Mullard Lockfit transistor failure survey

[kalee20]

Yes, I've seen some of those DIL IC's which have obviously been sawn apart. They do look confidence-inspiring.

Somehow, I would expect modern epoxies to be higher temperature resistant than those of a few decades ago, though. Not only are there vast amounts of surface mount devices used, where the whole package has to withstand the full soldering temperature, but we have moved to lead-free solders which need higher temperatures anyway.

[GW4FRX]

I'm currently restoring an amplifier which originally used BFW60 and BFW90 Lockfit devices. Any thoughts as to the reliability of these?

[paulsherwin]

I've no experience with those types, but I would treat them with suspicion and consider swapping them out. They don't appear to be anything special and superficially seem to be similar to the 2N2222/2N2907 or BC337/BC327.

[Maarten]

I'm still wondering if there's any difference in Mullard made and continental made lockfits.

On the other hand I've never seen these BFW types so I can't really comment on their reliability. BC635/BC636 (or higher) are possible replacements.

[ben]

I've had a few fail in Dual/Bettor record players and music centres (These were as common in Spain and Germany in the 70s and 80s as Thorn/Ferguson kit was in the UK). Fortunately my Tandberg language equipment, which is full of 'em, doesn't seem overly affected yet.

[David Simpson]

Originally Posted by kalee20

Yes, I've seen some of those DIL IC's which have obviously been sawn apart. They do look confidence-inspiring.

Somehow, I would expect modern epoxies to be higher temperature resistant than those of a few decades ago, though. Not only are there vast amounts of surface mount devices used, where the whole package has to withstand the full soldering temperature, but we have moved to lead-free solders which need higher temperatures anyway.

Who says we've all moved to Lead Free Solder ? I stick with good old "60/40". 60/40 & a good wee heat sink - made from a crock clip & added copper extension jaws - is ideal for most semi-conductor soldering. Using a modern soldering station, with careful selection of temperature & appropriate sized bit is ideal for working with vintage transistors & pcb's.
Before working with valuable vintage Mullard transistors - have a practice with some gash veroboard & some short lengths of tinned copper wire. You'll soon learn how long to leave the bit in contact, and at what temperature, before the perforated copper strip starts to lift &/or the wire's pvc insulation starts to shrink excessively or carbonise.
I can vaguely remember teaching RAF apprentices, back in the early 70's, soldering techniques with 14 pin DIL IC's, using small sized soldering bits & then leading up to spade bits. They all seem to master the technique after a while.
Most modern-ish DMM's can take a temperature probe attachment. Try applying your soldering iron to the leads of an old u/s transistor, whilst monitoring the temp at the point where the wire enters the encapsulation. Jesus - you've only a second or so. So speed is of the essence.

Regards, David

[kalee20]

Originally Posted by kalee20

Somehow, I would expect modern epoxies to be higher temperature resistant than those of a few decades ago, though....we have moved to lead-free solders which need higher temperatures anyway.

I don't think anyone has said we've ALL moved, Dave, I certainly hate the stuff! But most of the industry has, and hopefully this has resulted in components able to withstand high temperature excursions better.

So those of us who stick with 60/40, we are soldering components which are less susceptible to heat damage than before, but using the same trusty techniques. Win-win.

But the point is, WHY are Lockfits dying when TO92's and TO220's are seemingly OK?

[Refugee]

Originally Posted by kalee20

But the point is, WHY are Lockfits dying when TO92's and TO220's are seemingly OK?

They have thin flat lead outs with sharp corners on them.
TO92s are round and TO220s have softer lead outs that can be formed more easily.

[paulsherwin]

The general consensus does seem to be that it's something to do with the inflexible pins. It's a bit ironic that Mullard marketed the Lockfit standard as having improved reliability when it was introduced.

[Argus25]

Originally Posted by kalee20

Yes, I've seen some of those DIL IC's which have obviously been sawn apart. They do look confidence-inspiring.

I took a photo to demonstrate 3 types of epoxy resin.

Number 1 is the awe and confidence inspiring early Texas Instrument resin package, these were typical of late 1960's TTL from TI, you can see the saw cut marks at the end. Ignoring the fact that this specimen is a 16 pin type, the whole package is thicker, even the pins are tougher, its like a T.Rex.

Number 2 is a type of grey resin Signetics used, hard to see the label, its a 1972 vintage 7410. This is an extremely hard grey resin. When it is scratched it acts like granite, but its not as hard as ceramic but its extremely heat resistant. I always wondered if it contained some type of glass filler.

Number 3 is a late manufacture 7410 of the typical black epoxy on modern IC's. (Which after attempting to drill and burn it, I don't think its as tough as the other two).

Obviously its pretty difficult to beat ceramic, which is why all the mil spec TTL's were ceramic.

I don't really know how lockfits compare or if it is some sort of resin defect that causes them to fail at times.

[G6Tanuki]

I suspect that the Lockfits *would* have been overall more-reliable on a production-line using automatic-assembly/wave soldering machines: more-reliable, that is, in the sense that fewer failed boards would come off the line than with conventional wire-legged transistors whose legs would sometimes miss a hole.

[This ease-of-auto-insertion was also one of the touted reasons for those 1960s resistors which consisted of a carbon-stick with tubular brass caps-with-sideways-sticking-out tabs pressed on the ends. I've seen plenty of these where the brass has embrittled with age, cracked and lost its grip on the carbon-rod]

[David Simpson]

This debate has released another memory from the early 70's. At that time, the RAF had been getting fed up with the failure rate of some American DIL & other epoxy encapsulated semi-conductor circuitry. Transatlantic costs were getting higher & higher. Solution - send some avionics technicians from 30MU at RAF Sealand to the nearby Liverpool Dental School. These guys became experts at using dental equipment to repair connection failures between the wee legs or pins & the chip inside. Loose legs & epoxy breakdown, enhanced by vibration, being the main problem.
To digress slightly, wallets of cheapo dental picks, scrapers, scalpels, etc.can be bought on the internet. A must for folk working a lot with electronic components. Particularly the diddy dentists mirrors.

Regards, David

[Radiocruncher]

I repaired a Roberts Rambler last week with a failed BF195C lockfit in the IF section. I replaced it with a BF495C pulled from a later Rambler. I was surprised at the cost and lack of availability of a NOS BF495C. I have got a few MPSH10’s here but didn’t think of trying one so thanks Paul for the reminder.

Regards

Graham

[paulsherwin]

The 2SC2999 is also very cheap from Chinese eBay sellers and has always worked well on the few occasions I've used it. Watch the leadout though.

[G6Tanuki]

For the typical MW/LW/FM radio IFs, a humble BC109C has enough gain at the required frequencies to work just fine. For higher-Ft requirements like FM-broadcast-radio local-oscillators or RF amps you can use the likes of a BF115.

[David Simpson]

Perhaps Colin, to establish the cause of failure, you could have a shot at delving inside one of your duff Lockfit transistors. I believe that some Forum members in the past have attempted to remove encapsulation material using a Dremel & very thin small circular blades & grinding discs.
Once or twice in the past, I've attempted to repair broken leads out of TO5's etc., by using minute hobbyist drills & a dental pick, but it was extremely fiddly and not always successful.

Regards, David

[ukcol]

Originally Posted by David Simpson

Perhaps Colin, to establish the cause of failure, you could have a shot at delving inside one of your duff Lockfit transistors.

I don't have any faulty Lockfit transistors to work with at all. Part of the reason I started this thread is precisely that. A number of forum members have been reporting a high failure rate of Lockfits in sets in their collections but I have had none. As I have already indicated, the number of sets I have that are fitted with Lockfits is so small that I can't draw any conclusions from the zero failure rate that I have experienced. Years ago when Lockfits appeared in equipment in large numbers, the failure rate that I experienced was still low in percentage terms.

I am sorry that I have been repeating myself but I was trying to get an idea of failure rate from those of you who have seen, in recent years, significant numbers of this type of transistor.

[paulsherwin]

There is usually only one bad transistor in a radio, and replacing it cures any fault, but I usually swap them all out if replacement is easy. I haven't had a failure in a Mullard module yet, which is just as well as they're awkward to replace.

I reckon about half the radios with Lockfits I've encountered had some sort of transistor problem. I don't work on large numbers of sets though, so I don't know how typical that is. Obviously some people will see a lot of Lockfit faults and others few or none.

[David Simpson]

Hey - what a bunch of Silly Billys we all are - I've just Googled "Mullard Lockfit Transistors" & discovered a heap of failure info by Mark Hennessy :-

http://www.markhennessy.co.uk/articl...ransistors.htm

Well worth a good read.

Regards, David

[ukcol]

Thanks for the link David. As you say, anything from Mark is well worth reading.