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Old 3rd July 2012, 09:46 PM   #1
congole is offline congole  Serbia
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Question burning hot amp!

Looking for others' experience and/or advice ....
A friend of mine and myself are in the midst of renewing an ONKYO 817XD integrated amp. After finishing cca 60% of the work we let it play to burn-in the components ... and realized that it (still) runs very very hot! And when I say hot, I mean not being able to keep the hand on the heatsink for more than a fraction of second ... I would expect it maybe from a class A amp, but this one is not of a kind; the amp uses SANKEN 2SA1494/2SC3858 - pretty well known (and somewhat particular) transistors. We checked the base voltage and it is in line with the expectations we have from work on several other ONKYO models (either older or of similar age) - somewhere around 570-600mV. Other ref. measurements also are in line with expectations. Previous owner reported that it also ran very hot, so we might even expect this to be a normal manifestation, but still ' woul like to 'sleep well'. For reference, what remains still to be done on this one is changing the capacitors in the power section (which appear to be rather 'dry') and the terminals on the back.
Does anyone have an experience that could help? Can anyone perhaps provide a service manual for this model?
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Old 3rd July 2012, 10:35 PM   #2
RS232 is offline RS232  United Kingdom
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If you are saying that the bias is set to 570-600mV that is to high for a Class AB amplifier.
No wonder the flimsy heatsink are very hot.
I would say set bias to around 50-60mV
On the other end I'm no expert...
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Old 3rd July 2012, 11:15 PM   #3
Bone is offline Bone  United Kingdom
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You measured the 570/600 mV where?
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Old 4th July 2012, 12:00 AM   #4
juma is offline juma  Germany
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I understood that Vbe measured 570-600mV. To know the bias current the voltage drop on the output transistors' emitter resistors should be measured.
There is also a possibility of high freq. oscillations (as a cause of very high thermal dissipation) - if there are any, the scope will show it.
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Old 6th July 2012, 10:40 PM   #5
congole is offline congole  Serbia
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The voltage was measured between base and ground. And I would not say it is so high (as previous experience suggests). It still was not attached to oscilloscope, so I cannot confirm or deny the lack of oscillations, though if such were so present, I guess the amp would not sustain working very long; neverthless, we shall try to find if there were any suspicious components left in the circuit after the transistors (which is why it could be helpful if someone could point us to the service manual).
... or maybe it works is it was supposed to be (we are just paranoid!) - it has been left working for some time now and no so apparent problems yet manifested...
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Old 6th July 2012, 11:18 PM   #6
Spiny is offline Spiny  United Kingdom
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measure the bias current- the voltage across the emitter resistors , then calculate from the resistance value. I think you will find the bias needs adjusting or the drivers are gone.

VBE will be about the same no matter what current the transistors are passing its the diode drop voltage of the junction (0V its short, 0.6@ healthy, high voltage its open circuit)
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Old 10th July 2012, 11:34 PM   #7
congole is offline congole  Serbia
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Smile Update

I tried contacting ONKYO and got a reply in less than a day! They explained that the 817XD is more or less the international version of A-8690 (though I found out that the latter has also the DAC built in). Also, they were kind to point me to a private page where the tech. reference / service manuals could be downloaded from < Onkyo Integra - Online-Treff - Service Manual, Vollverstärker, Integrated Amp, Amplifier > :-).
To sum-up, the voltage we measured to be just under 600mV was marked also on the schematics as 0.6V. Also, most of other measurements were near to the references in the manual; the only difference we found was on collectors of driving transistors, where it should be -59.3 & +58.4 V while we measured just a drop over 60V. I guess we are to prepare for another session to finish work on this one, but for now it looks like it should be a hot one!
' hope this can help someone else, too.
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