The ultimate dual 500 "rising from the ashes" build

Driving 1 watt out (2.7v) with not load ran fine till I turned up the frequency to 20 k and loaded up and distroyed the the poss fuse again. I was watching scope for cross hatch distortion at the time but none there this time. I used the HP662a test oscillator this time. Wanted see is the issue was from the AP output. I guess that answers that- it's in the control board. Drivers are good so we get a fart and it blows. I will test again at high frequency.
 
laatsch55 said:
Still have an oscillation eh??
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I am beginning to think it is spikes on the input from static in the test equipment pots. Maybe I am pussy footing around with small fuses but when you hear it run away now twice I am walking slowly. At low power sine is clean of oscillations. Just letting run a bit
slowly
 
So is a runaway amp like a runaway diesel? Just goes uncontrolled full output until it blows?
 
laatsch55 said:
The AP has noisy pots??
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I just wanted to check that possibility. Lee you will be happy to here it dose not. I am hear to tell one and all the Audio Precision is one hell of a peice of equipment.

Something I found while screwing with this thing was the volume controls on the PL make some modulation in the signal at low volumes when not at full volume. It looks like a modulation over the whole sine wave. I will recreate and show some details later.
 
Just getting back to the runaway beast. It is stable but will blow rail fuses without any input or output connected. Lee has sujested a latch up somewhere. The 500 has the following differences on the backplate.
.39 ohm emitter resistors
only .05 uf caps on the rails to ground (700 has .33)
there are no fly back diodes

I have changed the power caps as the capacities were different on the ones it came with (15,000 and 18,00- which measured about 21,000 , to balanced 15,000 ones and will try first that change up.

Any my comments on fly back diodes anyone?
 
grapplesaw said:
Just getting back to the runaway beast. It is stable but will blow rail fuses without any input or output connected. Lee has sujested a latch up somewhere. The 500 has the following differences on the backplate.
.39 ohm emitter resistors
only .05 uf caps on the rails to ground (700 has .33)
there are no fly back diodes

I have changed the power caps as the capacities were different on the ones it came with (15,000 and 18,00- which measured about 21,000 , to balanced 15,000 ones and will try first that change up.

Any my comments on fly back diodes anyone?
Click to expand...

Doubtful flyback diodes are your cause Glenn. Zoebel network??
 
Today update
replaced power caps with matched 15,000 uf ones
replaced Zobel resistors both channels

After about 10-15 minuets under light load ( warming things up)

left channel running only 5 volts output

.132 mv at 1.2k input

N+THD .08%

initial warm Bias .3mv at 75volt both channels

8ohm load connected on driven left channel only (the problem child )

rail at 75v with 150v across caps ( low z setting)

no heat in heatsinks (80-90'F)

sitting and sipping a cup of tea and as I watch the unit do the following observation

Slight voltage drop in room ( slight loading sound)

Suddenly both vu meters rise( only one driven) to about -10 db on meters then driven left chann neg Rail fuse blows (only have 4 amp at this time as s little caution and last time the positive fuse blew)

rail voltage steady at 150v

blown driven Chan had minimal bias mv on left chann with right channel still at .310mv after fuse blew

turned off and now scratching my head

question what caused both channels to show high output on meters

did not have scope in sight when happened.

Something suddenly drove both channel with only one input connected
left channel is one blowing fuses wether under load or not.
Rise in output to excead the 4 amp fuse on loaded channel is obviously the case for fuse going out this time

unloaded channel into no load did not pass enough current to blow fuses yet this channel was suddenly driven

the indecent was not fast as it took about one second to rise and blow

are we any wiser ??

ideas anyone
 
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laatsch55 said:
Broken trace?
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I resoldered the underside of the board looking for such.

There re is a 4558 dual opamp in the centr of the board it is the high Fequecy attenuator to guard again feed back

its output is common to both 356 op amps input

here is the dicription of protection system and how it is built.
 

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grapplesaw said:
I resoldered the underside of the board looking for such.

There re is a 4558 dual opamp in the centr of the board it is the high Fequecy attenuator to guard again feed back

its output is common to both 356 op amps input

here is the dicription of protection system and how it is built.
Click to expand...

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