VynZ Modern Life

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How to identify and repair a cold solder joint

A cold solder joint refers to a soldered joint which has become loose over time due to vibrations, heating and cooling cycles, work hardening or other physical effects. I bought a nice but old Sony surround sound system on eBay. It worked really well until the centre channel began to cut out. This video shows the effect of pressing down on the heat sink indicating a cold solder.

In this case, the problem area was localised to the power amplifier board as a result of observing the results of flexing different areas of the board. It was also clear that the problem related to the relays which switched the channels on and off. Step one therefore was to get at the board. The STR-DB940 is a large heavy device and is assembled using modular boards to perform the different functions. It was therefore relatively easy to dismantle the device. The photograph below shows the internals of the unit with only the cover taken off. The power amplifier board is located at the bottom of the aluminium chassis.

The ribbon cables are disconnected by pulling carefully on the ends. Leaving one end connected makes it easier to re-assemble later. Once a board if free of cables, the screws are undone to free the board. The photograph below shows all cables detached from the top board which is the digital decoding board.

Underneath the digital decoding board are two analogue input/output boards. These are shown below and are removed in the same way.

Now we are left with just the power amplifier board. This unit also has an access panel located at the bottom. This means that for most repairs, the power amplifier board will not need to be removed completely.


Once the board is accessed, the job of reflowing the cold solders can begin. It important to keep a tidy and organised workplace. Keep the parts removed in order so they can be re-assembled in reverse. A grid of several small containers, in this case a Ferrero Roche packet, is useful to keep the screws similarly spaced and in order.

 

Often the offending joints can be easily identified by a dark ring around the joint indicating a crack. In my case, no such cracks could be observed. I thus proceeded to reflow the solder along the power traces, jumpers and of course the relays. I then reflowed the power transistors for good measure.

After reflowing the solder on several possible joints the amplifier was reassembled and tested. Result: centre channel now works perfectly with no pressure applied and there is no relay clicking even when pressure is applied to the heat sink.

The end. One working surround sound amplifier.

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This entry was posted on 1 April, 2013 by in Technology.
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