I first brought the soldering kit version, but couldnt find a heatsink suitable (this was about over a year ago when in the UK it was hard to get hold of it) so i brought the preassembled kit aswell. As i want stereo, i need two kits anyway.
I brought the N04BA (i think it is, not sure as its from top of my head) transformer from maplin at the same time. later brought a case for it.
The preassembled kit had never been out of the box (well should i say plastic packaging) and kept away from moisture, dust etc.
Sooo…
Last week when i finally got some time to make the amplifier, before i tried getting that to work, i thought i’d try the pre-assembled one.
Switched it on, about 10 seconds later (i hooked an CD player to the input so it was roughly 10 seconds on the elapsed time display), a single puff of smoke… and a nasty smell that lasted for along time!!
It appears R2 had burnt out. I checked all wiring and everything and the components everything else seemed OK, so i replaced the R2 with another one (checked the assembly manual to ensure getting the right value)
Switched it on again, this time instantly, puff of smoke… the same thing… everything looked ok apart from the R2 resistor.
So i tried a new plug and fuse (not that it would make any difference)… and replaced both R1 and R2 resistors.
Switched it on. No puff of smoke, the on light was on… but this time i saw a puff of smoke a few secodn later followed by alot of sparks, even a flame i believe, so again i isolated the power from the mains switch (at the plug) and let it cool down. R2 burnt out again, and the circuit board badly damaged too.
I dont know why this is happening, any ideas?
Before it went in smoke the first time the sound had a bit of hum, so presumely a ground problem,. and possibly the fact that it was in a aluminium rack case where the transformer was quite close to the amp?
i wouldnt think that was a reason fro it to blow (surely)?
Any help or suggestions will be appreciated. I have stopped trying to get it to work for the time being for obvious safety reasons.
Hard to tell what happened.
All amps are factory tested and the bias current is adjusted.
Do not touch the bias current adjust.
Transformer specs look ok.
It is possible that there has been an interruption in one side of the transformer AC connection (e.g. poor contact, …). This can cause a symmetric supplied amp to fail.
There are two options: either you return the product for repair (address can be found in manual) or you replace all transistors yourself. Make sure to use identical types (same gain factor for small signal transistor, isolated type for power transistors).
Thanks for your response. I have not modified the board at all and i have used a fuse. In fact everything setup as diagrams show. I was browsing this forum and i heard someone mention about a dodgy transformer from maplin with the same model number, do you think this would cause it?
The circuit board is basically beyond repair now considering part of it caught fire. i will have a look before i throw it away to see if i can salvage it.
I have thought about it and i dont think it could have been down to any errors of mine as i followed the instruction manual, used a suitable transformer and stuck the Y, R, B G connectors in the right way as stated on the PCB.
So it must be either:
A) the transformer from maplin, Or
B) a fault in the amp from velleman (but are tested so should be fine)
It was the R2 that burnt out, other components look fine.
I mean it was a potential fire problem… If i didnt test it outside the case i got for it, then it would have been in my bedroom… it could have been much worse!
You did keep the fuse in the circuit and the plastic wrapping on transformer?
When did you (roughly) buy the transformer? Mine was a year or so ago, maybe same batch? I brought a new one recently for the other side… hope that doesnt do that at all!
Please note that the Maplin transformer could have different colour code.
Please check :
about 30Vac between Red and Yellow
about 30Vac between Blue and Grey
About 0Vac between Blue and Red
IMPORTANT:
For safe testing, insert a 60W (about) lightbulb in series with the transformer mains voltage (see also the manual please).
The bulb shoult NOT light, or only breefly after connecting mains.
i have built 2 units both worked ok at switch on & still working now exept zobel network resistor 1ohm 0.6 w frys when any feedback from mic ect
please replace with 10 ohm 5w for reliable use