Acurus A250 and starground

[mondialfan]

So I cut the wire for nothing.

No. If the wire from the RCA ground went to the neg speaker binding post then you should have cut it and added a length of wire to it to give it its OWN dedicated pathway to star ground. It should not share the ground wire from the neg speaker terminal that travels to star ground. Although they're both going to the same place (star ground) there are issues that can arise from them sharing that same wire.

There's a second isolation that you can give the amp that should eliminate the hum or greatly minimize it. It involves adding an inrush protector between star ground and chassis earth ground to elevate the star ground from the Mains AC ground. You'll notice the green ground wire (earth ground) coming in from the wall power cord. This green wire attaches to the chassis and that connection point is shared with a dark colored wire that goes back to star ground. This dark colored wire will be cut and the inrush protector will be soldered into this wire between the chassis ground point and the star ground. This is a common tactic that many manufacturers use to isolate those grounds and minimize hum. I typically do it to all of the amps I work on. You want to use an inrush protector that'll handle all of the current the transformer can put out in the event of a failure and will blow the mains fuse without smoking the inrush protector, so this one from Mouser would work. It will give a good 10ohms of resistance between star ground and the chassis earth ground when cold and in the event of a failure the resistance will drop to nearly zero ohms and blow the mains fuse like its supposed to.

Just to make sure I'm clear on this for safety purposes. The green wire is left alone and stays attached to the chassis ground point. Only the dark colored wire is cut between the chassis ground and the star ground and the inrush protector in soldered into this cut wire. Before you do this if you measure resistance with your meter between star ground and the chassis ground bolt you'll measure nearly zero ohms. Once you install the inrush protector you should measure approximately 10 ohms between star ground and the chassis ground bolt. It might be a bit higher if the temps are colder in your house.

 

[Duck851]

No. If the wire from the RCA ground went to the neg speaker binding post then you should have cut it and added a length of wire to it to give it its OWN dedicated pathway to star ground. It should not share the ground wire from the neg speaker terminal that travels to star ground. Although they're both going to the same place (star ground) there are issues that can arise from them sharing that same wire.

The heat shrinked bundle in this line in the new amp is a lot bigger than the one in the old amp - could this be the pair of diodes that I see in the schematics on HiFiEngine? Could you recommend the specific parts if I were to make a new line instead of just splicing a wire onto the old one to reach the star ground?

There's a second isolation that you can give the amp that should eliminate the hum or greatly minimize it. It involves adding an inrush protector between star ground and chassis earth ground to elevate the star ground from the Mains AC ground. You'll notice the green ground wire (earth ground) coming in from the wall power cord. This green wire attaches to the chassis and that connection point is shared with a dark colored wire that goes back to star ground. This dark colored wire will be cut and the inrush protector will be soldered into this wire between the chassis ground point and the star ground. This is a common tactic that many manufacturers use to isolate those grounds and minimize hum. I typically do it to all of the amps I work on. You want to use an inrush protector that'll handle all of the current the transformer can put out in the event of a failure and will blow the mains fuse without smoking the inrush protector, so this one from Mouser would work. It will give a good 10ohms of resistance between star ground and the chassis earth ground when cold and in the event of a failure the resistance will drop to nearly zero ohms and blow the mains fuse like its supposed to.

Just to make sure I'm clear on this for safety purposes. The green wire is left alone and stays attached to the chassis ground point. Only the dark colored wire is cut between the chassis ground and the star ground and the inrush protector in soldered into this cut wire. Before you do this if you measure resistance with your meter between star ground and the chassis ground bolt you'll measure nearly zero ohms. Once you install the inrush protector you should measure approximately 10 ohms between star ground and the chassis ground bolt. It might be a bit higher if the temps are colder in your house.

In the schematic for the older amp, there is something that appears to be labeled "2A" between posts 1 & 2 but I don't know what the symbol is, and it is not present in my amp - would that be something to look at?

Thanks!
Jim

 

[mondialfan]

The A250/A200x3 schematic on hifiengine was provided by me. Its been modded to capture different revs and cleaned up for better viewing. Ignore that component scribbled between the output on the old schematic.

It's possible your very early A250 only has the resistor in the ground wire bundle. If so adding the diodes will not fix hum, as the 10ohms provides front end isolation. I would add the extra 10 ohm inrush protector as I outlined in the prior post. That's your best bet.

I've had customers that complained of having to use a ground defeater to get rid of hum that said the amp was fixed when I returned it to them. The only thing I did (beyond all the upgrade work) that would've solved that hum was to install the inrush prot in the star gnd to chassis earth ground connection

 

[Duck851]

I would add the extra 10 ohm inrush protector as I outlined in the prior post. That's your best bet.
I've had customers that complained of having to use a ground defeater to get rid of hum that said the amp was fixed when I returned it to them. The only thing I did (beyond all the upgrade work) that would've solved that hum was to install the inrush prot in the star gnd to chassis earth ground connection

Good enough proof for me! I'll post my results.

Thanks again
Jim

 

[Duck851]

Good enough proof for me! I'll post my results.

Thanks again
Jim

That inrush limiter alone did the trick for the hum, and as a bonus the switch led is softly, steadily glowing now also!

Do I want to do anything with DC offset and bias while I have all those tiny allen heads out of the lid?

 

[KNielsen]

That inrush limiter alone did the trick for the hum, and as a bonus the switch led is softly, steadily glowing now also!

Do I want to do anything with DC offset and bias while I have all those tiny allen heads out of the lid?

Good news with the hum

DC-offset can't be adjusted. BIAS is standard 6mV measured over the emitters.

 

[mondialfan]

That inrush limiter alone did the trick for the hum, and as a bonus the switch led is softly, steadily glowing now also!

Do I want to do anything with DC offset and bias while I have all those tiny allen heads out of the lid?

Unfortunately the switch lamp not flickering is just a coincidence. The inrush installed in the star ground will have no effect on that lamp in the switch.

 

[LeLyonnais]

Bonjour,
Je ne sais pas si je suis sur le bon forum mais les gars ici l'appellent sur l'acurus 200x3.
Je viens d'en acheter un aux USA que j'utiliserai en France.
Le transformateur à l'intérieur est un Tibi electronics 15-0057 est sa tension d'entrée 115V ou 115/230V ?
En plus il y a une petite pièce, ça ressemble à un Switch, à l'arrière sur l'alimentation et la prise fusible.
Il a 4 positions mais rien n'est écrit dessus.
Puis-je le brancher directement sur 230V ?

 

[LeLyonnais]

Merci d'avance

 

[mondialfan]

Bonjour,
Je ne sais pas si je suis sur le bon forum mais les gars ici l'appellent sur l'acurus 200x3.
Je viens d'en acheter un aux USA que j'utiliserai en France.
Le transformateur à l'intérieur est un Tibi electronics 15-0057 est sa tension d'entrée 115V ou 115/230V ?
En plus il y a une petite pièce, ça ressemble à un Switch, à l'arrière sur l'alimentation et la prise fusible.
Il a 4 positions mais rien n'est écrit dessus.
Puis-je le brancher directement sur 230V ?

It appears that you're asking questions about setting the rear power socket voltage selector. Your post would work much better in english.

 

[LeLyonnais]

Hello,
I don't know if I'm on the right forum but the guys here call it on the acurus 200x3.
I just bought one in the USA that I will use in France.
The transformer inside is a Tibi electronics 15-0057 is its input voltage 115V or 115/230V?
In addition there is a small room, it looks like a Switch, at the back on the power supply and the fuse socket.
It has 4 positions but nothing is written on it.
Can I plug it directly into 230V

 

[mondialfan]

Many times the voltage selector plugs lose their markings over time.

Here is a link to the datasheet for the Schurter KE10.6100.151 power entry module in the Acurus amplifiers. In the rear power entry module there are 6 pins numbered 5 - 10 that the voltage selector engages with. According to my notes for 220V operation you will insert the voltage selector in such a way as to short pins 6 to 7 and pins 8 to 9. Pins 5 and 10 are left floating (unconnected) in 220v operation. I attached a picture of my notes.

 

[mondialfan]

It has 4 positions but nothing is written on it.
Can I plug it directly into 230V

Absolutely not without setting the voltage selector properly. If you do connect it without changing from 110V operation to 220V you will very likely damage the amp.

 

[KNielsen]

Depending on the AC outlet voltage, 240VAC might be the way to go.
I have 238VAC from the wall (230V normally), making 92VDC on the rails if 220VAC is selected in the A250. It's not a big deal, but if bias is above 6mV, this could become a problem with extensive heat from the transistors....! Just an FYI..
The original schematic regarding Acurus shows quite well how to set the selector and colorcodes from xformer to AC matches the schematic.


Best regards

 

[LeLyonnais]

Thank you Mondialfan for your information.
I will finally be able to listen to this amp.
you guys are great.

thank you

 

[LeLyonnais]

Merci encore Mondialfan.
J'écoute avec plaisir ce 200*3 couplé à l'ACT 3.
Je suis heureux de savoir que si j'ai une question, je peux vous trouver ici.
Et aussi mettre mes petites connaissances à contribution.

 

[LeLyonnais]

Thank you again Mondialfan.
I enjoy listening to this 200*3 coupled to the ACT 3.
I'm glad to know that if I have a question I can find you here.
And also put my little knowledge to contribute.

 

[mondialfan]

Thank you again Mondialfan.
I enjoy listening to this 200*3 coupled to the ACT 3.
I'm glad to know that if I have a question I can find you here.
And also put my little knowledge to contribute.

I'm happy to have helped. If you have a way to measure your AC wall voltage then you may want to do so to make sure you've set the jumper plug correctly. As KNeilsen pointed out it may be better to be set for 240VAC if your voltage is closer to that.

 

[LeLyonnais]

I have something to measure this.
I will look.
Thanks to you KNelsen, for the information and the diagram

 

[Doug Minderh]

This thread seems to be the tome of Acurus Knowledge. Please let me know if I need to make a new thread.

Long story short, in the process of changing the electrolytics on one of the boards in one of my 200x5s I blew the output transistors. I am pretty confident that I blew more than that. I'm hoping that I can get some help on how I could have done this and how to go about repairing. I have one or more replacement parts for all the transistors on the board except the NPN/PNPs. I have some supposedly new old stock 2SC1302 and 2SC3281 coming from a USA seller on ebay... Hopefully they will be real.

So, short story long. I have three Gen 2 (oval faceplate) 200x5 amps, two of which are in active use in my home theater and one is a spare I picked up recently. I have five other acurus amps that I've re-capped the main filters with the recommended Kemets and added .1mfd panasonic bypass caps to without issue.

One of the channels in one of my in-use 200x5s started making a noise that sounds like wind on a mic without a windscreen. When I got the third 200x5 in, I swapped two of the boards from my in-use system with boards from the "new" amp. I wanted to take some time and go through and replace the two electrolytic caps, clean and re-apply heatsink compound, and re-bias each board, hoping to get another 20 years out of the "fleet". I also am adding .22mfd bypass caps in the provided locations on the +/- rails near the fuses. I would also like to add the 100mfd post fuse caps per Acurus design, however the boards are super cramped and I'm not confident about trying to stuff two additional large caps on the board. During this process I was hoping to fix whatever was causing the noise. I moved two boards over because when I couldn't remember which channel was making the noise after I took everything appart so I just swapped two of the boards over.

At this point in the process, I had disasembled the amp, removed the old main filter caps, cleaned and added snubber caps to the rectifier and re-installed the power supply. I checked it for proper voltage with a meter and all was well. I then took one of the boards that may have been making the noises from the old amp, desolderd the ground, desoldered the 10mfd input cap and replaced with a nichicon muse UES1V100MEM-ND 10mfd 35V bi-polar and replaced the 200mfd cap with a nichicon muse UES1V221MHM 220mfd 35V bi-polar. I checked both caps for shorts with a multimeter, both were fine. I then added the .22mfd 100V film bypass caps to CV1 and CV2. Again I used a meter to check my joints and they all seemed fine, or at least not shorted. At this point I re-greased the output transistors, added the mica insulators and re-attached to the heatsink. I was feeling pretty good because I really didn't mess with anything other than an input cap and some bypass caps. The only thing I was worried anbout was the 220mdf cap and that one seemed to have a good joints. I went ahead and connected the amp to the power rails and soldered the ground wire back in. At this point I powered the amp on and got no signal, not noise or hum...nothing. Fearing the worst I put my hand near the output transistors fearing they would be hot, they were room temp. I validated rail voltage was good, I put a tone generator on (laptop with USB audio output). I got nothing on the connected speaker. I had the amp running like this for quite a few mins, no sign of excessive heat from any components.

Not knowing what to do here I decided to use my scope to see how far into the circuit I was seeing a signal. I hooked one channel up to the RCA input and used a second channel to poke around the board. I had a clean copy of the signal on one side of the empty c11 solder pads. I wasnt able to locate it elsewhere and really wanted to get access to the bottom of the board.

This is where everything went to hell.

I thought, everything is running cool, I should be able to run the board off the heatsink so I can get access to probe the bottom of the board. I got a sheet of non-conductive plastic and put the board on the plastic outside the chassis. I had the amp on just a min or two and I felt heat and before I could kill power saw a spark from the neighborhood of the smaller to-220 transistors... All the output transistors and the to-220s were all rocket hot. Not a good feeling.

I ohmed out the 2SCXXXX transistors, the 1302s are dead short, the 3281s are short between two pins and .5 ohms between the other two. I'm just asuming all of those as well as all the TO-220 parts are bad at this point.

The board itself seems to be ok. I've attached pictures of the board immediately after all of this. I don't think I botched a joint, but perhaps there is something I don't know. I am suspicious of the 220mfd cap being a bi-polar. The original part seems to be a nichicon marked "NP" which I assumed mean "non-polar". The other Acurus amps seem to indicate a bi-polar part there. I don't see how the bypass caps could have caused this. I also don't understand how the fuses didn't blow before the output transistors blew. I am extremely confident that nothing was shorting. Perhaps there is something about not having the transistors mounted to the heatsinks?

I attached pictures of the aftermath. The one with the arrows indicate the joints I touched. The other joints that look touched are from heating of the components.

Please help me as these amps seem to be especially rare and I would like to get this one back together as well as get confident again on working through the other 24 channels worth of Acurus amps I have....

P.S. doing some tracing of the circuit, the topology seems a little closer to an A100 or DIA than an A200.