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Help me, i need one STK 3042

I've just tried to bridge the two STK0050 as you suggested, on the jumpers DMM reads 0.0V (good). Connecting headphones and audio source it sounds ok, so the stk3042 seems ok. Of course it is a little hot probably also because I am requesting some extra current to drive the headphones.
So I am wondering why the stk0050ii blows... I will let the amplifier go for some time but now I'm confused :no:

Here you can find the service manual / schematics I have found about AM-U04. May be useful to someone else.

full service manual (>5M):
http://alez.prodolon.it/docs/akai_am-u04_service.pdf
main schematics only (1,5M):
http://alez.prodolon.it/docs/akai_am-u04_main-schematics.pdf
 
The STK3042 seems good but one more test to confirm. There is a current source inside the STK3042. If this current source is too high it may possibly kill the bias voltage source inside the STK0050ii. If this is the situation then the output stage of the STK0050ii will have too much bias current and they will burn.

You have 100 Ohms resistors connected to pins 5, 6, 10 and 11 of the STK3042. Measure the DC voltage drop in the resistors. Compute the current in the resistor (i=V/R). You should have something of the order of 1~10mA. If you have much more than 10mA, that STK3042 may kill any STK0050ii that you will connect to.

I tried to download the service manual from your link and couldn't save it.
 
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Hi ecluser. The current test gave me good results, the current is about 6mA on each + or - output of each channel. So Stk3042 seems OK... then why the STK0050II burns? Power lines are ok... I will also check passive components looking for short-circuits but it shouldn't be the case, otherwise the amplifier would not have been working for a while after first replacement.
Your current theory is interesting because explains the little time between first replacement and second burn, are there any other reasons why current may increase?
Voltage at pin 1,2-14-15 (input of stk3042) is 9,2V, while the datasheets says 8,0V. But I don't think this makes the difference...

The link I have posted should work, if not I can send you by mail or try browsing http://alez.prodolon.it/docs/

Thank you for your patience.

Edit: when sound source and headphones connected the current goes to 7-8 mA on one of the + / -, of both channels. Does this mean that positive and negative branches are not equilibrated?
 
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I think that your STK3042 is good. The difference in the current (from 6 to 7-8mA)when headphones are connected may come from a small offset voltage at the output.

Why the STK0050ii burns? Maybe a poor STK0050ii (made in China) or high frequency oscillation. To see if there is HF oscillation you need to observe the output signal with an oscilloscope.
 
There is no HF oscillation.... All I can do now is replace again the STK0050II, and replace all its external components, in particular capacitors... after replace keep it under observation with oscilloscope and DMM to see what happens... and keep my fingers crossed!
 
Each channel has their own bridge rectifier and main filter caps. Is it possible to find a convenient place on the board to add protection fuses between the capacitors and the power IC, one fuse for each supply line and for each channel? Usually there are many jumpers on the top face of the board and tail gate fuses can be soldered to replace a jumper.
 
Yep there are two jumpers from the caps to the power amp packs... I will consider inserting 4 fuses there... thank you.
BTW: The STK0050II I bought is marked with a small label "AGS electronics" ..does anyone know this brand? Is it chinese or reliable? Or is it only a distributor?
 
Hello. Just a brief update: I've replaced with a new STK0050II, but this time checked with oscilloscope.... I have 3,5 MHz 1vpp sine out of it. The cause is a poor ceramic capacitor, so little but so important!
So I have 2 considerations.
1) God bless oscilloscope! :yes:
2) Never undervalue small capacitors and take care of them in amplifiers! :yes:
Hope this will be a warning for others.
Thanks to ecluser for patience and help.
 
It is early to be happy :sigh:
I thought it was the capacitor between pin 5 and 6 of the stk0050ii because I've swapped them between left and right channels, and found the HF in the channel that didn't burn (so I thought it was just this cap). But then swapped again the caps and HF is still in the good channel. So I'm again in a strange case, why the bad channel is now good and the good one is now bad? However the HF is only on pins 5,6 and 3,8. At input pins 4-7 there isn't. It goes through the double .47 R to the feedback R-C network. Beyond the RC, before the FET preamp, there are 1,5MHz 180mVpp but they are not stable, they come and go. Maybe the problem is one of the internal capacitors of the STK3042 so I will try to add one externally tomorrow...
But I need to sleep on it now!
 
I would try to add a cap (15pF to 30pF) between pins 4-6, and 10-12, on the STK3042

It is possible that you have no HF at pins 4-7 of the STK0050ii because the oscilloscope probe helps to stabilize the voltage amplifier.
 
ecluser said:
The STK3042II has internal compensation capacitor between pins 6 and 4. The STK3042III doesn't have this capacitor and may oscillate if you don't add an external capacitor. It's enough to blow a fuse. Try to add a 15pF cap between pins 6 and 4 (12 and 10 on the other channel).
Click to expand...

I know this is an older thread, but it is important enough to reiterate. The damping of the pre-drivers is critical. In applications where the STK-3042 II is being replaced by the STK-3042 III ( STK3042II / STK 3042III ) the supporting circuits may not include this damping. I found this to be true on a Yamaha R-300 / Yamaha R-500 type AF board as well. The oscillation caused the AF power amp module ( STK-2050 ) to heat rapidly to near destruction. It was only trying to do what the preamp told it to do.
 
The reason why HF oscillation caused the power amp module to heat rapidly is that the turn-off time of transistors is usually larger than the turn-on time, and at very high frequency both transistors (the NPN and the PNP) of the output stage are into conduction at the same time. This is a short between the rail supply lines, for a short period at each half cycle of the high frequency signal.

The STK0050II has provision for a capacitor, between the bases of the output stage. In the Akai AM-U04, this capacitor is very small, only 0.047uF. In some amplifiers this capacitor is as large as 10uF. If the power modules heat up, it would be a good idea to use a larger capacitor before the bases of the output stage (pins 5/6 on the STK0050II for instance).

The STK2050 doesn't have these pins but you can add a capacitor between pins 1 and 7 if no capacitor is implemented in the circuit.
 
You're right, ecluser. The oscilloscope probe capacitance was enough to kill the HF. I've noticed this using two oscilloscope channels: one connected to speaker output, the other touching medium points.
I've added two small 22p caps where you suggested and solved the problem! Actually I had tried before you wrote but with a too large capacitance that rised the offset voltage too much.
Thank you, I hope the AM-U04 will keep working now!
As you told the capacitor between basis of output stage is as small as 47nF, but the stk0050 datasheet asks for this value. I think that the maximum capacitance depends on the output resistance of the previous stage, too large can cut high audible frequency or be a load for the previous stage...
BTW I have noticed also that some cap values are not the same as service-manual schematics. C51 is 1.5 nF instead of 15nF, C46-C47 are 150pF instead of 22pF.
 
I'm sad to write that the AM-4U is dead again.
The component is always the same but this time it's not the output stage to be burnt but some input stage because output pins are 0V and pins 5,6 are up to rail voltage. Input pins instead are both to -47V ...
I really don't know what is happening with it...
 
A precisation: the amplifier worked half an hour with one speaker connected to "good" channel (only oscilloscope to both channels). When I connected the speaker to the other channel (the "bad" one) after a while it blew. Any ideas? :(
 
Yes the STK-0050 is burnt again, but this time not its output stage but its input stage. Input pins are both down to negative rail voltage, even if disconnected from the 3042. And blew only after attaching the speaker...
 
Paul C said:
There are a number of suppliers you can order from. If you can't buy these yourself, you may contact me directly.
Click to expand...

Hi Paul ,
I ordered two of these from an E-Bay member in china. I tried each of them only to burn out the STK-0050 outputs instantly. If I install new outputs & the original stk 3062 , the unit works ( but with a lot of noise ) ( if i spray the stk3062 with freeze , the unit plays fine for 10 min or so.)

I'm a little gun shy about buying another STK 3062 from China.
Will the NTE work correctly or are they ALL from China ? Would you recommend installing the stk-3062 III and doing the resistor/cap mod ?
Please advise.
 
Hi Jose Nice serero I have the same one the right chanel cuts out the amp so I replaced the stk0050II but it did not fix the cut out problem the 3042 has two amps in one chip and when the volume gets loud it cuts the amp out but if I put the balance to the left side I can crank it to the max no problem. I loved this amp
I wish I could get it back . I willl try the 3042 voltage amplifier chip.
Please let me know hat you think thank you.
 
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