NASA Navtex repair

[sahona]

Problem: Power on gives white screen (lit) but no characters. no button combinations alter anything.
Remove four screws at rear and split the unit like a book, hinged by the ribbon cables, see pic below.


The problem was the battery pack, which has been removed in the pic above. Applying power made the components to the right of the battery bay (especially the choke - a cylindrical object,) heat up, due to trying to charge a couple of shorted cells.
The proof of diagnosis ( I think this is true of all units with internal rechargeable memory batteries) is to remove the failed battery pack and replace it with a large electrolytic capacitor. - The black cylindrical component lying at an awkward angle ---> below.


The capacitor makes the circuit think it's got a battery and the unit boots up and the screen shows the start menu.
Theoretically I could just use the unit like this and leave it on all Summer, but the correct thing to do is locate a cell-pack like this:


The circuit board near the battery bay has been damaged by either water or fluid from the failed battery. The board was simply washed with isopropyl alcohol and will be re-lacquered to protect it in future.

[Rowana]

I'd be interested to hear if you manage to source a battery.

Where does one get isopropyl alcohol?

Hope you get it fixed.

[aquaplane]

Iso propanol should be available from chemists or here:
http://www.amazon.co.uk/ISOPROPANOL-Iso ... B003D8QFQO

It's the main ingredient in screem wash, no, it's the antifreeze on screen wash the main ingredient is water.

If you can't get IPA, ethanol has very similar properties, maybe even meths would do.

BTW, I'm dead impressed with the repair, electronics is way above my skill level.

[Pete Cooper]

BTW, I'm dead impressed with the repair, electronics is way above my skill level.

From what I hear on another forum sailing is way beyond your skill level!

[aquaplane]

BTW, I'm dead impressed with the repair, electronics is way above my skill level.

From what I hear on another forum sailing is way beyond your skill level!

I don't like the blokes "be reasonable, do it my way", "I'm right you're wrong", attitude.

[Silkie]

I've never liked his attitude but, in between bouts of sprayhood patching, I spent most of yesterday evening hanging out on the forums and he was really starting to annoy me in a big way. In particular, although it was by no means his only post to get up my nose, I had to excercise considerable restraint to avoid wading in after the comment in question. I see he hasn't yet replied to Phil's latest.

PS - I too am in awe of Bill's skills.

[Pete Cooper]

I don't like the blokes "be reasonable, do it my way", "I'm right you're wrong", attitude.

I have to say that he is a forumite who I don't think I would enjoy a drink with - he always knows best, and always has a story involving somebody well known, or famous in his eyes, carefully designed to impress.
Sorry - this should be in the 'Grumpy Old Sailors' forum not the 'Gear & Gadgets'

[sahona]

in between bouts of sprayhood patching, I spent most of yesterday evening hanging out on the forums and he was really starting to annoy me in a big way.
PS - I too am in awe of Bill's skills.

Who'se annoying you - not me I hope.

The marina parked me @rse to wind during part of the repair procees, resulting in the winter cover being shredded during the gales, so the car hasn't been in the garage for a day or two:


As for the Navtex battery, I have ordered a cordless phone battery from Maplin, which looks the same but has flying leads, which will be easier to solder. I will also "bag" the battery so it doesn't eat any more of the circuit board.
The isopropyl was issued to me in the days when we used to clean and change heads on disc drives, (capacity 5Meg!) Lasts better than some wine I've had recently...

[sahona]

Didn't log anything overnight. Will take aboard and check with real aerial.
May need to dig out the 'scope.

[wully]

D'ye do homers?

When I bought the boat I got a bag with 4 spare Autohelm 3000 controllers which might or might not work...Been to scared to plug them in incase they toast the thing...

[sahona]

Now installed aboard and logging happily. FYI the new battery is CP05JST by 'Energiser'
Don't know why my 16' whip on the shed didn't work, but no-matter.
Incident closed.

Wully - I think I've a couple of those autohelm units in the drawer, but no info on what to look for signal-wise.
I'm sure you could fit a low-value fuse and have a go, or, take a car battery and a copy of 'The Dangerous Book For Boys' into the shed and have a go there!

[wully]

Now installed aboard and logging happily. FYI the new battery is CP05JST by 'Energiser'
Don't know why my 16' whip on the shed didn't work, but no-matter.
Incident closed.

Wully - I think I've a couple of those autohelm units in the drawer, but no info on what to look for signal-wise.
I'm sure you could fit a low-value fuse and have a go, or, take a car battery and a copy of 'The Dangerous Book For Boys' into the shed and have a go there!

I've got an AVO and a soldering iron but not enough courage to blow up my working head... The last Autohelm head I took apart had failed in Barra as it leaked water from the buttons which weren't even slightly waterproof...I got it going for a while with meths and a cloth.
When I sent it back to who ever it was that owned Autohelm at the time they denied it was their fault that a waterproof cockpit auto pilot leaked because the was no sealant round the buttons that acted directly onto the circuit board...They did offer a 'free' goodwill repair if I sent them £40!

I bodged it up again as I had sold the boat.. and have never bought from them since but it seems there is no choice in the lower end of the market now Navico aren't making them any more

[bonny]

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.

[Telo]

Problem: Power on gives white screen (lit) but no characters. no button combinations alter anything.

Ours has gone the same way, but has been in a warm dry room all winter. Your solution looks effective, but I'm going to send it back, again, to Mr NASA. This is the second time that a NASA Navtex battery has packed in. An active life of less than one and a half sailing seasons doesn't seem very impressive for a rechargeable battery.

[DaveS]

Problem: Power on gives white screen (lit) but no characters. no button combinations alter anything.

Ours has gone the same way, but has been in a warm dry room all winter. Your solution looks effective, but I'm going to send it back, again, to Mr NASA. This is the second time that a NASA Navtex battery has packed in. An active life of less than one and a half sailing seasons doesn't seem very impressive for a rechargeable battery.

I would tend to agree. FWIW I leave mine connected aboard (it's the only load permanently on when the boat is left (fridge and dehumidifier only come on when either mains power is on or the engine is running). This was originally done so that on arrival on board the Navtex would already have all recent messages in memory, but a secondary benefit is that the internal battery stays on permanent charge.