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Tuesday, February 28, 2017

Beomaster 8000: FM & FM Interface Board Rework

The FM & FM Interface board assembly (the two boards connect together) appear to have been at the heart of the post burn-in failure that threw a circuit breaker in AC line powering the Beomaster. Once I had rechecked the power supply and output amplifier boards, I found that the Beomaster power up relays for the ±55 VDC rail voltages would fail to engage due to the Beomaster's fault protection circuitry. Unplugging power from the FM Interface board fixed the problem and I began using a spare FM/FM Interface set of boards for the Beomaster tuner.

My plan was to find the bad parts in the FM boards during the recap process and then re-install them. I discovered several defective transistors and a defective diode. This picture shows a couple of the damaged components on the FM Interface board. Since those components were bad I also replaced the three OpAmps on the FM Interface board.

After repairs were made to this board I re-installed it back into the Beomaster. Upon applying power and first selecting TP1 as the source input, I immediately started hearing a buzz in the speakers. Power was quickly removed as well as the FM board assembly. Something there is still not right. I am lucky nothing got damaged and installing it was not my best idea. This board needs to be wrung out on the bench by itself before testing it again in a Beomaster. 

To move forward with this project I decided to take one of my spare FM assemblies and use it. I selected the FM assembly that was closest in part number to the removed FM assembly.

Like the removed, faulty FM assembly I had replaced the electrolytic and tantalum capacitors plus the three TL072 OpAmps. One difference with this spare is that I used sockets for the OpAmps so I could experiment with different OpAmp types. The installed OpAmps here are OPA2134PA chips. The same kind I used in the Filter & Tone Control board.

To be consistent and to provide peace of mind, I decided to replace the TL072 OpAmps in the preamplifier board I completed earlier. Here is the preamplifier board with the OPA2134PA OpAmps installed.

This completes all but the Beomaster Display and Microcomputer boards. I will tackle the Display board next as it requires the most work in updating the displays modules with SMD LEDs.

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