Hi I'm new. I have this posted on Audio Karma too but I though I'd post in a couple of places for the most feedback.
I have a Sone TA-F448E that's a little poorly. It's a MOSFET based amp that'll produce between 55 and 85 watts ber channel depending on impedance. When I found it one of the power MOSFET's on the right channel had melted. I've replaced this and it's pair (2SK1529 melted and replaced, 2SJ200 looked fine but replaced anyway) as well as a couple of resistors that had gone out of spec or looked as if they'd gotten warm. I also tested all semiconductors upstream from the dead MOSFET's which seemed fine.
Powered it up with no drama and I got to re-adjusting the bias according to the service manual which can be found here: Download TA-F448E.pdf from Sendspace.com - send big files the easy way.
I've replaced Q453, Q454, R453 and R454 (See the scematic in the service manual)
After adjusting the bias I tested for DC offset on the speaker terminals before hooking up a pair of speakers. The left channel (undamaged) gave a very healthy reading of 1mV. The right channel however which I've supposedly repaired gave a reading of around 120mV when cold which would drop very slowly as the amp warmed up. And by warmed up I mean really warm.
I noticed that after a while the heatsink on Q453 and Q454 was giving off a lot of of heat so I switched it off. I've started it up again without the meter connected to the output in case the low resistance of the meter was causing it. Even without the meter there's still a lot of heat being developed.
Does anyone know what's going on here? My knowledge of amplifiers and electronics in general is pretty limited. I know about enough to understand how the circuit works but not enough to work out what's wrong.
Thanks.
I have a Sone TA-F448E that's a little poorly. It's a MOSFET based amp that'll produce between 55 and 85 watts ber channel depending on impedance. When I found it one of the power MOSFET's on the right channel had melted. I've replaced this and it's pair (2SK1529 melted and replaced, 2SJ200 looked fine but replaced anyway) as well as a couple of resistors that had gone out of spec or looked as if they'd gotten warm. I also tested all semiconductors upstream from the dead MOSFET's which seemed fine.
Powered it up with no drama and I got to re-adjusting the bias according to the service manual which can be found here: Download TA-F448E.pdf from Sendspace.com - send big files the easy way.
I've replaced Q453, Q454, R453 and R454 (See the scematic in the service manual)
After adjusting the bias I tested for DC offset on the speaker terminals before hooking up a pair of speakers. The left channel (undamaged) gave a very healthy reading of 1mV. The right channel however which I've supposedly repaired gave a reading of around 120mV when cold which would drop very slowly as the amp warmed up. And by warmed up I mean really warm.
I noticed that after a while the heatsink on Q453 and Q454 was giving off a lot of of heat so I switched it off. I've started it up again without the meter connected to the output in case the low resistance of the meter was causing it. Even without the meter there's still a lot of heat being developed.
Does anyone know what's going on here? My knowledge of amplifiers and electronics in general is pretty limited. I know about enough to understand how the circuit works but not enough to work out what's wrong.
Thanks.