Display Does Not Come On At All

Problem: The problem and solution apply if the display does not come up at all. The display on my 1995 vintage 'MP went dark tonight. The rig  still seems to function (I can computer control it)...and the front panel indicator lights are working.  I tried resetting the CPU...no luck.

Solution: If the problem is what I had, it's on the inverter board that provides some rather high voltage - a few kV?  - to the display.  The inverter board is not manufactured by Yaesu, so there's no schematic for it in the manuals and they do not repair it (at least based on my phone call to them a few months back) and so have no components for
it.  They will sell you an entire board (I don't recall the price).

The inverter is behind the front panel.  You need to remove at least the top cover (and I believe the bottom as well) in order to get access to the screws that hold the front panel on the main chassis. (No need to take the front panel itself apart - just disconnect it physically from the MP's main chassis.)  You *can* get it to kind of hinge down on
two screws though not enough, if I recall, to get full access to the inverter.

The inverter is a board about 25mm x 75mm (under a metal shield) mounted on the far right as you are looking at the MP from the front. It's not on the front panel assembly, but on the front surface of the main chassis.  If I recall correctly, a couple of screws hold the cover on - remove these and then you can get to the screws that hold the board to the chassis.

There's not much to the board - just a few components.  There are two fuses - a leaded over-current type (looks like a resistor) in series with a thermal fuse (mine looked like a square-package capacitor - about 5mm x 5mm x 2mm thick with radial leads).  The thermal fuse proper is mounted between and thermally coupled to two transistor (or
FET) devices.  My fuse was open circuited - I simply bypassed it and the display started working.  I am still looking (albeit less enthusiastically) for a suitable replacement.  It says 2A 102C.

In addition, apparently lots of folks have seen this happen (and I have as well): You turn the radio on, and the display takes an inordinate amount of time to come up (many seconds) but it does light up.  Next time you power it up, it may come up much faster.  No solution has been offered for this problem as far as I can tell.

Hope this helps. (Next time I open the MP, I'll try to think straight and shoot some digital photos.) Mike N2MG


Subequent Posting: Mike... Got the radio apart...found the board...even found the fuse.I realized that TF stood for thermal fuse. There seem to be six pins soldered into the board from the bottom of the fuse and two black wires that extend out of the side of the fuse that are soldered into the board. My initial guess was to cut the two wires...and solder them together. Is that anywhere close to correct? Ray ND8L

Response: The two black leads coming out are the fuse (the 6 pins going straight down are the leads for the two transistors I mentioned - the transistors whose temperatures are monitored by the TF.) Please note that the fuse is a separate physical device, that is bonded with some goop to the two transistors it is supposed to protect. The 2 transistors have 3 short leads each going into the board. The TF is "sky-wired" - the body is glued into the space between the two transistors and the TF's leads gently folded over and soldered to the board.

NO, don't cut the leads. You should first verify that the TF is the problem. So unsolder one of its leads from the PC board, and then run a continuity (ohm meter) check on the two leads (one still soldered on the board, one you just unsoldered). The TF should measure a VERY low ohm value. IF it's working properly. If, however, it's blown (as we suspect) then it should measure open (Megohms). If it is open, then place a jumper of wire on the board between the 2 pads that the TF was soldered to. You do not *have* to remove the TF physically from the transistors (it is "glued" in) for this "bypass" to work - only if you want to or want to replace it. But you probably should anyway just to get it out of the way. If the TF is not an open, then this will not work of course. Check the other fuse as well -it's wired in series right next to the TF - looks like a resistor on my board - a little brown leaded part. Test/bypass the same way if needed. Hope this is clear. 73 Mike N2MG


Additional Information: The description by Mike Gilmer N2MG states that there is a "...thermal fuse..". Possibly this thermal fuse is not a fuse, rather it is an NTC thermistor. This will explain the slow start up. When cold (room temperature), the thermistor will have higher resistance, limit the inrush current and the display will be dim.
As it warms up, the resistance drops, current increases and the display is bright. If indeed it is a thermistor (??), replacing it with a piece of wire may not be a good idea. There will be no inrush current limit!

As the saying goes, "if it is not broke, don't fix it". So, I don't like the idea opening my 1000MP and poke in. Possibly, someone who is working on the radio, can measure the room temperature resistance of the 'thermal fuse' and the current profile. If it is a thermistor it is possible to replace it with similar unit but higher ratings. The effect may be slower turn on of the display. Another possibility is that the device is a reset able solid state fuse. My vote is that the device is a thermistor. Mort, KB6BSN

It is a thermal fuse, not an NTC. It blew in my MkV and I had to replace it. It is a 2A fuse that opens at 102C.

This thermal fuse -topic was also discussed on the www.contesting.com Yaesu-reflector about a year ago, and it appears to be a very typical failure for the MP and MkV. 73, Esa OH7WV

Yesterday, I turned my Mark V on for the first time in about a month, and was appalled when the display did not come right on as usual. However, after about 5 seconds it did illuminate. I have since had the radio off overnight, and when I turned it on this morning it came on right away. This has me wondering whether the thermal fuse is the only issue involved, since overnight gives plenty of time to cool off, yet there was no display delay this time. 73, Pete N4ZR

Pete is right. Have your radio off overnight and it sure is cool in the morning. To me it sounds more like capacitor charge up time of thing. If you have the radio without power for days, all the capacitors will be completely
discharged and will require longer time to charge up again, which is not necessarily the case when you just have it rest overnight. 73, Esa OH7WV




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