthcoming, I just wanted to know if this had been an issue for anyone else. I was posting on the bus on my way to work!

Jaicen_solo · October 20, 2014, 05:15:53 PM

Ok, I had a quick look at the board which all seems to be exactly as the layout shows. It's all wired correctly and does have a lot of boost, it just doesn't have distortion.

Voltages on the TL082:

1: 4.5v
2: 4.5v
3: 2.8v
4: 0v
5: 8.9v
6: 4.5v
7: 4.5v
8: 4.5v

Q1 & Q2: 4.47 & 4.4v on all pins. There's 4.11v after D2 (having subbed in a 1n4148 to check of diode was issue)

Does anybody have any idea what's wrong???

Frank_NH · October 20, 2014, 09:27:24 PM

Quote from: midwayfair on October 20, 2014, 11:59:20 AM
Quote from: duck_arse on October 20, 2014, 10:05:35 AM
it is of interest that the ocd circuit on the linked page shows the 2N7000 clippers with the gate and drain tied, the perf layout shows the same, but the pcb layout shows source and gate tied. I wonder how many people are disappaointed with the pcb sound results.

It barely matters whether the clippers are even in the circuit the way it's done in the OCD. And only one of the MOSFETs does anything since the one by itself conducts in both directions -- without the germanium diode there on the other, it would also conduct as a body diode, in which case it's still not going to be any different.

I even recorded a sample with the MOSFET(s) on a switch and the difference with them in is so extremely subtle at all gain settings that I'm sure I couldn't pick them out in a blind test.

You know, I built an OCD clone and discovered the same thing - the MOSFET clippers did nothing! I had them socketed and so I pulled them out one day and...whoa, no change! I eventually shelved the project because the OCD was nothing special (in my opinion) as an overdrive.

If you want a great overdrive/distortion with LED clippers, build a Crunch Box (itself a variation on the Marshall Gov'nor). You can actually see the LEDs light up as you play!

duck_arse · October 21, 2014, 09:35:53 AM

jaicen - 2 things - the opamp pins number anticlockwise from the notch, so pin 8 is V+, not pin 5 as in your voltages.

and - pin 3 voltage. find out why it isn't also 4V5 or thereabouts.

anotherjim · October 21, 2014, 10:10:10 AM

"and - pin 3 voltage. find out why it isn't also 4V5 or thereabouts."
+1
You should have 4.5v at the junction of the 10k resistors and the 2x10uF caps (?)( wouldn't 1x 22uf do?) and the common ends of the MOSFETs and 1n cap. You probably will read a bit less than that 4.5v on pin 3 because your DMM input impedance will load it down against the 470k resistor.

Jaicen_solo · October 21, 2014, 11:15:37 AM

I think there is something wrong around the gain control?? I will have a look when I get home Thursday.

midwayfair · October 21, 2014, 11:37:39 AM

I'm suspecting the Op has mixed up the pots or connected them wrong. He gets clean boost, so his gain pot is doing something.

Jaicen_solo · October 22, 2014, 03:38:47 PM

So I got home for two hours before I have to go out and work again.
I can't see where the issue is, wiring is as per the layout but I still just get clean boost. Pin 2 is still stuck on 3v or there abouts, so I'm guessing the Plano is never hitting the rails hence the lack of grit.

Any idea why the voltage is low??

midwayfair · October 22, 2014, 03:47:10 PM

Post pictures. Audio probe. We're just guessing at this point.

Pyr0 · October 22, 2014, 06:35:54 PM

Pin 2 may not really be stuck at ~3v all the time, it could just be a false reading caused by the load your DMM is putting on the circuit when probing.
Best way to get help is post pictures of both sides and start testing with an audio probe. I've built that circuit from Haralds layout with no problems, so probably a short or wiring problem with the gain control.

Jaicen_solo · October 23, 2014, 02:50:56 AM

I will post pictures tonight. Not sure how useful an audio probe would be as it it passing signal. I'll try reflowing the joints , but it's pretty neat and I can't see anything obviously wrong. Very frustrating.

Jaicen_solo · October 24, 2014, 03:35:47 AM

As requested, here are some pictures:

Frank_NH · October 24, 2014, 09:32:58 AM

I've built the Harold Sabro layout and it works. I don't see anything obvious from your photos, but check for a solder bridge in the vicinity of the wire which connects Drive 2 to the board. Also check the voltages on the Drive pot lugs to see if they make sense. The pot should be in series with the nearby18K resistor in the op amp feedback loop, so when the pot is at minimum you should still have a boosted signal. Good luck!

duck_arse · October 24, 2014, 09:53:35 AM

^ like he says, I can't see, etc. maybe touch-up the solder on pin 4, and check with your meter that all those cuts are really cuts.

pull the IC, and measure the resistance from pin 3 to the 10k/10k Vref. also measure pin 3 resistance to V+ and ground.

Jaicen_solo · October 24, 2014, 12:56:06 PM

Ok, I still get signal at minimum gain. Voltage on pin 3 is 3.2v without ic, 2.8v with ic. Resistance to ground is 480k, to 9v is about 470k. To vref, resistance is 470k.

Can someone please post voltages of a working unit? Also, I'd like to hear one at full gain and minimum volume.

Frank_NH · October 24, 2014, 04:58:28 PM

Some debugging info here:

http://www.diystompboxes.com/smfforum/index.php?action=printpage;topic=71264.0

Some voltages from that thread:

U1
1=4,47 V
2=4,56V
3=4,25V
4=0V
5=4,45V
6=4,46V
7=4,46V
8=8,94V

katod on 1N34 (S on 2N7000)=4,36
Other (Bias networkside) of 2N7000=4,46

Sounds like there's an issue with the voltage at Pin 3 of the TL082. Check the board for any sign of a solder bridge or cold solder joint. It's not a big circuit so you will find the problem. Also possible that a component is bad. Hope you find the bug.

Jaicen_solo · October 24, 2014, 06:06:01 PM

Thanks, there's definitely an issue around pin three then. A couple of the components are nos, but I did test them with a multimeter prior to installation and all tested as good. I can't find any shorts with my meter either. I will rip out all the parts connected to pin three and try again I guess.

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OCD Clone issues