img pa-amp issue

gruni · 9th August 2009, 12:57 PM

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:
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.

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

Hi,
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?

9th August 2009, 12:57 PM	#1
gruni diyAudio Member Join Date: Feb 2008	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

9th August 2009, 05:46 PM	#4
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, the hissing could be a symptom of another fault. That same fault may be why the fuse/s blew (another symptom). __________________ regards Andrew T.

11th August 2009, 04:22 PM	#10
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	are the fuses T100mA or F100mA? __________________ regards Andrew T.

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9th August 2009, 05:17 PM	#2
megajocke diyAudio Member Join Date: Jan 2003 Location: Solna	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.

9th August 2009, 05:42 PM	#3
gruni diyAudio Member Join Date: Feb 2008	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.

9th August 2009, 05:57 PM	#5
gruni diyAudio Member Join Date: Feb 2008	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

9th August 2009, 06:12 PM	#6
megajocke diyAudio Member Join Date: Jan 2003 Location: Solna	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?

9th August 2009, 06:20 PM	#7
gruni diyAudio Member Join Date: Feb 2008	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?

9th August 2009, 06:36 PM	#8
megajocke diyAudio Member Join Date: Jan 2003 Location: Solna	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.

11th August 2009, 04:18 PM	#9
gruni diyAudio Member Join Date: Feb 2008	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.

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