Suggestions, Big muff pi 70's version batty or gated sound

[Wales]

Just can't get this project going. I've build a few other BMP's with no problems, but with the 70's version I've build two and had the same problem with both. The pedal only works when you are playing hard, soft notes don't register. I have triple checked the values of all the parts and can't see a poor solder joint. I read the voltages off the transistors and they are as follows in order of emitter, base, collector: Q1 .03 .54 3.31  Q2 .04 .62 3.05  Q3 .07 .68 .10  Q4 1.28 1.76 3.81. I can see that there is a problem in the Q3 area of the circut, but can't find a problem. Is it possible that the parts list off of GGG.com may have a typo? Oh yeah my battery voltage for these readings was 9.99V

[theehman]

What's the parts list?  Can you post it or a link?

[Wales]

Here is where I found the parts list. Sorry I didn't know how to make it a link.
http://www.generalguitargadgets.com/pdf/ggg_bmp_l70_bom.pdf

[theehman]

That helps.  Is there also a schematic somewhere?  It would handy to identify which part is which.

[killerkev]

Check the resistor that supplies the power to Q3, maybe you put  a too large value in there or desolder the leg of the resistor that connects to the collector and see if you get something in the area of 3.5 - 4 v. If you do and then solder it back in and you still have the low voltage on the collector then the problem lies elsewhere.

[Wales]

here is the schematic too
http://www.generalguitargadgets.com/pdf/ggg_bmp_l70_sc.pdf

[R.G.]

You're onto the area - Q3 is misbiased. ALL blatty, needs-hit-hard-to-get-through conditions are misbiasing.

In Q3's case, you've correctly identified the symptom, Q3 collector too low. This could be caused by the collector resistor being too big a value, or it could be defective open. Measure the DC voltage on both ends of the collector resistor. The upper end should be battery voltage. If it's not, the resistor or solder joint is open. The lower end should be collector voltage. If it's not, you have an open solder joint between the two.

This could also happen if the base is getting too much current. Since it's sitting at 0.68V, which is high for a small signal type, this is a distinct possibility. If C7 is shorted or leaky, this could cause it. Measure DC on both sides of C7; they should be different. If theyr'e the same, check C7 for open.

There are a couple of other "too-biased" conditions possible in there, but wth the collector already lower than the base, they don't seem likely. Check these first, then we can dig deeper.

[theehman]

here is the schematic too
http://www.generalguitargadgets.com/pdf/ggg_bmp_l70_sc.pdf

Well, according to what you posted earlier, there's definitely something funky (in a bad way) with Q3's voltages.  I'd check all the resistors in that stage and also make sure there aren't any solder shorts.
R18 in the tone control is a 100K, which is weird.  I'd make it a 39K.

[Wales]

Sorry It took so long, but I had a vacation since i last posted. I desoldered the resistor going to the Q3 and checked the voltage on each side. The Batt side of the resistor was at 9.15v the same as the batt and the other side was 9.01v. When put back on the voltage to the Q3 was still low. I then checked the resistor that was going to the Q2 which was working fine and got the same readings as on the Q3 resistor. I change the tone resistor R18 from a 100k to a 39K and same problem as before. I also checked the C7 capacitor and the voltage on the resistor side of the capacitor was 1.07V and the transistor side 3.10V, seems ok. So overall I still have no Idea what going on.

[mac]

Check the tone pot since there is a DC path from Q3 collector to ground via the pot.

mac

[petemoore]

  The capacitors block DC, the DC is used in the active stages, each stage independantly DC biased.
  The resistors in question are those 'inside Q3's..DC domain.
  For Q3 they are contained between C7 and C10.
  They're also contained or constrained to 9vDC, or 9v - 9v = 0.0v.
  Visual testing of color codes of resistors...it is hard to tell from here for sure, measuring them in circuit may throw the reading to lower value [since parallel resistances offer a lower resistance path, like adding a water channel between two channels: one with a higher higher potential and one with a lower potential.
  But only occasionally, and not in this circuit..unelss...well unless it ain't right.
  So...the base is biased with the 470k to collector, all the other paths are DC blocked.
  Collector is biased..through the CR,  R17, in this case it's marked at 15k.
  Emitter resistor, one side should read 0.0VDC = Gnd. The other side...a 'touch higher in potential...
  The collector voltages should all read 'somewhere towards the middle' of the supply, well you can see the bias resistors are identical except for Q4, which is not that far off, 'proportionally.
  Q3 isn't cooperating, Something must be pulling it down to near Gnd. Or it isn't being pulled toward V+, test the bottom of Q3R, then the actual Q3C lead.
  Inspect the connections with a close in light and Mag/Glass, look for cold solders or traces across the circuit.
  It's usually an open or closed circuit that shouldn't be, missread R-value, or some other little 'crosshook' that snags the bias.
 

[mac]

r19, r20 and r18 connect q3 collector to ground; there is a flow of DC current from q3c to gnd that can be neglected since r18+r19+r20>>r17.
But if there is an error they could bring q3c to near ground.

Some schem I saw have a big cap between the tone stack and q3c to effectively block DC.

mac

[Wales]

Thanks for the help guys. I'll keep checking it over and report back.

[Wales]

Finnally i found the problem it was a cold solder. I had checked them all by sight, but finally I just re-soldered all the joints in the problem area and presto. I want to thank everyone that posted to help me with my problem and hope to be able to return the favor one day.  Thanks again!