radioshack: Takes too long to turn on speakers

Fellow Radiophiles,

The Realistic STA-111 from Radio Shack (cat. 31-2002) is a very good receiver from the early 1980s and is still popular due to its performance.

Perhaps the most common problem that comes up on radio collector forums is that the speakers start to take longer and longer to turn on and eventually don't turn on anymore. The speakers are connected to the audio amplifiers through a timed relay after power turn-on. The relay is controlled by the Hitachi HA-12002 SIP for circuit and speaker protection in the small PC-board near the power transformer.

Click pictures to magnify

One big problem is that the schematic diagram is not available from the usual sources.

This application circuit came from the HA12002 data sheet and not from the STA111 schematic.

I was able to repair the problem by replacing C818=10uF,16V. This capacitor was open. It works as a carefully set time delay circuit in the HA12002 SIP,  that detects the removal of AC power. Fortunately, the data sheet for the SIP is available on-line and this was enough to debug the voltages around the SIP. 

The C818 capacitor on the STA111 pcb is marked as C102 in the HA12002 data sheet. It ties to pin 5 of the sip and to the anode of the AC-power detection diode D1. 

According to the HA12002 data sheet pin 5 has a trigger point of -3.5V. With C818/C102 open, the half wave rectified AC goes between -9V and -1V with every cycle and keeps the SIP disabled. After the cap was replaced, I got a negative DC level that kept the SIP enabled.

I can't be certain that the failed C818 is the only way for the speaker delay to keep increasing until the speakers. There are other caps with timed delays that should be checked if C818 is good. 

I checked several other capacitors in the protection board and they were good. The HA12002 also seems to be available on the market as a replacement.

Regards,

-Joe

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