QCX+ with Blown Finals - Repair

Bob, KB0**[1], called me in late July (2021) letting me know that he thought his QCX+ was dead as he could not transmit or receive with it and could I take a look at it. I was surprised as he only had it back several weeks since getting it finished and returned (refer to the previous page entitled "QCX+ 5W CW Transceiver..."). I suspected the finals were “blown” after talking with him for a few. He dropped the QCX+ rig off at my place later that afternoon.

[1] The name and call of “Bob” and “KB0**” are fictional and being used to protect the actual person’s identity. Yes, this is the same person as referred to in the previous page.

Got things figured-out pretty quick after getting into it a day or two later. The 3 FETs forming the final, Q1, Q2, and Q3, were removed and replaced with new BS170 FETs from my stock. Now, when checking things out, there was only about 29.5 Vpp coming out of the finals. Something seemed to be amiss as this QCX+ was running around 41.2 Vpp when finishing it up only some weeks before.

Further investigation found the transmit/receive FET shorted-out causing the RX (receive) signal line to stay at a logic low causing the rig to never go into receive. The transmit/receive FET (Q5) is right on the output from the final FETs too. Q5 was causing the lower-than-expected output power. After pulling Q5 and doing a quick test, the RX signal line was working correctly (which drives the TX line out of IC3D). That Q5 FET was found to have about a 29-ohm short between the drain and source and between the drain and gate doing a quick ohm check with the meter. A new BS170 FET was installed from my now dwindling stock.

Changing out Q5 made the difference. Receive was now working just as it should and had done previously. When getting the QCX into transmit, the output signal was right up there with where it was previously, at 41.2 Vpp. All was good after that with the rig operating normally. So, it was essentially just about ready to go. Performed the frequency calibration once again.

I had a hunch that with the finals “blown” (the 3 FETs in the finals), and the transmit/receive FET "blown", that there was something amiss operationally causing the rig to fail. The hunch was the QCX+ going into transmit without being connected to an antenna or dummy load creating a huge VSWR causing the FETs to fail. I could be wrong on that hunch, but that is what it looked like at the time.

That hunch was confirmed when taking the QCX+ back to Bob. Several probing questions were asked only to have them confirmed. Yes, Bob was moving cables around between his QRO rig and the QCX+ for connecting to his antenna jack at the window (he has one of those MFJ window inserts that has antenna and other connections in it). Somehow it seems, he lost track of which rig was connected to his antenna jack. He had gone into transmit with the QCX+, but then could not hear anything with it after that. The QRO rig could handle tranmitting into an open due to having VSWR protection for the finals. That is not the case with the QCX+ as there is not VSWR protection to throttle-back the power with increasing VSWR – the finals just “blew” transmitting into an open load.

A resolution for preventing the QCX+ from transmitting into an open load appeared to me pretty quick when talking with Bob before leaving to go home. I thought about his situation even more during the 20-minute drive home arriving at a potetnail resolution. That resolution is on the next page entitled "RF Transfer Switch".