Yes. Actually two different reputable places.
All the output mosfets in the left channel came from Allied electronics. Which I already used some in another RF amp I repaired and had no issues with them. They are vishay brand.
All the output mosfets in the right channel came from future electronics. They are also vishay brand.
I’ve never ordered from future electronics before but they were recommended to me by another person that repairs amps.
I usually get them from mouser but they were out of the 9640’s when I ordered them.
All the output mosfets in the left channel came from Allied electronics. Which I already used some in another RF amp I repaired and had no issues with them. They are vishay brand.
All the output mosfets in the right channel came from future electronics. They are also vishay brand.
I’ve never ordered from future electronics before but they were recommended to me by another person that repairs amps.
I usually get them from mouser but they were out of the 9640’s when I ordered them.
Good to know about future. They where actually cheaper than the others.
The rail voltage is measuring +24.01 vdc to ground and -24.03 to ground. +48.05 vdc rail to rail.
That seems about half of what it was measuring when I checked it with all the mosfets, source, and gate resistors out of the amp before I replaced them with new.
The rail voltage is measuring +24.01 vdc to ground and -24.03 to ground. +48.05 vdc rail to rail.
That seems about half of what it was measuring when I checked it with all the mosfets, source, and gate resistors out of the amp before I replaced them with new.
Yes I do. A 2 ohm 100 watt resistor. Forgot that.
So with the amp turned on and the current limiter in the b+ line I measure 6.23 vdc across the amps’ positive and negative terminals.
So the rail voltage measurements would be half or less with half the input voltage.
So with the amp turned on and the current limiter in the b+ line I measure 6.23 vdc across the amps’ positive and negative terminals.
So the rail voltage measurements would be half or less with half the input voltage.
No it doesn’t. I never thought to check with those components jumped. It draws .9 to 1 amps powered up with no limiter.
With no limiter in the b+ line the positive rail voltage is 51.30 vdc to ground.
The negative rail voltage is -51.34 vdc to ground. I measure 102.6 volts dc across the rails.
With no limiter in the b+ line the positive rail voltage is 51.30 vdc to ground.
The negative rail voltage is -51.34 vdc to ground. I measure 102.6 volts dc across the rails.
The problem may be that the new FETs have a lower threshold voltage than the originals.
I think it may be necessary to make a change to the bias circuit. Increasing the value of Rx20 or decreasing the value of Rx18 may give you the added range you need to adjust the bias. Try one channel at a time. Make small changes. The limiter will need to be in the B+ line. The bias jumper will be removed.
I think it may be necessary to make a change to the bias circuit. Increasing the value of Rx20 or decreasing the value of Rx18 may give you the added range you need to adjust the bias. Try one channel at a time. Make small changes. The limiter will need to be in the B+ line. The bias jumper will be removed.
Ok. Will do. Is 50-100 ohms at a time a good amount to try?
I will start with r220 in the left channel and increase it. How far should I go before I stop?
I will start with r220 in the left channel and increase it. How far should I go before I stop?
Rx18 may be easier since it will be easier to make small differences but either should make a difference.
With Rx20, changing it by 100 ohms will leave nothing.
With Rx20, changing it by 100 ohms will leave nothing.
The value of R218 and R318 in this amp are 1k and not 2k.
I decreased the value to the next resistor I had which was 680 ohm and no change in current draw at 3 amps with the inline limiter and the jumper off the bias transistor.
The next value I had was 470 ohm so I installed it and the current draw went down to 2.3 amps from 3.0 amps.
So I went down to a 330 ohm and now the current draw is .9 amps with the amp on and no jumper on the bias transistor.
I removed the limiter for a quick check and the current draw is still .9 amps.
Is this acceptable and can I move on to the right channel?
I decreased the value to the next resistor I had which was 680 ohm and no change in current draw at 3 amps with the inline limiter and the jumper off the bias transistor.
The next value I had was 470 ohm so I installed it and the current draw went down to 2.3 amps from 3.0 amps.
So I went down to a 330 ohm and now the current draw is .9 amps with the amp on and no jumper on the bias transistor.
I removed the limiter for a quick check and the current draw is still .9 amps.
Is this acceptable and can I move on to the right channel?
With the FETs clamped down, about how far from center is the bias pot when you get the bias set?
With the bias pot fully CCW, is the voltage across the source resistors 0.000v?
With the bias pot fully CCW, is the voltage across the source resistors 0.000v?
I can turn the bias pot all the way clockwise and the current draw never changes from .9 amps.
I usually set it by turning the pot clockwise until it raises the current draw about .4-.5 amps.
This is the correct way to set it right?
Yes, all the source resistors read 0.000 vdc across them with the bias pot fully counterclockwise or fully clockwise too.
I usually set it by turning the pot clockwise until it raises the current draw about .4-.5 amps.
This is the correct way to set it right?
Yes, all the source resistors read 0.000 vdc across them with the bias pot fully counterclockwise or fully clockwise too.
I set the bias to just where the current starts to increase. Like this:
http://www.bcae1.com/temp/ausettingbias.swf
You have to find a happy medium. You have to be able to get the bias to go from 0.000 across the source resistors at full CCW to where you can get the bias where you want it at about 1/2 of the range of the pot.
You have to use the voltage across the resistors more than the current from the 12v supply while trying to find the values that will work.
http://www.bcae1.com/temp/ausettingbias.swf
You have to find a happy medium. You have to be able to get the bias to go from 0.000 across the source resistors at full CCW to where you can get the bias where you want it at about 1/2 of the range of the pot.
You have to use the voltage across the resistors more than the current from the 12v supply while trying to find the values that will work.
Ok. I think I understand now.
I will have to do some scavenging of boards or order more values of smd resistors between 330 and 470 ohm to be able to set the bias close to halfway on the pot and then still have 0.000 vdc across the source resistors when the bias pot is fully counterclockwise.
I will have to do some scavenging of boards or order more values of smd resistors between 330 and 470 ohm to be able to set the bias close to halfway on the pot and then still have 0.000 vdc across the source resistors when the bias pot is fully counterclockwise.
Do you have a couple of pots that you can install in place of the two resistors to let you make changes quickly? After you find the settings that appear right, desolder the pots and measure the value. That should get you close.
If you can't get the right value with the limiter inline, insert a low value fuse (as low as 5 amps, if possible) to protect the amp.
If you can't get the right value with the limiter inline, insert a low value fuse (as low as 5 amps, if possible) to protect the amp.
Yes I do have a few pots laying around.
I think I have a few different values. Something like a 1k or 2k should do it.
Yes I have some 4 and 5 amp fuses I can put in the B+ line.
I think I have a few different values. Something like a 1k or 2k should do it.
Yes I have some 4 and 5 amp fuses I can put in the B+ line.
The ATC type hold better than the small glass fuses. You may have to jump the fuse with a wire at startup if it blows due to inrush current.