Panasonic Dect 'Phones KX-TG7120E S problem

  johndrew 17:01 02 Dec 2010

I have one of the subject 'phones (click here) which also has a second handset.

The whole lot was fine until a short while ago when the second (satellite?) handset seemed not to charge correctly - all the indications were that it was charging but when removed from its cradle it rapidly lost power.

I went through all the recommended battery terminal cleaning processes but this had no effect. However when I installed the handset into the main (as picture) unit it charged correctly and functioned normally.

I did a voltage check on the second base unit and found it to be supplying over 11VDC as opposed to the 9VDC at the terminals on the main unit. Both wall adaptor/transformer-rectifier units claim to supply 6.5VDC with a slightly higher power unit for the main base unit.

Contact with Panasonic has not helped particularly as they are unable to provide details of a voltage normally found at the terminals in charging units or to confirm if the handsets contain an over-volt protection circuit.

I have opened the satellite charging dock/unit which contains only a printed circuit board with four resistors located on it - nothing there to up the voltage that I can see. The resistors are supposed to be 9ohm and 10ohm but are down on these figures by 1ohm (at 8 & 9 respectively) assuming my meter is accurate. I have been unable to get a reading from the wall adaptor/transformer-rectifier unit even when plugged into the circuit board and taken at the input terminals of the circuit board mounted socket - even though I get a voltage at the output contacts as mentioned above and the 'phone appears to be charging correctly (strange).

Can anyone suggest any further checks/corrective measures for this problem and/or advise on the types of voltages to be expected at the dock/'phone terminals?

Many thanks in anticipation.

  woodchip 17:35 02 Dec 2010

You cannot read a resistor on board as it may be coupled in series or parallel with other components you have to de-solder one end to get correct measurements

  mooly 17:44 02 Dec 2010

Charging circuits are usually simple. The voltage may be just half wave rectified AC so measuring "off load" will give a false reading on a meter set on DC volts. That ties in with the readings you get... 6.5 volts so called DC is 6.5 times root 2 which is just over 9 volts. Off load drawing no current it's over 11.

You need to verify that the main unit correctly charges both handsets and that the satellite doesn't. If only one charges in the satellite then that's odd and points to an odd physical issue whereby connection is OK in one but not the other... if that makes sense.

  Pineman100 17:49 02 Dec 2010

We also have a Panasonic DECT phone kit - one main and one remote unit. Our model is KX-TG8421E. We've had it for about 18 months and it hasn't yet required any new rechargeable batteries.

I've just put my meter on the charging terminals. The main unit is delivering 5.5VDC and the rmote unit is delivering 6.5VDC.

I don't know whether that's any help.

  Pineman100 17:51 02 Dec 2010

mooly posted while I was typing. Having read his knowledgeable post I should perhaps add that I took the above readings with the handsets off their stations.

  johndrew 10:32 03 Dec 2010

Many thanks for coming back people.

When I read the resistor I was contacting only the two soldered ends of each in turn. Given there was nothing else connected to the PCB during the process (and I checked the values against a band chart) how should I get a false reading?

Your post tells me the voltages I saw are probably good (not that I fully understand the technicalities behind it). Can I assume the reason I get no reading at the socket in the satellite dock but do at the 'phone contact points when it is plugged in and switched on is something to do with the load applied by the resistors?

I have verified the main unit charges both handsets satisfactorily. But I fail to understand whether it is the satellite dock that is faulty or its power supply given that the voltages look good. Could it be the power supply is failing under load?

Many thanks for your efforts. I can see no reason why the voltage you see is so much lower than mine - as you say mooly may know - given our 'phone sets are very similar.

  mooly 11:38 03 Dec 2010

It's very hard to say what's going on without actually seeing it.

You say both handsets are OK in the main unit.

Now important question... are BOTH handsets giving the same no charge condition in the satellite station or only one of them ?
Thats's a vital clue, because if both handsets are OK (proved in the base unit) and yet one charges OK in the satellite and the other soes not, then the satellite must be electrically OK. Which leaves a physical problem, contact misalignment etc as a posible difference between handsets or an odd battery issue... see next para.

Just in case the charging circuits are different between base station and satellite would it be possible to swap the batteries over between handsets to eliminate that as a cause. There are sound technical reasons why that could occur although it sounds strange. The cells may have aged and still have good capacity but need charging with a higher voltage to force the current into them. That seems at odds with your readings but without knowing the circuit details it's possible... the main station could be smoothed DC while the satellite is just half wave rectified... and a meter doesn't show this, it just gives average readings.

Not sure if I am reading your first post correctly now... if you can not measure any output from the power supply try measuring on AC volts rather than DC. Are you sure the rectifier is in the PSU and not in the station ? If there is no voltage on either AC or DC volts then the PSU would appear faulty.
Can you measure DC voltage just across the charge contacts on the handset without it connected to anything... could that be confusing things.

  grey george 11:45 03 Dec 2010

You should be getting a reading from the jack plug load or no load. Is it the common centre positive female and outer negative male type? If so it may be worth using a paper clip or other wire to get right into the heart of the positive hole. It may be that the plug is badly made so your not getting a reading on the outside bit, which could mean only enough power is getting to the base to make it look like it's working when it has not enough to actually charge the phone. These little power supplies also often suffer with broken wires close to the ends. have you tried flexing the cable while trying to get a reading?

  johndrew 14:36 03 Dec 2010

Thanks for spending the time on this. I'll try to be as clear as I can.

The power supply is one of the 3 pin (13amp plug type) 'bricks' that plugs into the wall socket and has a simple round plug on the end of the supply wire. Details from the data plate are indicated as 250VAC with an output of 6.5V (doesn't actually say DV but gives the international solid over dotted line motif) at 350ma. Also confirms the centre connector as +ve with the outer as -ve.

The dock (station) is a simple plastic box with the PCB contaning only four resistors and a socket for the power supply connector.

If I connect the supply to the circuit board in the satellite dock and put the meter across the power supply socket solder points I get nothing. If I put the meter across the sprung loaded pins the handset terminals would contact I get slightly over 11VDC on the meter. I have used the probe points to ensure a good contact in both cases but find this very strange.

I have just put a meter on the 'phone handset contacts, having just removed it from charge, and there is no reading at all - this is the same for both handsets. I suppose it makes sense as otherwise it would be possible to short them out.

In the past I have swapped batteries between handsets and had both handsets in the satellite dock. Perhaps because the batteries were already charged I failed to confirm exactly what was happening as I must keep one handset, at least, fully functional for communication and was wary of allowing the one I was confident in running low on charge.

One thing I perhaps wasn't clear on is that when I put a handset in the satellite dock the LED at the top of the handset lights as it would in the main unit and the charge bar on the screen indicates it is charging. However when the charging indicator bar shows full and the handset is removed for use the handset will fail to a discharged (blank) condition almost immediately. Putting the handset in the main unit will cause it to charge and function correctly.

There is only one other point that may be relevant and that is the satellite dock is located near a DAB radio that is powered using a similar power 'brick' plugged into the other part of a double wall socket. Could this have any effect?

I hope this makes sense and clarifies all points you raised. If not I will need to try again.

  johndrew 14:37 03 Dec 2010

Thanks for your suggestions. I think I may have answered your points in my diatribe above.

  grey george 15:16 03 Dec 2010

Sounds as if the charger is flatening the battery. Can you check that the spring charging points are + & - the same way round in both chargers? Have you checked the faulty base on an ac setting? Might be the transformer is failing an putting out both.

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