Hello all,
Got this little amp for a few weeks. When at a feet distance I was able to hear some light buzz in one channel. One day the buzz suddenly became strong. I can still hear the music coming out of the speaker but this buzz is there. I opened the amp and checked for faulty input transistors, checked caps and ground - all fine. I replaced the main ps cap and the smoothing caps with new ones, same brand as original, no change. PS section is with one bridge, so I guess problem is not there. What could be the problem and where would you suggest that I should look now? Is it possible to be the pot grounding?
Got this little amp for a few weeks. When at a feet distance I was able to hear some light buzz in one channel. One day the buzz suddenly became strong. I can still hear the music coming out of the speaker but this buzz is there. I opened the amp and checked for faulty input transistors, checked caps and ground - all fine. I replaced the main ps cap and the smoothing caps with new ones, same brand as original, no change. PS section is with one bridge, so I guess problem is not there. What could be the problem and where would you suggest that I should look now? Is it possible to be the pot grounding?
It could be many things...
Does it buzz with NO inputs connected ?
Does it buzz on any input (CD, Tuner, Tape etc)
Yes, it may well be something like pot grounding. Anything where there is the possibility of mechanical stress on the PCB can be a problem. If in doubt resolder all the pot connections.
Does the buzz respond to any physical pressure on the PCB. That is, does it seem like an intermitant connection.
Does it buzz with NO inputs connected ?
Does it buzz on any input (CD, Tuner, Tape etc)
Yes, it may well be something like pot grounding. Anything where there is the possibility of mechanical stress on the PCB can be a problem. If in doubt resolder all the pot connections.
Does the buzz respond to any physical pressure on the PCB. That is, does it seem like an intermitant connection.
Hello and thanks for replying. Buzz is coming from onechannel only with nothing connected to the inputs. Output smoothing caps were replaced, so no AC could be passing through. I suspected a faulty input transistor but they seem to be fine. Even so, I replaced them with new ones and buzz is still there. Transformer had bad joints and was an R-core, I ordered a nice toroid which I`ll install today. So buzz iz when:
- nothing is connected to the input
- one channel only
- buzz is not affected by the volume pot position
- tested it on two different speakers with diferent cables and sofort
- Nothing seems burned on the inside, tested output darlingtons and replaced all input trannies
- grounding is a onepoint, resoldered it even if it looked fine
- nothing is connected to the input
- one channel only
- buzz is not affected by the volume pot position
- tested it on two different speakers with diferent cables and sofort
- Nothing seems burned on the inside, tested output darlingtons and replaced all input trannies
- grounding is a onepoint, resoldered it even if it looked fine
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make sure that you are working in an EMI free environment..... weird case but it is possible that you are picking up something
kind regards sakis
kind regards sakis
It doesn't sound like a transistor fault. Hum and buzzes have other causes.
To be able to help specifically we need to see a circuit. I suspect this will be something like cracked print somewhere.
Depending on test gear available (scope ??) I would suggest finding the input to the power amp in question on the PCB and shorting it to ground to see if it kills the buzz but you MUST do this correctly or it could cause more damage.
To be able to help specifically we need to see a circuit. I suspect this will be something like cracked print somewhere.
Depending on test gear available (scope ??) I would suggest finding the input to the power amp in question on the PCB and shorting it to ground to see if it kills the buzz but you MUST do this correctly or it could cause more damage.
Hello and thanks for the replies,
Yesterday I replaced the power transformer, as seen on the second photo it was not the original one but a R-core one. I bought a nice 160VA toroidal transformer ( a bit overrated than the original one ) and installed it yesterday. Now what is left is tosort out this problem with the channel.
Here is theschematics of the amp, hope Creek don`t mind that I post it here:
And a pic of the amp on the inside. Replaced the main ps cap with a Panasonic FM 6800uF and the output caps with some Rybucon 3300uF XYG for now, later I`ll drop in a pair of Elna Silmics.
Yesterday I replaced the power transformer, as seen on the second photo it was not the original one but a R-core one. I bought a nice 160VA toroidal transformer ( a bit overrated than the original one ) and installed it yesterday. Now what is left is tosort out this problem with the channel.
Here is theschematics of the amp, hope Creek don`t mind that I post it here:
An externally hosted image should be here but it was not working when we last tested it.
And a pic of the amp on the inside. Replaced the main ps cap with a Panasonic FM 6800uF and the output caps with some Rybucon 3300uF XYG for now, later I`ll drop in a pair of Elna Silmics.
An externally hosted image should be here but it was not working when we last tested it.
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Still a bit small to work with. If it's a pdf then you can put it in a zipped folder to post it.
Removing this cap will isolate the preamp from the power amp. I think the amp should be stable doing this from what I can see.
Or, you could short the left side of that cap to ground to remove all signal to see if hum disappears.
Removing this cap will isolate the preamp from the power amp. I think the amp should be stable doing this from what I can see.
Or, you could short the left side of that cap to ground to remove all signal to see if hum disappears.
Attachments
Didn`t know this, thanks for the input. Schematics is attached. Is the 4.7uF coupling cap the one that differentiates the pre-amp from the power amp section?
PS: Just saw you have highlighted it 🙂 It is an orange Rybucon electrolytic cap, I might replace them with Polypropylene caps instead, there`s plenty of space for that.
PS: Just saw you have highlighted it 🙂 It is an orange Rybucon electrolytic cap, I might replace them with Polypropylene caps instead, there`s plenty of space for that.
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Yes, it couples the pre amp to the power amp.
Initially you need to narrow down whether the problem is in the power anp or preamp or PSU.
Do you have an oscilloscope as that makes it so much easier ?
Initially you need to narrow down whether the problem is in the power anp or preamp or PSU.
Do you have an oscilloscope as that makes it so much easier ?
No oscilloscope, but have a multimeter and RLC meter...Butstarting point is good, I`ll isolate the preamp and power amp sections tonight and will post some results.
Hi Mooly,
Did what you advised, with the cap removed the amp still buzzed, so I guess it is an issue in the power amp section. Replaced the two coupling caps with some Wima MKS caps, one electrolyte less in the signal path should improve the sound a bit 🙂 Also ordered some Panasonic AM caps to replace all other on the board as they are quite old, no idea whether the AM is better than the FC ( its more expensive ), they state it is made for audio applications so should be good 🙂
Did what you advised, with the cap removed the amp still buzzed, so I guess it is an issue in the power amp section. Replaced the two coupling caps with some Wima MKS caps, one electrolyte less in the signal path should improve the sound a bit 🙂 Also ordered some Panasonic AM caps to replace all other on the board as they are quite old, no idea whether the AM is better than the FC ( its more expensive ), they state it is made for audio applications so should be good 🙂
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Some basic tests then to try and get a grip on what's going on.
Measure the DC voltage at the junction of the two 0.22 ohm resistors on the output transistors (can only make out R43 ??? as ref). It should be approximately half the voltage of the main supply rail.
Carefully measure the voltage across either of the 0.22 ohm resistors. It should be around 6 millivolts DC at a guess. Compare with the other channel.
Is C21A ???? (the 470uF above Q3) OK. Prove by substitution.
Measure the DC voltage at the junction of the two 0.22 ohm resistors on the output transistors (can only make out R43 ??? as ref). It should be approximately half the voltage of the main supply rail.
Carefully measure the voltage across either of the 0.22 ohm resistors. It should be around 6 millivolts DC at a guess. Compare with the other channel.
Is C21A ???? (the 470uF above Q3) OK. Prove by substitution.
Mooly,
I cannot express my gratitude, time and knowledge is an expensive asset, thank you for your patience, coaching and assistance! By a chance, I travel to London a few times per year, hope to be able to say thank you and have a cup of good quality tea one day there 🙂
So what happened, I measured the voltage and the drop - all as suggested by you. Then I exchanged the two 470uF caps and buzz went to the other channel which makes it clear to me 🙂 I have ordered a whole bunch of Panasonic caps, including the 470uF ones, so once they arrive, I`ll have this fixed and the amp will be up and running, restored to its original toroidal transformer ( a bit bigger indeed ) and with some higher quality caps. I also added an RCA connection to it. Nice piece of British engineering, now back in service, thanks to you 🙂
I cannot express my gratitude, time and knowledge is an expensive asset, thank you for your patience, coaching and assistance! By a chance, I travel to London a few times per year, hope to be able to say thank you and have a cup of good quality tea one day there 🙂
So what happened, I measured the voltage and the drop - all as suggested by you. Then I exchanged the two 470uF caps and buzz went to the other channel which makes it clear to me 🙂 I have ordered a whole bunch of Panasonic caps, including the 470uF ones, so once they arrive, I`ll have this fixed and the amp will be up and running, restored to its original toroidal transformer ( a bit bigger indeed ) and with some higher quality caps. I also added an RCA connection to it. Nice piece of British engineering, now back in service, thanks to you 🙂
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