Yamaha A400; bias too high after driver chip replacement - diyAudio
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Old 30th January 2005, 01:57 PM   #1
hiwiho is offline hiwiho  Germany
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Default Yamaha A400; bias too high after driver chip replacement

I exchanged the voltage driver chip in the amp, because one channel was dead. Original was a STK3042-II, which is now obsolete. So I took STK3042-III. I planned to change all semiconductors in the dead channel, too but didn't do cause I couldn't remember if it was left or right. Stupid me!
So I soldered the new driver chip in and turned it on. Relay switched on after a few seconds, no smoke, no smell. But damn, 2SA1104 and 2SC2579 got really warm on both channels! I switched it off, let it cool down and attached my voltmeter to measure bias. Manual says it should be 6.5mv, but I had 165mv on the next turn-on and 4mV in speaker-protect-mode before switching of relay! I can regulate between 120 and 190, but I wasn't planning in turning that old amp classA
Driver chip remained cold, at least during the few seconds i had it all turned on.

Why do I have such high bias?? Any ideas? All trannies measure ok, no shorts. I didn't measure the points given in the manual, cause i don't want to grill parts longer than necessary...

matthias

The schematic:
http://www.listmember.de/diyaudio/ya..._schematic.jpg
(380k)
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Old 30th January 2005, 04:53 PM   #2
hiwiho is offline hiwiho  Germany
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Update:

Bias is ok, when the machine is cold and relatively stable. Driver chip gets warm. However, when I switch on one speaker, even without one attached to the terminal, the bias rushes away. Doesn't matter if pair A or B. Once rushed away, the bias reacts very nervous on tries to regulate it. It will only work again if the machine cools off and then gets turned on again. I connected a speaker. Volume was zero, but i had a massive 50hz-hum through the speaker

What's wrong? I'm clueless!

matthias
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Old 30th January 2005, 05:08 PM   #3
Quattor is offline Quattor  France
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Hmmm, nervous bias current? No wonder – the trim pot is tied up between base and collector of TR201. Any dirt or damage in the trim pot leaves B-C open and TR201 switched off which means high bias current. The sanest thing to do is to reverse the circuit a bit and to tie a trim pot between base and emitter of TR201. In this case, open circuit in that trim pot will force TR201 to open and to reduce the bias current to minimum. If this is hard for you to do, try to replace the original trim pot with the new one – multi-turn is a must. This way, bias current is not going to be so jumpy. Solve this problem and see how it gets. Anyway, be warned – any game with the bias circuit is quite dangerous – in most cases it leads to smoke…
Regards!
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Old 30th January 2005, 05:21 PM   #4
hiwiho is offline hiwiho  Germany
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Thanks, I will try.

But any idea regarding speaker switch - bias rise??? Can that be some oscillation effect?? Some hf-blocking cap in the terminal gone conductive? They are rated 50volts - supply goes up to +/- 41volts...
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Old 30th January 2005, 05:28 PM   #5
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Quote:
Originally posted by hiwiho
But any idea regarding speaker switch - bias rise??? Can that be some oscillation effect?? Some hf-blocking cap in the terminal gone conductive? They are rated 50volts - supply goes up to +/- 41volts...
Hi

Inspect if there are any DC ofset at the output ...if so ,it can cause the increase in bias , when the speaker is connected.
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Old 30th January 2005, 07:55 PM   #6
hiwiho is offline hiwiho  Germany
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Yes, but the offset is only about 20mv.... not way too much. And I checked with my scope... no oscillation except about 12.5mV pp at around 2mhz.... at the limit of my old Hameg 10mhz-Scope.


I disconnected everything after the speaker switches, and bias stayed stable at about 8mV. With terminal plugged, it raised up to 230mV within a second... without any speaker connected!

I connected a speaker before the terminal. Bias stayed stable and low, the tested channel working...

matthias
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Old 31st January 2005, 12:01 AM   #7
Eva is offline Eva  Spain
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The amplifier is unstable with the new driver IC [this 2Mhz oscillation shouldn't be here] and something in the speaker switch circuit is making the oscillation worse, probably capacitive loading of the output
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Old 31st January 2005, 01:11 AM   #8
anatech is offline anatech  Canada
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Hi Matthias,
I agree with Eva, your amp is breaking into oscillation. Check the zobel resistor and capacitor on that channel. Have a look at the resistor on the other side while you are at it.
Of course the bias pot may be damaged / noisy also. It's in a bad spot (in the schematic).

-Chris
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Old 31st January 2005, 02:27 AM   #9
Eva is offline Eva  Spain
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The schematic reveals that the speaker switch sub-circuit has several capacitors connected in paralell with the output of the power amplifier. Obviously this translates a small oscillation into a huge one

Check that R257 and L201 are not shorted
Check that C225, C231, C233 are not open

Try to increase R257 to 10 ohms
Try to slightly increase the value of C225, C231, C233

I suspect that the new driver IC has higher gain-bandwidth product than the old one and requires more compensation to achieve stability with this output stage. Note that the new driver IC is not exactly the same model as the old one

I've experienced similar unstability problems from time to time when replacing obsolete or unavailable transistors with others having similar specs but lower capacitances. Adjusting compensation capacitors solved the problem

To check stability you have to look at the amplifier behavior when entering and leaving clipping when driving the rated load at high frequencies. It the circuit is stable when clipping a 10Khz sine wave then it's not likely to oscillate in any other circumstances. Sometimes the oscillation has less than 100mV amplitude so check carefully
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Old 31st January 2005, 03:33 PM   #10
K-amps is offline K-amps  United States
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Matthias,

I have had the same issue in the past with one amp I was repairing then let it go because of oscillations. (Still lying somewhere). It was a Sony ESD-110 (110wpc).

It had a blown STK-3042 and I replaced it with the Series III.

I see the amp breaking out into oscillation, I have played around with the compensation caps but no avail.... I do not have the schematics.....

Not sure if it helped, but now you know the problem was caused due to the new driver chip....
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