img pa-amp issue

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hello everybody!
i have a img stage line sta-160 pa amplifier. it works perfectly but when i opened it i realized that there are the two 100mA fuses of the 56v rails blown. what could the rail have been for?
perhaps anybody has a schematic for the amp?

thank you
 
You can find the schematic here:
http://www.abeltronics.co.uk/diagrams.php?z=stageline

Those fuses are for the higher front-end voltage. If blown the front-end is still powered through D11 and D33, so the amplifier will work but with lower output power as the rail loss will be higher. I'd guess they just became tired and blew because of the inrush current of the 1000µF 25V capacitors filtering this rail.
 
If anything, the filter caps, decoupling caps or rectifier. There isn't much else that can go wrong if the amplifiers are still working. I suggest you check those. If one of the decoupling capacitors or the bridge rectifier did short it would take out the front-end supply fuses and then the main power supply, backfeeding through the diodes, would probably destroy the shorted part leaving the amplifier in a still working condition.

The hissing might be another, non-related, fault. Is it dependent on level contol setting?
 
yes, a bit. if i set the left volume to 0, there is no hissing at all. but after about 9 o'clock the hissing doesnt increase... so i don't think its the preamplifier

edit: tomorrow i'll take the amp apart so i can measure for any short-circuits.

i realized another thing: in the schematic there are two output transistor pairs per channel. but my amp only has two transistors for each channel? are there different versions or so of the amp?
 
the hissing was produced by the preamp. just swapped the two connectors for left/right coming from the preamp, and the hissing was on the other channel. replaced the opamp in the preamp, now the amp is dead quiet :) i will disassmble it completely to look for short-circuits and then order new 100ma fuses, as i dont have such ones lying around.
 
i measured BR1, C54, C65, D33 and D11 and none of them has a short circuit. so i think the fuses really just blew because of the inrush current and i can replace them without destroying more of the amp?

ps: i realized that only one channel of the bad opamp was affected. so i just swapped the bridge inverter opamp and the receiver opamp for the left channel and now both is working without wasting a new opamp. this is cost effective repairing :D
 
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