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Member Since 02 Dec 2004
Offline Last Active Jul 02 2015 09:08 AM

Topics I've Started

MA3600VZ bi-level switch issue

07 July 2014 - 08:09 AM

Hello everyone,

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?