Setting Quiescent current through lightbulb limiter...

[noddyspuncture]

Hi folks,

I am struggling with an amplifier I have replace the output transistors in.
The service manual states that it is critical to set the quiescent current when any components have been replaced. Just incase I turn the unit on normally and they all blew again, I used the lightbulb trick and all was (seemed) fine.

The light bulb lit and then dimmed immediately. I also put a known good unit onto the light bulb and it behaved the exact same way - so I deduced that all was well.

When checking the quiescent current I had an imbalance. They say you should set in to measure 6.6 to 8.8mV across R's 4, 14, 44, 45.

One side read "zero" across the resistors ... the other read -76mV.  I could not set them and assumed it was because of the lightbulb.

So I removed the lightbulb - after all, it didn't "light" brightly and behaved in the same way as the known good unit...  I got sparks (from R42) and blew the output transistors again...!

My question is - should I be able to set the quiescent current whilst using the lightbulb trick...? I know voltages would be lower overall but should I be able to get an idea is setting the quiescent current will work...?

Appreciate any help,
Cheers,
Tom

[J M Fahey]

Sorry for what happened.

The lightbulb should let you set roughly close bias, if anything it can be touched up later, but I guess the amp was not yet fully repaired
Too busy now, will recheck this post tonight.

[Enzo]

I am going to guess you had a load on the amp.  You should always make sure the amp is stable and NOT producing DC before leaving teh bulb, then try UNLOADED with full mains before continuing.

[noddyspuncture]

I am going to guess you had a load on the amp.  You should always make sure the amp is stable and NOT producing DC before leaving teh bulb, then try UNLOADED with full mains before continuing.

No, it was not loaded and I checked and it was not producing DC either...!
I was OK on the lightbulb... bulb dimmed and stayed dim. But it was instant fireworks off the bulb...

Cheers,
Tom

[nosaj]

The load in this case would be a speaker or a dummy load.



nosaj

[phatt]

Try increasing the wattage of the bulb to find the offending part.
So if you have a 40Watt, go up to 60Watt, then try 100Watt. The higher the wattage the closer you get to full mains but still with some safety margin if it tries to smoke you will have enough time to power off and save the expensive stuff. A quick finger probe before it cools down will tell you what part is getting hot.
Being aware of any high voltage of course.  8|
Phil.