Kenwood KA-6100 BIAS issues - diyAudio
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Old 8th May 2013, 02:25 AM   #1
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Default Kenwood KA-6100 BIAS issues

Hello,
I was giving a KA-6100 a tune-up and while checking the BIAS found both channels were slowly cycling substantially above and below the 20mV spec. I checked for oscillations and there were none (.5V/div; .5mS/div). Also, the heat sink got warm but never hot and the unit sounds good to my ear and had no perceivable distortions on the scope. When I lifted a Thermistor (5TP41L) off the sink it still cycled And, when placed back on the sink it did change the BIAS (as it should). What do you think: bad thermistors?
Opinions solicited.
M.
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Old 8th May 2013, 06:50 AM   #2
Mooly is offline Mooly  United Kingdom
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Thermal inertia or lag is one possible reason. The sensing element/s simply don't accurately follow the temperature change of the outputs in the short term and so the system behaves like a servo with a large time constant.

Given enough time... perhaps 30 minutes at idle, and it should settle to a stable value. If it doesn't then I would be more inclined to consider the possibility that's its just a characteristic of the design. Both channels behaving the same suggests that possibility.
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Old 8th May 2013, 07:40 AM   #3
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It may not be the actual reason for instability in this instance but just taking the cover off an amplifier is enough to change the thermal environment substantially. Some amplifiers are just hopeless to service without attaching test leads and closing the case over them before settling the amplifier and then testing.

It also takes forever to set bias precisely when this is necessitated by weak thermal coupling between output device and sensor or interference from driver dissipation. This is all too common, even with designs where the sensor relies on case air temperature or is only positioned near the output stage for easy assembly reasons. However, cycling up and down suggests overcoupling with an inappropriate tempco, to me. Within limits, this may still be normal and quite unnoticeable for the model, as Mooly suggests.
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Old 8th May 2013, 08:05 AM   #4
Mooly is offline Mooly  United Kingdom
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Thats well put Ian, particularly regarding the change (sometimes huge change) that operating an amp with the covers on can make.

In fact I'm going to mention that to our friend with the Pioneer M3.
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Old 8th May 2013, 09:45 AM   #5
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bias and offset adjustment in an amplifier under repair will be the last thing to check but after the actual repair.

The unit needs a complete recaping given as a fact that is more than 30 years old

Tricky areas will be the secondary power supply and feedback and bootstrap capacitor

I would start from there

On the contrary to things said by Ian i could say that hardly ever consumer Japanese amplifiers had bias stability issues ... things like the above apply mostly to European or American designs and not really on the consumer side ...something like the more hi end side


Kind regards
Sakis
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Old 8th May 2013, 02:14 PM   #6
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I would generally agree with your experience, Sakis but I have to take the OP word for the bias behaviour.

If the problem was due to a bad boostrap cap, I would expect some sign in the audio too and I assumed any necessary replacements had already been carried out.
There are 2 possibilities; an RC or a thermal time constant. It would be interesting to hear what the frequency of the cycling is, and to see if there is any correlation to VAS bootstap voltage with or without a steady tone applied.
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Old 8th May 2013, 02:18 PM   #7
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Thank you all for your replies.
I'll try again with the cover on and maybe apply some thermal grease to the underside of the Thermistors and see what happens. I might even change the location closer to the base of the sink. I have never seen bias swings like this in any Japanese amp of this pedigree. My inner engineer tells me it's the thermistors and their inability to respond fast enough to stay on top of thermal transients (cover notwithstanding). I'll post my results.
Thanks again,
Mike
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Old 8th May 2013, 02:29 PM   #8
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Ian,
FYI - the cycle time is approximately 4-5 minutes. I will check the VAS BSv with, and without, tone and see if it tracks. I'm curious now.
Thanks again,
M.
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Old 8th May 2013, 02:50 PM   #9
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always open to hear weird problems ....

Been doing almost 30 Sanyo Plus 55 in my life and most of them happily working till the year 2013 ...with in this year i had to replace the mains switch to almost half of them

weird don't you think ? But i am in for more if it comes !!!

Athens is getting pretty hot ...summertime ...how things are going down under ?

Kind regards
Sakis
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Old 8th May 2013, 02:52 PM   #10
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Costumer called in few minutes ago to service a Kenwood 8100 .... I will monitor bias behavior and post a few words here ...
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