Akai AM-2200 transistor replacement & question - diyAudio
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Old 26th February 2015, 09:06 AM   #1
JohanH is offline JohanH  Netherlands
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Default Akai AM-2200 transistor replacement & question

Hello all,

I am having problems with my Akai AM-2200.
First off full DC on output.
Found out that TR4 and 5 were not putting out the required voltages.
Replaced those and most of the PS caps.
That fixed the TR4 and 5 voltages but not the DC on output.
In circuit the rest of the transistors show no unusual shortcircuits etc.
The fuse resistors all measure ok but I will replace those later on.
Disconnected the power transistors; still DC on output.
I have now decided to replace driver and power transistors.
Here's what i have come up with:

Original power transistors:
2SB616 100V, 5A, 60W, 22MHz hfe 40-200
2SD586 100V, 5A, 60W, 15MHz hfe 40-200

Replace with:
2SB688 150V, 8A, 80W, 10MHz hfe 55-160
2SD718 120V, 8A, 80W, 12MHz hfe 55-160

Original drivers:
2SB560 (E,F) 100V, 700mA, 0.9W, 100MHz, hfe 100-320 (E+F)
2SD438 (E,F) 100V, 700mA, 0.9W, 100MHz, hfe 100-320 (E+F)

Replace with:
2SA1837 230V 1A 20W 70MHz hfe 100-320
2SC4793 230V 1A 20W 100MHz hfe 100-320

Do these seem ok?
Choices were also dictated by what I can get locally for cheap.

Also I intend to replace the 0.47 ohm emitter resistors with 0.22 ones and adjust the bias accordingly.
Also possible and ok?

Thanks in advance for your input.
Cheers,
Johan
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Old 26th February 2015, 01:52 PM   #2
Junm is offline Junm  Philippines
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My akai 1135r has the same trannies...but one channel was burnt and can't find exact parts so i had replaced most of the parts that i already have..except the LTP and vas tranny....my output was B778/D998. driver was replaced with bd139/140 since my rails are only+/- 35v. also replaced the c945 vbe tranny with c3611 that was easily screwed in the heatsink but played a bit of the resistance in the vbe area to adjust the output bias normally to about 25ma..

Overall it was fine and no smoke....sounds good but can't compare to the previous trannies since it was brought to me not working.

As long as you comply to the ratings above you rails and dissipation...it will be okay but i guess you need some adjustment in the bias.
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Old 26th February 2015, 02:20 PM   #3
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To have full DC in the output will mean that one of your first stages transistors is shorted
another is that any of the transistors you replaced is oriented the wrong way

From experience i can tell you that even though the choice of drivers is very good these will not work in this amplifier since previous stages will not drive them properly and or compensation is designed fro the specific transistor and specs

Obtain original drivers its very possible

Notice that if you have oscillation coming from the choice drivers you are going to smell it before you hear it if you put load in the amplifier there you need a scope or original drivers

Regards
Sakis
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Old 26th February 2015, 02:30 PM   #4
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I would fault find first and then change the faulty component only.
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Old 26th February 2015, 02:39 PM   #5
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have some respect for the people that been there before John ...
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Old 26th February 2015, 04:12 PM   #6
JohanH is offline JohanH  Netherlands
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Ok,

thanks all for the input.
I was kind off afraid of the answers Sakis provides and of course Jon is right.
My intention was to replace as much as possible since most parts (that I chose) are not expensive, but tomorrow I will check first stage transistors first and report back.
If faulty would you think BC550C would be ok?

Thanks again,
Cheers,
Johan
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Old 26th February 2015, 04:46 PM   #7
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no it will not be OK
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Old 26th February 2015, 09:07 PM   #8
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ksc1845 would be a useable replacement for the ltp transistors and should have the same pinout. Remember that both of them must be replaced together.
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Old 26th February 2015, 09:21 PM   #9
anatech is offline anatech  Canada
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Hi Johan,
The input pair should be matched for beta for starters. You may not have a short to cause this. You also need to check the DC level shifter (TR3?) and have a look at the negative sides resistors that are supposed to work like a current source. An open here might run the output to one rail. You didn't say if the offset was positive or negative.

-Chris
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Old 26th February 2015, 09:28 PM   #10
llwhtt is offline llwhtt  United States
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Nice to see Anatech is back!

Craig
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