Radio Head Unit (EU 1991) Repair

[Heineken]

A direct message via the the German Motortalk forum reached me. A distressed NSX driver was reporting about trouble with his radio head unit. The volume was unusually low, noise was generated when changing the volume and another repair approach had already failed.
We talked a bit over the chat and since radios that haven't fully failed yet can often be repaired, decided to ship the unit. When it arrived I had a quick look inside and found an interesting graffiti that even the owner was not aware of, so It's source might stay a mystery.

The first assumption was that leaking capacitors and destroyed PCB traces where the reason for the issue and once repaired would provide a fully working radio. Fortunately or unfortunately, through a previous (and well executed) repair, the radio already gained new capacitors and multiple repaired traces.

This proved, on one hand, that the radio did suffer from leaking capacitors but also that it couldn't be the reason for the current issue – the search continued. All previously added repair bridges were connected correctly and the problem occurred on all audio sources (CD, Cassette, Radio) in exactly the same way. The potential reason therefore had to be near the end of the audio processing chain.

A check of the power supply for the last amplifiers before the speaker output (IC205 and IC206) yielded no results as the 10 V were absolutely stable (a problem detected in a another repair). Replacing the volume, tone and power supply PCB brought no change either. Out of suspicion, the four larger capacitors used to decouple IC205 and 206, were de-soldered and checked. One of them was a little off so all four where replace. No improvement whatsoever.

While working on the PCB, an interesting component came into focus - an analogue multiplexing chip named MC14066B, called IC304. He's probably responsible for the audio source selection. If he would be defective, all sources of audio would be affected. Installing a replacement from a broken radio PCB still made the same low and distorted sound appear - no luck again.

Right next to it is IC303, a four channel OP amplifier named JRC2060. While not 100 % sure what he was supposed to do, measurements where very strange. Correct voltage supply, correct input signal but almost no output .. .. after his replacement, loud and full static noise could finally be heard (the sleeves of the capacitors nearby took a bit of beating due to the hot air soldering but nothing serious).

Informing the owner, and he was just as happy as me to hear the the unit was finally fixed. Typical for such endeavours, finding the issue is the tricky part, not so much the repair itself.
Final test were all fine and the volume button that needed to be removed when testing with a replacement PCB was secured with thread locker to avoid a strange look of the radio front if the knob gets loose.

A significant amount of time went into the analysis but the gained knowledge should come in handy for future repairs. The exchanged ICs (MC14066B and JRC2060) are still available, via the usual channels, btw.