Replacement RF Transistor for ENI 3100LA Amplifier

[Weston]

The lab I work in has a ENI 3100LA amplifier which has clipping on the output stage before reaching maximum power output that I decided to try to fix.

This is one of those large linear class A PA's designed to operate into any load mismatch at full rated power. This one is rated for 100W at 250KHz - 100MHz. The inside is really modular and has a bunch of identical PA modules with a power combiner. Luckily the full service manual is online with schematics: http://manuals.repeater-builder.com/te-files/ENI/ENI%203100LA%20Operating%20&%20Service%20Instructions.pdf The only real issue with this amplifier and the other ENI boat anchor amplifiers is that all of the RF transistors are custom ENI part numbers.

After opening the PA and taking a look with the thermal camera I can see that one of the RF power transistors is damaged. Have not yet taken electrical measurements beyond checking the supply rails, but its reaching 200C+ while the other transistors idle at 60C, so I assume its close to a short or something. As the amplifiers are a push-pull pair this would explain the asymmetrical clipping at high powers.

Interestingly enough, this transistor seems to have already been replaced once by someone. I don't know the history of the amplifier, but I think it was impulse purchased off ebay a number of years ago by a lab member because these amplifiers are relatively rare and expensive new and very useful for the research we do. I am unsure if the amplifier ever worked with the replacement transistor.

The custom part number for the transistor is "ENI 16C". I can find a few posts on the forum cross referencing other ENI transistors, but not this one. I found a slightly dubious Chinese part listing which cross references this part to ASI PT9704 https://datasheetspdf.com/datasheet/PT9704.html . This transistor is also what the previous owner used to (try) to fix the amplifier, but that replacement transistor is now the blown part.  Just looking at the specs this transistor seems like it should be a good replacement, its designed for amplifiers up to 500MHz running on a ~28V supply. At this point my best hope is that something in the bias network is damaged which caused the replacement transistor to fail.

Looking at the schematic for the amplifier it seems like a replacement transistor should not be that critical (as long as I replace the other transistor its in a push pull pair with). The amplifier is class A with heavy emitter degeneration and collector feedback. Each transistor is only biased at ~0.9A with a 27V supply voltage, so power dissipation should be low, and the strong collector feedback should protect against load mismatch. There are no tuned circuits so the capacitance of the transistor should not be that critical. The heavy feedback should also reduce the dependence on the gain of the transistor.




Given that the blown transistor already seems to be a replacement transistor am I missing anything critical? Or am I right in assuming something else must have failed to kill the transistor?

I am not particularly interested in spending significant lab money on fixing this, so I was looking for low cost replacement transistors. The PT9704 seems a bit hard to get (or at the very least is not the cheapest option). Looking what is on ebay, would the BLV31 http://www.njsemi.com/datasheets/BLV31.pdf or USSR surplus KT922 https://datasheetspdf.com/datasheet/KT922.html work? They both have the correct voltage and current ratings and the similar maximum frequency and capacitances.

[Westrick]

Are you still having issues with the 3100L. I have a sick 3200L and am looking for replacement transistors. A colleague of mine at Fermi National Labs sent me a note from a repair facility that they use. They sub the ENI-16 with a GT4317 with some minor circuit changes. They did caution about using the PT9704.

If you haven't done anything with the 3100, I may be interested in acquiring it for a parts donor.