sx1500td main amp adjustment

john stumpf

Super Member
ive rebuilt my main amp board and as the service manual gives ZERO info im in search of adult supervision but first let me tell you what ive done all electrolytic caps are replaced 2sc497 and 2sa497's replaced with ksc2690 and ksa1220.2sc458's replaced with ksc1845. outputs replaced with mj21194g's all four trim pots replaced with bourn items 50 and 100k. ive researched this and it would seem that the consensus was to adjust the 50 ohm pots to achieve a 20millivolt reading on the test points to ground. the highest i can get is 10 millivolts on the left and 5 on the right. as im not at all sure what its supposed to be im not sure if im chasing my tail or not. all testing was done on aux. .with volume control set to nil.b+voltage is 71 vdc. it seems to play quite nicely.
 
From your description it sounds like you are setting 'bias' or 'idle' current, normally this would be across 2 test points, and very unlikely to be 'TP to ground'. 'TP to ground' is the usual instruction for setting DC offset which of course is a different adjustment. Be careful with this as you may be turning the bias current up too high - check the heatsink temperature regularly.

I am not familiar with the model you are working on, so I will defer to the experts in this forum to give you more advice.
 
From your description it sounds like you are setting 'bias' or 'idle' current, normally this would be across 2 test points, and very unlikely to be 'TP to ground'. 'TP to ground' is the usual instruction for setting DC offset which of course is a different adjustment. Be careful with this as you may be turning the bias current up too high - check the heatsink temperature regularly.

I am not familiar with the model you are working on, so I will defer to the experts in this forum to give you more advice.

The 1500TD is a capacitor coupled output with quasi complimentary output, so the bias can be checked by measuring across the lower transistors emitter resistor, which has one side going to ground, so procedure would be correct. The lack of adjustment is probably due to slightly higher Vbe of the mj21194G, which if you are really OCD, you can try different ones and find which ones give the best range, or change a resistor in the bias circuit to increase the range a tad... The fact that you have some bias means everything else is probably working fine, and sound should be good. So chase your tail if you like. :)
 
Shows just how wrong you can be - I thought some description and a warning might help as there seemed to be nobody around to help - thanks Steve. ;)
 
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