F5 power amplifier

[tinitus]

Originally Posted by AndrewT

and how do you propose to machine/grind/sand the spotface to make it flat enough to conduct heat from the device, with minimal Rth s-a?

You can make your own end sander bit

Ofcourse you need a minimum of tools
But point is, theres no need for advanced expencive tools

Try to hold something straight against the device
Look up against a window or other light source, and you will see how plane that thing is, or rather isnt

[prooptiki]

Originally Posted by AndrewT

how many channels are being powered by this transformer?
What is the transformer?
What is the continuous current draw from the PSU, i.e. output bias plus amplifier quiescent current?

I use a Talema toroidal for two channels, its getting hot when bias is 0.71V (when it had the insulators problems).
How can I check the psu continuous current and the amplifier quiescent current?

# Power rating: 300VA, 10A
# Core loss, typ: 1,7W
# Copper loss, typ (120 degrees celsius): 22,5W
# Size: 115x58mm
# Primary: 1x230V
# Secondary: 2x18V

[AndrewT]

Originally Posted by prooptiki

I use a Talema toroidal for two channels,
# Power rating: 300VA, 10A
# Secondary: 2x18V

300VA, 18+18Vac, is 8.3Aac

The maximum continuous DC current that can be drawn from this transformer is ~4.2Adc. It will run hot if worked this hard.
It is often recommended that the maximum continuous current should be around 50% of the transformer rating. This is 2.1Adc.
That is 1.05Adc of bias and quiescent current for each of the two channels. No wonder it is running hot on the exterior surface.
How much hotter will it be when inside the chassis/case when the heatsinks are running at 50degC?
How much hotter is the core of the transformer if the surface is already hot?
That 300VA transformer might run one channel and stay within maximum operating temperature. But that is just a guess, I don't know how hot the ambient will be inside your chassis and I don't know how hot the core is relative to the surface temp.

[tinitus]

If wont stick much more to this issue, but just note that if I should do any kind of smoothing, I would start with removing ALL the hard anodising with a hand sanding machine, BEFORE doing any attempts to the smoothing

[erpiii]

I just use a drill press and piece of wood bored out with a hole saw. I drill a hole in the center (or near center) and then insert a screw that has a smooth shank up near the head. Cut the head off the screw and mount sand paper to the other side of the cylinder of wood using double sided foam tape. Insert into the drill press and then bring it down onto sand paper to ensure it's flat (perpendicular) and then sand the aluminum; clearing often. I start out at 180 grit and then go to 320-350 wet dry. This will give a mirror like finish that is very flat as long as you don't apply too much pressure in the sanding process. I have also used this process with a hand drill that worked quite well when the drill was held in a fixed position and the heat sink brought in at perpendicular. The diameter has to be large enough to accommodate the device you are going to mount. You could also try using the bottom of the sanding cylinders sold at Harbor Freight.

I have tried using an endmill on a drill press at the highest speed using an x-y axis vice; disastrous! The finish was not very good and I got some bit or work deflection that lead to grooves. I think the use of an endmill is best left to a real machining station and not a drill press.

[tinitus]

[amp-guy]

While I have lapped sinks on a granite surface plate using compound which will make it flat to a few ten thousandths, I think that it is a point less effort (other than it looks coool).
if you place a sheet of 320 or 400 grit silicon carbide paper on your cast iron tablesaw top and lapp it that is plenty good.

[alazira]

Hi,

I finally reacquired the upgrade bug and built a new F5 module using sj201/sk1530. The first channel went smoothly, but on the second only R12 would bias up with P2. No amount of turning on P1 had any effect on R11 or DC offset (+21volts!). Finally replaced the Toshiba's with FQA's and it works now.

Obviously one of the mosfets is DOA. Can someone please tell me which one?

[Zen Mod]

perfect opportunity to replace crystal ball with little mosfet testing setup

[bear]

I think amp-guy's approach is closer to what i would suggest, if asked.

Making the surface shiny does not mean that it is flat.

_-_-bear