Acurus A250 and starground

The new multiturn foil potentiometers arrived :)
The black on is the new one, size is very impressive, almost half the original blue 1k pot....
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mondialfan said:
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.
Click to expand...
Selected 220V instead of 240V at the AC input, voltage raising to 92.1Vdc at the rails ;)
 
Pics for today...

Added the new potentiometers, preadjusted to 0.5k ohm like the old pots.
Also soldered new 10awg wires to the circuit and bridges. The circuit had to be drilled for the bigger cable to fit, here a 3.5mm drill was used and everything vent out perfectly.
I've added 100nf across the bridge rectifier.

Next shipment arrives in two days: screws, washers, nuts, techflex, signal cable, ect..... :)

Regards
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mondialfan said:
Looking very nice. The new paint job looks great. :thumbsup:
You'll have an amazing amp when you're finished.
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Thanks

My first project from scratch, so everything is a bit "nervous", but I think it start to come together very nicely and far better then I imagined when project started over two years ago :)
 
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Actually the part I struggle the most is getting the new AC wiring as tight looking as possible...... o_O:rolleyes:

The easy solution would be to disable the voltage selector at the AC plug, and just connect the two wires I'm using at the transformer, but I feel this part should be kept original.

Maby I buy some solidcore wire for this job, as it seems easier to handle with the angle between the cap and transformer to power switch??:cool:

Regards
 
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KNielsen said:
Maby I buy some solidcore wire for this job, as it seems easier to handle with the angle between the cap and transformer to power switch??:cool:
Click to expand...

No need for solidcore.. I'll post a picture as many times earlier in this thread as the layout progress:)

Mailman came by to day, only half the parts were in the package :yikes:!!! Enough to finish the wiring, but no screws, nuts ect for mounting the caps and so on....

Have a nice weekend;)
 
Not that good picture, but I did manage to do some work to night :)

It looks almost to tight right now on the picture, but with the psu-caps in place I can loosen the wires a bit so the transformer gets a bit more space.

Every wire to AC is in place, only need to be cut in correct length to the AC-input and connected... :beerchug:

Regards
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I'm struggling a bit how to manage the DC wires from bridge to caps....
Is it okay if the wires are close together, or should I separate +/- as much as possible and cross them if they meet???? Are there any rules that usually work at this point..?

Regards
 
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The DC wiring can be tied together no problem. You just want to keep the input signal wiring separate from the power wiring.
 
I'm very close to finishing this amplifier, my first project, started on paper 3 years ago :)

Only thing missing is the center ground point across the psu.

I'll guess the first thing to be aware of when powering up first time is the bias voltage across the emitter resistors??

Regards
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mondialfan said:
Wow,, it looks great. How does it sound?
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Thanks Kevin! And thanks for all your help! ;)

Haven't turned it on yet. I was hoping to find a person via some Danish forums, who could help with a "slow start transformer" - don't know the word in English :) Just to be on the safe side when powering up......

But actually I don't se any problems turning it on directly, just some nerves I guess....

Every component that's been changed has been measured and matched to one and other (and schematic)...! So I feel rather good with this build.

Also the PSB has alot of space between the components and I can't find anything here that looks out of place or purely connected.

I've been advised to check for DC leak at speakeroutput also.
 
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You need to connect it to a dim bulb tester (DBT) at a minimum. If you do not have one then its time to build one. Adding a variac would be good too, but a DBT (I use a 75W bulb for a A250) is mandatory for a first power up after the amount of work you've just completed. If you measured the old bias pots and preset the new ones to the old resistance then your bias should be very close to where it needs to be. If you didn't preset those pots then you should be ready to rapidly bring the bias down especially once you remove the DBT.

If you have a variac then bring it up to just a few volts the first time and make sure the polarity of the DC voltages across the new main caps is correct before proceeding. A couple of DC volts will not destroy the capacitors, but bringing it up to 20, 30+ volts in reverse polarity will be catastrophic. Once the polarity is verified then start ramping it up to operating voltage. Once you get to about 30 - 40 VDC then you should start to see the bias voltage coming up on the output stage. You've got a lot of capacitance in the PS so the DBT will light brightly for several seconds while those caps charge up. If the bias current is set too high then the DBT will never dim. If you have a Killawatt meter this is a good time to use it to check the power draw of the amp.
 
Thanks Mondialfan :)

You mean a light bulb in series with the power cord to the amplifier :) Actually I was planning a 100watt bulb if no one react to my posts for help in the danish forums.

If no one reacts.... - Should I power up without the PCB connected??? It would definitely leave out mistakes at PSU.

Thanks
 
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KNielsen said:
Thanks Mondialfan :)

You mean a light bulb in series with the power cord to the amplifier :) Actually I was planning a 100watt bulb if no one react to my posts for help in the danish forums.

If no one reacts.... - Should I power up without the PCB connected??? It would definitely leave out mistakes at PSU.

Thanks
Click to expand...
You don't have to answer this... o_O:beerchug::D
 
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