Tektronix AM503: Q230 Dual JFET Replacement

Some suggested 151-1032-00 replacements obviously won’t work, such as Tekwiki’s 2N5397 single JFET. Bonding a pair into a single heatsink might suffice, but two separate cans generally aren’t identical enough for the purpose.

Curiously, Tekwiki also lists the 2N5911 as a 151-1032-00 replacement, which (being an actual dual JFET) looks more promising. This agrees with another cross-reference, although the “Sim[ilar] to” suggests considerable caution.

The 2N5911 pinout, as taken from its datasheet:

2N5911 Dual JFET pinout
2N5911 Dual JFET pinout

The actual Tek 151-1032-00 can in its heatsink, oriented with the tab at the top (just visible to the right of the heatsink fin):

Tek 151-1032-00 - top view
Tek 151-1032-00 – top view

Testing one side (with the tab on the left):

Tek 151-1032-00 test side A
Tek 151-1032-00 test side A

And the other side (tab still on the left):

Tek 151-1032-00 test side B
Tek 151-1032-00 test side B

A picture being worth a kiloword:

Tek 151-1032-00 - measured pinout
Tek 151-1032-00 – measured pinout

The drain and source over on the left side seem to be swapped compared to the 2N5911, although both gates are on the proper pins. This being a JFET, the source and drain may be electrically identical and it’s possible the tester labelled them backwards. The only way to be sure Tek wasn’t tragically clever is to poke around the PCB to figure out which pins connect to which other components.

So take a picture of the component neighborhood around the Q230 sockets:

PXL_20220105_210538214
PXL_20220105_210538214

Overlay it with a similar picture of the solder side, suitably reversed / recolored / transformed to match:

Tek AM503 - 151-1032-00 area - X-ray traces
Tek AM503 – 151-1032-00 area – X-ray traces

The copper-side traces aren’t complete, as the red coloring marks only traces under the soldermask and omits bare solder-coated traces. Some traces on the component side run invisibly under parts. If I were doing it for money, not love, I’d pay more attention to the details.

Devote some time to tracing the traces and labeling the parts:

Tek AM503 - 151-1032-00 area - part IDs
Tek AM503 – 151-1032-00 area – part IDs

Then doodle out the actual connections:

Tek 151-1032-00 - part connections
Tek 151-1032-00 – part connections

R246 shows Q230B lives in the left side of the can, because it’s connected between the B gate and B source pins, and confirms the tester swapped the B source and B drain pins. Whew!

R236 connects the B drain and the A source, confirming the pinout matches the 2N5911.

Comfortingly, the A side gate goes to all those other parts as it should.

So a 2N5911 will drop right into the Q230 socket with the proper pins going to the proper places. Whether it’s electrically Close Enough™ to the Tek spec, whatever it might have been, remains to be seen, but a good transistor circuit won’t depend too much on the actual transistor parameters.

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