Correct voltages on sockets but only without transistors inserted

[patrick398]

My bassist dropped the harmonic transformer i built him and it stopped passing signal except in bypass. I thought it'd be an easy fix, broken connection somewhere, maybe a dislodged transistor or something but this one is driving me crazy.

With transistors out of the sockets the socket voltages are exactly as to be expected. Biased to around half supply on the base and 8ish v at the collector. When i insert a transistor the voltages drop to around 0.5v on the collector and it's got me totally stumped.

What could be happening here?

I've encountered this kind of problem before and never got to the bottom of it

[GibsonGM]

Got pics of the board, both sides, and a schematic?  A short or other fault somewhere that is not active until the transistor is inserted could be at fault, but that's just a guess....

[patrick398]

I built another board today as i just wasn't in the mood for debugging 
This one has the exact same issue though! Voltages look healthy until trannies are inserted then the pins are way too low.
If i strum very hard i just about get some farty mis-biased nastiness but the reason for the change in voltage is beyond me.

It would be an almighty coincidence for both boards to have the same short considering the first worked fine until dropped.
This leads me to think a dodgy transistor is at play. I've ordered some more but in the mean time any thoughts from you guys?

[thermionix]

Got pics of the board, both sides, and a schematic?

[patrick398]

Sorry i should have posted these before, i was in denial that human error was involved but i've made that mistake many many times before ha!

[patrick398]

Can't find the schem, i'll have a look over at FSB

EDIT: Can't seem to track one down. Here's the vero layout if that's any use

Also, i tried using 2N5088 instead but that yielded the same issue...so perhaps not a transistor issue

[amptramp]

One problem that crops up sometimes is the transistor pinout may be wrong.  Check the transistor data sheets against the layout.

[patrick398]

Yeah i checked the datasheet of the trannies from the tayda website and orientation seems fine. The weird thing is that the the first one worked perfectly until dropped and the second one doesn't work in exactly the same way...the only components that are common are the trannies

[thermionix]

[thermionix]

Wow, is this thing really supposed to have all the collectors at ground?

Edit:  I've seen on Madbean's forum others have built this IAW the layout above, so I guess it's correct.  But Patrick is somehow getting ~8V at the collector positions with the transistors removed.

[patrick398]

Sorry i forgot to link to the layout, long day yesterday ha, thanks for posting that up Yeah it's bizarre that all the transistors are reversed...it also means giving voltage readings is confusing.

So with nothing in the sockets i've got around 8v at the emitters of Q1-6. Collectors are all 0v and the bases sit at half supply as expected except for Q4 and Q5 which are also around 8v as they're directly coupled with the emitter of Q3

With transistors inserted:
Q1-Q3:
E - 0.03v
B - 0.52v
C - 0v

Q4:
E - 0.58v
B - 0.03
C - 0v

Q5:
E - 0v
B - 0.58
C - 0v

Q6:
E - 0.02
B - 0.54
C - 0v

If i place a trannie into Q1 socket Q2's empty socket voltages remain correct, this is true for all sockets except Q4 which had an E of 8.27v before Q5 was inserted and 0.58v afterwards.

Sorry if this is getting confusing. I'm totally stumped.

[antonis]

When dealing with Death by Audio fond reverse bias BJTs you have to expect everything.. 

[thermionix]

It sounds like you have a shorted transistor but you said you tried different ones.  So I'm lost.

[tonight, we ride]

It sounds like you have a shorted transistor but you said you tried different ones.  So I'm lost.

This is what I was initially thinking too...

Patrick398, any chance you have a multimeter with a "Diode" mode on it, so you can check for failed transistors? You should be able to check between the base-emitter, and base-collector combinations and get a "beep" or an indication of something in the 0.6 volts range. If there's no beep or voltage, try swapping the leads. If you get no beep or voltage for either polarity of your meter leads the transistor is failed.

It seems unlikely that you would have the same problem with different transistors, but if you have the ability to confirm your transistors as good then we can rule that out!

[patrick398]

More than happy to rule that out. I briefly did this yesterday but good to get some numbers written down.

Multimeter on diode test setting.

Black probe to base:
no reading (just says 1) on either collector or emitter with red probe.

Red probe to base:
collectors all read around 900/901/902...what is that measuring, 900 whats?
emitters all read between 904-908.

I'll be honest i have no idea what that's telling me 

[tonight, we ride]

Assuming those readings are good, it's most likely giving you a value in millivolts (mV). Those numbers are a little higher than I expected, but are definitely in the ballpark of being good. Unless anyone else chimes in with more info than I have I think you can consider the transistors to be good at this point.

Your like-for-like swap of different transistors basically had confirmed this already, but I always like to double check things with multiple methods when I'm stumped!

I'll be sure to let you know if I think of anything else to test!

[patrick398]

Thanks for the help, definitely good to rule out the transistors although i was kind of hoping it was this as now i am totally lost. I don't even know what else i can test to give me some indication of what's wrong. All track cuts have been checked with continuity test, in between tracks have been scored and scraped, component values and placement checked.

The strangest thing i'm finding is that something went wrong with the first circuit after being dropped and either i've somehow recreated the exact same fault in the rebuilt circuit (surely not) or something other than the transistors is common between both builds.

[thermionix]

EMP attack, coronal mass ejection.

[tonight, we ride]

Well, from a troubleshooting standpoint there are a few things that we can look at.

In my experience it is best to start troubleshooting where a fault can be consistently replicated. The first pedal failed after a drop, but the second build had the same issue without being dropped. There is an inconsistency there, so it is not necessarily useful information in terms of moving forward or eliminating possibilities.

However, you do have a consistent change in events when transistors are placed in sockets vs. pulled. If you can consistently replicate that change in events then I would look to anything surrounding that event: transistors (which we ruled out with the diode test and swapping transistors), the sockets, the pinouts, the associated components that would affect ALL transistors that replicate the change in voltage. Faults are almost always ONE thing that has gone wrong, but even when there are two things wrong it is best to approach troubleshooting assuming one fault, fixing that fault once identified, then moving onto the next fault.

I would look at your transistor sockets at this point. Without a transistor plugged in do you have continuity (remember to NEVER do a continuity/resistance test with power on the component/circuit!) between the two points that are shorting together with a transistor plugged in? Do you get the same result with the socket pulled and a transistor soldered directly to the board?

[patrick398]

EMP attack, coronal mass ejection.

I thought that 100uf cap across the rails was there to stop solar storm interference?