Introduction: Repair Samsung DS-24D Keyset (low Volume Fault)

By
About: Retired old fart with interest in fixing things, both electronic and mechanical. 3D printers, 3D CNC router, Laser Engraver, Laser Cutter and a huge quantity of spare parts.

Introduction

The Samsung DS-24D keyset phones form an essential part of a powerful domestic/commercial PABX phone system that is quite common worldwide. Unfortunately these phone keysets are ageing, and can suffer from the dreaded "loss of volume" affliction, particularly in warmer climates. This article is to show that it is possible to repair these phones successfully, and return them to full working condition.

Our Story

For many years our home has been fitted with a Samsung DX408 PABX phone system with four DS-24D keysets and 2 cordless handsets. Over the years we experience occasional failures in the DS-24D keysets, eventually purchasing two extra refurbished DS-24D units. However, the situation got a lot worse in late 2019 when our telephone system was connected to Australia’s NBN network, and we found ourselves with only one fully functional keyset, and five “faulty” ones.

After a bit of testing, including swapping handsets around between the keysets, we determined that we had only one type of fault common to all the defective keysets. They all appeared to be working OK digitally (display responded fine), but suffered varying degrees from loss of earpiece volume. The worst one had lost over 98% of earpiece output, while the best had lost "only" 70% volume. None of these keysets could be used for making or receiving calls

It was then found that our one “good” keyset had actually lost about 10% volume (but was still usable).

An Internet search turned up little useful information on these phones, except except where to buy second hand ones.

The attempt at diagnosis and repair was attempted on one of the “faulty” phones that only had about 10% maximum volume output. I figured it was hard to "break" something already broken and unusable

Diagnostics

The process of swapping handpieces between keysets proved that the fault was not in the handpiece (a very simple and rugged device), but stayed with the keyset itself. All handpieces worked fine on our one remaining "good" keyset.

Repair Process
The keyset with the second lowest volume was chosen as the first patient/victim. The process below was followed:-

Supplies

If you want to try this yourself, you will be dealing with small components in a surface-mount printed circuit board environment.

It helps to have access to access to:-

A:- A temperature-controlled soldering station with a fine tip in the iron,

B:- Some fine solder, a clip to help hold the small parts

C:- Small needle-nose pliers and small side-cutters for forming and trimming leads

D:- PCB cleaning fluid (spray pack) - (optional)

E:- A steady hand, good eyesight, bright light, patience and a some experience with small components.

Oh yes! - - - and F:- Some mini 10uF 35Volt capacitors (4 needed per phone).

Step 1: Dismantle DS-24D Keyset

All the work was carried out from the underside of the keyboard.

First the handset was unplugged and set aside. The plastic stand was un-clipped from the underside of the phone and also set aside. The 6 screws were removed from the underside of the case, and the plastic bottom cover lifted clear. It was found that there was no need to remove the circuit board from the case (a good thing actually as it helps to protect the components, and the keyboard pads, from accidental damage and fingerprint contamination).

Step 2: Trying to Find a Fault

At this point we carried out some tests on a few of the visible components associated with the power supply, but no obvious fault was detected. Next, the PCB itself was closely examined under magnification. The area outlined in the image above immediately came under suspicion when signs of green deposits were seen on some copper traces. This is usually a sign of failed capacitors.

It was suspected that the four tiny 10microfarad mini capacitors (C45, C46, C47 and C64) had failed and released the fluid onto the board. There was no evidence of distress with any other value capacitor on the board. The 4 components were all located close to the handpiece socket and are believed to act as “decoupling” capacitors for the microphone and speaker in the handpiece and “speakerphone” setup.

Step 3: Removal and Testing

As it was virtually impossible to test these components in place, it was decided to remove them for testing.

The four capacitors were carefully removed from the circuit board. A clip was used as a “handle” for a component, as a fine-tip temperature controlled iron was used to carefully melt the solder one side at a time as the component was carefully “rocked” off, as the solder melted. Extreme care was exercised to ensure the iron tip did not contact any other component, and ensuring the solder on the pad had fully melted before attempting to lift the side of the capacitor. This task may be made a little bit easier by adding a tiny amount of fine solder to each pad before attempting the remove each component.

Once the four capacitors were off the board, they were tested in a (fairly cheap) digital component tester. The result showed that all 10uF capacitors were well below their rated value. Two were just under 6uF and the other two were between 1uF and 2uF. BINGO!!! The worst value was C46, and the next worst was C47. Both these seem to trace to to the handset socket. If these components are low in value, less signal would pass to the earpiece, and volume would be drastically reduced in proportion to the reduced capacitance. It all seemed to Tally. It was decided to replace all 4 parts for safety.

Step 4: Sourcing Spare Parts

The removed components were surface mounted (smd) mini capacitors marked as 10uF x 16Volts (see left side of the image above)

Fortunately a small quantity of high-quality 10uF x 35V board-mount (leaded) mini capacitors were still in my stock, left over from a repair of a Bose 8 channel amplifier out of a S300 Mercedes (see right side of image above). It was felt that these might be a better substitute for the Samsung project as the leads could be easily adapted for surface mounting, and would allow easier soldering with a standard fine-point tip. It was also thought that the higher voltage rating may allow them to last longer than the original 16 volt parts under similar conditions (sub-tropical climate with summer heat).

These capacitors were originally bought from RS Components under the part number 475-9396. They are described as “10μF 35V dc Aluminium Electrolytic Capacitor, Through Hole 5 (Dia.) x 5mm (High)”.

Step 5: Preparing the New Parts

The circuit board solder pads for mounting the capacitors were first prepared by melting a dab of solder onto each one (see image).

The leads for each capacitor were bent outwards a short distance from the base (say 2mm), and cut short just beyond the edges of the case (see image).

The end of each lead was tinned with solder to make installation easier. This job is easier if the small component is held in a clip (see image in next step).

Step 6: Installing the Capacitors

An alligator clip was used to grip the small component to make installation simpler (see image above). Sometimes the capacitor needed to be rotated in the clip to achieve a better position for getting it into place on the PCB. (Please refer to the note below on component orientation)

Due to the small size of the pads and components, a high level of care was needed to solder each capacitor back in.

(NOTE) The adjacent pads on C45 and C46 are quite close, so care was taken to ensure the capacitor leads of these two parts don’t meet and short circuit. For safety, one lead was shortened a tiny bit more.

After soldering, the circuit board in the area of the repair was cleaned with a good quality PCB Cleaner to remove any traces of solder flux, and any residual corrosion products from the original failure.

(Note:- These components fit only one way round. The negative side is marked and it is vital that the parts are soldered in with the negative side to the bottom of the board (refer image above)).

Step 7: Re-assembling the Phone

This was just the reverse of the dismantling procedure.

(Hint) I like to avoid cutting extra threads, and losing material, wherever a screw locks into a plastic part. Eventually so many threads are cut that the thread strips and there is more for you to repair. The process I use is to rotate each screw anti-clockwise (undo) gently until I just feel the thread "click" into the previously cut thread. Now the screw can be turned clockwise down the original thread in the plastic part. Seems to work for me.

Step 8: Retesting

The DS-24D phone was re-installed and tested. Volume from the earpiece had returned to 100% normal. The entire phone was found to be fully operational.

Conclusion

Since then two more DS-24D keyset have been repaired in the same way (until my spare parts ran out). One started out with maximum 5% volume level. The C45 capacitor removed tested at only 5% of its marked value. A similar match-up was found with the second phone that had previously written off as completely useless (volume level almost undetectable at 1%, and the C45 capacitor value of only 0.09uF).

After undergoing the identical repairs both phones tested out with 100% volume. It was noted that one of the PCBs showed almost no evidence of capacitor leakage or corrosion damage.

Both handsets have now been returned to service and are working fine.

Now to lash out and spend some actual cash ordering spare capacitors for two more phones. I've decided to replace these capacitors anyway in the one phone that was still working (who knows when it will fail too, like all it's mates did).