Ge collector-base short

[whoisalhedges]

OK, so I'm building my oldest and dearest friend a (well-belated) Xmas/birthday prize; a one-of-a-kind guitar signal annihilation circuit. Without sharing the schematic (I fully intend this to remain one-of-a-kind ), it's a resonant lowpass filter smashed in the middle of a nonvariable gain (it's always filthy) Germanium fuzz.

Basic topology: common-source boost > filter > fuzz.

Except I keep ruining it.

Here's why I don't need to share the full schematic: I KNOW WHAT THE PROBLEM IS. I just don't know how it's happening or how to avoid it.

This thing lived on my breadboard for 2 weeks, and the second attempt at boxing it up worked for a couple of minutes, so I know the circuit is good. The dirt section (where the problem is) is a spin on the last transistor of a Mk III Bender/Buzzaround ('cept NPN). This thing sounds ridiculous, and if Tom doesn't want it, I'll gladly take it back. Anyway, that last transistor - I keep blowing it. The B-C junction keeps shorting out. It wasn't shorted before, then I measured the voltages (I don't remember exactly, but 6-7v on the collector and like 1v - or maybe 100mv - on the base. Soldered, boxed... dead. Troubleshooting, found B and C both at 100ish millivolts, and the continuity function on my DMM went BEEEEEEEEEEEEEEEP.

Take two, I not only got voltages which would suggest nice, loud, GROSS fuzz; but when I plugged it in (soldered, in box, but the back panel not screwed on) it sounded GREAT. Just like on the breadboard. Problem solved? No - screwed on the plate, and it's splatty, gated - the unmistakable sound of a misbiased transistor. Open it up, grab the DMM... and base is shorted to collector again.

Unsoldered and removed, the two trans are still shorted. So I've basically got a couple overpriced diodes on my hands now. Ge transistors are too rare and expensive for me to keep wasting like this! So... what the hell am I doing to keep destroying these transistors? I'm just trying to do something nice for a beloved friend, not take over the world - so why must the fates thwart me at every turn? Am I cooking it? I don't think so - I'm in and out quick, heatsinking with a tweezers; and besides, on the second try, I DID get good output - for a minute or two, before I put the back on. Am I somehow (I put bubble wrap on the inside of the back panel) touching one of the leads or the metal case (which is connected to base) to the grounded enclosure, and creating some kind of 9v ground loop that's killing the transistor?

I'm just trying to make a nice present! Please help me!

[thermionix]

Is the bubble wrap pink?  Or green?  I've seen those two colors in ESD-safe (conductive) bubble wrap.  There may be more colors available.  Check it with a meter.

What kind of Ge transistor?  Sourced from where?

[whoisalhedges]

Lol... it's clear.

They're 2N1306s from Small Bear. hFE around 200, leakage around 200uA. The second had a tiny bit more gain and leakage than the first.

Both worked and measured like good little transistors. Then I boxed them and they died.

[thermionix]

Can you post pics without giving away the circuit?  Is the bubble wrap pushing the board too far in, and causing a short between the board and the enclosure, or the body of a pot?

[whoisalhedges]

I only added the bubble wrap the second time around, so I doubt it's the culprit.

I've got to get up early tomorrow, and have a long Saturday ahead of me. But I can probably get a snap of the offending snippet.

Although I'm hardly an expert, and things can always go wrong; I am very confident in the circuit - and though I'm less *confident" in my soldering skills in general; I checked every connection, every cap and resistor (including pots) before applying power to the circuit; then checked voltages, which matched the values on the breadboard... it's solid.

My question goes WAY beyond this pedal, though; because if I'm killing the B-C junction on these transistors now, without having any idea how I'm doing it, I'm going to keep killing transistors. And man, who wants to do that?

[whoisalhedges]

Here's the stage in question. As you can tell, basic (NPN) Mk. III Bender/Buzzaround topology. Sound great on the breadboard, works for a few seconds after soldering; then close up the box and boom - transistor is now a diode, B-C junction permanently shorted.

[PRR]

I would assume the box is doing something bad.

I had a dandy and safe mike preamp. Boxed, I got a shock. Unboxed, no shock. I finally saw where a box cover screw was puncturing the power transformer (!!). Could have been serious if I had electrocuted a client.

[whoisalhedges]

I would assume the box is doing something bad.

I had a dandy and safe mike preamp. Boxed, I got a shock. Unboxed, no shock. I finally saw where a box cover screw was puncturing the power transformer (!!). Could have been serious if I had electrocuted a client.

OK - so, maybe I should try insulating the entire box (or wrapping the board in something nonconductive - like that handy bubble wrap I have lying around)?

I certainly hope it's something that simple. I really dig this circuit and how it sounds - if I were to make any more, I'd make the overall gain adjustable, but that's not the plan here. - and I can't afford to keep frying transistors.

[PRR]

> maybe I should try insulating the entire box

I would use my eyes first. Squint inside while you work it together. See what touches.

Blindly bubble-wrapping may work, but may wear-through and short-out at your Big Gig.

[whoisalhedges]

What I saw with my eyes on Thursday when I built it... first time I didn't have any bubble wrap on the back panel, so it absolutely touched the case of the transistor. Second time, I put the bubbles on the panel, but don't have any stand-offs; so the leads almost absolutely contacted the backs of the pots/inside of the enclosure.

I think I'll give bubble wrap a go tonight - then, if I don't fry my third xistor, I'll get some stand-offs to mount the board, as well as something a bit more durable for the inside of the panel, before sending it off.

[whoisalhedges]

OK!

Seems to be working, put it through the paces for about half an hour. I probably will want to re-build it anyway (soldering and de-soldering 3 transistors made the board kinda messy) before shipping it off to Tom; so I'll look for a more permanent solution than bubble wrap. But this is the good stuff. Love the nasty sound of the TONE FIST.

Thanks for the help.

[PRR]

So the transistors are not ruined?

Just don't-work when pressed against the box?

[Phoenix]

t's a resonant lowpass filter smashed in the middle of a nonvariable gain (it's always filthy) Germanium fuzz.

If it's a resonant circuit, you mean you have an inductor/choke/small signal transformer in there? Is there any DC current flowing through the inductor while the circuit is powered? If so, do you have a reverse-biased diode strapped across it to prevent flyback voltages? If not, when power is removed, the stored magnetic field in the inductor will produce a voltage spike to try to maintain the current flowing through it. Even a small value inductor can put out hundreds of volts, particularly if the circuit impedances are high, this is well and truly enough to zap a BC junction.
If the inductor is AC coupled, you may still have issues with flyback voltages (although it's less likely), and might need to add some protection diodes/zeners to the base of that transistor.

Hope this helps track down your problem.

[whoisalhedges]

t's a resonant lowpass filter smashed in the middle of a nonvariable gain (it's always filthy) Germanium fuzz.

If it's a resonant circuit, you mean you have an inductor/choke/small signal transformer in there? Is there any DC current flowing through the inductor while the circuit is powered? If so, do you have a reverse-biased diode strapped across it to prevent flyback voltages? If not, when power is removed, the stored magnetic field in the inductor will produce a voltage spike to try to maintain the current flowing through it. Even a small value inductor can put out hundreds of volts, particularly if the circuit impedances are high, this is well and truly enough to zap a BC junction.
If the inductor is AC coupled, you may still have issues with flyback voltages (although it's less likely), and might need to add some protection diodes/zeners to the base of that transistor.

Hope this helps track down your problem.

No inductor, I took my lead from Escobedo's Idiot Wah and the Colorsound inductorless wah... actually built it with a more traditional circuit first, with inductor - but was able to get the same sound with many fewer parts.

So the transistors are not ruined?

Just don't-work when pressed against the box?

Oh no, they're toast - B & C are still shorted - it's working now on transistor #3.

[anotherjim]

If the damaged Ge transistors have at least one diode junction working, don't throw them out -  try them as clipping diodes in something else.

[whoisalhedges]

If the damaged Ge transistors have at least one diode junction working, don't throw them out -  try them as clipping diodes in something else.

Oh I will, no worries there - it's just that I'm not thrilled of having $10 of diodes about 20 inches to my left... consisting of only two pieces.

I'm still curious as to how/why this occurred. The first time it happened, last Wednesday night, I was at a complete loss - that's why I posted here. I googled all sorts of terms, usually something like "Ge base-collector short" - and there is nothing out there, nor on a search of this forum. I've seen transistors in metal boxes all the time, with nothing insulating them from the enclosure (though most circuits of the past 40 years have had plastic TO-92 xistors). Why the hell is it happening with this circuit? I know I haven't posted a schematic; but is it conceivable that there's just something about this circuit that makes it a Q3 killer?

I'll take extra care on a new, neater board; and I'll come  up with a more durable insulation than bubble wrap. Does anyone out there know of anything that might work for this, like the OPPOSITE of stick-on shielding, or NON-conductive tape?[/i] Or am I basically looking for some kind of plastic-based insulation to line the enclosure with?

[PRR]

> I know I haven't posted a schematic;
> is it conceivable that there's just something about this circuit that makes it a Q3 killer?

Is this rhetorical? There are an infinite number of ways to assemble a few parts. You think we can figure out what you did (to kill) without figuring out what you did (to make it unique)?

However I propose a common Fuzz. Stock (Positive ground), if the case touches Q1 C, the 33K runs a bit warm. But built reverse-polarity Negative ground, if the case touches Q1 C, Q1 conducts all to hell with no limit, and dies.


--Actually there is a limit, and for the values I show it is just-barely higher than the rated dissipation of an older Ge transistor.

[anotherjim]

However the case shorted something out, I'd suggest doing a little more to prevent it happening anyway.
Having the board floating isn't a particularly "professional" approach, but it can be the most practical for prototypes & one-off builds. However, I don't like the idea of tape or bubble wrap - too thin - wire ends can cut through it.

You should trim the wire ends down, but not so far that you remove any of the solder joint as this weakens it.
Obtain plastic sheet. This can be cut with scissors from the flat areas of vacuum packaging if you want to be frugal -  in fact this is often the ideal type of plastic - tough and transparent. It's some kind of styrene probably. Obviously, don't solder near it.

If vero/stripboard, file the copper back from the edges to if the case touches at the edge, it don't touch copper.

Fit a piece over the back of the pots. You can cut oversize and fold the edges so it locates well without sliding about. The pot wires should loop around the plastic and back into the board. Fit the board. I prefer component side down facing the pots. Cut another piece of plastic so it covers the top side of the board.

The lid can have a sheet of plastic stuck on the inner side, but be very careful that the sealing lip around the lid can't short on anything.

[whoisalhedges]

> I know I haven't posted a schematic;
> is it conceivable that there's just something about this circuit that makes it a Q3 killer?

Is this rhetorical? There are an infinite number of ways to assemble a few parts. You think we can figure out what you did (to kill) without figuring out what you did (to make it unique)?

Actually, you gave me exactly the information I needed, thanks - that yes, in a common 9v circuit, there absolutely is a way, and it's not too difficult, to royally jam up the transistor.

The xistor case is tied to base, so I'm guessing (since the first go-round, there was no insulation on the back panel) that's my issue here. NPN, common emitter, somehow getting what would be the E-C current going B-C made it all go kablooey - I guess?