Sony HCD-HX80R Amplifier Volume Control Problem

[crackedprint]

I have a strange fault on a Sony amplifier. The volume control varies the display from MIN, then 1 up to 30 and then MAX as expected. At MIN there is no sound. However, at 1 it is quite loud and increasing above 1 does not change the sound level.

I have tried a Cold Reset as per the service manual.

I have looked at the I2C clock and data to volume control chip IC501 (BD3499FV). Data is only sent to IC501 when the volume is set to MIN or 1. At any other setting no data is sent to IC501.

I have looked all the way through the service manual and tried many online searches and come up blank. Does anybody have any suggestions please?

The CD player and FM tuner work other than the volume problem.

[crackedprint]

I have made some further progress with the Sony HCD-HX80R. The volume control does not work the way that I had assumed. A functional description to go with the block-diagram in the service manual would have been nice.

In the block-diagram IC501 (BD3499FV) is labelled 'INPUT SELECTOR, ELECTRICAL VOLUME, SURROUND / TONE CONTROL'. The data sheet shows that it is capable of performing a volume control function in dB. However, it is not used to control the volume. The System Controller always sets it to either -infinity or -12dB. This is what is causing the volume to go from nothing to loud when the volume is increased from MIN to 1.

This hi-fi unit does not have a conventional audio power-amplifier. The power amplifier is completely digital (presumably class D).

The actual control of volume is in the digital domain, in the Stream Processor of the power amplifier. The Stream Processor is IC705 (CXD9788AR). Unfortunately I can't find a datasheet for this part. However it has a serial interface and I have had a look at it with an oscilloscope. When taken out of standby, the System Controller sends a scree of data to IC705 via its SCLATCH, SCSHIFT and SCDT, pins 23, 22 and 21. The System Controller then sends it a little burst of data about once a second. I have no idea what that is meant to do. However, when I turn the volume knob I can see additional data being sent that is associated with the knob being turned.

I decided to stop control data to IC705 completely to see what the effect would be. There are resistors in series, so I grounded IC705 pin 22. Behavior is exactly the same with the serial communication disabled. I therefore suspect that the serial port on IC705 is faulty.

The System Controller IC301 in on the Main Board and the Stream Processor is on the AMP Board. They are connected together by a fairly long ribbon. The AMP board connects to external speakers via long lengths of flex. My guess is that this failure is the result of a thunder storm.

I have ordered a replacement CXD9788AR, but I am not sure how reputable the supplier is. We shall see...

[crackedprint]

I eventually managed to obtain a replacement CXD9788AR stream processor from Alltronic Computer Singapore. This has cured the fault. The volume control now operates correctly.

So, in summary:

Symptoms: The volume control varies the display from MIN, then 1 up to 30 and then MAX as expected. At MIN there is no sound. However, at 1 it is quite loud and increasing above 1 does not change the sound level.

Cause: Faulty CXD9788AR stream processor IC705.