Hi, I have a problem with an amplifier. It has a loud hum in both speakers that is not affected by the volume control or any controls or buttons. I am thinking that it is a power supply problem. It has about 180ma of ac present at the speaker terminals but only -1.8 mv of dc. I put a scope on the speaker terminal and I have uploaded a photo of the wave produced. I have also uploaded a photo of the speaker terminal with a 1khz tone injected at the tuner input.
Any help much appreciated.
Any help much appreciated.
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Hmmm I had one of those.
Spent too much time trying to 'fix' it.
Finally gave up and put the excreable thing in a Dumspter.
Bought a 'good' unit which is dead silent and sounds far better than the junky one Ever did.
Best thing I ever did... I'm happy.
Spent too much time trying to 'fix' it.
Finally gave up and put the excreable thing in a Dumspter.
Bought a 'good' unit which is dead silent and sounds far better than the junky one Ever did.
Best thing I ever did... I'm happy.
Hi,
What about checking the power supply with the scope. It will tell you if the problem it is with the PS.
What about checking the power supply with the scope. It will tell you if the problem it is with the PS.
It would also help to know some details like:
Is it a commercial product or known DIY amplifier design and which?
How old is it and when do you think it was last in use?
Has it always been like this or was it like this from the outset?
At first glance, it appears that the main smoothing cap(s) in a conventional transformer/rectifier power supply have died and need replacement. That suggestion would depend on the frequency of the waveform, whether there is also an output capacitor etc. These details are not shown but the waveform would then be 100 Hz frequency with a 50Hz mains supply. That may not be the whole story though and even the rectifier diodes could be bad.
Is it a commercial product or known DIY amplifier design and which?
How old is it and when do you think it was last in use?
Has it always been like this or was it like this from the outset?
At first glance, it appears that the main smoothing cap(s) in a conventional transformer/rectifier power supply have died and need replacement. That suggestion would depend on the frequency of the waveform, whether there is also an output capacitor etc. These details are not shown but the waveform would then be 100 Hz frequency with a 50Hz mains supply. That may not be the whole story though and even the rectifier diodes could be bad.
Thanks Ian, it's a Hitachi HA-6 from the 1980's. I bought it as a non working project. I replaced numerous transistors and resistors and it now produces sound on both channels. But it has this terrible hum on both channels slightly more on left than right. I'm guessing it has been out of use for many years.
Hi, it's most likely the power supply and decoupling caps then, did you power up gradually? Sometimes old electrolytics can be "reformed" this way?
Measure the hum at the output with the input shorted.
Measure the ripple on the supply rails again with the input shorted.
Give us some actual numbers. We can't do much with "a loud hum" !
Measure the ripple on the supply rails again with the input shorted.
Give us some actual numbers. We can't do much with "a loud hum" !
Have had a probe around the power supply today, It seems to be clean from the main supply to the rails, but it looks like there is more than one supply. there seems to be a supplementary supply (see picture) and the yellow wire has 21v of dc and 21v of half wave AC on it. I have pulled the nearby diodes and they check out ok, but I suppose they may only fail under load?
Any ideas?
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The service manual is available here it you haven't already got it https://www.hifiengine.com/manual_library/hitachi/ha-6.shtml
Can you isolate the preamp from the final stage I mean disconnect it ? to find out from where the humming is comming.
That would be an open electrolytic (power supply reservoir) capacitor, I bet. Look for a largish (>100uF) capacitor from that supply to ground. Bridge it with something like a 1000uF or so (35V or so), see if the waveform cleans up and the hum goes away.
Check the capacitors on the 21 volt rail.
If the amp is old its possible the electrolytics have died or partly died.
Thanks for the replies, I have ordered some new diodes to try and will check for bad caps. I will update in a few days.
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Good luck, it's most likely the caps from what you've said about the amp, and would be worth your while renewing them anyway
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