NASA Navtex repair

[john]

Hi,
I had a similar problem, where my battery died. I replaced it with an easily obtainable cordless phone battery 3.6V 760mah (original was about 400mah - anything upto about 1000mah will do), simply cut off the plug and soldered it in. Cost about £2 on ebay.
I also had a problem with corrosion in the bottom corner (below the battery) and cleaned this up then laquered the pc board (masking off the connector first). This was done in may 2010.

Last year 2011 I had a problem that the display went black and could not be adjusted with the contrast. This turned out to be a broken 39k resistor (bottom left corner near where the corrosion was. I spent some time figuring out the circuit and this works as follows.

The display board requires a negative supply of around -15V (3rd pin on the 16 pin connector) this is supplied by a pump charge circuit which uses a BC327 transistor and the inductor (which gets a bit warm). This inductor was mentioned in the original post and has nothing whatsoever to do with the battery. The transistor switches on and off rapidly, energising the inductor which provides a back emf which is rectified by the small diode on the bottom left edge of the board. This charges a capacitor (bottom left corner (10uF 63V). The voltage is monitored by the 39K resistor to the PIC microcontroller (big socketed 28 pin IC bottom left). The PIC monitors this voltage on pin 2. The normal voltage for the display is around -15V which results in about 1.7V pin 2 of the PIC. (The 39K is in series with a 10K to the +6V regulator).
The battery is charged by a 47R resistor via 2 1N4148 diodes from the 6V regulated supply. The battery also provides power through the same 47R resistor to the PIC and another IC. If the battery is short circuit this will pull down the supply to the PIC.

Problems that can occur with the display are:-
Too dark - display voltage is to high (-ve) if this is around -17V check the 39K resistor - could be open circuit - rare.
Too light - display voltage too low (-ve) if this is below about -13V and the inductor gets hot - (ouch hot) try replacing the 10uF 63v capacitor (may have gone leaky).
also try replacing the diode on the left edge (1N4148 should do) You could also try replacing the BC327 m- these are all low cost items - pence.
For information, the display normaly draws about 5mA from the -15V supply.

Hope this helps with anyone else having similar problems.

I too have a Navtex which has suffered from a leaking battery, I have repaired a couple of tracks, replaced the battery and a number of components.

But there is obviously something I have missed as the screen is illuminated without the menu and the 78L06 6V REG get blistering hot.

Can anyone suggest which component(s) could be the causing this!

[bonny]

I too have a Navtex which has suffered from a leaking battery, I have repaired a couple of tracks, replaced the battery and a number of components.

But there is obviously something I have missed as the screen is illuminated without the menu and the 78L06 6V REG get blistering hot.

Can anyone suggest which component(s) could be the causing this!

The 78L06 should not get hot, hardly even warm! This would tend to indicate that it is supplying too much current to something.
Can you measure how much current the unit is drawing from your 12V supply, I would guess about 50mA with the display light off and 100mA with the light on, anything more than double these figures would concern me and I would power off immediately.

Which components did you replace?

You say the display is illuminated, can you see anything on it, (even jibberish)?
Is the display all black dots or all white?
Do you get a thin horizontal line when you press all four buttons together?


Can you measure any voltages with a multimeter - be very careful not to short anything.
Try measuring battery voltage, output of 6V regulator to ground, voltage across the 10uF capacitor at the bottom corner (near the display connector).

[il_postino]

Hi,
I had a similar problem, where my battery died. I replaced it with an easily obtainable cordless phone battery 3.6V 760mah (original was about 400mah - anything upto about 1000mah will do), simply cut off the plug and soldered it in. Cost about £2 on ebay.
I also had a problem with corrosion in the bottom corner (below the battery) and cleaned this up then laquered the pc board (masking off the connector first). This was done in may 2010.

Last year 2011 I had a problem that the display went black and could not be adjusted with the contrast. This turned out to be a broken 39k resistor (bottom left corner near where the corrosion was. I spent some time figuring out the circuit and this works as follows.

The display board requires a negative supply of around -15V (3rd pin on the 16 pin connector) this is supplied by a pump charge circuit which uses a BC327 transistor and the inductor (which gets a bit warm). This inductor was mentioned in the original post and has nothing whatsoever to do with the battery. The transistor switches on and off rapidly, energising the inductor which provides a back emf which is rectified by the small diode on the bottom left edge of the board. This charges a capacitor (bottom left corner (10uF 63V). The voltage is monitored by the 39K resistor to the PIC microcontroller (big socketed 28 pin IC bottom left). The PIC monitors this voltage on pin 2. The normal voltage for the display is around -15V which results in about 1.7V pin 2 of the PIC. (The 39K is in series with a 10K to the +6V regulator).
The battery is charged by a 47R resistor via 2 1N4148 diodes from the 6V regulated supply. The battery also provides power through the same 47R resistor to the PIC and another IC. If the battery is short circuit this will pull down the supply to the PIC.

Problems that can occur with the display are:-
Too dark - display voltage is to high (-ve) if this is around -17V check the 39K resistor - could be open circuit - rare.
Too light - display voltage too low (-ve) if this is below about -13V and the inductor gets hot - (ouch hot) try replacing the 10uF 63v capacitor (may have gone leaky).
also try replacing the diode on the left edge (1N4148 should do) You could also try replacing the BC327 m- these are all low cost items - pence.
For information, the display normaly draws about 5mA from the -15V supply.

Hope this helps with anyone else having similar problems.

Yessss!!! After so many years I have found your post here. And it helped. After repairing 10 corroded tracks, cleaning PIC replacing socket, putting a new battery, replacing some eaten transistors I could only darken but not light up the display. I ended with absole dark...
And guess what, the 39k resistor was measured 52k... The metal film has been eaten partially under the cover... After replacing everything started to work.
So thank you very much

[bonny]

I sold my boat in 2021 and the navtex with it. Consequently I don't really look at this forum these days.
I just found your post in my junk mail.
So glad it was of help to you.
In my case the 39k resistor was physically broken and as such was supplying a -ve voltage to the pic (pin 2) and consequently damaged that input to the Pic. I was able to get a replacement Pic direct from NASA, the guy who designed it. I think he's a one man band, but was very helpful.

[encoread]

I have been dealing with the same white screen issue on my NASA Navtex, and your solution with the capacitor was a great help. It’s good to know these older units can still be fixed with a bit of effort.