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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

fewer through-hole transistors available



 
 
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  #1  
Old February 16th 17, 05:46 PM posted to sci.electronics.repair
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Posts: 76
Default fewer through-hole transistors available

On 17/02/17 09:03, Dave Platt wrote:
I was repairing a piece of old equipment, and needed a replacement RF
transistor. Checking at Digi-Key, I found very thin pickings, so i checked
a few other distributors, and found the same situation. They seemed to have
a modest choice of SMT transistors, but REALLY thin variety in T0-92 and
similar plastic packages. Not a good sign, as we have a lot of 30 - 40 year
old nuclear instrumentation here.


Yup. The last five years or so have been dire times for through-hole
semiconductors. A lot of the popular parts are now gone, past the
end of the "lifetime buy" cycle from their original manufacturers.

I've been trying to stock up my own (hobbyist-level) supplies of
useful TO-92 transistors...


Why? You're not doing production if 100 is lifetime. If it's for
prototyping, why not just learn to prototype in SMT? It's not that
hard. A lot of RF things are easier because stuff is physically
smaller.
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  #2  
Old February 16th 17, 07:57 PM posted to sci.electronics.repair
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Posts: 1,073
Default fewer through-hole transistors available

I was repairing a piece of old equipment, and needed a replacement RF
transistor. Checking at Digi-Key, I found very thin pickings, so i checked
a few other distributors, and found the same situation. They seemed to have
a modest choice of SMT transistors, but REALLY thin variety in T0-92 and
similar plastic packages. Not a good sign, as we have a lot of 30 - 40 year
old nuclear instrumentation here.

Another issue is the circuit is a differential NPN pair set up as a one-
shot. So, it was running about 23 mA through one transistor at idle, and
all of these on-at-idle transistors have failed. (The other transistors in
the pairs seem fine.) The part actually in the unit is an FMT1190, which
certainly seems like it should have been able to handle that current long-
term. After replacing it with the best thing I could find, the transistor
only has about 2V C-E, so the power dissipation is less than 50 mW,
shouldn't have burned them out. I'm wondering if somehow the startup
condition exceeded the base ratings.

Anyway, I'll replace the transistors and burn it in for a while and see if
it gives any more trouble.

Jon

  #4  
Old February 16th 17, 08:35 PM posted to sci.electronics.repair
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Posts: 630
Default fewer through-hole transistors available

On Thursday, February 16, 2017 at 2:57:41 PM UTC-5, Jon Elson wrote:
I was repairing a piece of old equipment, and needed a replacement RF
transistor. Checking at Digi-Key, I found very thin pickings, so i checked
a few other distributors, and found the same situation. They seemed to have
a modest choice of SMT transistors, but REALLY thin variety in T0-92 and
similar plastic packages. N


I have found that but-for a very few specialized transistors, most of the general purpose types are now lumped into a much smaller universe of replacement types. I have never had any problems, ultimately, with finding replacements - and I keep a variety of testers and checkers to make sure they will do - but they are not necessarily to OEM spec. Usually much better.

Then, I am a hobbyist dealing in onesie-twosies. So I can afford to go up a couple of notches if necessary.

Peter Wieck
Melrose Park, PA
  #5  
Old February 16th 17, 08:35 PM posted to sci.electronics.repair
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Posts: 1,073
Default fewer through-hole transistors available

Ralph Mowery wrote:


I worked in industry and had a lot of older instruments. Some were a
real pain to work on and the factory did not support them any more. We,
as the repair shop, told management they were just going to have to bite
the bullet and update them.

Well, this is modular signal processing modules, we have a wall of probably
400 of them of all different types. The companies that made this stuff in
the 1970s are mostly out of the business, and most of the modules we have
have no currently manufactured replacement. So, we don't have a lot of
choice. Some modules have been so troublesome we have, indeed, retired
them.

Jon
  #8  
Old February 16th 17, 10:03 PM posted to sci.electronics.repair
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Posts: 84
Default fewer through-hole transistors available

I was repairing a piece of old equipment, and needed a replacement RF
transistor. Checking at Digi-Key, I found very thin pickings, so i checked
a few other distributors, and found the same situation. They seemed to have
a modest choice of SMT transistors, but REALLY thin variety in T0-92 and
similar plastic packages. Not a good sign, as we have a lot of 30 - 40 year
old nuclear instrumentation here.


Yup. The last five years or so have been dire times for through-hole
semiconductors. A lot of the popular parts are now gone, past the
end of the "lifetime buy" cycle from their original manufacturers.

I've been trying to stock up my own (hobbyist-level) supplies of
useful TO-92 transistors - a mix of general-purpose jellybeans, RF
amps, JFETs, and low-noise audio parts - whenever I can. I figure
that a bag of 100 of any particular type constitutes a "lifetime buy"
for me.

For parts that have only recently gone obsolete, you might want to
check with Rochester Electronics. They seem to have a pretty good
stock of a lot of parts that Digi-Key and Mouser have dropped.
There's a $100 line-order minimum, but it might be worth selecting a
few useful parts and investing some $$ in a lifetime stock.

Another issue is the circuit is a differential NPN pair set up as a one-
shot. So, it was running about 23 mA through one transistor at idle, and
all of these on-at-idle transistors have failed. (The other transistors in
the pairs seem fine.) The part actually in the unit is an FMT1190, which
certainly seems like it should have been able to handle that current long-
term. After replacing it with the best thing I could find, the transistor
only has about 2V C-E, so the power dissipation is less than 50 mW,
shouldn't have burned them out. I'm wondering if somehow the startup
condition exceeded the base ratings.


Maybe a short inverse spike condition during powerup, which
reverse-biases the base and makes it avalanche?

Or, is there any chance that when the one-shot fires, the transitions
are slow enough that the transistor passes outside of its SOA while
turning on or off?


  #9  
Old February 16th 17, 11:02 PM posted to sci.electronics.repair
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Posts: 84
Default fewer through-hole transistors available

In article om,
Clifford Heath wrote:

I've been trying to stock up my own (hobbyist-level) supplies of
useful TO-92 transistors...


Why? You're not doing production if 100 is lifetime. If it's for
prototyping, why not just learn to prototype in SMT? It's not that
hard. A lot of RF things are easier because stuff is physically
smaller.


I do more prototyping in SMT these days when I'm puttering with a new
design of any complexity... have even put together a toaster-oven-
based setup with a ramp-and-soak controller.

For repairing older audio equipment and RF test gear which was
designed with through-hole parts, or for doing Manhattan-style
prototypes, I still find it easier to stick with through-hole
components.



 




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