F3 Builders Thread

[JeffYoung]

Single CL-60 or two of them?

 

[lordvader]

OK, so there are 2 300va transformers, with 66000uf per channel (in a clc setup). There's a 3a fuse at the mains, and each transformer gets 1.25a. Each channel has a thermistor to safety ground.

This configuration worked for ages.

Again, works fine with the bench supply, except one channel starts with a current spike. Haven't retested the real psu yet - wouldn't mind getting to the bottom of this spike (if it is a problem).

 

[JeffYoung]

So no thermistors across the transformer primary windings? I'd say the current surge is expected then -- it's the other one that's a bit of a head-scratcher.

 

[lordvader]

Sorry - I'm testing without the transformer. I'm getting the 2a spike with the bench supply.

 

[2 picoDumbs]

Inspect cap multiplier.
Maybe dead cap
Maybe cold solder joint.

 

[lordvader]

I'll check that.
Which components make up the capacitor multiplier? What should I look for?

On visual, there are no bulging caps, so could it be a transistor in the multiplier circuit?

 

[2 picoDumbs]

I'll check that.
Which components make up the capacitor multiplier? What should I look for?

On visual, there are no bulging caps, so could it be a transistor in the multiplier circuit?

If you post a copy of your circuit diagram I will list the part numbers.

 

[JeffYoung]

Cap multiplier is Q5/C5.

You might find my drawing of the schematic easier to follow:

 

[lordvader]

Looking at that circuit, it seems a failure of Q5, C5, or D1 will still result in a functional amp. I'll do some measurements when I get a chance.

 

[2 picoDumbs]

Yes it will be functional but could be the cause of the current spike.
R19/C5 would normally provide a soft start up.
If either of those parts are poorly soldered or faulty, then it will prevent the soft start up.

 

[lordvader]

Well I'm at a loss. I resoldered all the parts that made up the soft start, and still I get the current spike on startup.

Not sure what else to do.

Even the capacitor is measuring fine, so nothing wrong there

Did some quick voltage checks, and both channels measure similarly too...

 

[Zen Mod]

which means DC points are pretty much the same on both channels , or you meant that (now) both channels are showing same startup current spike ?

 

[lordvader]

Sorry!
I meant, even though both channels measure the same, only one channel has the current surge.

Think about attaching a small, sacrificial speaker to it, see if the current surge manifests on the output.

But short of rebuilding the channel from scratch ($$$), not sure what else to try...

 

[lordvader]

So...
Attaching the suspect channel to a sacrificial speaker was fine.
No explosive pops on power up, and a test signal played just fine.

I guess I'll run the amp normally with just a larger fuse, with the intent of one day rebuilding that channel (have a spare pcb).

 

[lordvader]

Quick rewind!

When I resoldered everything, I noticed that pad of the middle pin of Q5 was lifted, meaning that Q5 may not have been soldered in correctly (the drain I think? directly attached to V+?).

Don't want to redo everything, so might just run a wire from the drain of Q5 to V+.

 

[lordvader]

Argh!
No open circuit there...

 

[lordvader]

OK...

And sorry in advance for all the posts!
But replaced c5, and no change.

I checked voltages again, and the source on q5 is 43.5V, rather than 41V. Is this a sign of a busted transistor?

If so, where can I get an irfp240?

 

[JeffYoung]

What values of R6/R7/R8 are you using, and what is the voltage across one of them?

 

[lordvader]

Using 1 1 and 2 ohms.
0.66v

 

[JeffYoung]

So it's drawing full power (1.65A). What's the rail voltage above Q5?