I have an old Kenwood KAC-7285 and when I turn it on a medium loud steady hiss comes from the transformer. I tested all transistors, di0odes and resistors and all is good. I did inspect all caps and none are leaking or any signs of damage. If anyone who might be able to tell me what to check I will be so grateful.
Thanks
Jean
Thanks
Jean
Would this transformer be part of a switching power supply or class D amp? If so, I've sometimes heard hiss if the switching frequency is unstable. I suppose it could also happen if the transformer saturated (= is run at too high current).
Tom
Tom
Do you have a scope?
'Loud' is subjective. With the amp fully assembled in the trunk, would you easily be able to hear it?
Does the hiss get into the audio?
'Loud' is subjective. With the amp fully assembled in the trunk, would you easily be able to hear it?
Does the hiss get into the audio?
Tomchr: It's a Monoblock Class AB.
Perry: Yes I have a scope. No you would not hear it in the trunk and it doesn't get into the audio.
With the amp put together fully you can hear it clearly if TV is off. If you know what you're listening for you can hear it over the TV .
Perry: Yes I have a scope. No you would not hear it in the trunk and it doesn't get into the audio.
With the amp put together fully you can hear it clearly if TV is off. If you know what you're listening for you can hear it over the TV .
If you look at the drive pulses from the 494 driver IC, are they perfectly steady?
If you push on the inductor or transformer, does the hiss change?
If you push on the inductor or transformer, does the hiss change?
I found the problem, it was a transistor Q13. I thought I tested them all but these hang off the PCB and I didn’t notice it. It’s the most simple’s things that can be the hardest things to fix.
Thanks for your help.
Jean
Thanks for your help.
Jean
Did replacing it completely solve the problem? That's an odd (not impossible) fault to make the supply noisy but not causing noise in the audio.