Oldsansui441
Super Member
Hi, l am trying to diagnose a problem with this amplifier, it has VERY high dc offset, same on both channels.
After switching amp on the dc offset reads around 10.00V on both channels depending how cool unit is. After ten minutes of warm up the lowest the dc offset reads is 470-480mV both channels.
Bias as tested between test points and earth (l think this is correct as it is making it hard since l can't find a service manual anywhere) is 20.5mV and 26.5mV respectively. I did notice that the unit was set to 220V all of it's life as l bought it off orig owner, l have now set it to 240V as necessary, this did lower the bias to about 6.5mV and 10.5mV so l disconnected one end of factory jumpers as outlined in (sx-450) manual which did bring bias back up to 20.5 and 26.5mV.
I have checked all resistors, diodes and capacitors and have replaced all transistors, this wasn't to shot gun diag but just as good measure being many were the troublesome 2sc1312's.
One thing l did notice, hopefully it can be seen on my scanned diagram is in the centre of schematic there is a power supply of 22.6v after r34, l originally had 23.7v here so l subbed the 3.3k resistor for 3.9k (as the incoming voltage was to spec 31.5v), l now have 22.7v here but l still have 9v on both collectors of q2 and q3, the voltage shown on schematic is 6.8v?
Any help appreciated, l just can't see why the dc is out of control and exactly the same both channels, l have noticed there have been other enquiries about same amp with exact same concern.
After switching amp on the dc offset reads around 10.00V on both channels depending how cool unit is. After ten minutes of warm up the lowest the dc offset reads is 470-480mV both channels.
Bias as tested between test points and earth (l think this is correct as it is making it hard since l can't find a service manual anywhere) is 20.5mV and 26.5mV respectively. I did notice that the unit was set to 220V all of it's life as l bought it off orig owner, l have now set it to 240V as necessary, this did lower the bias to about 6.5mV and 10.5mV so l disconnected one end of factory jumpers as outlined in (sx-450) manual which did bring bias back up to 20.5 and 26.5mV.
I have checked all resistors, diodes and capacitors and have replaced all transistors, this wasn't to shot gun diag but just as good measure being many were the troublesome 2sc1312's.
One thing l did notice, hopefully it can be seen on my scanned diagram is in the centre of schematic there is a power supply of 22.6v after r34, l originally had 23.7v here so l subbed the 3.3k resistor for 3.9k (as the incoming voltage was to spec 31.5v), l now have 22.7v here but l still have 9v on both collectors of q2 and q3, the voltage shown on schematic is 6.8v?
Any help appreciated, l just can't see why the dc is out of control and exactly the same both channels, l have noticed there have been other enquiries about same amp with exact same concern.
. I have had about a dozen of these little 5300's now and l also leave the speakers switched on at all times to prevent the power on thump.