Hi.
The amp would shut down in protection mode, because of excessive heat in left channel.
The bias was way to high at 130mV instead of 38 mV at TP4 and TP8.
I adjusted down to 38mv and it seemed to stay there pretty stable. I let it go warm, shut it down, let it be cold, warm again and the bias where still 38mV.
This morning when I checked it was way down to about 10-12mV.
I'm not the first technician at this amp, and it seemes the former tech adjusted when it where down in bias, which resulted in way to much bias later on, because it not beeing stable.
My question is, what is causing this behavior?
It is not the most technician friendly amp to work on, I can't get to any meassuring when it is assembled.
I suspect Q8 2SA1869, which control the bias, but I'm not shure of anything.
Have some of you had the same problems.
The amp would shut down in protection mode, because of excessive heat in left channel.
The bias was way to high at 130mV instead of 38 mV at TP4 and TP8.
I adjusted down to 38mv and it seemed to stay there pretty stable. I let it go warm, shut it down, let it be cold, warm again and the bias where still 38mV.
This morning when I checked it was way down to about 10-12mV.
I'm not the first technician at this amp, and it seemes the former tech adjusted when it where down in bias, which resulted in way to much bias later on, because it not beeing stable.
My question is, what is causing this behavior?
It is not the most technician friendly amp to work on, I can't get to any meassuring when it is assembled.
I suspect Q8 2SA1869, which control the bias, but I'm not shure of anything.
Have some of you had the same problems.
You may have a lazy/damaged thermal-trak diode. I would (amp off) measure Vf on each and compare to other channels.
Hi.
I have to admit I didn't know it was a thermal track diode built into Q3 and Q14
It sounds like a logical way to start troubleshooting.
I can for shure compare them to the other channel.
Thank you very much for the advice...
I have to admit I didn't know it was a thermal track diode built into Q3 and Q14
It sounds like a logical way to start troubleshooting.
I can for shure compare them to the other channel.
Thank you very much for the advice...
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I know it's a little late to post the result, but it showed up to be the thermaltrak transistors Q3 and Q14 that was defect. Now it's completely stable with no hot issues.
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Hello, I know it has been a 1.5 years since your last posting and allow me to bump into this topic.
A story short, please share some light how to bias this amp. I have opened up top and side panels, and could only see the trimpot, the upper TP measurent points but could not access lower TP measurent point. Do I need to remove power amp board with the heat sinlk to get access? I was thinking to drill a hole at the cover panel to get access.
Please share the detail how to bias this amp.
Thanks in advance.
A story short, please share some light how to bias this amp. I have opened up top and side panels, and could only see the trimpot, the upper TP measurent points but could not access lower TP measurent point. Do I need to remove power amp board with the heat sinlk to get access? I was thinking to drill a hole at the cover panel to get access.
Please share the detail how to bias this amp.
Thanks in advance.
Hi,
Just wanted to let you know that I now can adjust the bias, with the help from @tonjen.
Many thanks.
Just wanted to let you know that I now can adjust the bias, with the help from @tonjen.
Many thanks.
Hello!
Could you maybe post the instructions on how to bias both channels here?
I've just got my hands one one of these and it seems to need a lot of work, it reportedly had thermal issues(I don't wanna push it) and someone changed a lot of components, the 220uF and 1000uF caps on the power amplifier boards to mention a few.. Oh and the 200 Ohm bourns potentiometer is gone and replaced on both channels with some crappy 100 Ohm variant that doesnt fit the PCB.
I thought my unit needed a bias tune, turns out I bought a frankenstein amp that someone did a real hack job with.. 🙁
I have ordered replacement parts though, I hope it can be made right again.
Could you maybe post the instructions on how to bias both channels here?
I've just got my hands one one of these and it seems to need a lot of work, it reportedly had thermal issues(I don't wanna push it) and someone changed a lot of components, the 220uF and 1000uF caps on the power amplifier boards to mention a few.. Oh and the 200 Ohm bourns potentiometer is gone and replaced on both channels with some crappy 100 Ohm variant that doesnt fit the PCB.
I thought my unit needed a bias tune, turns out I bought a frankenstein amp that someone did a real hack job with.. 🙁
I have ordered replacement parts though, I hope it can be made right again.
Biasing HK 990
Hello,
If you still need to bias this amp, please send the email to me at cgunawan23@gmail.com. Thanks.
Hello,
If you still need to bias this amp, please send the email to me at cgunawan23@gmail.com. Thanks.