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Old 2nd Apr 2011, 6:47 pm   #1
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Default Troubles with a tek 7A13

I may as well just come right out and admit it, I have managed to zap the + input of my beloved 7A13 plugin. I don't know how it happened, I suspect static, but the deed is done. Now the trace is right off the top of the screen.

I have removed the first transistor (Q120) from the input amp. and the trace reappears and is controllable. I tested the transistor with a multimeter and get strange readings, so I am hoping this is the only culprit. Now the real troubles begin. In the parts list, most transistors have an equivalent listed. This one just says FET, Tek spec. Part no. is 153-0559-00. I know this is a long shot, but does anyone out there have any comments or ideas as to where I might get one from? I have tried Telford Electronics and they have nothing available.

Any advice greatly appreciated.

Mark.
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Old 2nd Apr 2011, 6:55 pm   #2
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Default Re: Troubles with a tek 7A13

After having trouble with my scope CRT I've come across all sorts of sites for TEK goodies, this place is sold out at present, but you never know it might be worth contacting them, they might be able to source or know someone who can.

153-0559-00 = TO-18 (4 lead) SFE844M2 sets, factory matched and bagged in anti-static bags.


http://www.sphere.bc.ca/test/tek-parts/tekparts6.html

You could join the TEK forum and seek help too.

http://www.tek.com/forum/viewforum.php?f=5

HTH
Good luck

Chris

Last edited by oldticktock; 2nd Apr 2011 at 7:01 pm.
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Old 2nd Apr 2011, 10:09 pm   #3
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Default Re: Troubles with a tek 7A13

Mark,

I have a copy of a Tek parts catalogue, which at first glance suggests that 153-0559-00 is a "mtchd 2N4416". I'll have a closer look tomorrow to see if there's any more information to be had.

Hugh
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Old 3rd Apr 2011, 2:54 pm   #4
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Default Re: Troubles with a tek 7A13

Following up Stockden's post. 153-0559 is a pair of 2N4416, matched broadly for Idss. Really not very difficult to deal with. Unless you are determined to get original manufacturers performance over the whole temperature range of 0 to 50degC, and are working at differential inputs of 1mv.
Alternatives to 2N4416 are: the direct plastic equivalent 2N5245. Also produced by the same geometry and die process are 2N3819, 2N3823, 2N5284, BF244, BF245, BF256. Otherwise any N junction FET for RF amplifier use, or surface mount versions with a similar Idss & Vp.
But get several at the same time, and compare them. I have found it relatively easy to select a pair of FET matching well from 5 purchased at the same time from the same batch. The big variation with JFET is to get the Idss similar, but production processes have improved a lot.
I attach the Tek specification. And notes on how to measure Idss and Vp, using just a 15v supply and your trusty Avo. But your need is matched for Idss fairly widely, so just select on Idss in the higher range, and put them both in.
I believe we can make too much of a meal over replacement parts. When the unit was made, it might have been stretching the edges of 1980 technology, but that has now become common-or-garden cheap-o stuff. Bill
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Old 3rd Apr 2011, 3:50 pm   #5
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Default Re: Troubles with a tek 7A13

As Bill suggests this is not unduly difficult.

The last time I repaired one of these I used 2N3819s and the repaired unit gave an acceptable performance.

I don't recall trying many samples in the matching process.
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Old 3rd Apr 2011, 5:28 pm   #6
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Default Re: Troubles with a tek 7A13

Thank you all for your help and advice.

In the manual I have, there was no reference to a "real world" part number, only the Tek one. I had a feeling it must be a fairly normal FET, and you have confirmed that for me. I have ordered some 2N4416s and I will take it from there.

Bill, I agree. Most of us don't demand or expect our test gear to come up to original specs. I am just happy that it works at all and (usually) very reliably.

Thanks again,
Mark.
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