Q90 is the one that would fry if the antenna was hooked up and a nearby QRO station was operating.
Or a nearby electrical storm was operating.
Q13 of the Bitx40 often blow in this way.
The back-to-back diode fix of the Bitx40 is not such a good idea on the uBitx because
the uBitx is wideband up to the 45mhz filter, and the diodes can distort all that incoming RF.
We don't hear much about Q90 blowing on the uBitx,
likely because the diodes in the first mixer conduct before there's real trouble.
Here's an old thread on this: https://groups.io/g/BITX20/topic/7715656
On Tue, Jul 17, 2018 at 12:50 PM, Peter Parker wrote:
This fix may be helpful to someone.
* Symptom: Low then almost no transmit power. Power meter might move up to about 1w for a short time then drop down to very low. Tx signal audible in local receiver but weaker than it should be. PA transistor bias settings seem to be OK, indicating the
IRF510s are OK.
* Probing around with an RF probe indicated the burst from higher to low power was noticed on the PA transistor drain and back through the driver stage.
* DC voltage check found all were OK except the emitter of Q90. Instead of 3 volts there was only 0.9v.
* Q90 was removed and replaced with leaded transistor (I used a BC548 which is similar to a 2N3904). Used adjacent parts to solder leads to for better spacing.
Q90 turned out to be the problem. With the new transistor the emitter voltage reads normal and the rig now transmits.
73, Peter VK3YE