Yaesu FT-100D repair [1], Something is Screwed up

 I just purchased a "for Parts or repair" FT-100D, from the estate of a silent key. There are some things immediately identifiable as problems. 

1. The Microphone cord rubber has completely disintegrated. This is super common, it seems pretty much all of them do this. All that should be needed is a replacement cord.

2. The fans are tied together and connected to a 12V cigarette light plug. So the radio has either no fan or full power fan all the time. It's quite loud.
3. On power on, the unit draws 8 amps all the time.
4. There does not seem to be any transmit capability on any band.

The most concerning thing at the moment is the 8 amps on power on. I don't want to further damage the unit. I need to find the source of the short soon.

While opening the unit to inspect it one of the screws stripped and I could not remove it. Come in the reverse direction drill bits. They are very useful and have helped me remove a few very stripped out screws. I first drill a small hole in the head with small diameter bit. The hole makes space for a large bit to bite into the sidewalls of the new hole, then the screw will turn and can be removed without any damage to the casing. If you don’t have a set of reverse drill bits, I suggest you get some, you might now need them all the time but they are a really good tool when you need them. 
Above: Reverse Twist Drill Bit

Above: Removed Stripped out Screw


Once I was able to open up the case I started looking for any obvious issues. I quickly found one. There has been a temperature sensor,  a Kemet OHD3-40M which has one terminal soldered onto the Power Amplifier (PA) board of the FT-100D. The other terminal is floating in the case. Normally the FT-100D has 2 thermistors, TH3001, and TH3002 which measure the temperature of the PA Board. I have not yet measured the output to see if these are still working. Generally I believe the temperature sensor controls the fans for cooling as needed. Perhaps an issue happened when the previous owner was using the radio.

The power draw of 8 amps is will above the 1.2-1.6 amp typical value that the service manual states for receive only current draw. I am thinking there may be a pad output transistor which has shorted. This seems like the most reasonable thing as they are directly tied to ground and 13.8 v and 8 amps would mean a 6.8 amp abnormality. Which is a 2 ohm current path somewhere. 

I hope to figure out the issues with the obvious problems soon and then I can get working on the more tricky issues like the transmit capabilities. 



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