The ultimate dual 500 "rising from the ashes" build

Ok now that I bypassed the output relay. It has s bin a while so probably will make a few more mistakes.

D.C. Offset left channel 1.4mv at 160 and 240 rail voltage and .68mv on right at both voltages.

Did a test on both channels. Bias .350, no output. output increased to 6 volts with .035%THD+N nunweighted and bias increased to.470 mv at 160 rail voltage and.520 mv at 235 rail voltage. Both channels do the same. No heat in output transistors to speak of.

Should Bias increase this much?

Need to get hot hot to see if bias reduces. I may just change out the 2n3403s to 2n5088s
 
So the offset voltage problem is fixed......bias voltages are with no load correct?
 
Time to build my paint can load cells. I got Twelve 50 ohm noninductive resistors and will configure them in groups of six. This will give 8.3 ohm load and 500 watts dry and about 6000 watts when in oil bath.
 

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bias will rise as the load increases....350mv is perfect with no input...
 
Ed and I talked about it today....wasn't it a runaway heat problem?? Could it be caused by a high frequency oscillation?
 
laatsch55 said:
Ed and I talked about it today....wasn't it a runaway heat problem?? Could it be caused by a high frequency oscillation?
Click to expand...

Thats what I was thinking. It may be a whole lot of things. I will add the little mica caps to the out put transistors on the driver board. I got to build my dummy load and we are off to Hawaii soon so may take a little time. It was nice just to power it up. The phase 500 has a section on the pl32 to deal with that exact problem. It is an Attenuator. It has a ic chip that senses the distortion and nulls the input to stop the oscillation. The outputs are upgraded to mj21196 by Ed. The mystery continues.
 
I put the amp from ed back on the bench tonight. As noted the first test all looked good bias was set at .350mv both channels. So after running it up a bit I let it put out about 1 watt with only Meters connected no load. After a bit the bias dropped to 45 mv. So something wrong in the bias circuit. I ran the output frequency up to about 30 k and back down The sine wave started breaking up at only .5 mv output. Please see wave forms. Any other ideas are welcome but the chassis transistors are first to go then the 10 ohm on the outputs.
 

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laatsch55 said:
3403"s or 5088's?
Click to expand...
I will take out the 3403's and put in 5088's. I have been using #22 resently for leads. They are all more easy to place inbetween other cables that #20 gauge wire. I do not think they pass much current so unless some corrects me I will carry on with #22 leads
 
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So I spent 6 hours with my cable/internet provider to undo the mess they made with no success so i was now prepared to work on the problem child. I with back to the Mistery Amp , a dual 500 , to try to sort it out. After my last post I took the time to clean volume and bias pots. I further changed out the out drives on the Pl32 board to 2n3439 and 2n3516 pairs and changed out the slowing caps on these to a lesser .105 mica cap.

I fired up and and after some minor connect problems I ran the beast up to about 10 volts. DC offset was very good at 1 mv per channel
After running for some time on the supposedly good channel I found some minor crossover distortion as per photo. This should up at higher , 8k +, frequencies in both channels and more on the negotiate side. N+THD was ok though. I switched to the problem Channel and ran at 8 volts output into 8 ohms to check temps. All was well for a while then it started to pull some serious current and blew the rail fuse on the possitive leg. I had downsized to 4 amp so it must have put out some big number befo tilting. Unfortunately I was not looking and jumped to power it off when it heard it load up. All the transistors have high ohms between collector and emitters. Mistery???
does one driver short when warm?? Does ic go wild?? I do not think bias related. This was a sudden power surge

Ideas please
glen
 

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After a break I replaced the blown 4a fuses with 2a and powered up with the DBT. All good so far

Powered on with regular supply and took up to 160 rail voltage. No issues. Powered up to full rail on load alll ok

DC offset 1 mv or less

go figure!!!!
 
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