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Onkyo PE-C50 stereo graphic equalizer repair

A few months ago, I received this Onkyo PE-C50 stereo graphic equalizer from a friend because it was not working. He gave me this equalizer for free because he was planning to discard it due to some problems with its controls.

Front panel of the Onkyo PE-C50.

The Onkyo PE-C50 is a vintage, 7-channel, stereo digital graphic equalizer. This unit is manufactured in Japan and operates on 110V AC power only. Front panel controls allow me to adjust each channel separately or simultaneously. This unit also got 5-channel preset memory to save graphic equalizer settings. Besides a graphic equalizer, it comes with an attractive 2×7 channel spectrum analyzer.

At initial testing, I found that the unit's controls and spectrum analyzer would randomly stop working. After controls at the front panel freeze, I have to restart it to get it back in working condition.

PCB of the Onkyo PE-C50 graphic equalizer.

To troubleshoot, first, I try to find the service manual for this equalizer, but I couldn't find any references on the internet. After examining the boards, I found that the unit was designed around the LC6520 4-bit MPU.

I first checked the digital control inputs and analog output lines of the LC7522 digital potentiometer IC, and everything seemed to work fine. After a lengthy test, I found that all the core components of the main board are working fine, including LC6520, LC7565, μPA80, and LC7522 ICs.

After a few days of testing, it turns out that the problem is caused by some faulty tactile switches on the front panel of the equalizer. According to observation, some switches randomly short-circuit due to some problem in the internal mechanism. To remedy that, I replaced all the tactile switches on the front panel with 6mm × 6mm tactile switches, which are common in the market.

Replacing the tactile switches.

After this fix, the equalizer starts to work fine. The spectrum analyzer of this unit builds using a delicate VFD (Vacuum Fluorescent Display), so extra care must be taken when working on the front panel.

Testing the stereo graphic equalizer.

This equalizer also has a 3V lithium battery onboard. When repairing, I hot-swap this with a new CR2032 lithium battery.

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