ft tester

[DrStrangelove]

I'm rather tempted to build an ft tester like this one:

http://www.nd6t.com/test/Cutoff%20Tester.htm

I've found lots of bits of junk to build it out of down the shed and in the office cum junk room.

Then I could test those 1000s of unmarked untested transistors I bought back in the 1970s that reside unregarded in a selection of pipe tobacco tins in said office cum junk room.

Then again, it would probably reside in said office cum junk room, as unregarded as are said transistors.

[ppppenguin]

One genuine use for this is if you need old style, low fT, 2N3055 for a Quad 303.

[DrStrangelove]

Gosh. And after a mere 2 months: progress: found the bits.

Washed the dirt off the proposed case.

That's it.

[Craig]

This is essentially a dip meter. Except the oscillator transistor is the device under test.

[DrStrangelove]

Yes, I thought it a rather nice repurposing of an existing instrument.

YMMV.

[DrStrangelove]

Look out! Today I wound a coil, and found a rough value for it, not using the TF2700 (which seemed reluctant to do anything sensible) but by using a sig gen and a scope and the variable cap I found (not the one I intend to use of course, that would be much too easy).

Wonders shall never cease.

I've had to sit down before the shock overwhelms me.

At this rate the ft meter should be finished as soon as 2030.

That's the year 2030, not half past eight this evening.

[DrStrangelove]

Gosh. The TF2700 does work but you need to know how to use it., and the pots could do with cleaning

There was one in work: I wonder how the Esteemed Customers got on with it back in the day when it was used on some course or other.

Certainly not as idiot proof as the digital LCR meter varieties we used subsequently.

[Craig]

I regularly use my much older TF1313A, but even that is pretty useless with small value inductors. But like the TF2700 it uses audio frequencies, which are not a small inductor's friend.

I have a selection of various bridges, some RF jobs, which might make a better fist of measuring small value inductors.

[DrStrangelove]

The breadboard version has proved to work: 2N404 stopped working at about 4.5MHz.

OC44 and assorted others wouldn't work at all.

Getting there slowly.

[Mike Watterson]

The gain has to exceed the losses at the tuned frequency, so unless you add a buffer amp that slightly exceeds the losses, you can't actually measure "ft" at all.
Some transistors simply won't have enough gain.