Hi
This amp had 2 power transistors blown, one 2SC5200 and one 2SA1943. I replaced them and the amp is working fine, but the heatsink is getting very hot. I guess the bias setting is high. I downloaded the schematic for this amp from the net but it only has the preamp shown, the power amp schematic is missing. There are 3 trim pots on the power amp board and I guess these are for adjusting the bias. Does anyone knows what an how is the bias setting or has the power amp schematic . Please assist.
Thanks
Carlos
This amp had 2 power transistors blown, one 2SC5200 and one 2SA1943. I replaced them and the amp is working fine, but the heatsink is getting very hot. I guess the bias setting is high. I downloaded the schematic for this amp from the net but it only has the preamp shown, the power amp schematic is missing. There are 3 trim pots on the power amp board and I guess these are for adjusting the bias. Does anyone knows what an how is the bias setting or has the power amp schematic . Please assist.
Thanks
Carlos
I have had trouble putting new transistors in old amps.
Sometimes the new transistors have higher gain and/or bandwidth and oscillate.
I bought an old Maplin amp without output transistors.
I bought in a new set from RS Components and the amp oscillated badly.
I had to put a slightly bigger cap in the VAS to slow it down.
I tested the transistors on a semiconductor analyser and the gain was very high.
You need to scope it to see what is going on.
Could be as you say and the bias needs trimming.
Sometimes the new transistors have higher gain and/or bandwidth and oscillate.
I bought an old Maplin amp without output transistors.
I bought in a new set from RS Components and the amp oscillated badly.
I had to put a slightly bigger cap in the VAS to slow it down.
I tested the transistors on a semiconductor analyser and the gain was very high.
You need to scope it to see what is going on.
Could be as you say and the bias needs trimming.
This is not an old amp by any means. ten year old at last revision.
VR1 is the bias, and VR2, VR3 are for setting the output current limiters. I'd set the bias and leave the other two alone.
You set the bias on this amp like you would on any other solid state amp. Start with it at the cold end, and advance the bias until the crossover notch just disappears. There are four test points, X7,8,9,10, but they are across emitter resistors in the drivers which are 6.8 ohms. I am sure a procedure exists, but I have no idea what the specs might be.
The amplifier itself is model LPA1450, I don't offhand know any other models it was used in, but would suspect it was so used. You MIGHT have more success looking for that. It is indeed a separate file from the BX4500H schematic.
VR1 is the bias, and VR2, VR3 are for setting the output current limiters. I'd set the bias and leave the other two alone.
You set the bias on this amp like you would on any other solid state amp. Start with it at the cold end, and advance the bias until the crossover notch just disappears. There are four test points, X7,8,9,10, but they are across emitter resistors in the drivers which are 6.8 ohms. I am sure a procedure exists, but I have no idea what the specs might be.
The amplifier itself is model LPA1450, I don't offhand know any other models it was used in, but would suspect it was so used. You MIGHT have more success looking for that. It is indeed a separate file from the BX4500H schematic.
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