- This topic has 10 replies, 3 voices, and was last updated 1 year, 11 months ago by Anonymous.
October 12, 2018 at 5:18 pm #2440AnonymousInactive
Hey Guys, I need a little validation/help with my bass fuzz build. Using a production pedal of the same circuit (minus the boost section) as a baseline, my build just seems anemic, with lower output and less bass. With the boost and fuzz at 100%, unity is around 75%. I have compared components between both pedals and am 90% certain they are the same (all cap values are the same and I have measured the first gain stage resistors), but the production pedal is just thunderous compared to my build. I have gone through and verified all parts are in their correct places. I have checked for solder bridges and cold joints, but have not reflowed or audio probed yet pending your review of voltages. That said, here are the requisite voltage measurments: source voltage: 8.3v. Q1 BC549c hfe: 530 Cv: 16mv Bv: .56v Ev: 5.72v. Q2 BC549c hfe 535 Cv: 20.6mv Bv .57v Ev: 6.1v. Q3 2n5088 hfe 537 Cv: 8.7mv Bv: .54v Ev: 6.72v. Q4 mpf102 Dv: 6.45 (biased a tad hotter trying to milk more gain) Sv: 1.95v Gv: 0v. And the pics:
Any thoughts and help appreciated.October 12, 2018 at 5:37 pm #2441
Thank you VERY MUCH for a proper troubleshoot thread. It is almost like pulling teeth at times. 😉
So the first thing I notice is you say you have the Boost bypassed but your output wiring is coming from the Output T pad so you are using the Trimmer to control the Boost Volume which is then connected to the final Output T pad. My point is the Trimmer Boost Level needs to be set at 1:00 for unity gain. Otherwise it attenuates as you describe. From 1:00 position to full up is a boost of about 15-20 dB. Looking at the notch on the T2 Trimmer it is pointing toward about 10:00 which would attenuate. I do see that T1 is full up.
Make sure T2 is set at 1:00 or higher.
Having the option of Attenuation or Boost allows the user (guitar or bass) (active or passive pickups) to get a desired setting for nearly any rig or recording situation.
If that is not the issue in regards to the Trimmer setting my next guess is the MPF102
There are too many bad MPF102s saturating the market from the last 4 years. Can you show us a closeup of the Font for the MPF102? Many MPF102 “named” transistors have been saturating eBay etc.. for quite a few years now and are either inferior or fake. I can recognize the correct Font with a closeup of the transistor. I have seen 5 different fonts (4) of which are no good.
That is all I have at a glance for now and I hope it helps.October 12, 2018 at 7:13 pm #2444wilkie1Moderator
I think I read your post to indicate that you bypassed the Boost on the production pedal but, you are using the boost section on this build? I see that you did not install a pot for the Boost and are using T2 for the Boost.
Here are some other observations:
The source voltage is a bit low at 8.3. You may have a weak battery that is causing your readings to be low.
The Q4 drain voltage you reported is 0 V. You should be getting about 0.5 V there.
Q3 Collector voltage looks very low.
Notice that each of the following resistor pairs contain a 100K and a 100R (Ohm) resistor. It is easy to get these mixed up and insert them in the wrong position. Please double check these as this may cause an issue with these voltage readings.
R3/R6, R9/R12, R14/R17.
If these don’t solve your problem, then try doing a resolder of all joints, pads and lugs. You may have a cold joint that is invisible to the eye.October 13, 2018 at 4:38 pm #2454AnonymousInactive
Thanks for the replies. I work in IS, so I know the importance of complete information.😄To clarify, the production pedal I’m comparing with is a three transistor version of the circuit without the recovery gain/boost stage. I tried bypassing the boost stage in my build, running a jumper from lug 2 of the volume straight to the out, but that was even worse. I validated that all parts are correctly placed, used a fresh 9v source, and rebiased Q4 to 5.5v. There was a slight improvement. Using 9v, the values on the other three transistors changed a little (Gv on Q4 still 0v), but were still in proportion to the original values. Here is the mpf102
I sourced my parts from Mammoth, not that that means they don’t get duds too, so you be the judge. It is with a heavy sigh that I will resign myself to reflowing my joints with fingers crossed.October 13, 2018 at 11:23 pm #2455
The F font appears correct and while that won’t rule out a bad transistor at least it is a Fairchild. The problem Is occurring before that final stage anyway based on your report.
I suggest reflowing everything including all wiring and if that does not help replace the other transistors or even better use an audio probe to locate the problem area more easily.October 28, 2018 at 5:01 pm #2708AnonymousInactive
Final follow up. Finally got around to reflowing my solder, but there was no change. Then I got around to looking at the schematic a bit more, specifically r17, the emitter resistor for Q3, because I thought I had maybe lifted a trace on the back. However, the trace was fine since it connects on the top of the board. But that got me looking a harder and then, BAM!, I spotted it, r17 goes to the EMITTER of Q3 and my transistors were in backwards 🙄. To be fair, I was following the silk screened symbol on the board. Anyways, this fuzz tips now. Thanks for all the help.October 28, 2018 at 5:57 pm #2709
Thanks for the follow up and the silkscreen is always for the stock transistors per the build doc or BOM (since we could never cover all choices with one silkscreen) which is also a problem when troubleshooting since we cannot see what transistors are being used unless you are saying that that the stock transistors are backwards (which I would want to know) but I hope I would have noticed that in my pre-testing. I actually used to sell the transistor sets for this till i ran out.October 28, 2018 at 6:43 pm #2710AnonymousInactive
To be honest, I was slightly confused myself since the Q3 I was using was a 2n5088 as per the build doc. The other two were bc549c, which according to my googling have the opposite pinout, but I flipped them all and now it’s working as expected.October 28, 2018 at 6:47 pm #2711AnonymousInactive
Should add, the bc549c are in the correct orientation with the silkscreen, with the 2n5088 being switchedOctober 28, 2018 at 6:57 pm #2712
OK so the silkscreen is correct per the BC549C which are the stock transistors but as I see we recently changed it to the 2N5088 which are very much more available at more places so while I remembered to add an asterisk I see I forgot to add a note to the Build Doc regarding the transistor change when I ran out of stock transistors. By the way I had stock transistors for over a year and only ran out when I switched site hosts which was very unfortunate timing.
Sorry for the confusion. This was a tricky one as most want to use stock so that is what we went with for a stock build at the time but unfortunately it is harder to get them now. I will adjust the document to the orientation.
OK I do now see that I did add a note:“The original used BC549’s however *2N5088’s are much more readily available and sound fantastic. Feelfree to try similar types that suit your taste. Be sure to verify the pinout, which is yet another good reasonto socket first.”I do agree it needs to be much more specific so I will fix that. Again sorry for any inconvenience.October 28, 2018 at 8:35 pm #2714AnonymousInactive
No worries. I’m just glad that the pedal is working now. I guess when I read that side note, I was thinking further in the past to original production pedals, not necessarily the previous version of the PCB. Thanks again for the help.
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