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Old 23rd September 2005, 08:29 AM   #1
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Default STASIS 2 problem

Hello
i fixed a STASIS2 and did change all outputs in MJ15024/25 for one channel. This channel shows the following picture when feeded with 10khz rectangle wave, Sinus wave looks ok.
Do you know what is the reason for, i already did change the driver boards and the power rails and did a complete resoldering on this output board without effekt.
how i should interpret this picture, is it spikes or oszilation?
thanks for your helps
Wolfgang
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Old 23rd September 2005, 01:06 PM   #2
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Default bias value

hello
i found , i have to give more bias, what should be the right value in V/DC across the 1 Ohm resistor after warm up ?
docjoe wrote 0,7 to 0,8 A which means 700 to 800 mV is that ok ?
I am worry about to give so much bias, now its about 100mV and temperature is 46°C
Wolfgang
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Old 23rd September 2005, 04:05 PM   #3
mvollb is offline mvollb  Germany
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Hi Wolfgang,

there are many threads regarding this question, the search function can help... but to keep it simple for you:
remove the fuse of the negative supply voltage and connect a amperemeter - total bias should be around 700 to 750 mA, so that your heatsinks reach 40 to 50 degree celsius.

I guess your setting is ok but a good stasis2 will not have spices nor oscillation as shown on your picture, so check the condensers on the driver board, even the small tantalum pieces will make the problem with oscillation. Whats about the DC offset?

When you have a big silly sound during the switch on process - check the supply condensors.

best regards
Michael
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Old 23rd September 2005, 07:25 PM   #4
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That is oscillation, or at least a peaking condition stimulated
by the square wave. You'll end up screwing around with
compensation to get rid of it. I assume that the output RC
network to ground is still there.

I don't recall any tantalums on Stasis amps.

The bias should be set for about 25 deg C. above ambient,
and most of the time it works to about 50 deg.

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Old 24th September 2005, 12:32 AM   #5
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I found some tantalum caps in my S-500s. Without opening one up, I can't remember where. Front end CCS, perhaps? Not anything that would lead me to think in terms of a misbehaving output, though.

Grey
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Old 24th September 2005, 03:36 AM   #6
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Did you replace outputs on both channels, and do both
channels do it?
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Old 24th September 2005, 07:38 AM   #7
mvollb is offline mvollb  Germany
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"....I don't recall any tantalums on Stasis amps"

Hi all,

there are some small blue ones there - i have changed them at four amps during the past month...0,15uF/50V - one located at CCS, 2nd one at bias transistor.

I guess the regular types - 470uF/6,3 and the two 47uF/50 are worth changing too.

Mike
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Old 24th September 2005, 08:11 AM   #8
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I have exactly the same problem which gives me headache right now.

I assume the original Statis 2 output transistor is slower than MJ15024/25

http://www.diyaudio.com/forums/showt...969#post723969

A brief. For CFP output stage, it works OK for output transistors with slow speed (mine is OK with TIP35C ft=3Mhz).

But with the same CFP cct, just changing the output transistor to the faster one (I change TIP35C to 30Mhz C5200), it automaticly becomes an oscilator. It oscilates at Mhz region. It seems it oscilates by itself.

Mr. Pass suggest putting base stoopers with values to be tried. I haven't got the optimal R value here, but what I'm afraid that base stoppers do not sufficient to counter this problem.

Anyone has other suggestion what to do with CFP+highspeed device?

I haven't got the ultimate answer
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Old 24th September 2005, 01:39 PM   #9
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Hello all out there
thanks for your fast and active participation
The STASIS is running now for 4 hours on Audiostatic electrostatic speakers , temperature is ok, dc offset is 0,072 V and 0,124 V for the other channel . Is this ok ?, i took BC550 for differentialamp, matched with a cheap digital meter and bond them together for thermal coupling
Bias is about 0,130 V across 1 Ohm resistor
One last small problem left power fuse (6,3A 220V mittelträge) blows every second or third power on, perhabs i should take 8A fuse
The problem shown on the oscilloscope-picture was gone, when I give more bias, only bias to low thats all
Thanks for all the infos i found in this forum, i am a newby, without this forum i wouldn´t start repair this amp
best regards
Wolfgang
I am listening now
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Old 25th September 2005, 10:04 AM   #10
mvollb is offline mvollb  Germany
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I am listening now



By the way: do not switch "him" off, the sound is even better when the amp is powered up all the time, because he has all the time the right temperature and the right setpoint for the current...

My stasis is powered up most of the time during the past 15 years, no burned transistors, no problem at all. The only parts i have changed are these small caps i mentioned earlier. Because i run the amp for several years i do not notice that there was no proper control for the low frequencies. (2 pcs. 47/50 and 1 pc. 470/6.3).

But after comparing the stasis to a newer system, i start with the "caps restauration". Now the stasis is back again.

During the past month i have restored some other Stasis equipment. Blown transistors, weak caps and so on... The owners of these amps are switching on, switching off, switching on, switch......


Mike
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