Items Covered This Month Onkyo Surround Sound Home Theatre TXSR502 Cambridge Power Amplifier Yamaha RXV363 Home Theatre Amplifier Panasonic TC-60P22A (MX8 chassis) Trio Model CS-2110 Quad-Beam Oscilloscope

We have had a number of audio repairs come into the workshop recently, all of which have been irritatingly difficult.

The first was an Onkyo surround sound home-theatre system, model TXSR502, which was intermittently switching off. Unfortunately, the word "intermittent" often has different meanings for the customer and the repairer. To the former, it means that there obviously can’t be much wrong and therefore it will be cheap to repair an item that is actually working most of the time.

To the repairer, however, it means quite the opposite. For starters, it is often difficult to get the fault to occur to order so that the symptoms can be observed. An intermittent fault can also make it difficult to judge whether or not the fault has been really fixed, so that the set doesn’t come back under warranty.

Finally, tracking down an intermittent fault can be a time-consuming task and that adds to the expense. In addition, the unit has to be soak-tested for longer.

Unfortunately, the difference between these two positions usually has to be resolved into an estimate before you can start the job. And if guesswork is to be eliminated, you literally have to repair the set first, otherwise you could be hopelessly out.

Of course, you can always reduce an estimate but rarely are you able to increase it.

Fortunately, experience has shown that, after many years, the Onkyo TXSR502 can develop dry joints, especially in the pre-power amplifier driver stages. In fact, I had quickly concluded that the fault was probably due to the protection circuits switching the relays off, so this fitted in well with the known pattern of faults with this model.

As it happened, the board had quite a few dry joints so I was fairly confident that reworking the solder across the PC board would fix the fault. Eventually, after I had finally completed the work, I reassembled the set and put it aside to soak test.

Initially, all seemed well but my coffee fix had only just kicked in when, some 15 minutes later, I noticed that the same fault had started again. The speaker relays were clicking on and off and the set was locked onto the "Video 1" input and wouldn’t change.

This meant that it just had to be one of the two microprocessors the set uses. The main one, Q7502 on the display board, feeds the DSP (digital signal processor) which then feeds the protection circuits. I could measure the voltage changing on the output ports and could also see the data lines varying between the processors.

Being an Onkyo agent has its privileges and I was able to borrow another TXSR502 from another service agent. I then swapped the front display panels and found that the fault transferred from one set to the other.

OK, so the fault was on the display board, even though there was hardly anything on it apart from its microprocessor. This microprocessor (Q7502, µPD780232GE-030) is an 80-pin high-density square surface-mounted IC. It’s terrible to replace but cheap as chips so I ordered a new one, plus the duo-crystal X7501. They arrived fairly promptly and took a long time to install but I was relieved to think that this would almost certainly fix the fault.

Well, it didn’t!