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STK1080ii Devices
STK1080ii Devices
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Old 2nd February 2019, 06:22 AM   #11
Telnet100 is offline Telnet100  New Zealand
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Hello, All
Well, Progress! I constructed a small Veroboard circuit for which I have posted the schematic of. For the Devices, I used pulls from a Yamaha RX-V740 (which I am dismantling). For the front end I used an MPS-A14 Darlington Device, which is a replacement for the 1M SMD Device used in the STK. For the other preamp/driver devices, I used The pulled 2SC4614, and 2SA1770 which are complements to each
other. The Output Devices I used were also pulls and are 2SC4468 and 2SA1695, also complements. After Checking everything carefully, I Powered it up on the Variac and watched as many voltages as I could, all looked good, so I raised the AC Input Voltage to the correct level, and proceeded to take measurements with meter and scope.
No sign of any instability, and the rest of the circuit voltages closely matched what Akai had listed in the AM-U61 schematic. I noted that The Quiescent Current started off at around 30 Ma and gradually increased as the heatsink warmed up; I had not mounted that Input Darlington on the Heatsink, and as I warmed it up, I noted that the IQ immediately started dropping, so that will be a thing I will have to do before I start to load her up and test the Output Power. I see that many Amps of about this power seem to have IQs between 20 and 50 MA, which can be adjusted by altering the value of those R1 or R2 Resistors in the Schematic. Next Thing is to draw up and etch a suitable Pcb, which only needs to be small, but at least the proof of concept seems to be okay.
What sort of IQ would You suggest that I run this at?
Thanks so much to all that have helped!
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Old 2nd February 2019, 06:30 AM   #12
Telnet100 is offline Telnet100  New Zealand
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Default STK 1080 Replacement

https://www.diyaudio.com/forums/atta...1&d=1549092481
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File Type: jpg DSC00691.JPG (313.5 KB, 158 views)
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File Type: pdf STK 1080 Replacement.pdf (432.4 KB, 41 views)
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Old 2nd February 2019, 07:25 AM   #13
Mooly is offline Mooly  United Kingdom
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STK1080ii Devices
For the values you have in the pdf I'd say just over 100ma would be ideal but that may be considerably more than might be advisable given the heatsinking. As always these things are a compromise. The figure of 100ma is based on the comprehensive work done by Doug Self where optimal bias current is a function of Re.

In practice anything over a couple of milliamps will render the distortion totally inaudible and a figure of 30ma or so sounds more reasonable. You also have to be sure the Vbe multiplier tracks the temperature well if you want to push things. The last thing you want is thermal runaway as it heats up.

Good work though
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Old 2nd February 2019, 05:42 PM   #14
rsavas is offline rsavas  Canada
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FYI, I have designed a STK-0050 pcb, made it available free of charge for you to buy your own pcb. I have a link to the Mouser BOM for parts. Someone has made these available on eBay which includes the pcb and an Al heatsink.
It was originally designed for a Pioneer SX-780, but others have been using them to fix other amps like some Fishers' and others.
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Old 2nd February 2019, 09:30 PM   #15
Telnet100 is offline Telnet100  New Zealand
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Good Morning All--
That looks like a nice layout of your Pcb; I am looking to do something similar for this STK 1080ii replacement, but am considering one-sided at the moment.
After dismantling several of the older and newer STK1080ii devices, I was able to make an average of the component values they used to come up with a working circuit. The Problem is that these newer STK Devices just do not work properly in the Akai AM-U61 as I have posted before, and I just can't seem to tame the problem; whereas the circuit that I just built up and temporarily fitted seems to be stable with all the correct Akai
Schematic Voltages. Also, I found yet another Error in the Akai AM-U761 Schematic;
The Collector Voltage of that TR10 Driver Transistor is actually running at Minus 4.3 Volts, the exact opposite of the Plus 4.3v Collector Voltage of TR9, which you might expect. But the Akai Drawing shows Zero point 4? -- Interesting.
To Continue:
While measuring those dismantled STK 1080ii Devices, I was unable to find any real resistance in those Emitter Resistors marked as R7 and R8. There is just 2 copper tracks, which seem to measure between 0.01 and 0.02 Ohms on my Low R checker.
So in my mockup circuit, I used Point 22 Ohms in order to protect things a bit more.
What would You suggest I use in the final build?
Once again, Thanks to all for help thus far --
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Old 3rd February 2019, 11:15 AM   #16
Mooly is offline Mooly  United Kingdom
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STK1080ii Devices
0.22 ohm is pretty typical and was the value I based the bias current on (post above). Thermal stability I imagine will be poor or at least poorer if you omit them.
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Old 3rd February 2019, 08:21 PM   #17
Telnet100 is offline Telnet100  New Zealand
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Hi Mooly Thanks again --
Yes, looking at several Power Amp circuits by different manufacturers shows exactly that, .22 Ohms.
I wonder how Sanyo? and others who make these STK devices get away with it -- maybe this is the cause of some of the apparent STK unreliability that you read about?
Also, in this Akai Circuit (and probably others) the Voltage across these resistors is monitored by the Overdrive Limiting devices TR14 and TR15; so, how are they supposed to work properly when fed with such a low voltage which is developed across only .02 of an ohm? I guess I will have to look at this part of the circuit when I go to check the performance once I get that Vbe multiplier mounted to the heatsink.
I am also going to reconnect that older STK back into the circuit to measure the IQ, as I had not done that before; be interesting to see what IQ Akai ran that at.
Thanks for all!
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Old 4th February 2019, 07:27 AM   #18
Mooly is offline Mooly  United Kingdom
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STK1080ii Devices
0.02 ohm couldn't work at all as a sensing resistor, at least not just by using simple transistor circuitry. These type of resistors are often some kind of thick film type deposited onto the PCB but I can't really make anything out from the pictures and ss far as I know these types don't fail by going low in value.

Hard to say what is going on with these.
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Old 4th February 2019, 09:49 AM   #19
Telnet100 is offline Telnet100  New Zealand
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Hi Mooly
Yes, I agree; if You look at the dismantled unit's board, in both cases, there are only Copper Tracks less than an inch long. All of the STKs I have here (both dismantled and new) measure that .02 Ohm, or less, so like You, I can't see how any protection or signal limiting can work. Maybe in the very original STK there were resistors? That is why I queried what R Values to use there. I could send You closeups if you like --
Once Again, Thanks So Much for all --
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Old 4th February 2019, 11:05 AM   #20
Mooly is offline Mooly  United Kingdom
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STK1080ii Devices
Perhaps there is something incorporated into the output transistor mountings, under the white glue stuff... you would have to crack one off the board to see though.
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