So what is the Consensus
Below is a repost of my notes from 2013 on driver failures. I tried to gather more info from users that reported problems, but didn't get much useful detail. There seemed to be some connection to running to low TX power, antenna tuners into non-resonant antennas and questionable station grounding.
I have also noticed subtle changes in the pre-driver board layout and design as the Main PCB artwork changed. That circuit has always appeared somewhat unstable to me when trying trouble shoot it. I don't know if later rev Main board actually improved the pre-driver stability issues I observed.
I have two 7K's (circa 2006 and 2010) and have never had a driver failure. I have replaced several drivers for local hams over the years, but couldn't find a likely cause of the failure. Some had no/poor station grounding and I'm not aware of repeat failures after that was fixed, but I haven't stayed in touch with many of them either.
There have been several theories on why the driver board fails along with suggested "fixes", but none seemed based on sound technical analysis, in my opinion. Here are my rambling thoughts on what I think I know about the situation, with no solid conclusions at the end, unfortunately....
The board you replaced is the Driver. The pre-driver FET's are on the PA board and boost the IF output enough to drive the Driver FET.
Others have reported failures while running at lower power levels. This would seem to suggest a problem with the ALC loop perhaps. Several radios have a problem with the ALC control delay allowing full power for a few milliseconds before ALC reduces the gain of the IF stages. This is especially a problem when driving an amplifier. I haven't seen evidence of this in the 7K, except when compression is on perhaps.
The gain is constant in the pre-driver, driver and PA circuits, regardless of the TX PO. Power control is accomplished by the ALC signal changing the bias of the IF stages that supply the drive.
Whenever the Driver board or associated circuits on its input or output, you should perform the relevant TX Adjustments per the Service Manual. See Section 4-4 Drive/Final Idling Current.
The Driver runs as a class A amp and setting its idle current (as above) is essential for proper linearity AND to ensure it doesn't dissipate excessive heat.
I suspect there is some truth in owners saying that poor grounding contributes to the problem, especially when there is a high SWR being tuned with an autotuner that is rapidly switching the L-C networks trying to find a match. The early stages of tuning where the L-C matching may actually increase the mismatch can generate some pretty high voltages when the reactance is much higher than 50 ohms.
I only run resonant antennas and only use a tuner for portable operation -- and never have had a driver failure on either 7K after almost 7 years on one and 3 years on the other. In my opinion, a G5RV is a poor antenna technically. Yes, you can make contacts on it and just about anything else you string up in the air, but it can be more than a little unpredictable the way most hams have them installed.
I have tried looking for a weakness in the Driver chain several times and didn't find anything suspicious. But I didn't try to destroy my radio with high SWR and an autotuner. The ALC circuit does encompass multiple control inputs and the loop goes from the SWR detector near the antenna connector, back to the IF stages on the Main board. So poor grounding of the system (radio, tuner, lightning suppressor, and any equipment connected to the radio) is easier to fix than to figure out how or why it causes the problems.
Also critical is good grounding of the circuit board to the chassis via TIGHT mounting screws and to the outer case via the ground springs. It is easy for the ground springs to get mashed in and not contact the case adequately.
Another area to look at is excessive RF getting into your digital audio input, resulting in power spikes before the ALC can reduce the IF gain. The best cure for this is proper system grounding, but more importantly, minimizing RFI in the shack by using resonant, matched antennas with effective common-mode chokes at the feed point. Also, routing low-level signal cables away from RF and power cables and never in parallel. Use of proper ferrite chokes on the audio cables helps.
The best resource for analyzing these issues is at:
Read and reread the first document listed: " A Ham's Guide to RFI...
There was a subtle change in the Driver board in 2010 that removed a 10pf cap to ground on the FET's output. And there is now an OPTIONAL R103 (100 ohm)in series with the Driver FET's gate input, but the parts list says it is only for EUR, FRA and ESP country code models. Neither of my radios has either change installed.
I don't believe the driver is likely to fail due to ventilation problems in normal installations. 90% of the heat is dissipated into the chassis heat sink. Some owners have reported inadequate thermal compound on that interface, but I have not seen that in the 4-5 radios that I've looked at. The driver FET isn't being pushed hard at all, thermally. I do replace the white thermal compound with a very thin film of Arctic Silver compound if I have the Driver board removed. And I make sure the thermal spread, FET and chassis make "perfect" contact before screwing them back together.
It will probably be several months before I have time to revisit possible causes of Driver flame outs. I'm sure many others will add their experience and opinion with Driver failures. Unfortunately, I feel many of these problems are often related to the individual's unique installation of everything that makes up the radio system. Which is why there are some many different experiences and conclusions drawn among users.
The problem isn't that the SWR changes during tuning are reflected back to the driver, but how the SWR affects the ALC loop. Here are some of the contributing factors, but not all apply to all stations obviously.
All interesting stuff Steve and thanks to this group:
My 7000 is mainly used in the shack -- and because it used to run so warm I installed another little fan resting on top forcing air into the existing fan aperture -- and powered by the antenna plug on the back. It cost all of $3 and has zero impact on the fabric of the rig. No internal mods no resistors no nothing. It just sits there quietly doing its 'thing' and even on digital modes the rig never gets out of the 'green' (I only run 50 watts max)
Its great to have a resource like this.