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gruni 9th August 2009 12:57 PM

img pa-amp issue
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

megajocke 9th August 2009 05:17 PM

You can find the schematic here:

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 1000F 25V capacitors filtering this rail.

gruni 9th August 2009 05:42 PM

hi megajoke!

thank you very much for the answer and the schematic!
i hope it will help me fixing the bit of hissing on the left channel.

AndrewT 9th August 2009 05:46 PM

the hissing could be a symptom of another fault.
That same fault may be why the fuse/s blew (another symptom).

gruni 9th August 2009 05:57 PM

what could it be? something in the 55v-part?
i think it cant be any bad damage, because the channel works perfectly except for he hissing

megajocke 9th August 2009 06:12 PM

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?

gruni 9th August 2009 06:20 PM

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?

megajocke 9th August 2009 06:36 PM

The board is probably shared with a higher power model. I didn't notice at first, but the schematic says "N.F." (not fitted) besides one pair of outputs so it's not supposed to have two pairs.

gruni 11th August 2009 04:18 PM

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.

AndrewT 11th August 2009 04:22 PM

are the fuses T100mA or F100mA?

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