The ultimate dual 500 "rising from the ashes" build

[grapplesaw]

That high frequency attenuation circuit is quite bogus. Evidently the output stage suffers from cross conduction.

Thank you Joe.
I pulled it out just before this test.
I will try a different probe

I think I will also change out the volume pots. I tried to clean them but I think it part of my problem.

now that high frequency section is out I may change out the ic to lf351 from lf356 and up the power resistors to 7.5k

 

[grapplesaw]

Updated crash report. From today test

High frequecy curcuit removed

2k signal steady at 4 volts out into 8 ohms

two tests
first crashed at about 18 minutes run time
second started 3 minutes later. And ran only 10 min

heat seams to be low but maybe the root of issue.

Results

7:52 start2k
4 volts out
.110 volts in
.4688mv bias
Right
.65mv out
.374mv bias


Temp 77 F start both



5 min run time temp 82 .0701% N THD


10 min run time temp 89.5 left 80 right .0709 N THD
Left bias .4635. Right bias .379
Left out 4 volts Right output .634 mv
153.6 rail voltage


15 min run time temp 90.4 left 83.5 right .070 N THD
Left bias .4616 Right bias .376
Left out 4 volts Right output .634 mv
153.2 rail voltage





16 minutes in crash occurred !!!!

Temp on heat sink at crash
92 deg left
85 deg right



Blown fuse Positive fuse 4 amp buss 250v



23 min in restart
Temp 88 left 85 right
.0839N THD
Left bias .449 Right bias .339
Left out 4 volts Right output .634 mv
153.6 rail voltage



5 min 92.8 left 86 right .0752 N THD
Left bias .455 Right bias .360
Left out 4 volts Right output .634 mv
154 rail voltage



10 min crash positive fuse again

so is it ic lf356 or driver positive rail?? Ideas anyone

 

[Gepetto]

Updated crash report. From today test

High frequecy curcuit removed

2k signal steady at 4 volts out into 8 ohms

two tests
first crashed at about 18 minutes run time
second started 3 minutes later. And ran only 10 min

heat seams to be low but maybe the root of issue.

Results

7:52 start2k
4 volts out
.110 volts in
.4688mv bias
Right
.65mv out
.374mv bias


Temp 77 F start both



5 min run time temp 82 .0701% N THD


10 min run time temp 89.5 left 80 right .0709 N THD
Left bias .4635. Right bias .379
Left out 4 volts Right output .634 mv
153.6 rail voltage


15 min run time temp 90.4 left 83.5 right .070 N THD
Left bias .4616 Right bias .376
Left out 4 volts Right output .634 mv
153.2 rail voltage





16 minutes in crash occurred !!!!

Temp on heat sink at crash
92 deg left
85 deg right



Blown fuse Positive fuse 4 amp buss 250v



23 min in restart
Temp 88 left 85 right
.0839N THD
Left bias .449 Right bias .339
Left out 4 volts Right output .634 mv
153.6 rail voltage



5 min 92.8 left 86 right .0752 N THD
Left bias .455 Right bias .360
Left out 4 volts Right output .634 mv
154 rail voltage



10 min crash positive fuse again

so is it ic lf356 or driver positive rail?? Ideas anyone

Why is the bias so high on the left channel Glenn? Should be 0.35V

 

[grapplesaw]

Why is the bias so high on the left channel Glenn? Should be 0.35V

That is reading I get when the driven channel is under load with 4 volts output. I record it to see the changes as it heats up. The channels are both set at .360 mv on the low voltage rails after initial warm up. This will confirm bias resistor is functioning properly I hope.

 

[Gepetto]

That is reading I get when the driven channel is under load with 4 volts output. I record it to see the changes as it heats up. The channels are both set at .360 mv on the low voltage rails after initial warm up. This will confirm bias resistor is functioning properly I hope.

You have a bias circuit problem on the left channel Glenn. This creeping up to .46 leads to cross conduction problems in the L channel output as the predriver and output drivers warm up.

Also what is the base emitter resistor used on the predriver? 150 ohms? 180 ohms? Other?

 

[grapplesaw]

You have a bias circuit problem on the left channel Glenn. This creeping up to .46 leads to cross conduction problems in the L channel output as the predriver and output drivers warm up.

Also what is the base emitter resistor used on the predriver? 150 ohms? 180 ohms? Other?

Ok Joe
I will I'll check and report back in 30 min

 

[grapplesaw]

Joe here is info please let me know hat you think.

as you are aware this has a high and low rail voltage these earlier reports were under run condition passing current


The bias mv left channel reported earlier in the .400's was with 4 volt 8 ohm load connected and input of .110 mv . The bias was set at .360mv at low voltage setting with out input or output connected prior Testing



To answer your question the Bias left resistor 185 ohm right 180.2 ohm

I have put in new fuse and running again under No output to input connected either channel

bias voltage cold start heat sink 77F



Low power L and R channel bias at .330 mv at low rail voltage of 154 voltage across caps

High power left bias .460mv and right .495mv at high rail voltage. Of 228 volts across caps



after 3 minutes warm up no input or output connected.



low voltage bias left .382mv right .395
high voltage bias left .465mv and right .495mv

******i cycled the high low switch a couple times to confirm these voltage differences and on the second cycle that negative rail fuse blew again. No input or output connected. Something does not like change in there. !!!!!!

So can the transformer momentarily bridge the high low windings to cause this? I dought it. My volt meter across the power caps seamed to be steady at either setting but not witnessing any spike. This switching is through a relay. I think Ed worked the relay socket in the past.

 

[grapplesaw]

Bias voltage suddenly doubles with high output distortion

I got a little more info

diabled the high low so now only high voltage

balanced both channel to .350 mv at stabilized heat

left fuses in only and increased to 5 amp

run with warm unit with 1.5 volt out 94'f run temp in 77' room

run for three minutes and bias jumped to over 1 volt.

Turned off off no fuse blown

restart and run 30 seconds and distortion goes to the moon and high bias again.

No high temp light

turn on fan to cool down

start again no input yet

bias .250 volts at start and rising

start signal. All god heat sink down to 86'

bias steady .412 loaded

86' temp

bias resistor 88 'f when fan cooling

turn off fan and bias jumps to .9 volts.


Bias resistor 91'f

Issue.

so so increase bias voltage with full distortion

col off and repeats

temp related Bias curcuit.

 

[Northwinds]

Glenn, I would be pulling my hair out right now

 

[grapplesaw]

Bias resistor looks good

Removed the vari resistor and heated to 150+ and no change in resistance

next test bias transistor

 

[laatsch55]

I got a little more info

diabled the high low so now only high voltage

balanced both channel to .350 mv at stabilized heat

left fuses in only and increased to 5 amp

run with warm unit with 1.5 volt out 94'f run temp in 77' room

run for three minutes and bias jumped to over 1 volt.

Turned off off no fuse blown

restart and run 30 seconds and distortion goes to the moon and high bias again.

No high temp light

turn on fan to cool down

start again no input yet

bias .250 volts at start and rising

start signal. All god heat sink down to 86'

bias steady .412 loaded

86' temp

bias resistor 88 'f when fan cooling

turn off fan and bias jumps to .9 volts.


Bias resistor 91'f

Issue.

so so increase bias voltage with full distortion

col off and repeats

temp related Bias curcuit.

Bias jumped to . (point) 9 volts?? Like 900 millivolts??

 

[grapplesaw]

Bias jumped to . (point) 9 volts?? Like 900 millivolts??

Eyeh .9 volts

is it worth testing bias transistor or just change it and the 180 ohm resistor on the first driver?.

 

[grapplesaw]

Bias jumped to . (point) 9 volts?? Like 900 millivolts??

So is this short or a break in the curcuit to cause the rise

 

[laatsch55]

Eyeh .9 volts

is it worth testing bias transistor or just change it and the 180 ohm resistor on the first driver?.

I would measure all 3 legs of each bias transistor to see if the inputs to them are the same....

 

[grapplesaw]

I would measure all 3 legs of each bias transistor to see if the inputs to them are the same....

ok I will pull it and test. Ed had replaced it but it was loose when I got it. This thing gos to full bias like a switch is turned at what ever temp it goes at

 

[grapplesaw]

I would measure all 3 legs of each bias transistor to see if the inputs to them are the same....

Leakage test ok hot and cold

you say measure inputs when turned on????

i will put in a 5088 I guess

 

[laatsch55]

If you have the 5088's, yeah I'd be putting them in...

 

[grapplesaw]

If you have the 5088's, yeah I'd be putting them in...

Transistor tester said ok

volt meter readings. Red probe on black. 5.139m both legs. With switched to diode setting 68.65 ohms red to black. And 69.24 ohms brown to black

 

[laatsch55]

Transistor tester said ok

volt meter readings. Red probe on black. 5.139m both legs. With switched to diode setting 68.65 ohms red to black. And 69.24 ohms brown to black

I meant in and powered, black test lead on ground, see what you have for voltage on each leg....

 

[grapplesaw]

So the issue is a temp increase opens the bias to a high output ( or closes) to give high bias to base of outputs transistors. Will a failure like this happen in a bias transistor then recover when cooled off. I can see a resistor maybe doing that on a repetitive biases. As to if there is a cracked trace I cannot find it

the trace path is short so easy to follow it

I guess I an not clear on Which one of raising or lowering resistance will lower the bias output

i thought we are we reducing resistance in the vari resistor to increase bias. I may have this reversed. Please clear up this one if you can