advertisement


Turn on surge blowing fuses?

As it's passed those tests you could wind it up to full mains whilst testing voltages and especially for output DC offset... expect a few mV. Check for any burning smells and keep an eye out for any overheating of heatsinks, TX etc.

I did say put the lottery on if both fuses were so tired as to blow one after another.... but you never know:)
 
Thanks for the fuse feedback - I will get some ordered.

I will "obviously" not be risking any bold experiments until I have spare fuses - my current input fuse is the only means I have of making any further measurements!

BugBear
 
Run up to 240V on the Variac, I show 0.2 mV DC offset on the left channel, and 0 (within the limit of my meter) on the R.

I love DC servos (at least I think that's what I've got)

All PSU voltages and waveforms look good.

BugBear
 
Last edited:
The next test will be run up to 240V on the Variac to charge the caps, turn off, change over plug to mains socket, switch on. That should (if I'm quick) reduce the cap charging aspect of the surge.

BugBear
 
indeed. I want to confirm that it is (or isn't) an inrush failure. If my "trick" works, I can try running the amplifier. If that works (if it sounds OK), I know I've got an inrush current limiter problem.

(comments from the wise, up to and including "you're a loony", welcome as always)

BugBear
 
OK. With thanks to "Electrosmart Limited" (88dp on eBay); I ordered a 10 pack of fuses Monday 9 AM, and they were waiting on my door step last night.

On to testing this morning; I preloaded the caps via the variac, switched the lead to the mains socket, turned on. As everyone (?) expected, this worked, although the audible transformer thump was louder than I expected, and continued to hum audibly (albeit with the lid off).

I then waited 15 mins (is that long enough to discharge the caps?) and tried again, straight from the mains.

The fuses held, and all principle PSU voltages are good.

I'm starting to consider that lottery ticket, although the real test is this evening - I'm guessing a full day will be adequate discharge time.

BugBear
 
It depends on the actual power supply circuit how quickly they discharge, some amps have a resistor across the power supply that will discharge the caps when you switch off ?.
If there is no load on the power supply it can take a while to completely discharge
You could measure the DC voltage across the Caps, if its zero they are discharged.
You can physically discharge them with a 1K x 3w resistor

Alan
 
I have one last question - should I get a lucky dip, or pick my own numbers? :)

It seems to be working.

BugBear (still nervous)
 
Note there is to some extent a 'lottery' here. The surge behaviour may depend on exactly when you switch on the amp in terms of the phase of the mains waveform at that instant. If I turn on one of my Armstrong 700 power amps, sometimes the lights in the house flicker, sometimes they don't. All depends on which part of the mains cycle it 'eats' to charge up the caps. 8-] In the days of CRT TVs ours used to roll a frame...
 
Is there any scope for ADDING some kind of anti-surge? What are my options (if any) ?

I mean, it DOES dim the lights when it turns on, always has.

BugBear
 
The most idiot-simple way is to add a series resistor to the charging loop. But this has snags unless you can bypass it once the amp is on. And is problematic for a class-A or high bias amp. Beyond that we/I'd need to see the circuit diagram. The old Armstrong 600s orginally had a series resistor that was bypassed after a few seconds by a 'thermal delay'. However these were infuriatingly unreliable, so in that case I just upped the current ratings of the relevant items and discarded the surge limiting for later versions.

It is also possible that slightly higher value slow fuses will simply last a lot longer than the ones you had.
 


advertisement


Back
Top