Blackfire help please.

[theundeadelvis]

So I put together a Blackfire today with the only substitutions being linear instead of audio taper pots and Q2,Q3, and Q4 are 2N5088 (biased to 4.4v at  the collector). The problem is I can only use it at the most minimum gain setting without there being unusable levels of gain and gating. When the gain is even slightly up there is so much gain present it sounds like static and the level immediately cuts out like it is being gated? Any suggestion on what this might be or at least where I might begin looking first? Thanks in advance.

[HOTGUIT]

are there trim pots?
If there are, check trim settings; usually the "gate" effect is due to wrong trim setting
bye

[theundeadelvis]

Unfortunately there are no trim pots.

[theundeadelvis]

Something strange is happening at the collector of Q5. I have been trying to bias it to 4-5volts as Joe Davisson recommended in a previous thread, but I swear the voltage keeps jumping all over the place. I had to take the 10k resistor up to a 47k in order to get it to 4.9 volts? I am reaching maximum frustration! Somebody save me please!!!!!

[theundeadelvis]

Ok, I just measured the voltages to the transistors. I feel like something has gone awry around Q5?

Q1 2N5089
C-9.29
B-1.95
E-1.66

Q2 2N5088
C-4.47
B-1.65
E-1.05

Q3 2N5088
C-9.29
B-1.96
E-1.66

Q4 2N5088
C-4.55
B-1.57
E-1.05

Q5 2N5088
C-4.25
B-(-2.66)?
E-220mV and continuously drops?

[theundeadelvis]

Thought I'd try a bump.

[theundeadelvis]

I'm wondering if mixing 5089s and 5088s could have this gating effect? I think I will build an audio probe and see what I can find out. On another note, does anyone know how to test hfe? I have a Craftsman DMM that supposedly does it, but there's no socket for a a tranny?

[Dragonfly]

Do you have a link to the schematic ? Its tough to troubleshoot without it...maybe we can figure it out if you can link it or pm me with it.

AC

[theundeadelvis]

Thanks Dragonfly! Here is the schem:

[PerroGrande]

Are the voltages you posted quiescent?

[PerroGrande]

Okay -- I don't have time to breadboard one, so I fired it up in my Sim... Here are the voltages that I get with Vcc set to 9.3 volts and the pots set to 50%:


Q1
B 2.15
E 1.61
C 9.30

Q2
B 1.60
E 0.956
C 4.96

Q3
B 2.15
E 1.61
C 9.3

Q4
B 1.6
E 0.956
C 4.96

Q5
B 1.99
E 1.34
C 3.21

I noted several things.  Several of your voltages showed less than a diode drop between Base and Emitter.  This is probably not good...  Assuming your numbers were quiescent (and probably even if they weren't), your Q5 numbers do not make a lot of sense.

Also -- I noted that the first pot in the circuit (gain) seemed to be kind of an all-or-nothing deal.  I changed it to 1K and it seemed to have a more interesting effect on the waveform.  YMMV, after all it is on a sim.

Lastly, I ran the sim with 5088's in the places where you have them.  The output waveform looked essentially the same, and the voltages only varied by a few hundredths of a volt.  This is probably not your issue.

[theundeadelvis]

PerroGrande-I replaced the 1 meg pot with a 1k and that made a huge difference! Still has a bit of gating (mainly when picking individual notes and not when chording). When it gates it sounds sort of like my Jawari sitar pedal.

By quiescent, do you mean without any signal be fed through the circuit? If so, then yes.

Thank you very much for taking the time to run that in your sim  (What program do you use?)

[PerroGrande]

Correct -- Quiescent voltages are measured with the inputs "quiet" -- I like to short the input to ground when taking quiescent voltages.  That way, you're sure that the initial stage is not getting spurious hum and stuff from an attached cable, etc.

As for the sim program -- I'm using MultiSim. 

I'm still concerned about some of the voltage readings you posted.  Some of the voltages you posted could easily explain the gating effect you're hearing. 

[theundeadelvis]

PerroGrande-You are my hero! Mentioning the 1k pot led me to try a 1k which helped. Which then made think if the 1k was better something must be affecting the 1meg. So, I did what you're supposed to do from the start of a debug-I rechecked my component values. Holy cow!  I found 2 resistors that were supposed to be 470k but were 4.7k instead. Problem solved and the pedal rocks! Now to decide on an appropriate tone stack to put with it. Thanks again!

[PerroGrande]

Sweet!!!!!  Glad to hear you've got it working.  I'd be interested to hear what you finally decide on for a tone stack to complement this pedal. 

Rock on!