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Author Topic: Kenwood TS-820 FET  (Read 3977 times)

Posts: 5030

« on: May 04, 2012, 03:11:21 PM »

I am trying to help an 85 year old ham with his TS-820 that seems to have lost receive. He seems to think it is a FET on the RF PCB. The only 2 FETs ate a 3SK35 and 3SK41. Of course, they are obsolete, and the only reference I could find was an NTE222. Can someone tell me why the NTE222 is the same cross for both FETs. I have not checked the specs yet, but figured if Kenwood wanted to use the same FET, they would have done so back then.

Posts: 2083

« Reply #1 on: May 04, 2012, 03:42:20 PM »

dual-gate MOSFETS of approximately the same voltages and capabilities is why.  most silicon MOSFETS are no longer made... in RCA/Intersil/Harris' case, it was obsolescence of the 2-inch wafer process that doomed them, and not enough business case to make new masks.

there are a number of GaAS MOSFETS about, and some fiddling of bias resistor values could make them work.

a signal injection probe... or sig gen cranked way down with a nice isolation cap, maybe even a harmonic-making diode on the output... can be walked back from the IFs to confirm the suspicion.  but while you can, swallow hard, hit Dan's Small Parts or someplace and get a couple spares.

learned way to much about this in the 7-odd years I've been barely poking at a CX7a project radio, which is screaming' full of silicon dual-gates.
« Last Edit: May 04, 2012, 03:45:16 PM by KD0REQ » Logged

Posts: 17476

« Reply #2 on: May 04, 2012, 05:08:11 PM »

In some cases the same basic part is graded into different performance bins when tested,
and the prime parts are sold for a higher price.  Often one is has lower gain or doesn't work
as well at higher frequencies, etc.  The lessor part may be slightly cheaper, so it makes sense
to use it in a circuit where the full capabilities aren't needed.  But both could be replaced
with the better part without any reduction in performance.

There are a lot of parts that have only minor differences.  For example the common 2N3904
bipolar transistor is good though the HF range, and can be used to replace many other parts
in audio stages, or where the original had lower voltage or current ratings.  A properly-designed
circuit can use many different transistor types and still work well.  Some parameters are
important to match - the replacement has to be able to handle at least the maximum current
and voltage, for example, but in most cases it doesn't matter how much more.

Looking through the RCA MOSFET manual, it is surprising how similar the parameters are for
all their different offerings.  Some are qualified for higher frequencies, some are tested for
performance as a mixer rather than an amplifier, some might have higher reliability standards
than others, but for basic functionality most appear to be pretty much interchangeable.
Any of the common types, such as 3N200, 3N211, 40673, 40822, etc. should work well enough
in your application.  (All are rated for operation up to 100 MHz or more.)  If you look them
up they probably all cross to the NTE 222 as well.  Dan's Small Parts is a good source.

So don't worry about an exact match - use what you can find and chances are pretty good
that it will still work fine.

Posts: 3160

« Reply #3 on: May 04, 2012, 06:12:34 PM »

A Hong Kong supplier resells on eBay and had these available recently.
There are supplies (NOS), Yaesu also used these parts in their radios of this period.
NTE 222 is the substitute

Same questions on Kenwood Hybrids -- Yahoo Group
« Last Edit: May 04, 2012, 06:17:20 PM by W9GB » Logged
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