Hi I have recently constructed a Velleman 200W power amplifier, tested and adjusted it as per manual instructions, however when using it for the first time it worked for approx 45seconds then blew the 1A slow blow fuse, I noticed that R19 & R 20 were quite hot. I have seen a few comments on the forum of people having had similar problems in the past with this kit, could someone please advise as to the possible problem to save me a bit of time fault finding?
Sorry to read that you experience problems with this popular kit.
Make no mistake: People come to this forum if they have a problem, they don’t come here to tell us that everything is working fine 
This might give the wrong impression that there is something wrong with the kit.
Basically, there are a number of issues which need to be considered with this project:
Power supply: Make sure it is within specs and correctly connected.
Do not modify the kit in any way, this can lead to HF oscillation and thermal runaway.
Make sure it is correctly heatsinked (no shorts between any parts and heatsink, sensing diode and transistor in good thermal contact with heatsink).
Make sure bias is correctly adjusted.
Speakers: avoid really low impedances (< 4 ohm) and too capacitive speakers (piezo tweeters without series resistor).
In most cases, if the amp is defective, we recommend to simply replace all semiconductors.
This is the fastest and most reliable solution.
[quote=“VEL417”]Sorry to read that you experience problems with this popular kit.
Make no mistake: People come to this forum if they have a problem, they don’t come here to tell us that everything is working fine
This might give the wrong impression that there is something wrong with the kit.
Basically, there are a number of issues which need to be considered with this project:
Power supply: Make sure it is within specs and correctly connected.
Do not modify the kit in any way, this can lead to HF oscillation and thermal runaway.
Make sure it is correctly heatsinked (no shorts between any parts and heatsink, sensing diode and transistor in good thermal contact with heatsink).
Make sure bias is correctly adjusted.
Speakers: avoid really low impedances (< 4 ohm) and too capacitive speakers (piezo tweeters without series resistor).
In most cases, if the amp is defective, we recommend to simply replace all semiconductors.
This is the fastest and most reliable solution.[/quote]
Just one caveat to my original post it was only when I turned up the volume via the 47K ohm potentiometer recommended in the manual that the fuse blew prior to that at low volume everything seemed fine including stable bias voltage of 10mv.
Sorry, this forum does not allow picture uploads
If you can provide LINKS (e.g. by using a free online picture base) to HI-RES and SHARP pics of your assembly (both solder- and component side)
then we can take a look and provide some feedback.
Also: what power supply do you use?
[quote=“VEL417”]Sorry, this forum does not allow picture uploads
If you can provide LINKS (e.g. by using a free online picture base) to HI-RES and SHARP pics of your assembly (both solder- and component side)
then we can take a look and provide some feedback.
Also: what power supply do you use?[/quote]
The toroidal transformer I was sold for this amplifier is a model Vigortronix VTX-146-120-230 120VA Primary 0-115 0-115 50-60hz
secondary 0 - 30v 2.0 a
" 0 - 30v 2.0 a.
I have checked the cct board solder side there are no shorts and I am currently checking the components.