Power amplifier SMPS problems - Page 1
Forum section: electronics / general

author: Anks
author_rank: Frequent Contributor
timestamp: July 31, 2013, 01:17:14 am
content: Hi I fix power amplifiers and lots of sound equipment and I'm struggling with on particular amplifier.The amp in question is a Bell Auidio CS2000a and its driving me mad. The service documents for the control boards are pretty much unreadable. I got it working and decided to give it a test to make sure it was good to go. 10mins after I started using it at reasonable level it started shutting down again.At first the symptoms seemed to point at the protection circuit and hence I found 10 bad caps and portion of the board seemed to have been really hot around a custom hybrid chip. I assume that by replacing the caps Ive just restarted the initial fault. Ive check all components that I can see that may correspond to this point in the circuit but it mat be the custom.Now the amp wont even engage power up and seems as if it missing control voltage (not checked its to late here now). Ive taken all the plugs off the SMPS and two resistors seem to be getting hot even with no load. This to me says that something is loading the supply even when the load is disconnected.This is really doing my nut in. It worked flawlessly after I recapped the control board for 10 mins but now seems deader that before. I'm thinking what made the caps go is the main problem and needs finding first.Ill attach schematics but the control schematics are shocking. Ive also got loads of pictures that may help if anyone is up for lending a hand.

author: Rerouter
author_rank: Super Contributor
timestamp: July 31, 2013, 02:34:35 am
content: if you believe something is overloading your voltage rails, perhaps throw an ohm meter across the rail and see if there is any excessive resistive load. e.g. a shorted transistor or capacitor, equally if it has the older carbon slug type resistors (dark brown with flat ends), see that none are open circuit,

author: Anks
author_rank: Frequent Contributor
timestamp: August 01, 2013, 11:06:55 am
content: Well it wasn't overloading it appears to be the soft start not fully engaging and dropping out from the 3525 controller. the soft start signal comes from a custom on the control board. its obviously been getting very hot. I'm hoping this custom can be replaced with another circuit if possible.Now I want to be 100% sure that the control board isn't seeing something wrong and shutting the amplifier down for safety. Normally when a amplifier like this shuts itself off to protect either itself from catastrophic failure of to protect the speakers from dc the amplifier with show this on the front by displaying a protect light and the relays will click as it pulls the speakers out of circuit. The control board is not even engaging the relays and starting the smps.The initial start up supply comes from a separate transformer rectified and regulated by a lm7812. The voltages from the regulator look good and clean on the scope. Ive tried to ignore the custom but everything keep coming back to this.Anyone got and Ideas.

author: Anks
author_rank: Frequent Contributor
timestamp: August 01, 2013, 03:49:59 pm
content: Well its back from the dead but its not tip top just yet got a few niggles with it shutting down at random saying this its been on now for 30 minuets without issue but at least I'm getting somewhere it was a tiny burned resistor hiding under the custom.