JVC JA-S310 Bias setup

[stofbj]

Hi,

Just a quick question, in the service manual for the JVC JA-S310 Bias setup it says left channel 10mA and then in brackets 20mA for the right channel, is this a mistake, never seen an amp with different bias setting for left and right channels?


Thanks in advance.

 

[sgrossklass]

Me neither. Do you spot any differences between the two channels in part values or heatsinking? All I can imagine is that for some reason ventilation may be worse for the left channel and this was a bodge to stop things from overheating. Or if the bias has a negative TC, maybe it's the right channel that's the problematic one.

That's an odd one for sure.

 

[patrick101]

The manual tell you to measure the voltage, but it show you currents values, so that there is mistake in the values

 

[patrick101]

it measure the voltage of one output power transistor's emitter resistor 0.5 ohm, so that the correct value is 10mV and means a 20mA bias current for both channel

 

[stofbj]

Ah yes was thinking that might be the case, i did not notice any difference between the two sides, so 10mV for both test points it is then, thanks for clearing that up. I left them at 10mA each for the time being, as I found it strange alright. They were quite a bit out, more in the 30mV range, but the DC bias was well out too, might not have been serviced since the 70s !
I stripped it apart, inspected solder joints under magnification, cleaned all switches with super 10, set dc offset and bias, replaced 4 x 1uf audio coupling caps as ESR was about 4.4ohms a bit high all the rest measure really good. Thanks for your help