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Radio Head Unit - Repair II

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.. continuation of Radio - Repair

Too little Volume and Distortions increasing with Volume

Observed on: 1 EUDM Radio
Fixed by: OP amp replacement
State: Fully understood

The audio signal path on these radios is fully analogue with several operational amplifiers at various stages. One of them is IC303, a 20 V, 10 MHz quad operational amplifier from JRC/NJM: NJM2060M.

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IC303 at the rear end of the lower PCB

According to the data sheet, voltage supply pins are 11 (V-) and 4 (V+) (the middle pin on each side of the IC). Output pins are 1, 7, 8 and 14 (the outer edge pins). All remaining pins (2, 3, 5, 6, 9, 10, 12, 13) are input pins.

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NJM/JRC2060M Pin-out

Without going into details about specific OP amp circuits, the first check should be if the supply voltage is correct at 0V (pin 11) and +10V (pin 4) since a too low power supply will distort signals at higher volumes. If correct, check the input signals.

Alpine configured this OP amp to work as a voltage follower. The output pin of each amplifier (e.g. pin 14) is directly fed back into the negative input pin. Example visible on the photo above: Pin 14 (D OUTPUT) connected to pin 13 (D-INPUT). That leaves the positive input pin (+INPUT) as where the audio signal arrives.

At this stage of audio processing it will be a 1 V audio signal riding on 5V DC that can be checked with an oscilloscope. If the input signals are implausible that could be an indicator of issues further upstream or a defective IC303 pulling down the input signal.

If all looks good, check the output signals on pin 1, 7, 8, and 14. These should be identical to the input signals (typical for a voltage follower circuit). If these signals are implausible, replace IC303.

Phaser-Effect-like Sound
Observed on: 2 EUDM Radio
Fixed by: Trace repair
State: Fully understood

With the balance control knob in the middle position listen to music in stereo. If the right and left channel sound like they are cancelling each other out and the only way to make it sound "normal" is to turn the balance fully to either side than it's very likely an issue with the PCB tracks under the last stage amplifier power supply stabilization capacitor E818.

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E818 with acid damage

E818 often starts leaking together with its neighbours E816 and E817. When doing so, the acid starts to eat away on the underlying traces. Once the lowest trace is dissolved, the amplifiers start to float and cause above sound effect that can sound like an echo or voice cancellation effect.

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broken trace underneath E818

The operation of the output amplifiers return to normal once the broken connection is repaired - no further steps are needed.

Ticking sound even with volume at zero, disappearing over time (warm-up)
Observed on: 1 EUDM Radio
Fixed by: Replacement of flex cable connector, re-soldering
of VIAs
State: Partly understood

A ticking or thumping sound is observed on all inputs and is unaffected by the volume control. In addition, the area between the right flex cable connector (connecting to the middle PCB) is sensitive to temperature changes (usage of freeze spray on the underside of the PCB):

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the temperature sensitive area

Even though the effect wasn't fully understood, it's probably to PCB issues. After replacement of the flex cable connector and re-soldering all VIAs the issue wasn't observed any more and cooling it with freeze spray did not longer have any effect.

Distortions on Radio and Cassette input disappearing over time (warm-up)
Observed on: 1 EUDM Radio
Fixed by: Adding a pull-down resistor on the muting control line

State: Partly understood

When starting the radio after ~24 hours of being disconnected from power, both cassette and radio (but not CD) have audible distortions. The wave form at the speakers show a compression effect in the lower half of a test tone (1 kHz tone recorded to cassette):

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red line shows the compression of the lower half on a test tone sine wave

The effect fades away with time and can't be measured or heard after several hours of continuous use. It does not show any sensitivity to temperature when spraying the PCB with freeze spray.

Audio from cassette and radio pass through the Dolby Noise reduction chip before entering the muting circuit, followed by the digital noise reduction chip:

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The signal is distortion free when measured at the Dolby chip test points but distorted when measuring at the muting circuit input and output pins:

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corresponding test points

The issue is caused by a too high DC voltage (several 100 mV) on the muting control line when there is no muting request. This voltage drops slowly and reaches normal levels (~60 mV) after several hours of radio usage and the distortions disappear.

Confirmation: Connecting the muting control line to ground removes all distortions.

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If the source of the DC offset can't be detected (tracing the muting control line across the flex cable connector next to the CD port and continuing on the middle PCB) adding a 1 kΩ pull-down resistor can mitigate the issue:

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installation of the pull down resistor near D301

It sufficiently reduces the DC offset while keeping the muting functionality working. Since muting is only activated sporadically (while tuning or cassette operation) there shouldn't be an issue with the increased load.

Note: Page limit reached - continued on Radio Head Unit Repair III

Updated 12-07-2023 at 09:28 PM by Heineken

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  1. Heineken's Avatar
    Added information about Phazer-like-sound and how to fix it.
  2. Heineken's Avatar
    Added information about issues with the muting control line.