My Yaesu FT-840 “Project”

After the K3, I had no real intentions of obtaining another HF rig. But when a deal comes by, I couldn’t resist jumping on it. There was an SK estate sale that I found out through another ham. However the estate was not located near by so everything had to be shipped. Among the items, the Yaesu FT-840 stuck out at me as the perfect HF for someone who is getting involved in the hobby. The guy was asking for reasonable offers. I didn’t want some commercial seller to swipe it. Since I had no need for the radio, I put in a low bid and sure enough it was accepted.

The horrors of buying before trying

Since the price was so tempting, I made a foolish mistake of buying the radio without seeing a picture of the radio, without asking questions and placing my trust in people I don’t know. I’ve done this a couple times in the past and thankfully I never had a problem. This time wasn’t the case.

When I received the radio, it didn’t look good. Knobs were bent and there was a giant dent in the corner. The dented corner had some very shiny metal expose with no rust/oxidation. However the box it was shipped in was in excellent condition. It led me to believe that the radio was dropped just prior to shipment. There was also a missing part. I’ve contacted the middle man explaining what happened and that’s when I’ve learned that other shipments from the estate were just as bad, if not worse than my radio. According to the middle man, the handler of the estate didn’t seem to care and that I would have to take it up with the shipper’s insurance company. I wasn’t even close to being happy with the situation.

Mic Gain / PWR adjustment pots after some attempt at strightening. Still rubbed and was bent so I had to strip it to the chassis

I’ve learned from this though. I will never jump on anything until I am certain that I am getting what exactly I want. Even if the deal seems too good to be true, It’s better to pass up the unknown rather than getting stuck with an expensive brick. I guess I am keeping the radio

Fixing the unknown

Soon as I started tearing into the FT-840, It started to look promising. The front panel chassis was bent and thought I can strip it down and straighten everything out. I was able to do just that. While I was in there, I cleaned everything I could and replaced the internal battery. When I thought I was finished, everything went back together nicely and now all the knobs move smoothly. the VFO wasn’t sticking and it looked much better than when it came in. I thought I was done. But once again, I was wrong.

Who needs ALC?

While checking power output, I found that when I went the SSB modes, the ALC meter was showing  either almost nothing or full scale with only a minor adjustment to the pot. I thought something was wrong with the mic at first but it turned out to be okay. Once again I contacted the middle man and was informed that the seller was aware of the issue but didn’t think anything of it. So now on SSB, I will either be barely heard or my signal will be overdriven. It made me more angry knowing that seller was aware.

I ended up tracing it to the pot that controlled the mic gain being faulty. It was also the same pot / knob that was damaged during shipping. With a DMM I checked the resistance while adjusting the pot. There wasn’t a consistent change throughout the adjustment. The service manuals were online and I was able to find the part numbers of the pot

 The hunt for the dual pot

Pulling the number from the pot itself found many hits online. However the part sources didn’t have any in stock and were not planning to put them in stock unless there was a 10,000pc minimum order. I thought I was out of luck and called Yaesu USA in California and contacted their parts dept as a last resort. Sure enough they had them in stock. They had the pots and they had the sub assembly.  I must say that it was wonderful dealing with Yaesu. The parts were here in just a couple days.

It’s FIXED!!! YESSSS! Or so I thought.

The pot was easy to replace and I thought the radio was finally fixed. I turned on the radio and I am now able to adjust the mic gain to where I am no longer over driving the ALC while outputting the proper wattage. I was happy! Until I heard it.

What’s going on now? 

While playing around with the radio I noticed something weird. I would tune into a conversation and while I was listening, I would notice the conversation start to move around and sound off frequency. OH WHAT NOW!!! Now I am back to being angry. Thinking something was up with the oscillator, I left the radio on and took a walk with the family. Upon return I notice the drifting was no longer happening, That led me to believe that the changes in temperature both in the air and in the case caused the drifting to happen. It would drift up to 400hz. The new operator in me never had a radio without some kind of controlled LO so I never had to deal with drift.

Put a jacket on that LO!

Why should I care about a slight drift? Well if you are into digital modes. If you are into low noise modes like JT65 and WSPR, having a stable LO would be beneficial. There is an TCXO or Temperature controlled oscillator that would help keep drift at bay but it’s not easy to get due to the age of the radio. There are not many 10.48576Mhz TCXOs out there either. So the next best thing is to insulate the LO to keep it from changing temperature drastically. I’ve do stuff similar to icom rigs using cotton balls. It appears FT-840 owners used styrofoam. I guess I will follow suite and make a nice little foam block to protect the sensitive LO.

I took a nice little block of foam, placed it on top of the reference LO and press down a little bit. I would remove the box and hollow out the impression. I kept doing that until the block sat flushed with the PC board. The crystal and trimmer is now shielded from quick changes in temperature. I’ve also adjusted the reference trimmer using WWV to make sure it was close to being on frequency as possible.

Much improvement

The LO insulator is doing it’s job quite well. The radio doesn’t noticeably drift around anymore. Been able to make many contacts with it. Too bad I didn’t have a soundcard interface because I would like to try out WSPR to see what I was getting back from those who could hear.

I think it’s safe to say that the FT-840 is now back in working order. However I may not hang on to it long enough to enjoy it. But I sort of grown attached to it now.

Thanks for reading,
Jeff (NT1K)