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STK1080ii Devices
STK1080ii Devices
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Old 4th February 2019, 08:30 PM   #21
Telnet100 is offline Telnet100  New Zealand
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Morning Mooly -- Thanks for that --
I was just measuring between pins 2 and 3, and then between pins 4 and 5, so wouldn't you think that this should find R7 and R8 as on the pdf? It appears that the emitter leads from the output dies are spot welded to pads which then go directly to the output terminals, so that would eliminate anything under those devices.
And, all the STK 1080ii devices I have here measure the same -- Interesting, when R7 and R8 are shown and numbered on the pdf. I have attached the original Sanyo Datasheet for some of the genuine parts, which could be of interest for You.
In my Replacement, I'll stick to those .22 Ohm values and closely monitor what happens.
Can You tell me the purpose of that D10 Diode which is connected to the Collector of Limiting Transistor TR14?
Thanks Again for all!https://www.diyaudio.com/forums/atta...1&d=1549315819
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File Type: pdf STK Data 2.pdf (416.2 KB, 34 views)
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Old 4th February 2019, 08:40 PM   #22
Telnet100 is offline Telnet100  New Zealand
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Just a followup thought: Maybe when this Akai AM-U61 was built, the devices would have been the Genuine STKs which would have contained those Emitter Resistors --
Maybe since then, (with it's recent history of unreliability and blowing up STKs), that these new STKs that have been fitted lack those resistors? - As the STK I removed definitely does not have any that I can find/measure.
From What I read on the 'net these Akai AM-U61 are very reliable. -- Interesting.
Thanks for all --
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Old 5th February 2019, 03:11 AM   #23
PRR is online now PRR  United States
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> purpose of that D10 Diode which is connected to the Collector of Limiting Transistor TR14?

And its twin.

The output is a Darlington. Needs 1.2V to turn it on.

Clamp transistor can pull-down to <0.1V. That's a hard turn-off.

Clamp plus diode pulls down to 0.6V. This is sufficient to to turn-off the Darlington without getting so hard-off that it recovers badly.
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Old 5th February 2019, 07:53 AM   #24
Mooly is offline Mooly  United Kingdom
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STK1080ii Devices
The original STK's have to have the 0.22ohms somewhere. There is no other way to sense the output stage current.

Try passing a fixed current of say 1 amp between pins 2 and 3 and then measure the volt drop between those pins. If you don't see a meaningful volt drop then I'd have to suggest all the devices you have perhaps aren't genuine.

The two diodes are as PRR describes.
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Old 8th February 2019, 08:21 AM   #25
Telnet100 is offline Telnet100  New Zealand
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Hi Mooly
Of Course, You are absolutely right -- how can the Limiters sense output Device Current if there is no Emitter Resistance; Looks like what You said is correct; these devices I have are ALL virtually a Short Circuit between Pins 2 and 3, and 4 and 5. This then explains the recent unreliability of this Amp.
Did what You suggested, and there is practically NO Voltage drop.
In that Datasheet I sent You, Sanyo state that there should be .22 Ohms in each case, so I will go with that value in my replacement circuit. I fiddled with those Vbe Multiplier Base Values in order to use standard values of resistors for the final build; the Original STK Device looks like it would have run between about 10 to 15 Ma IQ, so I'll shoot for that at the end. Small problem is that it will probably need a Trimpot in the Base/emitter side of the circuit to be able to set it up to that; I decided if I put it there, and it goes Open Circuit in the future, that the IQ will only drop and therefore protect the rest.
Once again, Thanks for All the help!
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Old 8th February 2019, 12:16 PM   #26
Mooly is offline Mooly  United Kingdom
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STK1080ii Devices
That all sounds like a plan and its interesting that the devices you have seem to be missing this resistance... which might explain a lot actually. Its not just current limiting that is affected but perhaps more importantly the thermal stability of the stage and likelihood of thermal runaway.
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Old 18th February 2019, 05:44 AM   #27
Telnet100 is offline Telnet100  New Zealand
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Default STK1080ii Device Replacement

Thanks for that --

I have now completed the Pc Boards and are now assembling them; see attachment;
I Need some advice; Due to the fact that I don't know how this amp is going to be treated, Do You think it could be an idea to increase the values of those Emitter Resistors to say .33 of an Ohm to make sure that the Current Limiting Circuit of TR14 and TR15 comes into play a bit earlier -- Or is it just best to leave them at .22 of an Ohm?

Thanks for all, and Kindest Regards --
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File Type: jpg DSC00701.JPG (294.6 KB, 104 views)
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Old 18th February 2019, 07:59 AM   #28
Mooly is offline Mooly  United Kingdom
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STK1080ii Devices
I can't answer that one I'm afraid... because many current limiting schemes would not protect an amp from major abuse. Thermal issues would still come into play under sustained overload and probably see the module off.

All you can do is make a judgement call. Try 0.33 (or 0.47) and see how loud you can play the amp and then decide if it will be OK for the customer. Try adding a fixed a 8 ohm load across the speaker to make things a bit tougher and see if it still plays OK without noticeable clipping.
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Old 2nd March 2019, 08:40 AM   #29
Telnet100 is offline Telnet100  New Zealand
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Default STK1080 Devices

Well, I seem to have had significant success.
I mounted the Output Devices onto a piece of 5 mm Thick Aluminum which I have attached to the Amp's Heatsink with Thermal Paste and screws.
After powering it up on the Variac, I slowly Increased the supply voltage up to our normal 230 Vac while monitoring the Quiescent Current, which I have set to 15 Ma in both channels. I have also attached the Vbe Multipliers to the heatsink and these appear to be doing what they should. After watching it for a while to see where things were going, I connected up the Oscilloscope and lo and behold, it seems to be Rock Stable, which is good considering there is probably some extra wire length in there between the mother board and my additions. Then I connected a set of speakers while still monitoring the outputs, all good. Then, a Signal Source to the input; and it sounds great.
Tomorrow I will give it a Power Output Test into my Fan Cooled Resistor Stack and see what is what. In the Final Build, I used .33 of an Ohm Emitter Resistors as I believe this might help the Thermal Stability and Current Limiting to activate a little earlier. I have attached some shots of the exercise. Thanks to All who have given help!
Attached Images
File Type: jpg One Channel.JPG (287.4 KB, 73 views)
File Type: jpg Ready for Install.JPG (325.3 KB, 79 views)
File Type: jpg Installation in Akai.JPG (346.3 KB, 80 views)
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Old 2nd March 2019, 02:06 PM   #30
Mooly is offline Mooly  United Kingdom
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That's excellent, well done

(neat boards as well)
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