FT-8900r repair - pre-driver

This particular Yaesu FT-8900r came from a repeat customer of the Cly Institute.  He delivered the bare board for the repair saying to change the final transistor, and even supplied the correct part.  The process was done, but that did not cure the problem, there was still no output on transmit.

With the now fully re-assembled transceiver, the debugging process started by stepping back from final transistor gate to discover where the transmit signal was being lost.  It didn't take long to find the gate of the pre-driver transistor had the transmit signal. This meant that the pre-driver was not functioning.  Surprisingly pressing down on the chip re-established output.  This turned into an easy fix by re-sweating some solder under the chip.  In our experience at the Cly Institute, the RoHS solder they are using today is pretty poor for higher vibration applications.  We only use Lead solder for repairs trusting in its age old reliability in soldering applications.

This would have been the end of the repair story, however with the transmitter set to HIGH output, the final stage began to ring, causing a tone on the received audio, and likely some spurious emissions.  It had to go back to the bench for investigation.  In the process of replacing the final transistor, the chip capacitors had to be removed surrounding it.  This was the likely culprit that one, if not all were overheated and no longer performing their designed task.

The original capacitors were rated 50pf at 500vdc.  They were not ceramic, and were likely heat-damaged during the process of replacing the final transistor.  New ones were only a eBay order away.

The areas circled in red are the ones that were providing trouble.  The solder joints around Q1137 were an issue.  I had to add some serious heat to this transistor to get it re-flowed, but trying to keep the body cool by holding a cold metallic tool on the top of the chip.

Note the new ceramic capacitors around the final transistor Q1134 in the picture above.  These are much higher quality than the original capacitors.  They can take a good bit more heat and should be more stable throughout their lifetime.

The radio was held in transmit attached to a dummy load for an excessive time.  It did not overheat on high, and it did not drop power.  I listened to it through the radio I keep in the shop, and it did not oscillate or produce any spurious tones.  Its time to box it back up and return it to the customer.