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Acurus A250 and starground

A triangle wave of < 3Vpp shouldn't have hurt anything.

I do not use the triangle output for anything. Square waves allow you to look for many issues that show up as excessive overshoot, ringing, or freq response (sloped).
I'll also use high freq sine waves to look for signs of oscillation. When viewed on my analog scope the much higher freq oscillations are more readily apparent on a 100kHz sine wave IMO than looking for them on a 1kHz sine.
 
I also didn't understand the Zobel was burned. All squarewaves were spot on, and measuring fall/raise times, ringing ect afterwards didn't show anything unusual. It was the visual inspection that took me by surprise.
Should be mentioned that I completed the measurements with a traingel, after the square.......

Anyway, thanks for the hint on about the 100k sine, now I just have to learn more about my scope. Analogue scopes are just better to display abnormal behaviour then the digi-versions (or so I've heard) if adjustments isn't set up correctly to the taske, and even the it can be hard to detect. Don't know??, but will find out soon :)

For now I've ordered some new Dale's wirewound 1.5Ω 2W for the Zobel, then I'll try the scope once again with the WG as source

Thanks.
 
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Changed the Zobel and tried test once again. No doubt the amp is oscillating. At 1khz sinewave I see glimps on the scope and 100khz it's all bended, very easy to see even at very very low Vpp.. I had to discontinue because of heat raising to much at the new Zobel resistor, and didn't want to burn the hole network.

BIAS is dead straight and stable, no remarkable DC at outputs (roughly 4mV on both channels), the 10Ω at inputground is also ok.

Should be mentioned that I haven't measured all emitters and noted their mV yet....

Funny thing is, it's apparently both channels, but didn't dig that much into the second channel before pulling the plug from the wall when left channel began to show signs of heating up on R34. If it were ground related I would assume that more DC could be measured at outputs.

Have to dig further into this tomorrow

EDIT.... I'll try disconnecting the left channel tomorrow and continue only with the right channel to get this one cleared. Simply cannot see how these two channels could show the same signs of glimps
 
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KNielsen said:
Changed the Zobel and tried test once again. No doubt the amp is oscillating. At 1khz sinewave I see glimps on the scope and 100khz it's all bended, very easy to see even at very very low Vpp.. I had to discontinue because of heat raising to much at the new Zobel resistor, and didn't want to burn the hole network.

BIAS is dead straight and stable, no remarkable DC at outputs (roughly 4mV on both channels), the 10Ω at inputground is also ok.

Should be mentioned that I haven't measured all emitters and noted their mV yet....

Funny thing is, it's apparently both channels, but didn't dig that much into the second channel before pulling the plug from the wall when left channel began to show signs of heating up on R34. If it were ground related I would assume that more DC could be measured at outputs.

Have to dig further into this tomorrow

EDIT.... I'll try disconnecting the left channel tomorrow and continue only with the right channel to get this one cleared. Simply cannot see how these two channels could show the same signs of glimps
Click to expand...
PROBLEM SOLVED!!!:
- Since it were both channels that behaved odd, it turned my attention to the testgear.. I shited the the face 180° at the wall outlet, and voilà, it's all good.
No magic smoke from the Zobel, and curves are like magic even at 150kHz sine :)
 
Finished this one today.
Before soldering and new caps the amp put out 2x256watt 8Ω at 1kHz when arrived...

Today when finished, the amp output had claimed to 2x290watts under same conditions. Still a bit lower then what we normally see, but I'm quite pleased that even simpel mods can have this positive effect:)
IMG_20201101_171040_134.jpg
 
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I have some "pops" comming from the left speaker-channel in my own A250. It been like this for some month now but lots of work has taken my time, so I haven't had enough hours on the clock to do some research until now :)

When I turn on the amp, DC-offset is always around a healthy 5mV on a cold start... And no pops....
After playing some music DC-offset raises to a very unstable ~47mV (+/-2mV, some times more) when the music is turned off and after some times the speaker will begin to make a hizz noise from the treble and a quick pop in the bass now and then. This is where I disconnect the amp ;)

I decided to put my amp under test today, but there were no oscillations to spot as high as 150kHz sinewave. I tried many different kHz to provoke at various Vpp..

But when I turned off the signal generator, the amp instantly claimed from 5mV to ~47mv DC-offset, just like when I turn off my music. I was lucky to spot this hizz/pop on the scope and my meter after about 20mim of waiting and recording with no signal at the input, the DC-offset went from 47mV to over 1000mV in a glimpse..!
I printed the o-scope results to the USB but sadly the .PNG-file is not working, so I cannot upload the results found for now :(

As soon as I turn on the signal generator again at 1/5/10/20kHz ect the DC drops instantly to rock steady 5mV!?? Only thing that had my attention was that after about 20kHz and up the DC-offset started to raise slowly and at 100kHz it was about 150mV if I remember correctly, but the sinewave is nice and clean.
-I didn't have time to try a squarewave.
-The channel don't show signs of overheating.

Any idea what is happening?

Best regards
 
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KNielsen said:
I have some "pops" comming from the left speaker-channel in my own A250. It been like this for some month now but lots of work has taken my time, so I haven't had enough hours on the clock to do some research until now :)

When I turn on the amp, DC-offset is always around a healthy 5mV on a cold start... And no pops....
After playing some music DC-offset raises to a very unstable ~47mV (+/-2mV, some times more) when the music is turned off and after some times the speaker will begin to make a hizz noise from the treble and a quick pop in the bass now and then. This is where I disconnect the amp ;)

I decided to put my amp under test today, but there were no oscillations to spot as high as 150kHz sinewave. I tried many different kHz to provoke at various Vpp..

But when I turned off the signal generator, the amp instantly claimed from 5mV to ~47mv DC-offset, just like when I turn off my music. I was lucky to spot this hizz/pop on the scope and my meter after about 20mim of waiting and recording with no signal at the input, the DC-offset went from 47mV to over 1000mV in a glimpse..!
I printed the o-scope results to the USB but sadly the .PNG-file is not working, so I cannot upload the results found for now :(

As soon as I turn on the signal generator again at 1/5/10/20kHz ect the DC drops instantly to rock steady 5mV!?? Only thing that had my attention was that after about 20kHz and up the DC-offset started to raise slowly and at 100kHz it was about 150mV if I remember correctly, but the sinewave is nice and clean.
-I didn't have time to try a squarewave.
-The channel don't show signs of overheating.

Any idea what is happening?

Best regards
Click to expand...
Ok.... Performed a squarewave, and didn't catch any unusual data..
Measured the right "good" channel and It does NOT change DC-offset that much depending on Hz and Vpp, much more stable overall even pass 200khz and more verify the problems to solve...
I'll be looking at the diff section for now in the left channel, got new transistors, but still find this behaviour odd??

Best regards
 
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Well just talking to my self ;)

To make mathers worse I just discovered that BIAS is drifting. 32mV at the emitters, and the amp was hot. Normally it's set to roughly 12mV

Last evening I measured 4mV and thought this was a mistake and raised to 12mV on the emitters. Today it didn't pay off and readjusted. Dame :)
 
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My question is:
How high frequence can I set the sinewave to see what could be oscillations? Is it possible that oscillations is happening way above 100kHz?

Thanks
 
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KNielsen said:
My question is:
How high frequence can I set the sinewave to see what could be oscillations? Is it possible that oscillations is happening way above 100kHz?

Thanks
Click to expand...

Sure oscillations can occur much above 100kHz. You do not need to set the signal generator higher to see them though. They should show up on your 100kHz sine wave on the o-scope. Instead of clean lines you'll have additional sine (or even HF hash/grass) riding on top of your 100kHz signal.
 
mondialfan said:
Did you eliminate the input coupling caps in the amp that has the popping noises?
Click to expand...
Yes. Removed them 3 month ago when I tried to fix the "pop" the first time. The amp has been running fine without input caps when I first build it. I reinstalled the caps to hear the difference a year ago I think. So the hizz/pop has also been present with the inputcaps in place.
I had my focus 3 month ago on D3/D6. It was a long shot, but didn't find anything that didn't measure as it should at the time, only D3 was a bit off, so changed both D3/D6 with new parts and Panasonic bypass across as I originally built it. I also desoldered the hole diff-section to measure faulty transistors, but they all looked good and reacted as a diode and hfe were pretty close too.
I thought this was the solution with D3, but the amp still doesn't behave as I want to. The main problem is that it sort of swinging in the DC until it becomes unstable and return to "normal" again. This is repeated every once pr 10-20min, so a lot of waiting and luck to record just this information spot on when happening. And some times it can take hours before the first pop occurs. Measuring showed it's when the swing becomes to big, that DC runs out of control.
I attached a pic drawn in hand, just to show what I see when it occurs. The peak is the "pop"
IMG_20201115_114742.jpg
 
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The more I think about it, the more it seams like a ground-problem regarding the DC-offset. The fact that BIAS isn't stable also, makes me wonder if the copper traces are OK, making poor connection? It's not drifting, but more like it's jumping coursed by a sudden change in resistance.
It's the same circuit that made me start drawing a new PCB since the copper was quite picky and much care must be taken when soldering. I never seen other Acurus PCB's like this one, where even small amounts of heat courses the cobber to lift from the PCB.

I made repair to the complete feedback circuit, a small repair to the BIAS-transistor network, and the input were also in bad shape, making new routing to R2 and R3. All repairs were hardwired P2P

I can not rule out that while the amp heats up, this might cause a bad connection in the copper trace it self. In cold disassembled condition, all connections are spot on. I've looked under a magnifying glass with light, but couldn't find anything that points to copper traces being faulty??! :(

Anyway it's plays fantastic, so I'm a bit lost where to start...... ;)
 
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Well, since I'm going to take it apart and dick deeper into this matter, I decided to order a new groundplate while working on the amp anyway.
240x60x3mm metal to cover all E.caps instead of the solution thats in the amp now (picture).
-A mistake I made when building and better fix this to eliminate potential loops, while digging into other problems that may occur.

I'll get the costs tomorrow on different materials for the groundplate before making the final decision/order.
IMG_20201115_175012.jpg
 
Left PCB draw finished:)... Just need to mirror everything to make the right side also:oops::p

A250 Amoded.jpg

EDIT: dammm, I forgot the mountning holes for the screws, need to move the decoupling caps a bit lower
 
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OK.. So I moved the postfuse-caps.
I centered them much more around the the fuse position, enabling much shorter groundtraces between the caps to GND, but I'm still uncertain if a 3mm matric coppertrace is enough (that's what the space on the PCB allow). The A200 uses aprox 2mm for the small unused caps at the PCB, but doesn't include a trace that return to GND

A250 new measurments.jpg
 
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mondialfan said:
Sure oscillations can occur much above 100kHz. You do not need to set the signal generator higher to see them though. They should show up on your 100kHz sine wave on the o-scope. Instead of clean lines you'll have additional sine (or even HF hash/grass) riding on top of your 100kHz signal.
Click to expand...
Ok. Also my thoughts as you describe, thanks.
I didn't see anything unusual.
I'll post my findings when I return to my amp some day hopefully in the near future:)

Regards
 
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mondialfan said:
Your new channel boards are coming along nicely. Well done!
Click to expand...
Thanks!!:).... Still I'm unclear about the the groundtrace between the de-caps if it's wide enough.. The A200 surgest 2mm should be enough. As the layout is above, 4mm is the maximum I can do for now.
Someone surgested SMD's as this would provide a lot of free space for grounds and this is correct... I'm just not that much into SMD, like the old school through hole for audio
 
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