Heineken
14-03-2021, 06:59 PM
The radio that came with the three amplifiers (http://www.nsxcb.co.uk/showthread.php?15905-Speaker-Amplifer-Repair-and-Capacitor-Replacement) was in a pretty bad shape. Capacitor leakage had even penetrated to the bottom of the board. The design of the 1992 EU radio utilizes several large capacitors on the lower PCB that can cause significant issues when leaking. Since the car spent about ten years in storage the electrolyte had plenty of time to do its damage.
14218
lower PCB, top view
14219
lower PCB, bottom view
As the owner already reported that it couldn't be turned on no attempt was made to try before removal of the old capacitors as well as cleaning and repair of the PCB. To get rid of the acid one of the flex cable connectors and the CD changer port was removed. The flex cable was stuck inside the connector so both connector and cable had to be replaced. Broken traces required two repair wires to be added and potentially more later.
14220
cleaned and repaired
14221
2nd repair wire
After installation of the new capacitors the radio came back to life - at least partially. It started and was able to tune to stations but audio output was muffled and needed a few minutes of warm-up. The CD changer was working but absolutely no sound even though it could be observed to arrive at the input pin.
Looking at the top of the board, the small Operational Amplifier (JR4560) near the CD changer port was heavily affected by capacitor acid and is responsible for connecting CD audio. It's no longer available as a direct replacement but an improved (and otherwise compatible) version (JR4580) is still sold, in a slightly narrower package (8-SOIC), though. The mounting pads of the old part are sufficiently large to work with the smaller package, too.
14222
new OP amp installed
14223
more repair wires
Getting the replacement installed required another repair wire and resulted in a working CD changer port but Audio was still subject to a warm up period. Poking around the PCB with an oscilloscope probe showed that the output amplifier's supply voltage was slowly changing and the muffled sound seemed to correlate. The voltage is created by a Z-diode in the acid affected area - a quick check with cooling spray and the soldering iron showed that the corresponding diode ZD807 strongly reacting to temperature changes.
After replacing it with a new Z-diode of 11.3 V the reference voltage returned back to 10 V and audio output was stable again. Unfortunately, a new issues became audible: Whenever muting was activated, the right channel made a popping noise. It took me until Saturday morning to find the reason in a defective capacitor on the equalizer board. It was introducing a DC offset that changed by 10 mV when the muting circuit was activated.
14224
the output amp reference voltage diode ZD807
14225
a working radio - back from the dead
A last issue with the tape deck seems to point to a defective motor. The speed control constantly switches between high and low speed creating a strange sound effect. Since the belt was fine and tape isn't planned to be used it was decided not to analyse it further. A replacement tape unit worked fine, though.
Quite a challenging but rewarding experience. More detailed information will be provided on a corresponding blog entry at a later time - need to finish another gauge cluster tester :)
14218
lower PCB, top view
14219
lower PCB, bottom view
As the owner already reported that it couldn't be turned on no attempt was made to try before removal of the old capacitors as well as cleaning and repair of the PCB. To get rid of the acid one of the flex cable connectors and the CD changer port was removed. The flex cable was stuck inside the connector so both connector and cable had to be replaced. Broken traces required two repair wires to be added and potentially more later.
14220
cleaned and repaired
14221
2nd repair wire
After installation of the new capacitors the radio came back to life - at least partially. It started and was able to tune to stations but audio output was muffled and needed a few minutes of warm-up. The CD changer was working but absolutely no sound even though it could be observed to arrive at the input pin.
Looking at the top of the board, the small Operational Amplifier (JR4560) near the CD changer port was heavily affected by capacitor acid and is responsible for connecting CD audio. It's no longer available as a direct replacement but an improved (and otherwise compatible) version (JR4580) is still sold, in a slightly narrower package (8-SOIC), though. The mounting pads of the old part are sufficiently large to work with the smaller package, too.
14222
new OP amp installed
14223
more repair wires
Getting the replacement installed required another repair wire and resulted in a working CD changer port but Audio was still subject to a warm up period. Poking around the PCB with an oscilloscope probe showed that the output amplifier's supply voltage was slowly changing and the muffled sound seemed to correlate. The voltage is created by a Z-diode in the acid affected area - a quick check with cooling spray and the soldering iron showed that the corresponding diode ZD807 strongly reacting to temperature changes.
After replacing it with a new Z-diode of 11.3 V the reference voltage returned back to 10 V and audio output was stable again. Unfortunately, a new issues became audible: Whenever muting was activated, the right channel made a popping noise. It took me until Saturday morning to find the reason in a defective capacitor on the equalizer board. It was introducing a DC offset that changed by 10 mV when the muting circuit was activated.
14224
the output amp reference voltage diode ZD807
14225
a working radio - back from the dead
A last issue with the tape deck seems to point to a defective motor. The speed control constantly switches between high and low speed creating a strange sound effect. Since the belt was fine and tape isn't planned to be used it was decided not to analyse it further. A replacement tape unit worked fine, though.
Quite a challenging but rewarding experience. More detailed information will be provided on a corresponding blog entry at a later time - need to finish another gauge cluster tester :)