Signal One CX-7 bias transistor

[Phil]

Phil Patton via groups.io <philwpatton=gmail.com@groups.io>

3:22 PM (2 minutes ago)

to signal-one
I've been fighting with a strange bias issue in my CX-7 and finally found the solution so I thought I would post it to perhaps save others pulling their hair out with the same problem.
My rig had what appears to be a common problem with them - namely Q5 working ok for some period of time and then shorting CE and dropping the bias to about -3 volts.
I had replaced every resistor in the circuit and replaced the transistor with a 2N3440. It "sort of" worked in that the bias would switch and drop to a lower value in transmit and return to cut-off in receive, however the collector voltage, which is supposed to be adjusted to zero volts in transmit with no drive, stayed at 11-12 volts no matter what. I had gone over and over the circuit and couldn't find anything wrong. Well, I had ordered another transistor that also appeared to be a reasonable replacement for the 2N5184, a MPSA42, so in a desperate move I chopped out the 2N3440 and tacked on the MPSA42. Success! With the MPSA42 the circuit works exactly as it should. Transmit bias can be adjusted to just about anything you want in transmit, it returns to -60 in receive and the collector easily adjusted to zero volts. So the lesson learned is the MPSA42 works in this circuit and others which are very similar in hfe do not.

I hope this information will prove useful to other CX-7 owners. Most of the information I could find online about this was years old and the parts they suggested as replacements are now no longer available.

73,
K1FB

P.S.- I AM going to pull the driver board out again and replace the transistor properly!
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