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Acurus A250 and starground

I'm pretty close to finishing the boards. Only new potentiometers and some wires are left to do.
I've added 0.47uf across D3/D6 and finished the postfuse mod.

I ran into some problems with cobber on the circuit slipping, that's why the wirring on the back isn't the same on both boards, as I had to fix this also.

As Mondialfan recommended, I've changed C7 to the originale 220uf instead of the 470uf that I first installed.

Also, had some Duelund 0.8 cooper/cotton wire at home, installed it at signal input, don't know how it behaves in the amplifier... Will see:)

Regardsreceived_209446529993818-1008x756.jpegreceived_267177253961044-1008x756.jpeg
 
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Recived the newly welded chassis today.
Here is a layout pic before it goes for paint
 
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Looking good there. You decided to go all out redesigning the rear panel is a lot of added work for sure, but It'll look nice. You should drill the capacitor mounting bracket holes before sending the lower chassis out for painting.
 
Thanks. The AC had to be moved or the caps didn't fit in :D
mondialfan said:
You should drill the capacitor mounting bracket holes before sending the lower chassis out for painting.
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Is there any rules about this or just good practice??

Actually I was planning drilling all the holes for caps, chassis gnd and bridges afterwards.

I'm a carpainter (or what it's called in English) my self so I'll do the paintjob at work:)

Regards
 
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Anyway the project is stalled a couple of days, waiting for my potentiometers and the above mentioned "cap-clips" to arrive with the mail:)

I've ordered Viskay foil mulitturn 1k potentiometers
 
I think its a good practice when extensive changes are being made to the layout. The reason is that more modifications to the chassis may be needed and I have found that many times I do not allow for them all. I have built a few Class A amplifier cases by hand, I did the machining myself and took all of the finished cases to a local shop for anodizing. I also have had a chassis professionally powder coated with textured black finish before. If the nice final finish is in place then there is a much higher probability that it'll get damaged if large changes are needed. Its always best to have everything mounted and looking the way you want it, then disassemble and send out for the final finish. Cars aren't really so different I've restored a few over the years, although I had someone else paint them for me.
 
mondialfan said:
I think its a good practice when extensive changes are being made to the layout.
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Makes good sense. Clips arrives today, so maby I get some time this weekend to mark all the points that needs to be drilled.

I've ordered a 230v 15a Audiophonics (relay-type) Softstart for this build, although the space in the chassis is becoming very limited. Think with 104000uf it may be a good idea?
 
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My Class A mono amps have 132k uF in each one of them and no dedicated softstart. However I installed an NTC thermistor to minimize in-rush current just like in the Acurus amps. You may want to consider at least replacing the thermistor due to the fact it has taken a lot of turn on surges over the years and installing over 2X the capacitance may cause it to fail eventually. Ametherm makes some very large amperage versions, of course they're physically large too.
 
mondialfan said:
My Class A mono amps have 132k uF in each one of them and no dedicated softstart. However I installed an NTC thermistor to minimize in-rush current just like in the Acurus amps. You may want to consider at least replacing the thermistor due to the fact it has taken a lot of turn on surges over the years and installing over 2X the capacitance may cause it to fail eventually. Ametherm makes some very large amperage versions, of course they're physically large too.
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Can you provide me with a link or partnumber for a NTC thermistor that would suit this project??
Normally I use mouser

Regards // thanks for reply
 
Sorry I meant to reply and forgot.
The stock A200X3/A250 typically uses a 2R510 or something similar which has a 10A 2.5ohm rating. I would recommend the SL32 2R025 as long as you're comfortable with the much larger diameter.
 
One other thing, make sure you leave the arc suppression caps in place to protect your power switch. These are the blue discs mounted directly to the rear mains AC plug in your amp. Some of the Acurus amps had them mounted directly to the front panel switch, others (like yours) to the rear panel mains plug.
 
I think the thermister solution is the way to go. Although the softstart doesn't take mush space, it's hard to get everything right with this module added :)
 
Mondialfan: Should I stick with the 5.6k resistor above each cap in the PSU???
My rails are about 86.5v.. the original resistors are 3.9k.
 
I typically see >90V on the A250's I work on however my house has 124 - 126VAC at the wall. With your 86V rails and 3.9k you're close to 2W of dissipation which even on a 3W resistor seems excessively high to me. That's why I went to a 5.6k which brings it down to a more acceptable level which is < 50% of rated power. You'll see a slightly slower bleed off of the capacitors after powering off, but I personally could care less about that.
 
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Just finished the layout, took lot of time to get it right... tryed to get the wiring as short as possible, and AC away from the rest.. not easy!
21 holes had to be drilled. Also I made one new hole near each RCA input for the new sheelded input cable (connected to chassis, only in one ende).

That's it for today. Hope it's painted this weekend :)
 
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Painted :)

Now I can begin assembly, in small steps...

Need new wires and other signal cables.
The potentiometers are coming tomorrow, and hopefully my new mica insulation will arrive very soon ;)
 
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