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Old 23rd May 2005, 12:56 PM   #1
dmx2020 is offline dmx2020  New Zealand
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Default ONKYO amp trans. problem

hi all,
i got this onkyo amp recently and the left channel in it was not working. (It just gives a deep bass sound and goes in to protect mode) The right channel's working fine. It has mn2488/mp1620 transistor pair for the amp and it has few other small transistor before it goes in to the mn1620/mp1620.
I measured the resistance across the emitter/collector in them and all of them show open circuits(for both Rch and Lch). So i'm guessing they are ok. But when i mesure voltage across the same pins, right ch trans. stay contant at 57v but in the left ch. one of them is at 47 and the other is at 70v.

So does this mean that the left trans. pair is blown or could it be some other problem. (there are 3 more transistors between the preamp input and main transistor input, i chekd them for resistance but they seem to be ok as well.

hope im making sense. Juz wanna figure out if its the mn1620/mp2488 that has the problem or not. also is there anything else i cud measre to check.

thanks for any help...:>
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Old 23rd May 2005, 02:31 PM   #2
anatech is offline anatech  Canada
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Hi dmx2020,
There is not enough information to give you an answer, but I'll bet there are some other problems. An output device failure will mean heavy current flow unless the emitter resistors are open - and some others as well.

-Chris
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Old 23rd May 2005, 02:58 PM   #3
Netlist is offline Netlist  Belgium
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As a start the type# could bring some help as well as a schematic.
Also, do you have experience with troubleshooting/repair?
In case you havenít: beware of the Big power supply capacitors.
Discharge before measuring.

/Hugo
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Old 23rd May 2005, 03:08 PM   #4
anatech is offline anatech  Canada
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Hey Hugo,
Everyone should have a handtool with a chunk missing! Of course, Hugo is correct. THere are many gotcha's in servicing since electricity is invisable. You can feel it and it can kill you.
-Chris
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Old 23rd May 2005, 03:56 PM   #5
Netlist is offline Netlist  Belgium
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I'm still recovering from a discharging cap who hit me good two days ago despite many years of experience and one moment of carelessness. Hence my post.

/Hugo
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Old 23rd May 2005, 08:10 PM   #6
anatech is offline anatech  Canada
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Yeah, I still get nailed from time to time. Then it bugs me how stupid I was. Most of the damage can come from jerking your arm back (involuntary reaction). So you still have the jitters?

It's easy to take these things for granted.
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Old 23rd May 2005, 08:34 PM   #7
Netlist is offline Netlist  Belgium
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Apart from a burned finger, all seems back to normal.
320VDC from a TV primary is quite a lot, luckily I had the one-hand-in-the-pocket.
Meanwhile we're going off topic but... the thread stays on top.

/Hugo
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Old 24th May 2005, 10:04 AM   #8
dmx2020 is offline dmx2020  New Zealand
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thanx 4 the advice guys,
The model number's onkyo TX866 discrete amp tuner.

I have attached a schematic of the Left channel of the amp pcb( its the same for the right ch).
On it u can see the peramp input coming in from the left side, and then it goes to the 3 transistors i was talkin about(Q515, Q517, Q519). And then it goes to the big power transistors( Q521, Q523).(These are the MN2488/MP1620 pair) In btween these 2 pnp/npn combination the output comes to the speaker(L out)
Im not sure what all the other transistors are for(Q601-Q605) but i think they are 4 the protection circuit.
The power comes in from +B1 and -B1 of JL504 and its at about 100v.

As i said above, at coll/emmiter of Q521 is around 70v and Q523 is at around 47v whereas on the right side they are both at about 57v.

I'm sure this time its a lot clearer, so is there a way to check if the fault is with Q521/523 or any of the other transistors.

The part numbers are:
Q515: 2sc1740 (npn)
Q517: 2sd1763 (npn)
Q519: 2sb1186 (pnp)

and mn2488/mp1620 are the Q521/Q523.

Hpoe this is enuf info. Thnx in advance...
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File Type: jpg amp_lch1.jpg (71.7 KB, 422 views)
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Old 24th May 2005, 06:35 PM   #9
Netlist is offline Netlist  Belgium
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As far as I can see all the Q6xx are protection circuit and a link to the Rch.
Taking out the 5 Q5xx transistors is probably easy to do for measuring. Have a look at R537 and 539 as well as the emitter R's.

/Hugo
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Old 24th May 2005, 09:40 PM   #10
anatech is offline anatech  Canada
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Hi dmx2020,
I think we need to see the voltage amp section as well. It sounds like you have a problem further up. The outputs in there belong in a Sony. I suspect it has been serviced before. The original #'s are on your schematic.

-Chris
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