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EGG 5210 Hang at power up

cchu2k

Jul 24, 2019
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Joined
Jul 24, 2019
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I have an EGG 5210. It hangs on power up with analog and digital meters stuck at some random number.

I did check all the power supply voltages. They are all within limit. I don't see any ringing or dipping in power supplies.

I borrowed a spare working EGG 5210 for a day. I took bunch of pictures of its blue jumpers. Somehow the website refuses to accept the pictures. Is there a limit on image size?

Anyway, I mirrored all the blue jumpers per the working amplifier. It was still the same. The amplifier hanged at power up. Also, the analog and digital meters got stuck at some strange but fixed value.

By probing and comparing the working and non-working 5210, I noticed the i8254 clock generator right next to the main micro-controller 68008 having different clock rate. In the working 5210, about 100ms after power up, the output of clock switched from 4MHz to about 100kHz. While in the non-working 5210, the clock stayed at 4MHz. I traced back to another TTL 74157 right next to the large crystal can. It is supposed to do switching between the two frequencies. That is how far I got before I had to return the loaner unit.

It looked like the bad 5210 68008 cleared its own Power On Reset (POR), but the 74157 clock selector chip got stuck in the pre-POR state. Do you know if there is an RC type of POR circuit for the clock distribution?

Thanks for your help.
 

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