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.

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Old February 17th 17, 12:05 AM posted to sci.electronics.repair
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Dave Platt wrote:



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

Analyzing the bias supply for the base, I can't really SEE how it could get
a reverse voltage spike. I certainly thought about this on power-up.
OHHH, wait! Maybe it could be on power DOWN! There is a bias network on
the base with a capacitor to ground, and the supply to the emitters is
connected to a reference supply that might dump VERY quickly when power is
cut off. That could maybe put about -6 V bias on the base when the emitter
supply drains back toward zero. I could probably stick a diode there to
limit the reverse bias.

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?

Well, maybe. But, it is only 23 mA, and differential pairs are usually
pretty benign when they transition. We just got this module out of a pile
of stuff, it is the only one we have, and we have no history on it. It
might have sat in some setup for 30 years powered on. Or, it might have
experienced some kind of transient failure of the power supply that powered
it in the past. This one-shot can't go over 100 ns, and the transition
should be REALLY sharp, just a couple of ns.

So, the only way to know if it will be reliable is to test it a bit.

Thanks,

Jon

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Old February 17th 17, 12:47 AM posted to sci.electronics.repair
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Default fewer through-hole transistors available

An A430 crosses to an NTE 161 which is a UHF-VHF amp. Since the FMT1190 is an RF-IF AMP, this should work as well. I know that these are available (NTE160).

While I am not a fan of NTE devices, they will work in a pinch. Give this a try.

If you can't find one, let me know. I have some in stock.

Dan
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Old February 17th 17, 01:30 AM posted to sci.electronics.repair
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Default fewer through-hole transistors available

In article ,
Jon Elson wrote:

Analyzing the bias supply for the base, I can't really SEE how it could get
a reverse voltage spike. I certainly thought about this on power-up.
OHHH, wait! Maybe it could be on power DOWN! There is a bias network on
the base with a capacitor to ground, and the supply to the emitters is
connected to a reference supply that might dump VERY quickly when power is
cut off. That could maybe put about -6 V bias on the base when the emitter
supply drains back toward zero. I could probably stick a diode there to
limit the reverse bias.


Probably a good idea, and (assuming that the diode's capacitance when
biased-off isn't a problem) it should be good cheap protection.

Thinking about it... I wonder whether those poor old transistors might
not just have been Plumb Tuckered Out? A friend of mine has told
stories about the early generations of Fairchild transistors - the
mushroom-shaped ones with a plastic case - and said that a lot of them
he'd dealt with had failed over the years as a result of problems with
the encapsulations. Apparently the plastic didn't seal to the leads
very well, allowed oxygen to infiltrate, and the internal wire-bonds
or metallization would oxidize and fail over time.

Possibly this is something which might have affected those old parts
of yours, with the (modest) amount of heat in the "on at idle"
transistors having the effect of accelerating the aging process.


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Old February 17th 17, 03:29 PM posted to sci.electronics.repair
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On 02/16/2017 02:57 PM, 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. 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


A whole lot of TO92s went away in the last couple of years. I bought
several thousand as a lifetime prototyping supply.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net
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Old February 17th 17, 03:38 PM posted to sci.electronics.repair
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On 02/16/2017 12:46 PM, Clifford Heath wrote:
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.


SMT prototyping is about a factor of 10 slower than dead bug using
through-hole parts. Every DIP package gets you a bunch of nice strong
standoffs.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net
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Old February 17th 17, 05:29 PM posted to sci.electronics.repair
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Dave Platt wrote:


Thinking about it... I wonder whether those poor old transistors might
not just have been Plumb Tuckered Out?

Yes, that is another possibility I considered. If the module had been in
use for a long time, with no cooling, these are the highest power
dissipation of all the signal transistors in the unit. Always on at 23 mA.

I repaired an HP synthesizer that had about 400 Germanium transistors in it,
and about a dozen had failed. I replaced them with Silicon transistors and
then sold it on eBay before any more units went out.

Jon
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Old February 17th 17, 07:28 PM posted to sci.electronics.repair
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On 02/17/2017 02:31 PM, Michael Black wrote:
On Fri, 17 Feb 2017, Jon Elson wrote:

Dave Platt wrote:


Thinking about it... I wonder whether those poor old transistors might
not just have been Plumb Tuckered Out?

Yes, that is another possibility I considered. If the module had been in
use for a long time, with no cooling, these are the highest power
dissipation of all the signal transistors in the unit. Always on at
23 mA.

I repaired an HP synthesizer that had about 400 Germanium transistors
in it,
and about a dozen had failed. I replaced them with Silicon
transistors and
then sold it on eBay before any more units went out.

I thought germanium transistors in general had a tendency to go bad over
time. They'd start leaking. Maybe manufacturing improved over time,
but then at some point, silicon took over completely, except for a
handful of uses. So you can't easily get replacements, because few
germanium devices are made now.

Michael

Point contact devices especially.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net
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Old February 17th 17, 07:31 PM posted to sci.electronics.repair
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Default fewer through-hole transistors available

On Fri, 17 Feb 2017, Jon Elson wrote:

Dave Platt wrote:


Thinking about it... I wonder whether those poor old transistors might
not just have been Plumb Tuckered Out?

Yes, that is another possibility I considered. If the module had been in
use for a long time, with no cooling, these are the highest power
dissipation of all the signal transistors in the unit. Always on at 23 mA.

I repaired an HP synthesizer that had about 400 Germanium transistors in it,
and about a dozen had failed. I replaced them with Silicon transistors and
then sold it on eBay before any more units went out.

I thought germanium transistors in general had a tendency to go bad over
time. They'd start leaking. Maybe manufacturing improved over time, but
then at some point, silicon took over completely, except for a handful of
uses. So you can't easily get replacements, because few germanium devices
are made now.

Michael

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