I have a couple vintage amps that use 2sk270A and 2sj90A JFETs in the input sections. The JFETs are fine, but I may need to do some de-soldering/soldering of components connected to them.
I am seeking opinions on how vulnerable are these 40 year old JFETs to heat, say in comparison to BJT transistors?
Thanks in advance...
I am seeking opinions on how vulnerable are these 40 year old JFETs to heat, say in comparison to BJT transistors?
Thanks in advance...
Watch for static electricity, especially on the gate, that will kill them. Wear a grounded wrist strap,
and use a static safe work surface. Hopefully your soldering iron tip is also grounded.
and use a static safe work surface. Hopefully your soldering iron tip is also grounded.
Your advice is good; but it is MOSFETs that die easy, J-FETs take a lot of abuse and survive. At least as much as a BJT. Although large or repeated shocks may raise the leakage (typically moot in audio) or the hiss level(!!).
If you can put this off until summer, you'll be better off. Indoor % humidity here in a Maine winter runs 26%RH or dryer, and huge static if we don't use spray. Indiana in the summer is "only" 80% (NJ does 98F/98%) and there's no static at all.
If you can put this off until summer, you'll be better off. Indoor % humidity here in a Maine winter runs 26%RH or dryer, and huge static if we don't use spray. Indiana in the summer is "only" 80% (NJ does 98F/98%) and there's no static at all.
Static charge can easily go to hundreds or even thousands of volts, so I'd be cautious with any
semiconductor. When the humidity is low in the winter, it's just terrible.
semiconductor. When the humidity is low in the winter, it's just terrible.
Thanks to rayma and PRR 🙂 Yes, good advice about static electricity. And I do intend to wait until summer to do the work. I get static here in winter, so I work on speakers during winter, amps during summer.
But I am planning now, and ordering parts. Trying to decide now, based on soldering iron heat alone and not static electric charge, how much work I will do in close proximity to those precious JFETS when the time comes.
So would it be correct to say that vulnerability of JFETs to heat is similar to, or less than, BJTs? (I phrase it this way because I have worked around BJTs a lot with no issues, but not much experience with JFET devices, so I am looking for a comparison)
But I am planning now, and ordering parts. Trying to decide now, based on soldering iron heat alone and not static electric charge, how much work I will do in close proximity to those precious JFETS when the time comes.
So would it be correct to say that vulnerability of JFETs to heat is similar to, or less than, BJTs? (I phrase it this way because I have worked around BJTs a lot with no issues, but not much experience with JFET devices, so I am looking for a comparison)
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There is nothing in the physics of silicon jfets vs bjts that is more or less temperature sensitive
IME, JFETs are more rugged as input devices. Self-clamping when driven too hard in one direction. And can take way more than bipolars in reverse.
Damage due to soldering iron heat was definitely a thing for old germanium devices. Silicon devices are much more robust. Its normally the package softening that does any damage, so solder technique is important - iron at the right temperature, quick and efficient action.
An iron that's too cold is more likely to cause damage ironically, due to the longer soldering times involved. If a solder joint takes more than 2 or 3 seconds its time to practice on some scrap electronics.
For static earthed soldering iron and bench should be standard practice, don't wear synthetic, cotton and wool are conductive enough to prevent static build up. And never handle sensitive components by the pins or remove from their anti-static package until used.
An iron that's too cold is more likely to cause damage ironically, due to the longer soldering times involved. If a solder joint takes more than 2 or 3 seconds its time to practice on some scrap electronics.
For static earthed soldering iron and bench should be standard practice, don't wear synthetic, cotton and wool are conductive enough to prevent static build up. And never handle sensitive components by the pins or remove from their anti-static package until used.
Take a piece of toilet paper or paper kitchen towel , drench it with rubbing alcohol or IPA on check the resistance of the mixture , wrap it arround the Fet's (it will usually stick to it) . It will be ESD protection and heat protection , keeping the component , here JFets , cool while soldering and its pens shorted by arround 10 to 100K , depending on your alcohol .
In the past leaded mosfets were supplied with a shorting ring to be removed after soldering and before applying power
You can do the same for removal of any semi, to deal with static and absorb a little heat in the process
You can do the same for removal of any semi, to deal with static and absorb a little heat in the process
I usually solder a bridge across the leads on the solder side and pull the device once the solder reflows in all leads. It is a quick process. Deal with cleaning the holes,pads, plated barrels after the device is removed
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BJTs and JFETs are very similar in their physical structures. A BJT is two PN junctions whereas the JFET is only one. So they're similar in terms of ESD susceptibility and heat susceptibility - at least assuming similar die sizes.
Tom
Tom
They make some small clamp-like aluminum devices that are heatsinks that you can clamp onto the leads of a component to stop the heat from traveling up the lead into the device. They look like a small scissors with a spring to close the two halves, and they do work pretty well. They're also useful for soldering sensitive film capacitors, or assembling PCBs with other sensitive components, so it's worth having a handful around the bench for random uses. Clamp them away from the solder joint, so you allow the joint to heat up.
I'm not sure where I bought mine, but they look like these, and you can probably find them in a lot of places: https://www.robotshop.com/en/soldering-heat-sink.html
I'm not sure where I bought mine, but they look like these, and you can probably find them in a lot of places: https://www.robotshop.com/en/soldering-heat-sink.html
There is also smaller surgical hemostats that can be used to clip on leads. I have some but do not use them as I find I can safely removed devices using other methods
I don’t think I’ve ever heat damaged a 3 legged device de-soldering it, unless there was some complicating factor in the disassembly like a tab soldered to a heat sink. As long as the part comes out (or goes in) relatively quickly, you dont transfer enough heat from the iron to ruin in. Time seems to be the enemy - and pulling heat out with some clamp on device make the process take longer. Damaged my share of boards taking too long desoldering, especially old fiberglass ones in 1970’s amps. Traces will peel off just looking at it the wrong way.