Rotel RT850L Tuner

[WaveyDipole]

I have had this tuner for some time having picked it up for next to nothing with a known problem. The original problem was a rumbling noise that appeared some time after the turner was turned on and got gradually worse until sound disappeared altogether. The rumble was traced to a couple of trimmer capacitors which were removed, cleaned and retuned to the PCB. This cured the rumble, but the tuner would still suddenly stop working - no audio, no signal on the meter. A tap in the vicinity of the antenna tuner seemed to restore the signal. I re-flowed a number of joints in the vicinity and this seemed to cure the problem. However, it was while working on this that the third problem came up.

The problem I have now is that the frequency is no longer shown on the digital display. I can change between AM and FM and the radio still tunes, I can receive stations and the tuning band is shown, but I have no idea which frequency I am tuned into.

I have established that I have both an FM and AM frequency signal going to the TC9175AP PLL/control chip. The AM frequency signal goes directly from the LA1245 AM receiver chip to pin 39 of the TC9175AP. The FM frequency signal goes first to an FM pre-scaler (TD6104P) and the output of that goes to pin 38 of the TC9175AP. The correct voltage is present on pin 28 of the display driver.

Voltages around the TC9175AP seem to be as per circuit diagram but I am uncertain as to how to check that data is being sent from pin 24/25/26/27 to the display driver. I couldn't find a datasheet for the TC9175AP (if anyone has one a copy would be appreciated) but according to the datasheet for the TC6301AP display driver chip, data transfer is accomplished using a serial system via 3 wires - DATA, CK1 and CK2 (and I would imagine a GND). Assuming that it is being sent, is it possible to read this data with anything?

[WaveyDipole]

Finally got around to probing the serial data line between the two ICs with a logic analyzer and got absolutely nothing on it. Changed tuning frequencies, bands etc and still no change, just a flat line. On the oscilloscope I am seeing a steady 40mV sine wave signal at approximately 45-70MHz with some 120mV peaks at around 25kHz. What I am not seeing is any square data signal.

I have measured the frequency signal between the IC and the AM section and it is consistent with the tuned frequency plus a 450kHz IF. The tuner is tuned to 106.5FM and I was picking up a frequency signal of 106.3xx at the pre-scaler which became 49.995 after the pre-scaler.

I guess that the TC9157A IC has failed in some way and is not outputting data to the driver anymore. Strange that the PLL part is still working as the tuner still receives OK.