I’m modding the Philips AZ5737 boombox from 2006 to support Bluetooth playback, it also need some repairs; the left speaker has no sound and the tape drive doesn’t move when pressing play button.
The CD/DVD player can’t read any discs either, not sure if it needs calibration. I did find two potentiometers which can be adjusted on the head assembly but more likely the laser diode is dead. Unfortunately the flat flex cable (FFC) to the CD player had some leads damaged when removing it, so I will skip repairing CD player for now until the FFC cable on order arrives.
I analyzed the missing speaker output by following schematics in the service manual while measuring the output and eventually figured out the problem was the volume potentiometer which read really high resistance for left audio channel.
The volume potentiometer labelled “B503” has six pins with support for left and right channel audio, both variable resistance for speakers and fixed resistance for audio output jack, and a ground pin. This pot turned out to be hard to get, I ordered from two different shops which sent me the wrong one with smaller foot print, despite showing the correct one in product photo (as seen above). On the third try from yet another shop I finally got the right pot but it had slightly longer shaft which turned out to be OK. Replacing the pot fixed the problem with left channel audio missing and both speakers now sounded fine.
The tape drive has never been used until now, so it’s generally in good shape, but over time the two rubber belts had dried up and were too loose which prevented the motor from turning the tape. I replaced the belts, cleaned the head with IPA, greased up the plastic gears and moving parts with silicon grease. After this the tape drive started working.
I ordered a cheap Bluetooth receiver board called “VHM 314”, it’s the small black board (PCB) seen on the right in image above wired up to the green mainboard on the left. While browsing the schematics in the service manual I found a suitable 5V and ground point for the Bluetooth board close to the connector for the CD player and the big switch selecting between CD/FM/AM/Tape had the right and left audio points for CD needed. After connecting the Bluetooth board the power LED lit up and it worked to connect with the phone to it via Bluetooth but no sound could be heard when playing music on the phone. I tried connecting headphones directly to the jack on the Bluetooth board and the music could be heard there so the problem was something related to left and right audio connections to mainboard.
After some head scratching and spending more time reading the schematics I found a mute circuit (“L-MUTE” on image above) which turns off the amplifier when no CD is playing which explains why I couldn’t hear anything, so I desoldered R724 (circled in red) in the path to disconnect the mute circuit and with amplifier running again there was sound from the Bluetooth board!
Besides the CD/DVD player (to be continued), the rest of the boombox is working fine and it now has Bluetooth support when “CD” operation is selected.
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