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Sunday, February 19, 2017

Beomaster 8000: Power Startup Circuit

Since the post burn-in test failure I have reworked the right and left channel output amplifier boards and while I found I needed to update the heatsink component mounting, I didn't find any faults on those boards. That left me with the power start up circuit. That circuit has two relays that control the ±55 VDC rail voltages. The startup circuit worked during the output amplifier burn-in test but failed right after the test. Initially I observed some intermittent power problems where only one of the relays on the startup circuit would energize (RL2). RL1 would not energize and it is the relay controlled by the delay circuit on the power supply board. The RL1 problem finally settled where it never energized.






































I spent some time checking components (relay RL1, its D1 diode and the power supply board). As I removed components for this part of the circuit to measure I found the problem underneath the capacitor C21 (22uF, 63V). This was an electrolytic capacitor I recapped at the beginning of the restoration. C21 charges up to 25VDC when the Beomaster turns on (goes from Standby to some play function).  That should result in a short delay between RL2 clicking on and RL1 clicking on.  When I measured the voltage across C21 I wasn't getting anything. When I removed C21 from the board to check it again I saw that the solder pad on the positive terminal was now gone.




























I repaired the connection here by running a short jumper from the C21 positive terminal to the D21 cathode terminal.























That fixed the problem with the RL2 relay not engaging when the Beomaster turns on.  Now I have to put the Beomaster boards I removed for this trouble-shooting exercise back in and get back where I can re-run the burn-in test. That is necessary to make sure the original, post burn-in test problem is fixed.

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