Hi everyone, I am currently working on a JVC VN-700. I have just finished replacing all the caps, output transistors, and the driver transistors etc and upon testing it, one of the channels has excessive DC offset. I have attached the schematic of the amplifier module for reference.
One of the channels tuned up fine, with about 1 mV offset and correctly set bias, but the other channel does not agree.
The DC offset trimpot (R412) had little effect, so I tried changing resistor 413 from a 2.2K to a 3.3K which helped a little bit but only by about 40 mV and it is not the ideal way to solve the problem. I have circled these two resistors in red, as the labelling is a bit fuzzy and unclear
Could it be as simple as swapping out the two differential pair transistors X402 and X403 for a better matched pair? They are both 2SC1345. I can buy BC550 locally for around $0.50 each, so might buy ten or so and find a good matched pair.
What else could be a potential cause for the measured offset? I had no problems adjusting the bias etc, though I will likely need to re-do the bias once I sort the offset problem. Any ideas would be welcome, I'm looking forward to see what this amp sounds like, its a big ol' beast.
One of the channels tuned up fine, with about 1 mV offset and correctly set bias, but the other channel does not agree.
The DC offset trimpot (R412) had little effect, so I tried changing resistor 413 from a 2.2K to a 3.3K which helped a little bit but only by about 40 mV and it is not the ideal way to solve the problem. I have circled these two resistors in red, as the labelling is a bit fuzzy and unclear
Could it be as simple as swapping out the two differential pair transistors X402 and X403 for a better matched pair? They are both 2SC1345. I can buy BC550 locally for around $0.50 each, so might buy ten or so and find a good matched pair.
What else could be a potential cause for the measured offset? I had no problems adjusting the bias etc, though I will likely need to re-do the bias once I sort the offset problem. Any ideas would be welcome, I'm looking forward to see what this amp sounds like, its a big ol' beast.
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