Lost a whole afternoon due to fake/faulty/mislabelled transistors

[Mick Bailey]

I replaced a transistor with a base-emitter short as well as some associated components that are part of a protection circuit. It still went into protect. No DC on the output, no short, no thermal issues. The problem was that the 2n5551 transistor I'd replaced appeared to be conducting, but had no base voltage. I removed it and the pads read collector 68v, emitter 0v, base 0v. So I thought it must be shorted. It read good out of circuit, but I replaced it anyhow. So the same problem persisted. Can't be the transistor, I thought. Vce is stated as 160v, so 68v won't be a problem. These are transistors from a reputable source and bought recently. So then I'm thinking "have I lost the plot?" and traced everything I'd done repeatedly to try to find what could be wrong. It still came back to the transistor, so I randomly picked another from the bandolier and still the same thing. The 68v pulled down to 0.6v and switched the amp into protect.

So a last ditch was to sub in an MPSA42 and the amp fired up fine. Checked the 2N5551s and they all break down with a Vce of 40v. So either fake, faulty or mis-labelled.

So I'm pretty fuming right now.

[TomCarlos]

I would love to have a cheapo curve tracer - one that could test for Vce. But I just don't have the room and budget for an old Tektronix!

[nevetslab]

I would love to have a cheapo curve tracer - one that could test for Vce. But I just don't have the room and budget for an old Tektronix!

I too never allocated funds for a Tek 576 or better Curve Tracer. I had bought a B & K Precision 501A, one of the only two B & K Precision instruments I ever bought, and while it's limited to 100V in collector/drain sweep voltage, it has served me well since the mid-70's. I did buy a Hameg 8042 that was loaded into their 2-slot 8001 mainframe. Both instruments need an XY scope, though the Hameg can run without one, it having a display to yield info on the DUT. The Hameg, though, can only sweep out to 40V. Neither will get you out to where the DUT begins breaking down, but, adequate for most uses. The 'soft wheel' on the Hameg used in dialing in parameters during set-up, tends to be eratic...something I've never delved into to try and cure. Neither instrument cost me much. I bought the B & K 501a new, might have been $200 or thereabouts. Paid less for the Hameg, an ebay purchase.

I'd think by now, there should be current technology low-$ curve tracers, though I haven't been looking.

[nevetslab]

I replaced a transistor with a base-emitter short as well as some associated components that are part of a protection circuit. It still went into protect. No DC on the output, no short, no thermal issues. The problem was that the 2n5551 transistor I'd replaced appeared to be conducting, but had no base voltage. I removed it and the pads read collector 68v, emitter 0v, base 0v. So I thought it must be shorted. It read good out of circuit, but I replaced it anyhow. So the same problem persisted. Can't be the transistor, I thought. Vce is stated as 160v, so 68v won't be a problem. These are transistors from a reputable source and bought recently. So then I'm thinking "have I lost the plot?" and traced everything I'd done repeatedly to try to find what could be wrong. It still came back to the transistor, so I randomly picked another from the bandolier and still the same thing. The 68v pulled down to 0.6v and switched the amp into protect.

So a last ditch was to sub in an MPSA42 and the amp fired up fine. Checked the 2N5551s and they all break down with a Vce of 40v. So either fake, faulty or mis-labelled.

So I'm pretty fuming right now.

I hear ya on that! I gather you didn't buy those parts out of normal distribution. 40V sure is way shy of their rated 160V Vceo!

[nickb]

This is worrying. I expect it if I buy off eBay but hold big distributors as reliable. Was this one of the biggest i.e. RS, Farnell, Rapid? I don't expect you'll ever get to the bottom of it but if you do please let us know.

As an aside I wonder if a forum for "Useful Projects" might be of interest. I've lost count of the number of times a x100 HV scope probe has been posted. Other designs from amps to pedals, probes to curve tracers could all live there. I bet there is a plethora of cunning circuits for testing that would right at home there too.

[Mick Bailey]

I hear ya on that! I gather you didn't buy those parts out of normal distribution. 40V sure is way shy of their rated 160V Vceo!

I got them from a long-standing supplier of components here in the UK. They mainly specialise in out-of production tubes and semiconductors and I've been buying from them for a long time and never had a single problem before. My main supplier (RS components) now has a minimum quantity of 200 on this part and I doubt if I would use these up before retirement.

The problem with second-tier suppliers is there's no traceability.

[nevetslab]

As an aside I wonder if a forum for "Useful Projects" might be of interest. I've lost count of the number of times a x100 HV scope probe has been posted. Other designs from amps to pedals, probes to curve tracers could all live there. I bet there is a plethora of cunning circuits for testing that would right at home there too.

I've always thought a separate MEF Forum category such as this would be useful. It would certainly make it faster to search for something you came across in one of the maintenance or design forums related to such listings.

[J M Fahey]

You sure you have the correct pinout?

I use a lot of 2N5401/5551 , they are in my "universal replacement" spares box, but I found that, depending on manufacturer, they use 2 different pinouts.


"Normal" on most transistors is EBC left to right, legs towards you, flat side down but standard 2N54x1 is EBC flat side UP.

That said, a few are made EBC flat side down, so it always pays to check pinout, you might have gotten these instead.

Since most transistors are "reversible", sort of, but with much poorer specs, and a diode check will NOT show that (both BE and BC *are* diodes) best test but slightly complex is Vce so second best is to test Hfe on a multimeter Hfe tester socket.

Proper oriented one will show Hfe around 100-150 while reversed ones will "still" work , sort of, but show POOR Hfe, from 10 to 30-40 which is too low for them.

Just for curiosity sake, test those you suspect , both ways.

[drewl]

I always check pinout to see if they're good.

[Mick Bailey]

You know something - I did check the transistors beforehand on my component analyzer - Hfe (158) and leakage. Just rechecked and what I didn't spot originally was the Japanese pin orientation - these are ECB. So they're a 2N5551-C (centre collector) but marked without the C suffix.

[J M Fahey]

I told you these transistors in particular come with varying pinouts, so that can easily make you waste a lot of time.
Donīt ask how I found out

I found two versions: EBC and CBE ... now you found ECB (or BCE) .... sweet

[nevetslab]

I told you these transistors in particular come with varying pinouts, so that can easily make you waste a lot of time.
Donīt ask how I found out

I found two versions: EBC and CBE ... now you found ECB (or BCE) .... sweet

Ah, ain't that grand to hear a common 2N5551 has 3 flavors, without being a different prefix. I've never encountered any of the non-EBC pinout versions with the 2N prefix. We'll see how long that tidbit remains in my head for a frightful day later.

[TomCarlos]

Updating this thread to note that NCC 5551 is a different pinout compared to the 2N5551 - something I recently learned.