Philips FA-91: One channel no sound

This time on the workbench, a Philips FA-91 midi set amplifier, which is part of the FW-91 set.

Philips FA-91

The problems with this amplifier: Only sound from one channel, other channel is completely silent.

At first I expected blown output transistors in the power amplifier section, so I started to disassemble the device to get to the power amplifier, which is not an STK-type amplifier, what some would expect with small midi sets, but this is a conventionally build class A/B amplifier. Definitely a higher quality than other midi sets.

I measured the transistors quickly for short circuits using the continuity tester on the multimeter. All seemed fine. So powered up the amplifier using a series lamp. I provided a test signal to the amplifier and discovered that there was only audio going into one channel, the other was complete silence.


So that looks like a pre-amplifier problem.
Time to take a look at the service manual, which was a bit hard to find. It is registered as “70FA091”.
The error becomes a bit more challenging since all audio is controlled by several audio ICs.
First an audio selector LC7821, then an equaliser (LC7520) and balance control (LC7536), analog volume control and finally to the power amplifier.

When going through the menus of the equaliser, I discovered that the equaliser was set to -10db for all frequencies. After that I had a look at the balance setting, which is also controlled by the equaliser. This one was also set to one channel only.

This can only be caused by memory settings, not deliberately set by a user, so a factory reset is a good start.
To do that, we need to enter the Service Mode:

Entering Service Mode

In the service mode we can reset (CLEAR) the EEPROM.
Default parameters will then be stored in memory.


At the same time we can also test the EEPROM…


That gave a PROM – DEF error message. So it seems like the EEPROM need to be replaced…

Replace the EEPROM

The EEPROM is a small 8-pins IC that is located on the front PCB.

Disassembly of the amplifier
EEPROM removed
New EEPROM installed

After a reset and successful test of the EEPROM, the amplifier is fully working again. 🙂