Discussion in 'Amateur Radio Amplifiers' started by KB3SQW, Oct 12, 2019.
Where do i get this part . It looks burnt.
That is the meter shunt for measuring plate current. Location R3 in the schematic. It is in the milliohm range and should check as a short with a conventional ohm meter. I think they are somewhere around 0.1 ohms. If it is open, your plate current meter is destroyed if metering switch was in the Ip position when the amp was operated. A fault current can destroy the meter shunt (tube flashover - HV shorted to ground). No value is given in the manual for it and if replacement is needed you will have to calculate a value based on the meter movement in the amplifier.
Yes, all Yaesu Meters from that era that read PA current (whether in Transmitters or Amplifiers) were supplied with their own hand-wound shunt resistor.
Each one was individually wound to calibrate that actual meter, so it would read correctly! If you ordered a new Meter from Yaesu, it came with it's own custom shunt, so you had to replace that too !
The fabric insulation on the resistance wire on that shunt may have turned a little brown, but I expect it's actually fine.
You can test whether your PA Meter is reading correctly with a simple test - with the rig off (and capacitors discharged!) connect a multimeter on a 1 amp range in series with an AA cell and connect it to the top of the shunt resistor, the other lead to chassis. Check whether your Multimeter reads the same as the PA Meter.
If you DO need to replace the shunt, you can use this method to calibrate the new one - I had to do this on my Amp when the meter went faulty and started reading abour 50% of the correct value. I just used a couple of big wirewound resistors in parallel, trial and error to get the right value.