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ARC Classic 30 problem
ARC Classic 30 problem
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Old 18th September 2006, 01:42 PM   #1
TonyE is offline TonyE  Italy
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Join Date: Dec 2004
Location: Italy
Angry ARC Classic 30 problem

Hi all
If you can help me please.
I'm going to be creazy with a problem on my ARC CL30 tube amplifier.I have purchased it used no more time ago'and after some listening I'found no very good the sound, so I've tried to make the biasing procedure but the bias current is out of range and I'm not able to adj it by multiturn trimmer existing on both side of the unit,so after some chek I've found the -55 voltage no stable, floating around -48 -:- -54v (refer to chassis also first of R72)and the trimmer RV2 is turned to max voltage but obtain the upper indicated value.You can see the power supply schematic diagram at the attachment. So I've tried to replace in this order ZD16(TL431CLP), ZD15(51v zener),Q8 (P-Channel mosfet),Q11 (P-Channel Mosfet),DZ7,8(LM329DZ),U2(MC34071)and checked C41,40 and 19)becouse I've seen little bit ripple and mybe some small spikes, the caps result good.The problem still again , also without load.I've also check more resistors, another info is that the floating voltage appear only accross R81,R57 and R70, the u2 supply voltage,DZ7+8,DZ16,DZ15 voltage still locked.
I don't know what I can do! Can you help me please.

For schematic and other :

Many Thanks.
Toni E
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Old 18th September 2006, 06:44 PM   #2
richwalters is offline richwalters
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Location: Alps:Tube amp designs over 150W, SMPS guru.
This one of those overcomplicated murderous hybrid designs for a modest output power. Many components are 1% so was from a relatively modern build era.The o/p stage cathodes are connected to the sec o/p tranny (attempted copy Quad) and if one side is LS grounded to another earth then one can have serious instability problems of driving into an earth loop. Solid state won't like that.
My first temptation is to change the optocoupler U6 (fades with age) and coupling caps. If the -55V varies check value of R71, C19 and associate diode components. It's the old story of replacing old electrolytics in critical locations i.e C6/8. can't make out wattage of R71 but a decent rating is wise.
The -55V rail must be running correctly otherwise the 6550's will be strained. The TL431 (ZD16) used in non-inv input to op-amp is unlikely to be the problem.
Anodes V1/2 should sit around 200V mark.

Generally I'm pretty scathing about these designs....If on ehas experience then a rebuild on much simpler lines would last longer.

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Old 2nd January 2007, 10:48 AM   #3
TonyE is offline TonyE  Italy
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Join Date: Dec 2004
Location: Italy
Hi my friend, sorry for my very late answare, I still have the same problem.
I`ve tried to test the regulator only, without load, by desoldering the wire that connect -55v regulated power supply to r73/74.Still to float. Why the other line voltages positive +220v and +330v are very stable also if it are very high voltage ? Make I, maybe, somthing wrong when I refer always to common ground plane to measure all tree voltages, positive and negative?
Can someone help please?
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Old 2nd January 2007, 11:05 AM   #4
sajti is offline sajti  Hungary
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Join Date: Apr 2003
Location: Százhalombatta, Hungary
Check the grounding of the regulator! You have to see 0V on the common points of R81, R57, and R70

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