First breadboard attempt, I need help (Rangepig)

[Herr Masel]

I am building Will Firstbrook's rangepig, still on the breadboard (for the first time), I'm doing the newer version of the two. I measured the voltage at the positive side of the battery/circuit - 8.7. At the negative, nothing and no reading from the bias point, so thinking I haven't stuck a component in deep enough I put the dmm on continuity mode to see that everything is connected. Now at the positive side there is no reading while the negative shows continuity. I checked it out, I had a wrong value cap and the trimpot was sitting the wrong way around, I corrected those but still same problem.

There are a few places where I strayed from the schematic: the trimpot is 500k and not 1meg, I know that's a big difference but that's what the guy gave me and I didn't have any higher ones lying around. Also the transistors are not 2n5210, or so I think. They have three stripes on the front, green white and black (they might of been drawn by hand though) and the back has two ~ signs on top of eachother, like a flag, then 938, under that 975, and under that e b c, which is the only way I knew how to put them in because I couldn't find a datasheet. Maybe I should replace them, but I can't find that old thread saying which ones I could use.

Ok so I know those are big variations, do you think that's the source of it all?

[niftydog]

the back has two ~ signs

Like one of these?

It's made by National Semiconductor.

the trimpot is 500k and not 1meg, I know that's a big difference but...

it is in the bias circuit, so any small change like that could upset the circuit.

Sounds like you're not even sure if the transistors are NPN or PNP. Definitely go for something that you know is NPN. If what you have there is a 2SB938, that's PNP and it'll never work in that circuit.

[will]

There are a few places where I strayed from the schematic: the trimpot is 500k and not 1meg, I know that's a big difference but that's what the guy gave me and I didn't have any higher ones lying around.

That shouldnt make any difference a 200K pot will work just as well for R6. The idea was to create a variable bias voltage that range from 0 to 9volts for the base of the transistors. When you adjust this control you can control how the transistors are biased. 

Also the transistors are not 2n5210, or so I think. They have three stripes on the front, green white and black (they might of been drawn by hand though) and the back has two ~ signs on top of eachother, like a flag, then 938, under that 975, and under that e b c, which is the only way I knew how to put them in because I couldn't find a datasheet. Maybe I should replace them, but I can't find that old thread saying which ones I could use.

Ok so I know those are big variations, do you think that's the source of it all?

As long as the transistors are NPN they should work, Ideally they would not have super high gain. NPN Transistors that have a gain simular to the 2N5210 would work great.   I would initally set the input bias to around 3volts then adjust it up or down till the collector on the 2nd transistor reads approx 6.5 to 7 volts. I'm not sure what the emitter voltage will be exactly as it depends on the transistor used and I havent checked it for quite a while, although I use it as my treble boost in an already clipping amp for leads ever so often. I would expect it to be above zero and less than 3 volts.

If you cant get the collector voltage to around 6.5 volt range, either there is a wiring error, cold soldering joint,  blown transistor, or PNP transistor, shorted C4 cap, or incorrecly installed Transistors.

Good luck!

Regards,
Will Firstbrook

[Herr Masel]

Oh the might be PNP I never thought of that  . But the guy from the shop put them in a baggy and wrote on it =2n5210, I guess he didn't think of that either. The gain wasn't that low either, 180 on one of them. I'll try a different pair. Thanks.

[Herr Masel]

No, actually that cannot be true. I measured one of the transistors for gain, and it only worked in the NPN slots, so that's not it. How can I check to find if a cap is blown? I tried a continuity test and nothing came up, is that it?

Edited to add that I changed the capacitor but the still no reading. I changed the transistors to 2n2222a, I went over the connections again. Sill nothing on the negative side. And when I plug in the guitar comes trhough, just without the effect.

[Herr Masel]

Hate to nag, but any other ideas would be appreciated. I just checked and the guitar signal still comes through (the effect bypassed) even when the battery is not connected Is that something that happens with breadboards? it doesn't seem right.

[will]

Hi

Hate to nag, but any other ideas would be appreciated. I just checked and the guitar signal still comes through (the effect bypassed) even when the battery is not connected Is that something that happens with breadboards? it doesn't seem right.

I'm sure the problem is something simple once you find it. I mess up someting on just about every circuit I build. Its often in an area I've checked superficially at first then kept skipping over when trying to find the problem. Breadboards should basicly work as well as the finished circuit so if something seems odd, its how the circuit was put together.

The bypass is a true bypass it should work in bypass mode even if the circuit is dead. Is the bypass switch S1 wired correctly? Is it effectively bypassed all the time?  Did you use a 2 channel switch?

As for other ideas:

Are you getting the transistors biased?
Are you sure the transistors are correctly wired to the circuit? I know it often gets confusing translating a schematic to a working circuit, 

In the schematic the orientation is:
B (base) = left side of each transistors
E (emitter) = bottom of each transistors
C (collector) = top of each transistors

You could remove Q1 from the circuit to get it going. It will operate just fine with Q2. Q1 makes the circuit more germanium like.

With the circuit turned off with a resistance meter quickly Check that the resistance from E for Q2 to ground is approx 5K.  It should start to rise /or lower due to the caps in the circuit but start around 5K. Also check that C to the 9v is approx 10K.

Turn on the circuit:

If you set the bias at 3 volts measured at B on Q2
Set the drive pot R4 to 0 for minimum gain. This puts C4 is effectively out of the circuit.

What do you get for E and C for Q2?

If you adjust the bias pot R6, can you get 6.5 â€" 7 volts at C for Q2?
If so what is the voltage at B & E for Q2?

That’s all I can think of right now. Good luck!

Regards,
Will

[Herr Masel]

Ok, thanks alot for your help Will! I got it working now, but there is hardly any volume, and not much of an effect. As far as biasing goes, The voltage on C of Q2 ranges from 1.8 to about 2, by adjusting the bias pot. That's not a very big range at all and it is not close to the 7v it should be at. This could be the trimpot (I used 500k instead of 1M) but I don't think so, plus you said yourself it should work fine. Something else is starving the circuit, because of the low volume.

[will]

Hi,

Ok, thanks alot for your help Will! I got it working now, but there is hardly any volume, and not much of an effect. As far as biasing goes, The voltage on C of Q2 ranges from 1.8 to about 2, by adjusting the bias pot. That's not a very big range at all and it is not close to the 7v it should be at. This could be the trimpot (I used 500k instead of 1M) but I don't think so, plus you said yourself it should work fine. Something else is starving the circuit, because of the low volume.

Sounds like there is still something wrong with your circuit, as you should be able to make voltage at C Q2 range from around 1 to around 8.5 volts by adjusting the bias pot.

Does the bias pot vary the input voltage to the base of Q1 & Q2 from 0 to 9V?  If so the bias pot is working correctly.

The gain control at minimum should be close to unity gain (same level as bypass) then go up from there. It doesn’t have huge gain but enough to push the 1st stage of a tube amp into clipping quite nicely. Especially a tube amp already clipping. If desired you could easily double the gain by connecting C3 to the other side of R8 where it connects with the collector of Q2.

Regards,
Will

[Herr Masel]

Ok, first of all I changed the transistors to 2n2222, I mentioned it before but I'll say it again. The bias pot is improvised - I soldered a 1M resistor across the 1 and three leads of a 10k pot. The voltage on the bases of the transistors go from 0 to about 1.6 volts. I removed the .022 cap that was on the collector of Q1 and left the collector out of the circuit, when it was working it sounded better that way. If it is not the transistors then I don't know what could be starving the circuit of voltage........ frustration ensues.

P.S. the 2n5210s don't give any different readings..

[will]

The bias pot is improvised - I soldered a 1M resistor across the 1 and three leads of a 10k pot. The voltage on the bases of the transistors go from 0 to about 1.6 volts.

That doesn’t look right at all.  A 1 M resistor across a 10K pot is still virtually 10K. The voltage to the base of the transistors should be able to swing from 0 to at least 8volts, with a turn of the bias trimpot.

Having said that, if the collector is sitting at 2 volts it will clip about the same as if it was at 7 volts. It will just clip the other side of the waveform. Which will sound similar but not identical.

The  .022 cap C7 just rolled off some more highs to make the effect less bright it was optional anyway.

Regards,
Will

[Herr Masel]

Oops, I should of know about the trimpot....
Out of interest, how do you clip the other part of the wavefrom? How did it happen if I used the right parts?

[will]

Hi,

Out of interest, how do you clip the other part of the wavefrom? How did it happen if I used the right parts?

Assuming you have a 9 volts. If you set the bias so the collector for Q2 is around 7 volts you will clip the top part of the wave form. Say the transistor is trying to swing a 6 volt signal. When the bias point is set at 7 volts,  It will try to swing 3 volts above the bias point and 3v below the bias point. The circuit will try to reproduce 7 +3 = 10 volts at the top of the wave and 7-3+4 volt at the bottom. The bottom is no problem but the circuit can not produce a voltage higher than the battery voltage. So it will limit the max voltage to slightly less voltage than the battery puts out.

If you set the bias so the collector for Q2 is around 2 volts you will clip the bottom part of the waveform. If you set the bias so the collector is 4.5 volts the circuit will clip both tob and bottom portions of the wave evenly assuming you have enough gain and an input signal large enough to clip.

Hope that helps.

Regards,
Will