The drive voltage is constant with the bench supply between 8.5 and 14 volts.
Above 14 volts the drive voltage increases as does the voltage on pin 12 of the 494.
The amp draws slightly more current as the supply voltage increases. 0.85 amp at 8.5 volts and 1.00 amp at 14 volts. From 14 to 16 it fluctuates from 1.00 to 0.96.
With a supply voltage of 16v I get (+)35.13 and (-)35.08 referenced to secondary center tap.
16v is the limit, above that and it shuts down.
At no point does the rail voltage stop increasing as the supply voltage increases.
If I misspoke somewhere and muddied the waters, I apologize.
Above 14 volts the drive voltage increases as does the voltage on pin 12 of the 494.
The amp draws slightly more current as the supply voltage increases. 0.85 amp at 8.5 volts and 1.00 amp at 14 volts. From 14 to 16 it fluctuates from 1.00 to 0.96.
With a supply voltage of 16v I get (+)35.13 and (-)35.08 referenced to secondary center tap.
16v is the limit, above that and it shuts down.
At no point does the rail voltage stop increasing as the supply voltage increases.
If I misspoke somewhere and muddied the waters, I apologize.
Monitor the DC voltage across the pins 1 and 2 of the 494 (red on 1). At 8.5v, they should be about 5v apart. Increase the supply voltage slowly. Do they ever reach 0v or go negative? If not, what's their voltage at the point where the amp shuts down?
At 8.5v supply, red probe on pin 1 I measure (-)2.353
As I increase the supply voltage the voltage between the pins gradually moves toward 0 reaching (-)0.267 at a supply voltage of 16.
As I increase the supply voltage the voltage between the pins gradually moves toward 0 reaching (-)0.267 at a supply voltage of 16.
The information on post 43 isn't what I expected. Does it make a difference in the bias current as you go from 8.5v to 14v?
I knew it wouldn't be considering it started off in the negative. I actually triple checked the polarity of my probes and tested it four times knowing it wasn't what you were expecting.
How should I test if the supply voltage effects the bias current? Set the potentiometer to a position above 0mv on the source resistor, increase/decrease the supply voltage while monitoring the voltage across the resistor?
How should I test if the supply voltage effects the bias current? Set the potentiometer to a position above 0mv on the source resistor, increase/decrease the supply voltage while monitoring the voltage across the resistor?
Yes. Set the bias to what you'd want it to be on the good channel and vary the DC voltage to see how much it varies.
With the supply voltage at 8.6v I set the bias of the good channel to 1.3mv. As I increased the supply voltage to 16v the bias voltage steadily increased to 1.8mv. The voltage across the resistor is directly proportional to the supply voltage.
(8.5v is right on the edge of being too low to function and I didn't feel like fighting with it.)
(8.5v is right on the edge of being too low to function and I didn't feel like fighting with it.)
The other 3 channels: I set the pot to about the half way point (where voltage is present across the source resistors) and it behaves the exact opposite. As the supply voltage increases the voltage across the source resistor drops. As the supply voltage decreases the voltage across the source resistors rises.
I should have asked this earlier... Were the outputs clamped tightly to the heatsink when you were trying to set the bias?
If you touch rub the tip of your scope probe on the board near the TL594, Do you see any deflection (due to contamination)? Scope set to 2ms and 1v/div.
If you touch rub the tip of your scope probe on the board near the TL594, Do you see any deflection (due to contamination)? Scope set to 2ms and 1v/div.
The first few times I made adjustments to the bias everything was clamped down. The more I've worked on it I've gotten tired of taking the board in and out of the heatsink and now monitor the temperature with my hand any time it's powered up. Nothing has gotten too hot to touch. The MEHSA boards seem to be keeping things from running away.
No deflections while circling the 494 and components in that area aside from bumping into solder joints.
One curiosity that may be unrelated: when I probe pin 3 of Q9 the power supply makes a clicking sound. It kinda sounds like static or sparks or a very small relay opening and closing rapidly and erratically. It's hard to describe.
No deflections while circling the 494 and components in that area aside from bumping into solder joints.
One curiosity that may be unrelated: when I probe pin 3 of Q9 the power supply makes a clicking sound. It kinda sounds like static or sparks or a very small relay opening and closing rapidly and erratically. It's hard to describe.
If it's your scope, setting the probe to 10x, it may not do this.
When the supply is at full duty cycle, the deadtime pin of the 594 is sensitive. In some amps you have to solder a small capacitor between 4 and 7 to be able to measure the voltage with a multimeter.
This isn't really part of your problem but I'm having trouble with the negative voltage on pin1 of the 594. Look at the diagram. Where could the negative voltage be coming from? The voltage divider connects from the positive rail to ground and then to pin 1. None of this is negative voltage.
When the supply is at full duty cycle, the deadtime pin of the 594 is sensitive. In some amps you have to solder a small capacitor between 4 and 7 to be able to measure the voltage with a multimeter.
This isn't really part of your problem but I'm having trouble with the negative voltage on pin1 of the 594. Look at the diagram. Where could the negative voltage be coming from? The voltage divider connects from the positive rail to ground and then to pin 1. None of this is negative voltage.
Ah that makes sense. If I had looked at the schematic a bit closer I'd have seen it was tied to the dead time pin. At least now I know that circuit is sensitive to probing.
I measured pins 1 and 2 referenced to ground:
Pin 1) 1.255
Pin 2) 3.473
There is no negative voltage there it's just that pin 2 is a higher voltage than pin 1 and measuring the voltage across those pins will show a negative voltage when the reference point is the higher voltage (pin 2.)
I thought I followed your instructions correctly but I'm afraid I may have done something incorrectly.
I measured pins 1 and 2 referenced to ground:
Pin 1) 1.255
Pin 2) 3.473
There is no negative voltage there it's just that pin 2 is a higher voltage than pin 1 and measuring the voltage across those pins will show a negative voltage when the reference point is the higher voltage (pin 2.)
I thought I followed your instructions correctly but I'm afraid I may have done something incorrectly.
Unless I read wrong, you stated that pin 1 had -2.? volts. There was no reference to pin 2 so I assumed that you were measuring to ground.
What was the rail voltage when you measured the voltages above?
What was the rail voltage when you measured the voltages above?
I interpreted that to mean red probe on 1, black probe on 2 in the same way measuring voltage across a resistor is a probe on either end unless I'm doing that incorrectly too which is a real possibility. My apologies, I should have included the location of the black probe.
The voltages listed in post 53 are with a supply voltage of 9.0 and the rail voltages are (+)19.55 and (-)19.53 measured at the tab of the rectifiers ref. to secondary center tap.
R21 measures 10.15k out of circuit
R28 measures 149.4k out of circuit
The diagram I've been referencing says R27 and R28 should be 150k for the 400a4. Top left of the first page.
R28 measures 149.4k out of circuit
The diagram I've been referencing says R27 and R28 should be 150k for the 400a4. Top left of the first page.
I specifically looked for a note regarding that and couldn't find it. It's clearly there. I have no excuse.
I think the regulation is OK.
For the bias, I think you stated that if the bias wouldn't come down as you turned the pot slightly that it would be lower after you cycled the power. Is that right?
I think the regulation is OK.
For the bias, I think you stated that if the bias wouldn't come down as you turned the pot slightly that it would be lower after you cycled the power. Is that right?
That is correct for channels 2-4
Turning the pot CW (starting from fully CCW) does nothing till about the half way point where the voltage across the source resistors jumps to between 1.2 and 2.0mv. From this point turning the pot back down (CCW) either does nothing or increases the voltage across the source resistor. Each channel jumps to a different voltage across the resistor but it's impossible to get back down to 0mv without shutting down the amp.
It's mostly repeatable but as things warm up it becomes a little erratic. Twice I've measured voltage across channel 3's low side resistor at startup with the pot fully CCW.
I'm going to have to call it there for now. I'll check back in about 12 hours
Turning the pot CW (starting from fully CCW) does nothing till about the half way point where the voltage across the source resistors jumps to between 1.2 and 2.0mv. From this point turning the pot back down (CCW) either does nothing or increases the voltage across the source resistor. Each channel jumps to a different voltage across the resistor but it's impossible to get back down to 0mv without shutting down the amp.
It's mostly repeatable but as things warm up it becomes a little erratic. Twice I've measured voltage across channel 3's low side resistor at startup with the pot fully CCW.
I'm going to have to call it there for now. I'll check back in about 12 hours
Can you set it and power down and re-check to see if it is where you want it?
Does it hold consistently if you don't readjust the pot?
How does the bias hold when you run the temperature up?
Does it hold consistently if you don't readjust the pot?
How does the bias hold when you run the temperature up?