Problem with my ramsa

[jimmyT]

Right im a student studying electronic engineering and have reasonable knowledge on the subject, but by no means extensive.

Ive been given some amps by a friend as a job lot to have a go at fixing, I find i learn quite well by opening things up.

The amp in question is a Panasonic Ramsa WP9110E. It was given to me with one channel in permenant protection mode. After opening it up and probing with a voltmeter i found the emitters of all powerstage transistors at -56v (rail voltage +56/0/-56). I assumed a problem with the group of transistors that deal with the negative cycle so probed them and found that the smaller transistor driving the power ones had shorted collector to emitter. This caused the power transistors for the positve cycle to run very hot (i guess as they had 112v accross collector and emitter).

Anyways i ordered and refitted the offending component and wehey! It came out of protection mode But... there is a noticable hissing noise now comming from this fixed channel and loudly crackles randomly every minute or so. I could see the speaker cone being pulled inward quite violently so assume some -ve DC is making its way to the output when this happens? If a play music through this channel its a little quieter than the other channel but both channels seem to increase at the same rate. Unlike the working channel, the dodgy one starts to distort one over 50% volume.

When left to idle with no signal the part of the heatsink dealing with -ve part of the wave is noticably warmer than other parts. I have no schematics so have spent hours following tracks on the boards and beginning to figure out how these things work.

On a big learning curve here sorry if im being stupid. Also this is probably a bit long winded but trying to give as much info as poss. Spent quite a while on this now and i hate being defeted, will end up in the bin otherwise n seems such a waste.

Also added a picture just in case it helps

Thanks in advance

[sakis]

----replace all small electrolytics on drive and main board
---- double check both pcb for soldering

even from the picture is obvious that you have electrolytics that dont look good

[djk]

Always go back one stage further and replace. If the outputs are bad, replace the drivers. If the drivers are bad, replace the predrivers. If it doesn't have predrivers, replace the Vas and CCS devices.

If the amplifier is more than 10 years old, it is worthwhile to replace all the small electrolytics.

It is also a good idea to always replace the cap in the feedback loop to ground when the channel blows, as it puts full supply voltage on this cap through the feedback resistor.