Help me please!

[Anonymous]

Can anyone tell me what type of transistor can I use instead of JFET Chopper transistor (J113) from the GOLDSCAN IV circuit.
J113 can't be found in Bulgaria and I meed to replace it with anoder transistor.
Thanks!

 

[Anonymous]

I will post this for your benefit, Reg would have to go into detail, he is working, thought it might help you.
One might try a TIS75. It should work I would think, but the pinout is different. As for a different FET, one would just have to look for a JFET that has a very low turn on voltage. Most take a voltage difference of maybe -9V or so and will turn on. The J113 will turn on with as low as maybe -1V and a max of about -3V. This allows it to be used with power supplies of + and - 5V. The TIS 75 has a max of -4V which is close.

 

[Anonymous]

Mouser electronics in US has J113 fets for 23 cents
each.My CS Eats them like pop-corn.You might wont to
get a match set. www.mouser.com

 

[Anonymous]

For over 30 years, I have used E113's, J113's, J112's and NPD5566's (matched dual J113). Must run into many thousands of devices. I can recall only a couple of failures. No reports of failures from other manufacturers, who I have done designs for, including C.Scope. Can't understand how your CS eats them like popcorn <img src="/metal/html/confused.gif" border=0 width=15 height=22 alt=":?">
Eric.

 

[Anonymous]

Hi Eric,
Answer to your question.I dont know.I installed
sockets at two main fets.for easy change.normal in
volt at fet should be .85 to 1.25v. out volt about
1.80v on my meter.when it gets over 4volts.I have been replaceing 2 main fets.TO get TP-3 below 4 volts
I increased R from 2k to 9k on in volt to fets,this works.I am still experminting with it.Hope I did-ent
hurt your feelings.Did-ent mean to.
Fet E113,J112, NPD5566,Witch one has lowest volt
input and highets volt out put.Like to try them.
Stoiov, in Bulgaria J-fets might not be bad.If you
can tell him what the right voltage at TP test point
for his detector.
its easy to tell.
Frank.

 

[Anonymous]

Hi Don,
I probably should have been more clear when suggesting the TIS 75. This is an analog switch that most likely would meet the criteria if one were trying to build something similar to the GS or if all JFets were to be replaced. The TIS 75 has been used as an analog switch in other metal detectors operating at the + and -5V level.
It is best to try to match what is already in place if only one jfet is to be replaced because of a failure. There is sufficient ON resistance difference between jfets so many analog switches may work but create some other problems.
One way to try to find a suitable subsitute is to go to a website such as the Fairchild website and search through the jfets for another with identical characteristics as the J113. For example, the PN 4393 appears to be quite close at first glance. I didn't carefully compare the two so there may still be some minor problems if one is substituted for the other.
Reg

 

[Anonymous]

Hi Frank,
No hurt feelings, but,you will never get your CS6 working properly, without using a scope to monitor the waveforms. Using just a dc meter on the test points is of little value. True, the voltage on TP3 is a dc level, but unless the waveform on TP2 is monitored with a scope, it is difficult to get the right setting with the bias preset on the front end amplifier. Also, you need a scope to check that the sample and delay timings are right. No way can you do this with a meter. If the input signal, bias level, sample timing etc. is wrong, then the J113's, or any other fet, can't do their job properly.
Eric.

 

[Anonymous]

Eric,
You are right about the scope.I am 65,when you pass 50 time goes by fast.If I get a scope,I would use up all my free time useing it,and never get out of the house.That's why I use the Libary for my E-mail.It gets me out of the house.
I figure that if I put all new parts in, same as schematic and move R up or down it get me close.Thats how I stoped it from pulseing.In my detector the area you mentioned (J-FET)is main cause of pulseing.
Threshold has me stumped.I never ben able to get a threshold,it has threshold gain.when I increase gain trying to get a low sound.it gives off pecking noise.
When I solve threshold.it should increase dept 3 or 4"
Frank.

 

[Anonymous]

clever <img src="/metal/html/grin.gif" border=0 width=15 height=15 alt=":grin">