Hi All
I've experimented the following:
swapped left to right channels (did not remove PCBs from their place, since they are mirrored prints and do not fit in eachothers' place) by rewiring with no results;
removed ground wires that run from RCAs inputs to star ground and then reconnected one at a time, with no result;
measured transfo output voltage and is exactly the same.
At last I did a test that may be meaningless but, well, I report it here: powered up the amp with right channel (the noisy one) fuse removed and replaced by a provision wire. Then, once the anti-bump relais was engaged, removed the wire thus removing supply to right channel. As I expected the background noise suddenly stopped (the high current absorbed immediately depletes capacitors and channel goes silent) but I heard the hum coming back about two times, very very weak, with a, say, "waving" volume.... It's just difficult to describe. Power cut, silence, hum coming up and down but very weak, hum coming up and down even weaker and then total silence.
I don't know what this could mean but tha fact that I hear hum with cutted power makes me think that it's not a power induced hum but it's air catched.
I more and more think that the problem is in the circuit...but no answer by Sugden even thru Facebook...and I doubt there is official technical service in Italy for them,
Bye
Raf
I've experimented the following:
swapped left to right channels (did not remove PCBs from their place, since they are mirrored prints and do not fit in eachothers' place) by rewiring with no results;
removed ground wires that run from RCAs inputs to star ground and then reconnected one at a time, with no result;
measured transfo output voltage and is exactly the same.
At last I did a test that may be meaningless but, well, I report it here: powered up the amp with right channel (the noisy one) fuse removed and replaced by a provision wire. Then, once the anti-bump relais was engaged, removed the wire thus removing supply to right channel. As I expected the background noise suddenly stopped (the high current absorbed immediately depletes capacitors and channel goes silent) but I heard the hum coming back about two times, very very weak, with a, say, "waving" volume.... It's just difficult to describe. Power cut, silence, hum coming up and down but very weak, hum coming up and down even weaker and then total silence.
I don't know what this could mean but tha fact that I hear hum with cutted power makes me think that it's not a power induced hum but it's air catched.
I more and more think that the problem is in the circuit...but no answer by Sugden even thru Facebook...and I doubt there is official technical service in Italy for them,
Bye
Raf
To be clear, when you say you swapped left to right channels, I assume you swapped only the power supply leads "with no results" in the above post. Can you confirm that the hum remained in the right channel?
As a hint to finding the hum source, mains AC hum has a deep, fairly clean sounding 50Hz note and originates from the mains wiring, transformer etc. Raw DC noise, as you hear radiated from the rectifier, transformer secondary leads and even from the power leads to the amplifiers, PCB tracks etc. has considerable harmonic content and has an obvious 'buzzz' sound to it. 'Hope that helps a little.
As a hint to finding the hum source, mains AC hum has a deep, fairly clean sounding 50Hz note and originates from the mains wiring, transformer etc. Raw DC noise, as you hear radiated from the rectifier, transformer secondary leads and even from the power leads to the amplifiers, PCB tracks etc. has considerable harmonic content and has an obvious 'buzzz' sound to it. 'Hope that helps a little.
Hello Raf,
Do you have access to an oscilloscope?
I suspect your problem is RF related...
I saw this problem happen many times on Sugden amplifiers like the Au41, A28B and Masterclass. I have a vague souvenir of having had the problem with an A21A but can't find any documentation about it. On some amplifiers, like the A41P, I had to short 2 pcb tracks and on others, like the A28B, I had to change the value of the feedback capacitor. The A21A schematic shows the feedback capacitor at 150 pF or 160 pF. If you can confirm your amplifier is oscillating, I suggest you change the value of this capacitor. On the A28B the value was changed from 150 pF to 47 pF and it fixed the problem permanently. I suggest you go slow in dropping the value and stop when you reach the right value.
Hope it can help you!
Good luck,
Michel
Do you have access to an oscilloscope?
I suspect your problem is RF related...
I saw this problem happen many times on Sugden amplifiers like the Au41, A28B and Masterclass. I have a vague souvenir of having had the problem with an A21A but can't find any documentation about it. On some amplifiers, like the A41P, I had to short 2 pcb tracks and on others, like the A28B, I had to change the value of the feedback capacitor. The A21A schematic shows the feedback capacitor at 150 pF or 160 pF. If you can confirm your amplifier is oscillating, I suggest you change the value of this capacitor. On the A28B the value was changed from 150 pF to 47 pF and it fixed the problem permanently. I suggest you go slow in dropping the value and stop when you reach the right value.
Hope it can help you!
Good luck,
Michel
Raf
Your description matches my version of this amplifier.
I'm having some issues with it though.
Would you be able to let me know what the exact model of the relay is (I can't tell what mine is but the dimensions do not match those of the OMRON G2R)?
I also need to know what the values of the two capacitors are and whether there are any resistors connected across the screw terminals at the back of the pcb.
Cheers
Stelios
It's closer to the A51 circuit rather than the A21 circuit though both share the same basic topology of course.
The power stage and the power supply regulation circuit one can consider them to a large extent unchanged.
The input stage and the VA stage are quite different.
Also the main PS and the output-coupling capacitors are larger than the original - 4 times the capacity in fact.
In summary, the A51 circuit is a good starting guide for troubleshooting the amplifier version of that age shown in the photos. Later versions I would not know about. I'd expect all A21 versions produced this far to probably share the same basic topology.
Now, having said that, I did have an A21 with a wooden sleeve which did not appear to be class A. It did not have the words "class A" on the front panel - as its siblings did - nor did it get warm. I did not check the circuit inside though so I would not know what the circuit topology was.
Stelios