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US Radio Head Unit (USDM 1991) - Repair

5 August 2014
While working on the radio from France the unit from Another radio question came in. Just as described, all inputs and functions were fine but the speaker output was intermittent and worked only for a few seconds before cutting in and out randomly.

Since there was obvious damage by leaking capacitors, their replacement and taking care of the PCB was the first item on the list.


traces near E509

With the large capacitors removed, it became more obvious.


no massive damage but on a good way to cause further problems

In addition, the flex cable connector next to the CD port showed suspicious greenish marks, usually related to being contaminated, too.


that color ain't supposed to be there

After removal of the flex cable it became rather obvious why there was intermittent sound. The pins were corroded up to a level where they potentially lost contact completely. This requires a new connector and a new cable before continuing any further. After removal of the connector, some pins measured a resistance of less than 200 kΩ against other pins - in an OK connector they should be >2 MΩ.


not looking good

Since the whole area was contaminated, it required the usual treatment: Cleaning the PCB down to bare metal and strengthening the VIAs by soldering a thin wire through them - about 30 of them were reworked like that, including the capacitor area.


that should be better

A first test run and and one cold solder point later the radio was working stable and the two remaining capacitors on the power PCB were replaced. Not much damage to traces there but the capacitors already started leaking, too.


on the bench

The US PCB layout seems to be more robust against acid damage. The usual path of the capacitor liquid does not cross too many components and as there is no extra processor one the lower PCB, required for the RDS decoding on EU radios, that one can't be damaged either. Happy to have a more-or-less regular repair instead of the usual, more difficult cases.
His posts make me way more scared of doing the caps in my radio.
The earlier you start, the lesser the issues .. the longer you wait, the bigger the problems.
One radio from NZ was just diagnosed to be damaged beyond repair - a processor pin has an internal short and overloads a data line. Not much that can be done, unfortunately since it's an Alpine proprietary chip.