measuring low voltage to tune transistor for Dallas Rangemaster clone

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e.r.stevens
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Post by e.r.stevens »

Hello,

This is my first pedal build and seemed to have hit a snag and not sure how to follow through or what to change from what I have. I'm following an old PDF from GEO Fex http://www.geofex.com/Article_Folders/R ... tboost.pdf
I'm trying to do this Terminal Strip method and have gotten as far as the tuning process, inserting 100k and 10k pot to get a proper voltage reading from the collector and ground to set the transistor gain properly.

The PDF states a -6.8 to -7.2 voltage from the collector and ground is what is usually desired to get the proper sound. I believe I have everything hooked up properly and the most voltage I can get is around -1.16 V. The battery I used was measuring a strong 9.5 V at first but I might have initially hooked up something wrong and the battery was really warming up and measured it after that and seems to be 8.8 V now. The PDF recommends a transistor gain of 75-100. The MP42B PNP resistor I'm using is around 60-70 hfe, someone else stated that this would be fine.

I forgot to heatsink the leads to the transistor maybe twice now, do you think it's cooked and that's the reason why it's not showing a voltage in range with the pots? I have another of the same one and different PNP transistor that's 45-100 hfe I believe. Would that get me to a closer voltage range?
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IMG_5133.JPG (64.36 KiB) Viewed 991 times
The lead with the dot and backwards C I used as the collector initially which I had connected to the collector wire to Lug 2. I swapped the Emitter and Collector leads just in case I had it wrong and it still measured the same voltage. I'm about to try the other PNP transistor and see how that differs, being cautions to heatsink the leads before every solder.

I'll see how that goes but wouldn't mind any insight to what's maybe going on with the voltage being off.

Thanks for any advice/help, muchly appreciated in this first build.

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e.r.stevens
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Post by e.r.stevens »

So I tried the other PNP transistor of the same hfe value that I tried originally (60-70 hfe) and got pretty much the same voltage reading of -1.44 V. I tried the other type of PNP transistor that I have that should be 45-100 hfe range and also still got the same -1.44 V reading. So I'm not sure what's going on or what to check for now. The 47uf caps seemed to measure close and fine before putting them in. Maybe it's the cheek and nose being off for the pots for tuning? I have the 100k pot cheek to Lug 1 and the nose to Lug 3. The 10K pot cheek is to Lug 1 and the nose is to Lug 4. The PDF didn't specify how to put the pot in so I thought as long as I didn't put the pot in by both cheeks I would be good. Feeling a little lost here.

Also I tried taking out the 10K pot and putting that in place of the 100K pot and seemed to get a higher voltage rating of around 4 from collector to ground, but didn't know if this was recommended or would hurt the transistor in some way taking a resistor out.

I think the PDF suggests putting in a 25k pot but not sure if that was in place of the 100k or the 10k, it's not really clear.

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Post by bmxguitarsbmx »

He says to have -6.8V to -7.2V collector voltages in reference to the -9V. So, it depends on where you put your meter. It seems to me that you have it right, just your probes are in the wrong place. -1.44V = (9v - 1.44v = -7.55V). If you have -7.55V, then lower the value of your collector voltage until it gets in the 6.8V to 7.2V range.

Just remember to put your RED test lead to the -9V supply rail and your BLACK test lead to the collector of your transistors Collector.

It seems to me that your circuit is good though. Transistor boosts sound best with voltages where you have them. With a NPN transistor boost, measured to ground, collector voltages of around 2V is the sweet spot (imo).

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e.r.stevens
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Post by e.r.stevens »

bmxguitarsbmx wrote: 15 Apr 2022, 17:24 He says to have -6.8V to -7.2V collector voltages in reference to the -9V. So, it depends on where you put your meter. It seems to me that you have it right, just your probes are in the wrong place. -1.44V = (9v - 1.44v = -7.55V). If you have -7.55V, then lower the value of your collector voltage until it gets in the 6.8V to 7.2V range.

Just remember to put your RED test lead to the -9V supply rail and your BLACK test lead to the collector of your transistors Collector.

It seems to me that your circuit is good though. Transistor boosts sound best with voltages where you have them. With a NPN transistor boost, measured to ground, collector voltages of around 2V is the sweet spot (imo).
Ok, I probed this way and am getting 7.35v right now from RED probe to positive (+) side of the 9v battery and BLACK probe to the collector leg of the PNP. The problem now is that 7.35 is the lowest I can get the reading with the pots. My 100k pot is turned all the way to the right and turning it left gets me closer to 9v. Also my 10k pot doesn't seem to be doing anything when turning it which has me confused/concerned if it's in the circuit properly. Would using a different value pot help in replace of the 10k? Is this 7.35 reading close enough to 7.2 to get pretty good tone still? Or would it still be undesirable in some way? Thank you for pointing out the misprobe.

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Post by bmxguitarsbmx »

Take your measurement between the end of the Emitter resistor that connects to the battery and the Collector of the transistor.

Does it sound good to you?

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Post by e.r.stevens »

bmxguitarsbmx wrote: 20 Apr 2022, 01:16 Take your measurement between the end of the Emitter resistor that connects to the battery and the Collector of the transistor.

Does it sound good to you?
I'm not sure what you mean by the "Emitter resistor". The Emitter leg is connected L4 and the two ends of the resistor are connected to L3 and L5. L5 connects to the battery so that's where I've been probing. Also I'm not sure how it sounds because I've only hooked it up to this point without putting in the jacks. I could add the jacks at this point to see how it sounds but wanted to do it in a way that was less permanent than soldering by the instructions.

I'm also confused why the 10K pot isn't doing anything when I turn it. Should I try a different pot or no?

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Post by bmxguitarsbmx »

Red test lead on L5.
Black test lead on Collector of transistor.

Then you won't have to subtract your measurement form the battery voltage.

The 10k pot is for volume control. It won't change the bias point to turn it, but it should change the output level.

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Post by e.r.stevens »

I had some other projects but made some time to come back to this. I'm trying to just hook it up to see how it sounds so far but feel like the directions from the Geo FEX site are kind of vague for a beginner to follow these instructions. http://www.geofex.com/article_folders/r ... tboost.pdf

(Steps of terminal strip build on Page 5) Basically I've gotten to the end of Step 12 without placing the terminal strip in any casing yet. I'm trying to figure out which lugs to attach different things to for the DPDT switch. The instructions don't refer to any lug in particular and it's stressing me out. Tried to educate myself on DPDT switches and seems that I should have either the top two or bottom two wired together to bypass the circuit.

At the end of Step 12, it says "Base Capacitor to the bypass switch" I'm assuming this is the capacitor I have connected to L3 (which is also connected to the Base of the transistor). Which lug on the DPDT switch do I connect this to?

Step 13 says "Solder the output capacitor from the wiper of the boost set pot to the bypass switch" There's one cap I didn't use yet that's the .01 uf one. Also is one lead of this cap going to the same leg of the DPDT switch as the base capacitor previously mentioned? Or does it go in the leg across?

Step 14 says "Battery connector (-, black) wire to the bypass switch" Again, a leg isn't specified.

Step 15 says "Battery connector (+, red) wire to the input jack stereo signal lug" Which lug of the stereo input jack do I connect this to?

Step 16 says "Bypass switch output wire to the output jack" I take it I make a wire and run it from the bypass switch to the output jack. Which leg of the DPDT switch do I connect this wire to and where on the output jack do I connect the other end?

Feels like some of this would be common sense to anyone pretty familiar with electronics but I'm just starting out as you can see so some of this is coming off a little vague and not direct and easy to follow and obviously not trying to wire this up wrong.

Thank you to anyone with the patience to clear some of the confusion on what legs on the DPDT switch and input and output jacks to connect to.

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