LA-2A Build

[Spencerleehorton]

Hi All,

Im revisiting a couple of LA-2A builds i did a while ago and seem to be getting quite a hot 4.7k R29, it looked pretty totalledm it was a 2W.
Ive replaced and the unit seem to function fine now but there is a slight smell of hot resistors after a few minutes turned on?
My B+ at the diodes is 361v and after the 4.7k it is 257v.

Any ideas why this 4.7k is heating up?

 

[Rob Flinn]

This resistor is disipating 2w. You need at least 5w resistor here.

 

[Harpo]

My B+ at the diodes is 361v and after the 4.7k it is 257v.

Ohm's law ?
P=U^2/R
(361V - 257V)^2 / 4700 = 2.3W

 

[Spencerleehorton]

Oh right!!! On all the schematics it says 2W!!

OK order some 5w ones and check the power rating on the other resistors

 

[Spencerleehorton]

Thanks guys, I'm always **** with remembering these equations

 

[Spencerleehorton]

4.7k 5watt resistors ordered, now Ive been reminded of the way to work it out I'll check the other resistors down the line from there!!

I've got two toroidal in mine and have a diode bridge rather than just the 2 x diodes? Need to investigate why I've used two!!! It's been so long, will probably make a couple of new turret boards and have a nice clean layout as it's a bit of a mess in there!!!

Great help thank you.

 

[Spencerleehorton]

I didn't put in the variable capacitors even though I do have a could of them, or I did and changed them? So long ago I can't remember what I did but it seems I have 2 x 150pf mica in parallel now, which should be around the 75pf mark!!
It ask for 80pf on the layout and is vague on the schematic.

Any idea of the exact values?

 

[Spencerleehorton]

From what I've read in the la2a thread, C4 and C14 variable caps should be 150pf fixed?

Can anyone confirm this please?

 

[Rob Flinn]

I didn't put in the variable capacitors even though I do have a could of them, or I did and changed them? So long ago I can't remember what I did but it seems I have 2 x 150pf mica in parallel now, which should be around the 75pf mark!!
It ask for 80pf on the layout and is vague on the schematic.

Any idea of the exact values?

Caps in parallel add up therefore 2 x 150pf + 300pf

 

[Spencerleehorton]

Yep sorry my mistake!!, I'll remove one of the 150pf and see how it sounds along with changing the 4.7k 2w for 5w.