advertisement


Resistor to ground - suspected inrush current problem

booja30 said:
Also, something that wasn't obvious to me, but if you have a loop through the transformer via the mounting hardware (e.g. bolt touching case bottom and also contacting something metallic up top) then you can have a big surprise. Here's some diagrams:

49977992061_17778680a3_c.jpg


I'm not sure how this could cause what was observed above but I guess if PSU ground touched the toroid mounting hardware something like this could happen.
Click to expand...
A very good diagram, making the potential errors clear
 
Same amp, similar, but slightly different problem.

Back with the original US spec traffo and a step down 110V transformer

After playing all day sound suddenly dropped off followed by a rustling noise on both speakers and sadly a faint buning smell. I jumped out of my seat and switched everythig off, removed the cover to find that the heat shrink on either end of the 1R 10W resistor between the traffo centre tap and the PSU caps had burned it's insulation to a crisp. Traffo was too hot to touch.

I assume that' an overheated / saturated transformer?

Next day I disconnected the traffo and checked it for continuity between the frame and windings, measured the resistance of the various circuits and all seems totally fine. Connected it back up with a new 1R 10W resistor and the amp biased perfectly and warmed up nicely, DC offset all normal as before.

Connected back in to system and the resistor stated getting hot again - now paused for a rethink

What's going on?
 
Something seriously nasty & saturated; at the simplest explanation. Something deeply wrong/badly engineered, more likely overall.

(original trafo on nominal 110VAC/60hz remains >20% undersized if run on 50Hz at 110VAC )



...Isn't that where the thread started?
 
Especially when the step down is likely outputting 120V+ in the UK, if it is a simple 2:1 and UK mains is often higher than 240V.
 
Thanks Chaps. Step down is putting out 109V this morning. Perhaps I should add that I tested it yesterday morning and pluged it back into the system last night. Perhaps the mains was up at 250V? and the step down putting out 120V - shouldn't make a difference

Mains is measuring 238V - very low
 
Also of note is that I removed the resistor and cap to ground from the PSU caps centre point, That was the one that previously got very hot and also mented it's insulation. Looking like I've just moved the same problem to a different location...possibly
 
Dan, this whole thing sounds very, very unhappy somewhere. I can't help thinking that the amp, or the way you are using it as part of a system, is leading to a massive leakage current - which should not be there.

Do try checking all DC impedances between all inputs (both + and 0v), the chassis, and any available pins on the mains input socket.
Do try checking for any unintended (or omitted!/loose!) connections between signal 0v, mains earth reference, the case etc....

happy to assist/review with you if you think it useful - I'm pretty local.
 
I never had these issues before I fitted the HUGE Epcos 40V 68,000uF PSU caps, these have an ESR of 5 milli Ohms. The rectifier is a std 25A metal bodied item, although I've not been able to detect the rectifier overheating.

My theory (for today at least) is that these monster PSU caps are 'faster' than the rectifier diodes and given the right conditions (in-rush/ saturation) they are able to defeat the rectifier.

I've checked the impedance of everything again and NFF still persists

Thoughts please :)
 
Dan K said:
My theory (for today at least) is that these monster PSU caps are 'faster' than the rectifier diodes and given the right conditions (in-rush/ saturation) they are able to defeat the rectifier.
Click to expand...
Sorry, but that doesn't make sense.
 
My theory, you have an 8
intermitant short of unknown origin, could be thermal in the Traffo. I still have that spare if you want to try it?
 
I had this problem with the std USA traffo and the 240V 550VA UK toroidal one. The big UK traffo toasted the zero volt to ground resistor during power up. The USA traffo did the centre tap to zero resistor after maybe 10hrs of constant playback, the resistor to ground having been removed
 
sq225917 said:
My theory, you have an 8
intermitant short of unknown origin, could be thermal in the Traffo. I still have that spare if you want to try it?
Click to expand...

Si, are you suggesting that I have 2 faulty traffos? Perfectly possible. Would a fault have to be between either of the windings and ground, or between windings?
 
It massively unlikely to be on two different traffos.

If it was an earthing ground fault later in the amp you'd blow the op stage.

What's the diode Bridge rated at?
 
The resistor burning out has to be a symptom rather than causal.

You need some data logging..
 
This is an odd thread / fault?
I agree, some numbers would be good.
You say you fitted 68,000uF caps is that a typo? Best I can see on the web is 6,800uF caps.
Do you know the idle current draw of each channel ? Have you monitored the voltage across the 1 ohm / 7 watt resistor? It will tell you the total current draw. It must be over 3 volts/ 3A to cook the 7 watt resistor...
Are there any fuses in the power supply? What are they rated at?
Best guess is it bursts into oscillation at times or there is a faulty replacement cap.
 
You must log in or register to reply here.


advertisement


Back
Top