PNP Rangemaster passing signal without power

[patricks]

Hi all, happy cheese week (a friend calls this week cheese week cos you eat too much cheese and forget what day it is  )

I need some help troubleshooting a Rangemaster variant, please. It's a design made by one of the forum members (Darron, from Dazytronix) who's very kindly sent me a couple of PCBs to play with to use up some germanium transistors I rescued from a friend's old record player.
I don't really want to post a full schematic because it's his IP, and posting here for general troubleshooting advice because he's been lovely with email responses but I don't want to pester him too much
It's very much based on the Rangemaster circuit, with a tweak to the biasing (Rangemaster circuit posted for reference below)

The trouble I'm having is that there's signal even when the pedal isn't powered. The volume pot works, too, so something's getting through the circuit somehow.
It's a positive ground circuit, I'm powering it from a TrueTone CS7 which has isolated outputs. One thing that came to mind is that the amp I'm testing it through is a Ruby that I usually power via a OneSpot daisy chain; I'm powering this through a separate output from the CS7, but it's not grounded through its own power cord so perhaps the two circuits are sharing a ground plane even though they're on separate isolated outputs?
The transistors I've tried in it are an AC152 and an AC154, with gains of 65 and 89, respectively, and leakage of around 200 uA (possibly more for the AC154).

I've checked the polarity of the diode and capacitors and all's good (though I should check again after a bit of a break). Should I be hunting for a solder bridge, or could the transistors be bad enough that they're shorted?

Thanks heaps
Pat

[PRR]

> The trouble I'm having is that there's signal even when the pedal isn't powered.

Why is that a "problem"? IMHO it is kinda normal for super-simple circuits.

[patricks]

Only because I thought that it's meant to be silent when it's engaged but not powered. Bypass works just fine, tho.
I'll probably pop it on the breadboard next and test it out that way

[patricks]

I desoldered the transistor and it still passed signal with no power applied, so I think you're right and it's normal behaviour.
I suspect the transistors aren't up to the task, I'll try breadboarding it and also try soldering in a PNP silicon transistor and see how it goes

[Lino22]

How does the signal pass when the tranny is out? Is it a crosstalk between wires close together?

[duck_arse]

I'd hide that circuit diagram, with +9V and ground, and +9V on the jack sleeves.

[pacealot]

This is what happens when no-one can seem to agree to a convention for drawing positive ground circuits.

[patricks]

How does the signal pass when the tranny is out? Is it a crosstalk between wires close together?

It keeps going when the tranny is out. I wouldn't have thought there'd be enough signal strength in crosstalk to be audible without the transistor in to amplify it but it's a good thought.

I'd hide that circuit diagram, with +9V and ground, and +9V on the jack sleeves.

This is what happens when no-one can seem to agree to a convention for drawing positive ground circuits.

In the pedal I've got it wired up as +9v to the tip/ground lug of the DC Jack, and Ground to the sleeve, so in my head/the pedal it's GND and -9v 

[PRR]

> I wouldn't have thought there'd be enough signal strength

Most guitar-amp inputs are very high impedance. It does not take much sneakage to be audible. This has several paths and I don't know which dominates. Or why we care.

If you actually NEEDED high isolation, such as wire-tapping, you would not expect it from an unpowered box of resistors.

[idy]

If the battery and transistor is out, you basically have a 470K resistor and 10k pot in series with your signal. Should sound weaker, depends on impedance of following stage. Signal is also attached to negstive terminal of battery, but that is not grounded, and since +9v is not hooked up.. is irrelevant.

[patricks]

Thanks very much PRR and idy. Makes perfect sense, and that's exactly what it does.
I'll hunt down some alternative transistors (or try a silicon PNP in there temporarily to test).

Love learning things here 🙂

[Lino22]

Signal is also attached to negstive terminal of battery, but that is not grounded, and since +9v is not hooked up.. is irrelevant.

How come it is not grounded?

[Lino22]

Oh i see, it is a PNP tranny...

[patricks]

Yup. This is my first time building up a positive ground circuit, and double checking I had the polarised components around the right way and thinking through how to wire up the DC jack (tip positive and the jack wired "normally" with ground on the tip, or tip negative with the jack wired "backward") gave me a brain strain 

[patricks]

Resurrecting the thread with one more question.
I got some unmeasured PNP germanium transistors (mil-spec IT308B) from DIY Guitar Projects in Australia.
They measured out at between 54 and 62hFE, with leakage below 33uA.
I've tried one in the booster and it works but it's weak, as expected. Rather than hunt for an in-spec germ, in going to experiment with using another in a Darlington pair.
I've found the article on the Smallbear website that steps through the rangemaster and outlines a variant using a Darlington pair - are there any calculators around online that'll get me ballpark values for the two resistors, since the gains I'm dealing with are larger than the 20-40 described in the article?

[R.G.]

As opposed to an online calculator, how about using an analog computer - the transistors themselves and your ears.

You could build as shown in  the small bear article and put in a 100K resistor instead of the Rc (2.2K). Then temporarily hook a 10K or 50K pot up as a variable resistor in parallel with the 100K Rc, twist the pot knob until you like it. Measure the pot setting and solder in the nearest 5% resistor value over the top of the 100K resistor.

[pacealot]

between 54 and 62hFE, with leakage below 33uA.
I've tried one in the booster and it works but it's weak, as expected

At the risk of sounding contrarian, I would not necessarily assume that those gains would be expected to be "weak" (by which I assume you mean low-output?) in an RM-type circuit...

[patricks]

As opposed to an online calculator, how about using an analog computer - the transistors themselves and your ears.

You could build as shown in  the small bear article and put in a 100K resistor instead of the Rc (2.2K). Then temporarily hook a 10K or 50K pot up as a variable resistor in parallel with the 100K Rc, twist the pot knob until you like it. Measure the pot setting and solder in the nearest 5% resistor value over the top of the 100K resistor.

That's a great suggestion, thanks very much 
The breadboard was my next stop, but I didn't have a good idea of values or approach.

between 54 and 62hFE, with leakage below 33uA.
I've tried one in the booster and it works but it's weak, as expected

At the risk of sounding contrarian, I would not necessarily assume that those gains would be expected to be "weak" (by which I assume you mean low-output?) in an RM-type circuit...

It's not super-weak, and it's definitely doing a "thing", I can hear it colouring the sound a bit when it's biased. The output is slightly less than unity, though, even with the transistor biased and the pot maxed.

[pacealot]

Hmm – interesting. I would've thought that, even at those gains, it'd be able to put out decently more than unity. But I suppose with germanium, all things are possible...

[patricks]

Hmm – interesting. I would've thought that, even at those gains, it'd be able to put out decently more than unity. But I suppose with germanium, all things are possible...

Me too, but it could be that my measurements are a little off. I'm using RG Keen's germanium tester with a multi-turn trimpot to set the resistance as close to 2.472k as possible, but my multimeter only reads to 2dp and if I bumped it while handling then that could mess things up.
Or maybe I've got a solder bridge somewhere and need to take a closer look at the board, check my fundamentals again