I know this is an old thread, but wanted to add that "necessity is the mama of invention" sometimes. I run a Heil GM5 mic which has the built-in TX/RX stitch in it, but found it to be unusable because the mic is mounted in a shock mount which makes getting at that little switch a real pain.
So I wired-in a small SPST toggle switch on 3ft of shielded wire to the mic connector and mounted it in a pill bottle. Works great! And with the mic on a boom, it's "hands-free" during keydowns.
Also, you only need to compare frequency response and impedance to any proposed application here. I run a "work" DM-45 (an SM58 clone) in the garage and it sounds better on that radio than the heil does on that radio (FT-840), given no external EQ changes. My point being, you wont know for certain until you try.

I don't get it either. My best friend does this and he is far from alone. I've made him aware of my disapproval, but he is a headstrong dude. He just keys-up whenever, wherever, and even belts-out his call on top of a QSO -- very rude. I've heard more courtesy on channel six during a 'shootout'!

At the point that it would be actually TRANSMITTED, yes. Until then, the FCC you,me the pope can do nothing to prevent them.
At the point that they would use this "code", the point of legality becomes moot because anarchy would be the government du jour, or a government which would go against the constitutional and God-given freedoms to do intervene. And yes, I realize we are already there with the current regime. But that wasn't the question. You could try to get your operatives on the inside to learn the code first anyhow. Is it against the law to formulate a code? No. It's against the law to use it in communications.
I think I'm going to check into this further. I might want to learn it now while I have the chance.

I know this thread is ancient, but because this is a common fault with the 840, I thought I would add my two cents, Just in case anyone else has had this problem;
The blinking display would indicate it has no data supply to the VCO or DDS circuits. These radios have two voltage regulator IC's mounted on a small PC board, which plugs into the local board near the audio output ic. They are Q2901 (a 5V regulator ic ) and Q2902, (a 9volt regulator ic).

The 9Volt ic can outright FAIL, but this board is 'free-floating' -- simply "plugged-into" the local pcb much like the option boards are, pin-n-socket style while the IC's themselves are heat-sunk to the base of the chassis. Vibration can provide a lot of "fun" little quirks here. The main problem is that the soldered pads on the PC are subject to break if the rig was banged-around a lot, or causing this sub-board's pins to lose contact... And to think, they actually made (and sold a boat load of) mobile mounting kits for the 840.. LOL.
IC replacement tip: Mount the new ic's WITHOUT soldering them to the pc board. Mount the ic board into the local board, then mount the IC's to the chassis using heatsink compound -- THEN solder the ic legs to their board. This way, no "pre-stress" is placed on the connections when it is buttoned-down....... Found this out the hard way.

Happy hunting.
 

Problem solved: I appreciate all the responses.
I told the guy that I probably wouldn't be able to crack the cover on the radio until sometime late in October or November, to which, he replied, "that's fine. It isn't werking now,so take as long as you need."
He called me this weekend to ask if it was ready yet and that he needed it soon. I told him no, it isn't done -- Then I told him to give me two hours to put it back together, then come get it out of my sight.
He really tweaked me with this one.

Hmmm.. You're right! I have been attaching ground to a nice, "safe" chassis ground. I'll backtrack my measurements asap. Thanks.

No need to apologize, just know I appreciate your efforts any time that you are able, but even I haven't had much time to address this one much further anyhow myself. I won't give up on it,but I'm in the middle of remodeling the house and have had some hectic hours at my job lately, too... I have a feeling this rig is going to be on the bench a lonnng time
But that's the way the fortune cookie crumbles..
I agree it is a supply issue, but not likely to be the PA stages, because the supply disappears before it gets there. No juice to kick the relays,at least, but I haven't ruled-out anything but the 78xx's so far. Even though they have been checked and have proper output, it could be a blown zener, one of those funky,obsolete chips,or an open coil someplace, I guess. I did  get time to lift the main PCB -- No easy feat (took me all last Saturday). All traces look 'factory-fresh'.I haven't nailed it back down yet, as I will probably need access to that side to do any further component checking or replacement under there. All I can add is WoW, was this old rig well-made!

With the storm churning out there, I had a chance to tear-into this thing a bit more. All the regulator IC's check good. Additionally, I also checked and can rule-out Q-56, Q72.

Definitely molested, with half the original case screws missing, the other half with stripped heads... But the price was right. It was GIVEN to him. It is complete, but all else appears in order, and the 'thump-test' turned-up no anomalies.
Tried that, same result. the RX switches-out, the indicatator goes out, but no indication of TX in any mode. That does narrow it down to after the key, but before the driver.... But that is a lot of real estate. I appreciate the input so far, gents. I am lost without a readable schematic. For instance -- I want to check a component that appears in the block diagram labled the "PTT CNTL", but I can't make-out the part number, or location in the radio, or schematic. It's a 2SC458B (i think, as it is hard to make out also).It's Q20-something.  Also, a 7805 regulator delivers 5V in TX, but i again, can't find it.

This rig is not mine, but a brand new ham's first HF rig, brought to me (of all people) to see if I could get some life out it. It wasn't receiving Either, but it truned out to be an unplugged "CAR IN" coax connection and badda-bing, got great ears.
HOWEVER, I cannot get TX, or any indication of TX from PTT from mike, or when switching the MOX, and no TX led indicator.
It does cancel RX when attempting TX, but nodda other than that.
I have the schematics available online, but those of you who have seen them, know I might as well toss some tea leaves into the air and read them when they hit the floor.
Any direction given would be greatly appreciated.

So much "food for thought", I had to copy/paste it to a notebook file! Thanks. I might try that when I get my towers back up (lost them to a storm in June). 160 was useless in the summer months with my previous setup. 80 wasn't much better, but it sure sounds great in RX lately -- wish I could TX, but my makeshift dipole wont even tune. I cut them way long for future clipping, but they are touching the roof,10 ft off the ground right now.

LOL. I hear the doc may let me return to werk the next day.

Man am I glad I ran into THIS topic! I just purchased a nice (looking) MFJ-962D from a local hamfest, only to find the "resonance killer" burnt up. The little epoxy pc board "pawls" were charcoal, as was a portion of the center PVC frame. I cleaned it up as best I could, removing every trace of carbon (remembering lessons learned in the HV stages of TV sets), but those pawls are toast -- nuthin left. I was going to call MFJ tomorrow morning to see if I could purchase just the pawls. But if there is no harm done in totally eliminating this mouse trap assembly in it's entirety. it is HISTORY.

Thanks, Tisha. It is a 'lousy design' ,for sure. When it comes to RF sig generators, it would appear that 'you get what you pay for' , for sure. I'm more or less stuck with my poor choice now, but if I had it all to do over, I would've saved my cabbage until I have enough to buy a REAL SG..  The only thing worse than the poor design and construction practices found in this device, is the schematic -- because one cannot be found. It's hard trying to figure out 'what is what' without a road map, but I did find a 5K Ohm, fixed resistor in series with all of the band tuning coils. I took it out and tacked-in a 5K pot and bingo! 'fine' tuning! I ordered a 10-turn pot for a permanent install, but the steady drift-down seams uncontrollable. I would steer everybody I could away from paying more than $20 for one of these train - "REKs".

I cut out a slot in the front panel and installed one of those freq counter modules found on ebay for $34 (including S&H) in this poc.
It gives me something pretty to look at, as it "counts-down" like a NASA clock.... "T-minus 10:240:121 and counting." After about an hour or so powered-up, the 'countdown' ACTUALLY DOES slow down to something I can work with, but it never totally quits.  

73

Thanks, y'all, for your input. The main board looks like it was properly assembled, but the assembly of the  peripheral components looks HORRIBLE! My grandson could do better and I'm not exaggerating.
I've noticed that when I just get close to it, the frequency shifts worse! And very very coarse! I have a DDS-60 that is much more accurate, but it's not very convenient. I may just end-up installing an adjustable attenuator and audio generator to it, but I've been having too much fun using for a SD VFO. But if I could get this thing to not drift so drastically, I'd use it instead.
I'll go-over all the solder joints to see if I can clean it up some. Any ideas on how to install a fine-tuning capability to this thing?