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Lokata marine receiver

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Lokata marine receiver

Post by Niall » Thu Dec 26, 2013 6:31 pm

Another "spares or repair" ham rally bargain, this.

HF AM / SSB and VHF FM band II receiver, synthesised with memories and RDF capability.

When powered up displayed garbage on the LCD display. Lokata appears to have carefully guarded their service data, then merged with another company and moved out of the leisure sailing market. I was able to find references to a defunct website which used to be run by an ex Lokata engineer who fixed them. Although the site is dead the Wayback Machine (www.archive.org) was able to retrieve it. The only fault listed for the marine RX was failure to start up, suggesting the memory back up battery or the power on reset circuit.

I wasn't surprised to find the NiCad backup battery on the board dead. Rather than buy a replacement which would be wasted if I couldn't get the thing working, I bought some AAA battery holders from Maplin and some AAA NiMhs from the pound shop and wired them in. Still wouldn't start.

This is not the easiest set to work on, it consists of a stack of boards which basically have to be plugged together in a stack before it can be powered in which form it is virtually inaccessible. I assume the official service workshops would have had extender cables to work on it. I attached a length of wire to the reset pin on the microprocessor to allow access to it with the unit powered up. Sure enough, the reset line sits a few millivolts above 0V, touch it to the + of the backup battery after switch on, it latches at +5V and the receiver starts up.

So the fault is indeed in the power up reset circuit. I have checked for dry joints, as far as I can tell the BJT and FET which appear to be involved in this circuit are OK, but it still won't work. This gives me a dilemma. As I said the processor board is impossible to test in situ, the PCB is double sided and really difficult to follow, there is no data available whatsoever. I can easily lash up a little circuit to detect switch on and pull the reset line to +5V which would make it work, but it would be a non original bodge and somehow unsatisfying. On the other hand I could continue to work at it and risk rendering it completely non functional.

I suppose one option would be to do the bodge and come back to it at a later date if I get any bright ideas or come across a set of extender cables and / or a diagram.

 
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Re: Lokata marine receiver

Post by Refugee » Thu Dec 26, 2013 11:34 pm

Would that not be a dead capacitor that pulls the reset line either high directly or down by a transistor acting as an inverter and latch?
There must be some kind of R/C timer to allow power to come up first and then activate the reset.
There should be few parts to check to fix it.

 
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Re: Lokata marine receiver

Post by Niall » Fri Dec 27, 2013 12:42 am

The tech notes say:

"The circuit senses the power supply voltage and allows the processor to start when the voltage reaches 10.5Vdc approx. There is a preset pot which sets this threshold."

It is really difficult with this board to work out what does what. There are tracks and through holes everywhere including under ICs and the power on reset seems unnecessarily complicated.

I have found a 3 terminal device designed to do the same job, the MC34064. I think I will pick one up in RS tomorrow and just disconnect the reset line from the old circuitry and connect it to the 34064. It will sense the 5 volts from the 7805 on the processor board rather than the main 12V supply but I don't think that will matter. I'm calling this an upgrade rather than a bodge!

 
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Re: Lokata marine receiver

Post by Refugee » Fri Dec 27, 2013 2:32 am

That FET and BJT is telling me that there is a bad timing capacitor to be found and replaced to fix it with the original parts doing the job.
It only has to pulse it a few seconds after power is applied and that is all.

 
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Re: Lokata marine receiver

Post by Niall » Tue Dec 31, 2013 6:26 pm

I did have another go, changed a capacitor in the start up circuit, tested everything which seemed to be associated with it, no go.

I cut a track and removed a resistor to isolate the reset line, connected the MC34064 + and ground terminals to the appropriate pins of the 7805 and ran a link from the reset terminal of the 34064 to the reset line.

It looks from the info I have as if the original start up circuit was never very reliable.

The RX now powers up every time, currently on soak tuned to Shannon VOLMET on 5505.

Whatever goes on the bench next, I think I will try to make sure its something for which documentation is available!

 
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Re: Lokata marine receiver

Post by CTV » Tue Aug 26, 2014 2:08 pm

Hi Niall,

The designer of the Lokata marine receiver contacted me this morning, he's joined the forum and supplied me with the schematic for his receiver. I've turned it into a 16 page PDF and It is now loading into the Technical Library, its fairly large.

Thanks to Cosmo for taking the time to make contact and provide the data.

Regards
Chris

 
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Re: Lokata marine receiver

Post by Niall » Tue Aug 26, 2014 5:41 pm

Chris, that's great as it has gone again in the interim. I suspect I may have been asking too much of the reset IC and have bought a different one but not got around to fitting it. I may not get it back on the bench until the dark and cold evenings return.

 

Re: Lokata marine receiver

Post by cosmo » Wed Aug 27, 2014 10:28 am

Dear Niall,

My posts yesterday did not get through, so I am repeating some comments on the reset circuit.

This proved unreliable due to leakage from the nicad battery soldered on the board. The board should be cleaned as well as possible with isopropyl alcohol, and a new battery fitted off the board. The components associated with the reset circuit are rather high resistance values. These can all be reduced by a factor of 10, which will make the reset circuit less prone to leakage paths.
The receiver will work without the battery, however the clock will not run, and the station and status memories will be lost.
I have quite a lot of other design information in my 30 year old file. Please ask if you need anything else.
I still have one of these radios on my yacht, also a working Lokata Navtex receiver.

regards

Cosmo Little

 
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Re: Lokata marine receiver

Post by Niall » Sun Mar 01, 2015 11:14 pm

Many thanks for this information and the diagrams which I have now downloaded as I decided to have another go at this.
I see the diagram has the amended values for the reset circuit, so my first move will be to remove my modifications and replace the appropriate components.

 
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Re: Lokata marine receiver

Post by Niall » Wed Mar 04, 2015 6:48 pm

I have now replaced all the resistors in the startup circuit with the modified values as marked on the diagram in the tech. library.
The unit now starts up although it sometimes needs a few switch ons. I will try adjusting the preset in the start up circuit.
Currently on soak.

If I was doing it again I would work out the values for parallel resistors soldered across the existing ones as getting the old ones out of the double sided plated through board wasn't easy. For one which was 10M originally to be reduced to 100K I just soldered the 100K in parallel as the 10M won't make much difference to the final value.


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