I had previously posted this in Amplifier General.
For those interested, I recently had a rail change issue with an MA3600VZ bi-level switch module :
I had just completed a refurbishment of the output modules and was testing out the amp. Loaded the amp at 4 ohms and passed music. After 10 minutesd, Ch1 started to sputter and I noticed the ODEP indicator fail. I had just refurbished the output section! It shouldn't have happened : Ch2 was pumping away, gracefully.
Verified the rail voltages and discovered that Ch1 was stuck in high rail mode (+/-90v). Ch2 rails were cool, locked at +/-45V. I just had to investigate the bi-level switch. It is usually very reliable. The MA5000 uses a similar bi-level switch circuit. Note : The bi-level switch assembly includes the rail changer circuit, fets and the hi-voltage caps.
I was a little hesitant to investigate this fault : high voltages, cramped spaces, barely space to stick a probe. After quite a bit of trying to find ways to troubleshoot this module, I flashed on the timer chip (LM555) : It has a support! I proceded to remove both timers from the two bi-level modules and power up : Now both sides were stuck in high rail mode! Since Ch2 stepped from low rails to high rails, I cogitated that the fault was located pre-timer; And yes, the fault was with the preceding chip, a comparator! On pin1, it should have been a high level signal (as per Ch2); I found a low level signal instead! It most probably was the culprit. I replaced U03 (LM393, comparator) and tested the psu : Yeah! Triumph! The comparator signal was back to normal. I put back the timers, reinstalled the bi-level switch module and powered up : Yes! Both channels are now operating normally.
I thought this could be a nice topic to discuss : If others have troubleshooting tips please make them available!
Perhaps, we should do this more often, discuss our troubleshooting tips and how to resolve intriguing issues. What do YOU think?
joustMember Since 02 Dec 2004
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