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.