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STK1080ii Devices
STK1080ii Devices
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Old 16th January 2019, 12:30 AM   #1
Telnet100 is offline Telnet100  New Zealand
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Default STK1080ii Devices

Hello, All --
At present, I have been working on a Customer's Akai AM-U61 Amplifier.
This device came to me and had been previously been worked on by another Tech, apparently without much success.
The Amp has had a recent history of blowing up the STK1080ii Output Devices.
After removing the suspect STKs from the board, I decided to then check everything for correct Component Values to ascertain what had been done in the past.
Firsty, I found that the Protection Zener Diode named as D23 was Short Circuited in both channels, no suprises here. Looking Further, I found that One of the NPN Driver Devices, TR10, (2SC2704) in one channel was suspect with a very low B to E Voltage drop, so I replaced it with the same type. To Fool the Va Stage into thinking that the STKs were in place, I made up a small Pcb with some resistors and a Darlington Transistor to take the place of the Input Stage of the STK, in each channel.
After Powering it all up off of my Variac, I increased the Mains Voltage to the correct value and proceeded to do measurements with Meter and Oscilloscope. Everything looked pretty good, so I then mounted one of the Original STK devices on an outboard heatsink and connected it to the motherboard.
Powered her up again on the Variac: Lo and behold, all voltages were almost exactly what the Akai Manual showed. No Sign of Instability at all. Did a Load Check into a Resistor Stack at 1Khz and all looked good.
Then: Inserted a New STK1080ii into the other Channel, and did the same Power up.
As soon as the Speaker Protection Relay closed, The Inner Loop between the
Va Stage output and the input Terminals of the STK went absolutely unstable, looked like bad noise on the scope. No sign of this anywhere else in the circuit, from the preamp stage through to the input of the Final Va stage was clean, even the STK Output Terminal was stable.

There is talk of Counterfeit STK Devices; how is it that the Original STK Device sets up fine, and Yet the New Device is like this? Maybe it is because
the New/current STKs are ALL Counterfeit?

I proved this by swapping the Output STKs between channels; the Fault followed the New One.

Is there any Way I can tame this?

Sorry to go on, but when people are trying to help, it pays to try and be accurate.

I will have a go to post the Circuit Schematic and Scope results.

Thanks in Advance --
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Old 16th January 2019, 01:39 AM   #2
Telnet100 is offline Telnet100  New Zealand
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Here is the Schematichttps://www.diyaudio.com/forums/attachment.php?attachmentid=729360&stc=1&d=1547605 580 for the Akai AM-U61
Attached Files
File Type: pdf Akai AM-U61 Schematic.pdf (767.1 KB, 72 views)
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Old 16th January 2019, 02:08 AM   #3
Telnet100 is offline Telnet100  New Zealand
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Here is a shot of the Scope Trace while connected to The Input Pin 12 of the STK:
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File Type: jpg DSC00635.JPG (334.2 KB, 413 views)
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Old 16th January 2019, 02:14 AM   #4
Telnet100 is offline Telnet100  New Zealand
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Here are shots showing as the Input of 1Khz is increased into the Amp's input; the last shows the Output Signal which look clean --
Attached Images
File Type: jpg DSC00639.JPG (325.6 KB, 414 views)
File Type: jpg DSC00640.JPG (347.2 KB, 401 views)
File Type: jpg DSC00641.JPG (334.2 KB, 392 views)
File Type: jpg DSC00643 (2).JPG (347.6 KB, 382 views)
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Old 16th January 2019, 02:52 AM   #5
Telnet100 is offline Telnet100  New Zealand
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Here are the shots of the 'Counterfeit' STK1080ii

To Me, this looks to be far more robust, with it's Driver Transistors in parallel.

This is the Device which works properly in the Akai.
Attached Images
File Type: jpg DSC00671.JPG (335.0 KB, 174 views)
File Type: jpg DSC00668.JPG (330.7 KB, 206 views)
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Old 16th January 2019, 02:55 AM   #6
Telnet100 is offline Telnet100  New Zealand
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This is the Only STK1080ii Device Currently Available, and this is the Device which has the Noise/oscillation? problem when connected in the Akai Circuit.

When compared to the other, makes one wonder 'What's going on here?'
Attached Images
File Type: jpg DSC00672.JPG (324.0 KB, 185 views)
File Type: jpg DSC00667.JPG (316.4 KB, 150 views)
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Old 21st January 2019, 11:35 AM   #7
Mooly is offline Mooly  United Kingdom
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STK1080ii Devices
Its very hard to say what is going but you mustn't lose sight of the fact that you have confirmed that the fault follows the 'new' modules wheras the old original one appears to work.

D23 short circuit. Either there has been a 'local' problem within the STK that has killed the Zener or the fault has allowed excess current to flow through the Zener and via (at least) D2 and through TR10.

Could that explain the suspect TR10?

All that you describe does sound like there is some major difference in the modules affecting stability.

You could try a small cap across D23, something like 1uF or 10uF and also try tweaking C19 to see if it changes things. C23 and C24 could be worth tweaking as well (one a 10pF and one a 100pF ???).

Its all trial and error I'm afraid but it sounds like the bottom line is that the modules are causing the problem.
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Old 21st January 2019, 01:35 PM   #8
Kay Pirinha is offline Kay Pirinha  Germany
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We mustn't forget that Sanyo, the only (!) manufacturer of these thick film hybrid modules, had to close it's semiconductor division in 2004 after the Chuetsu earthquake, which ruined their semiconductor plant almost completely. So it has to be assumed that any of these devices that is being marketed as from new production is conterfeit.
Sad but true .
Best regards!
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"Bless you, Sister. May all your sons be bishops." (Brendan Behan)
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Old 28th January 2019, 09:27 AM   #9
Telnet100 is offline Telnet100  New Zealand
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Hello

Thanks so much for the help. Unfortunately Trying Capacitance across D23 does not alter anything. This effect Seems to be Power Supply Voltage sensitive. The Instability/noise problem only seems to be only in the inner loop between the Outputs of TR9 -TR10 and the Input Terminals of the STK.

I discovered a Circuit Error in the Akai Drawing for the AM-U61.

The Emitter of TR10 is Connected to the Collectors of TR6, and TR8, NOT the Collector of TR12.
The Collector of TR12 is connected to the Emitter of TR13 and the Junctions of C20, and R52.
The Drawing of the AM-U61 is A Misprint, and the Akai Drawing of the similar AM-U41 is correct in this instance, but of course the AM-U41 lacks the two earlier TR5 and TR6 Devices.

I am looking at Constructing a Replacement for the STK1080. I have done a lot of research into the layout/construction of both the Original and Newer
currently availalable STK Devices, and am convinced that I can engineer a
Discrete Replacement using currently available Bi-polar devices. After all, it looks like only a Double Darlington Amp driving an ordinary Bi-polar Output Stage with a complementary pair of Output Devices. I can do this on a small Pcb, and mount the output devices onto the original heatsink.

I have already made up a trial board to test and check all voltages and currents.
I am about to mount the Output Devices and test run it from the existing circuitry of the AM-U61. It will be interesting to see what happens.
I will keep You informed on progress, because this could help a lot of Techs with a similar problem.
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Old 28th January 2019, 11:19 AM   #10
Mooly is offline Mooly  United Kingdom
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STK1080ii Devices
I'm sure engineering a replacement is very do-able and will prove to be an interesting exercise.

Good luck... keep us posted.
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