EHX Electro-Harmonix Doctor Q

[michael_krell]

I am trying to get a EHX Doctor Q to work. The problem is basically that I have no range at all. I can find a small notch on the range trimpot that will make it work somewhat, but there is no headroom at all. Not quite sure what the problem is here. Maybe you can help. Heres the schem.

[Paul Perry (Frostwave)]

Maybe the range trimpot is tanished (if itis one of those shitty ones).
Maybe the 10uF electro is dried out.

[Mark Hammer]

It's not exactly clear what you mean by "headroom".  Do you mean that it clips or do you mean that you simply get very little sweep?

The sensible thing to do is to pull out your voltmeter, setit to AC and measure what you get at the junction of the 10uf cao and the 1N914.  It should be at least a half a volt.  If not, then the chances are very good you are suffering from undersignalitis.  The cure is a heftier guitar signal and an increase in the 2.2Meg resistor.

[michael_krell]

I should have been a little more clear when i said head room. When the trimpot is turned there is an instance where the pedal actually works, but does not work very well. At other spots on the trimpot it either has no volume or no effect at all. When I can get it working it just does not sound good and the range knob on top does not effect the sound at all.

[Mark Hammer]

Okay, that helps.

What transistor are you using?

What value feedback resistor are you using in the gain stage of the envelope follower/detector?

If the sensitivity is turned down, can you hear "re-tuning" the filter by tweaking the trimpot or its panel equivalent?

[michael_krell]

I took a picture of the transistor that is in the pedal.

It looks like it says B C 238.

I am not really surewhat you mean by the rest of your questions. All the parts should be stock.

[AzzR]

It's not exactly clear what you mean by "headroom".  Do you mean that it clips or do you mean that you simply get very little sweep?

If not, then the chances are very good you are suffering from "undersignalitis".

That one actually made me laugh out loud!

[Mark Hammer]

I hadn't realized it was stock.  When you posted the schematic, I thought you had built it yourself.

I'd recommend reading the paper called "Technology of Autowahs" at my site (hammer.ampage.org  -  the one at geofex is a slightly earlier version), but in the meantime, let's try and fix your headache.

Envelope controlled filters need a big enough input signal to result in audible sweep.  Although the pedal is designed to be able to anticipate a reasonable range of possible input levels and picking styles, it often does not accommodate as big a range as might be really needed for player/instrument X.  That's what I meant by "undersignalitis".  Aside from the usual sorts of errors committed in building things, I find many of the complaints about such pedals stem from simply not having enough "oomph" to make a sweep.

There are two ways to address this, which can be used individually or in combination.  One is to simply provide a larger amplitude input.  The buffer section on the Dr. Quack redesign from Jack Orman does this, but just about any boost in front of the pedal will work, whether fuzzbox, clean booster, compressor, etc.  A second strategy is to boost the gain of the subcircuit that detects the amplitude of the note (the op-amp/triangle) at the bottom of the screen.  The stock version uses a 2.2M resistor (red-red-green) between the input and output of that op-amp.  Increasing the value of that resistor will make the stage more sensitive.  A standard recommendation is to replace it with 2.7M (red-purple-green).  That often ends up providing the needed extra gain to yield a satisfying sweep, even with weaker pickups and gentler picking style.

[michael_krell]

I will read your article, I am sure that will help a bit.

It does not seem to me that there is a problem with not enough signal. I think the problem may be component failure. Once i understand how it works I can make a better call.

Thanks

[michael_krell]

welp sorry it has taken so long forme to reply. I have replaced all the caps and the IC and the transistor, just to return everything to spec. It did not really change anything. I did notice that the 100k pot on top does not change the sound at all. It seems like when i turn the trim pot there is a small small range where it sounds like it wants to work, but it does not sound good at all. Maybe this info might help. i am sure the problem is with the envelope detector. gracias

[michael_krell]

OK i traced the envelope detector and i think i might have found something. When i input a 1kHz sine wave and i trace it, I get the 1khz wave riding on 2.5 VDC. At the output of the op amp i am just getting solid 9VDC. Obviously this is not right. I am beginning to think this is an opamp problem. Let me know what you think.

[michael_krell]

Does anyone know if the 1N914 Diodes are required or will any diodes do?

[Mark Hammer]

Any silicon diode with a forward voltage of around 500mv is fine.  1N914 and 1N4148 are the most common, but I'm sure there are others.  The critical aspects are that: a) it be oriented the right way, b) that it not subtract more than about half a volt from the envelope signal.  If one was desparate, a pair of Ge diodes in series could be substituted.

[michael_krell]

Ok, then I am sure there is not a problem with the diodes. what kind of output should I be getting from the opamp in the envelope detector if i am driving a 1khz sine wave at the input?

[Mark Hammer]

You can calculate the output of the op-amp by calculating the gain of the stage (feedback resistance, divided by input resistance), and multiplying the input signal amplitude by that factor.  The diode on the output will subtract whatever its forward voltage drop is from that amplitude.  So, if the gain is x57 (2.7meg, divided by 47k) and the input signal amplitude is 50mv, then you should see 2.85VAC at the output.  If the diode subtracts 630mv, then you should see about 800vAC on the stripe side of the diode.  Why only 800mv?  Because 2.85VAC swing is achieved when BOTH half-cycles are taken into consideration.  The diode only permits ONE half-cycle to pass, and only allows that portion of the half-cycle greater than its forward voltage to do so.  So, we start with 2.85v, we chop that in half to 1.425, and deduct .63v from that.

The good news is that is you whack the strings real good, keep the sensitivity up high, and deliver 200mv to the follower gain stage, then a gain of x57 gets you the absolute maximum deliverable from that stage which is just shy of 9v.  Chop that in half and subtract a half volt, and your transistor sees a couple of volts for a moment or two.  Understand that the brief full rail-to-rail swing IS brief, and subsides quickly to something more modest.

[michael_krell]

well i guess the problem is at that gain stage, because even if i lift the trace at the output of the op amp and sense I I am not getting anything that resembles a 1kHz sine wave after a gain stage. so, maybe ill focus on that.

[Mark Hammer]

Just for the heck of it, take the wire that goes from the wiper of your sensitivity pot, and solder it to the input lug.  This is what that pot would be electronically equivalent to if turned up all the way, except that any contact issues internal to the pot itself will be ruled out.

[michael_krell]

ok, ill try that when I get home today, however I have no problem getting signal after the pot or at the neg input of the op amp. the problem is signal disapears at the output of the opamp.

ill let you know what happens.

[michael_krell]

I am sorry, I have not had a chance to look at this yet. I will try to look at it soon

[michael_krell]

Ok,

welp it seems that no matter what I do I cannot get the right signal on the output of the gain stage in the envelope detector circuit. I think im just getting a DC voltage at the output. would it be a good idea to take out the opamp and check the input impedance and the feedback impedance to make sure I am getting the right Ri and Rf?