A club member brought a Yaesu FT920 with the fault, no transmit output.
On connecting it up on the bench I found that a small amount of transmit power did show on my wattmeter (about 10 watts on FM) and the signal could be heard on my FRG7,however the current drain from my PSU was about 18 amps and the wattmeter on the radio display showed 100 watts indicating that the radio PA was producing 100 watts but thiswas not getting through either the low pass filter, the tx/rx switching relay or the automatic ATU.
Experience has shown me that the relays in the low pass filter and the tx/rx switching relay can be a source of trouble and as they are all on the filter PCB on this radio I removedthe filter pcb from the radio with a view to powering it up “on the bench” (it's underneath the main pcb on the underside of the radio in this rig so would be difficult to fault find on withthe board in situ.)
Once the appropriate section of the low pass filter and the aerial changeover relay have been energised there should be a DC path from the input to the filter pcb to the output (thatfeeds to the auto atu board) so should be easy to find the problem.
I energised the filter relays (for 40 metres) and the aerial changeover relay to simulate the radio being keyed to transmit
A check through the selected filter relay contacts and aerial changeover relay contacts revealed that the changeover relay had failed (open circuit contacts.)
I obtained a replacement (these are an industry standard so are not too difficult to obtain) and fitted it, after reassembly the radio produced 100 watts fine so after a check on allbands it was returned to it's owner.Tony g0mqg