Kenwood battery charger - repairable or worth repairing?

[Sparks]

The charging dock in these two pictures has stopped working today. Neither the red nor green lights are illuminating when the power adaptor is plugged in and my multimeter is showing 0 volts across the charging terminals. The loose grey wire on the left may be a red herring as it is cleanly cut and doesn't appear to have been connected to anything whereas the red and yellow are connected to the charging terminals.

I'd appreciate any clues or opinions. I don't want to spend around £30 for a replacement if it's just a simple issue. Thankyou.

[Cobaltblue]

I assume there is a wall wart that powers the charger possibly a transformer or SMP.

Are you sure that has an output.

It used to be a problem on the old budget Nicad battery drill chargers that the cable that feeds the actual charging station gets damaged and shorts the output of the wall wart.

Is this the case here?

Cheers

Mike T

[paulsherwin]

Yes, check the wall wart before doing anything else.

Is the charger for a LiIon device?

Have a look at the component side of the PCB and see if anything's gone bang.

[Sparks]

The power adaptors - I've used several different ones - are all functioning. Whatever the problem is, it's inside the charging dock itself.

[Terry_VK5TM]

Have you measured on the pcb across the input jack terminals for voltage?

Also model number of the charger would help to look for a schematic (or model number of the radio).

[Sparks]

As far as I can tell it is a KSC-17, although there is no model number on the dock. The radio itself is a Kenwood Tk359.

[G6Tanuki]

I see three contacts in the 'dock' where the battery goes; this would suggest some degree of intelligence in the charger circuit.

Testing with a simple meter might not present the correct information to the charger circuit for it to kick in; some of these sorts of things will not charge unless the battery presents a certain minimum voltage to the charge circuit [intended to stop the thing trying to charge a totally-dead/short-circuit battery pack], others use the third terminal for a temperature-sensing circuit [thermistor, or some forward-biased diodes] to moniotor the pack temperature and prevent overcharging; yet others use a fixed resistor in the battery-pack which essentially 'identifies' the battery-pack's capacity to the charger so it can apply the correct charge-rate.

Without knowing what the third terminal is for, diagnostics could be tricky!

[paulsherwin]

The third terminal will be for the temperature sensor, but it isn't connected to anything in the charger. That isn't unusual - most charge contoller chips don't use it.

[G6Tanuki]

Originally Posted by paulsherwin

The third terminal will be for the temperature sensor, but it isn't connected to anything in the charger. That isn't unusual - most charge contoller chips don't use it.

I wonder if - perhaps - the third terminal is where the trailing grey wire _should_ be connected to?

[paulsherwin]

Maybe, but it doesn't look as if it's ever been soldered.

[Sparks]

Thankyou for your replies. With a power adaptor plugged in - 11.5 volts at 600 mA - I'm measuring about 15 volts across the socket board terminals! I suspect that is my inexpensive meter overreading rather than the actual voltage; it's done it before. I've tried tracing +15V beyond the solder terminal but that's as far as it seems to get. So, the socket is getting power but not proceeding beyond it, if you see what I mean. I've also tried an audible continuity test beyond the + solder terminal and getting nothing.

Picture of the board.

[paulsherwin]

If the adaptor is unregulated then the output voltage will increase offload.

Is it something as simple as a dry joint on the socket? Try reflowing the joints.

[Sparks]

Could well be dry joints. When I finish my next work shift I'll hopefully have time to look further. Thankyou.

[SiriusHardware]

As already mentioned, 'simple' unregulated supplies which contain little more than a transformer, four diodes and a capacitor always output a significantly higher voltage than their marked voltage when they are not loaded or lightly loaded, so the 15V+ output you are seeing will be correct, and the meter will be blameless.

You could assist us by taking two more well-lit, well focused photos of the PCB from directly overhead and directly underneath - try to take the shots such that the PCB exactly fills the screen on your phone or camera, whichever you are using to take the shots, to give us a chance of seeing enough detail to be able to trace / understand the circuit.

After it leaves the DC power socket the +ve supply goes to two capacitors, one electrolytic, one non-electrolytic, both down to GND. The only other direction it goes in is through the diode which is next to the DC socket, so measure the voltage (with respect to 0V) on the 'bar' or 'ringed' end of that diode and see what you have there.

[Terry_VK5TM]

The hunt for the schematic for that particular charger has been fruitless.

A quick squizz at the top side photo appears to show it might be microprocessor controlled re what appears to be a ceramic resonator to the left of the verticallly oriented chip.

[Sparks]

Thankyou for the advice, Sirius, especially regarding the multimeter! I'll take more pictures next time I'm off work, in a few days.

[David G4EBT]

The grey wire clearly has a purpose in life, and so does the middle contact for the battery pack, which presumably also has a middle mating contact. Looking closely at the poor quality of the soldering of the yellow and red wires onto the contacts, and the fact that the loose grey wire seems to be cut flush, my guess would be that when the insulation was stripped, the stranded conductors were 'nicked' and the grey wire has come adrift from the central contact.

The yellow wire is almost a right angles to the solder pad, so to my eyes, looks kinked at the point where the insulation was stripped. Maybe an indication that it too was 'nicked' when stripped. If an iron is applied to the central contact, when the solder melts, if it has the remnants of wire strands, that would confirm any suspicions.

That central contact simply must have a purpose in life - maybe to tell the charging circuitry when the battery pack is fully charged? If the grey wire doesn't belong to the central contact, where does it belong? It makes no sense. I'd be inclined to connect it to the central contact and check the charger with the battery pack plugged in, and I'd remake the poorly soldered joints on the yellow and red wires.

Just my thoughts.

[paulsherwin]

You may well be right David, but it's not unusual for LiIon chargers to not use the battery temperature sensor. I agree that carefully checking if the contact has ever been soldered would be sensible. It doesn't look like it to me.

[Nickthedentist]

It doesn’t, but I’ve just repaired a 1985 desktop calculator which was dead after being dropped. The wire from the PCB to the -ve terminal of the battery holder had pulled off the terminal, leaving it looking as if it had never been soldered.

[Sparks]

Thankyou once again folks. Closer inspection and more pictures to follow asap.