Hi Guys,
I just finished putting together my Aleph2 and started to bring it up. Smoke!!!
Here is what I did.
1) No inputs
2) No output connected.
3) Variable transformer (10A)
First, I tested the power supply. Brought voltage up slowly, with no driver or output fets connected.
Voltages measured fine +/- 48V (No load)
Connected driver board and output boards.
Brought voltage up slowly. Voltages across R14/R11 were 4.5V
As I ramped up the voltage, everything seemed OK.
As I approached the full +/-45V, smoke poured out of one resistor on each output board...
Any ideas, what other voltages should I look at?
Best Regards,
Dale
I just finished putting together my Aleph2 and started to bring it up. Smoke!!!
Here is what I did.
1) No inputs
2) No output connected.
3) Variable transformer (10A)
First, I tested the power supply. Brought voltage up slowly, with no driver or output fets connected.
Voltages measured fine +/- 48V (No load)
Connected driver board and output boards.
Brought voltage up slowly. Voltages across R14/R11 were 4.5V
As I ramped up the voltage, everything seemed OK.
As I approached the full +/-45V, smoke poured out of one resistor on each output board...
Any ideas, what other voltages should I look at?
Best Regards,
Dale
Member
Joined 2002
More information...
The smoking resistors were the 1.0 Ohm 5W source resistors.
On careful inspection, I have Z4 backwards. Would this cause a problem. I thought it was for transient ESD protection.
Best Regards,
Dale
The smoking resistors were the 1.0 Ohm 5W source resistors.
On careful inspection, I have Z4 backwards. Would this cause a problem. I thought it was for transient ESD protection.
Best Regards,
Dale
Member
Joined 2002
Member
Joined 2002
Guys,
The amp does not go up in smoke right away. I have had the amp at 50% (+/-25V rails) for 30 minutes.
The voltages across the 1 Ohm resistors vary from .43V to .53V
Does this mean that the fets are not sharing current very nicely?
I have
3.7V on the Q1/Q2 Source.
4.5V across R14
4.5V across R11
Q3 Gate is 25-9 = ~16V
Q5 VCE = 3.9V
Gate voltage on the 6 CS output fets is 3.9V
Gate Voltage on the 6 switching fets is -21V
DC offset is <6mV
All look OK?
Bad current sharing? Bad heat sinking?
Thanks,
Dale
The amp does not go up in smoke right away. I have had the amp at 50% (+/-25V rails) for 30 minutes.
The voltages across the 1 Ohm resistors vary from .43V to .53V
Does this mean that the fets are not sharing current very nicely?
I have
3.7V on the Q1/Q2 Source.
4.5V across R14
4.5V across R11
Q3 Gate is 25-9 = ~16V
Q5 VCE = 3.9V
Gate voltage on the 6 CS output fets is 3.9V
Gate Voltage on the 6 switching fets is -21V
DC offset is <6mV
All look OK?
Bad current sharing? Bad heat sinking?
Thanks,
Dale
Dale,
Sounds like there is some mismatch between devices. were these all fomr the same batch of 240's? Also double check and make sure the 1 ohm resistors have not gone up in value slightly...they should be exctly 1 ohm, plus or minus .1 ohm!! When I fired up my forst 2 I had the rails reversed(how stipid am I) and I blew one output and one one ohm resistor. Replacing them and setting the rails correctly straightened it out on the next attempt. The second Aleph worked perfectky from first turn on. Both have hardly been powered down since I got them both up and running last december.
How exactly did you go about matching devices? Also I noticed that you have seven resistors stuffed into the board at R-22 to 27. The drawing only calls for 6 to be in circuit for Aleph 2...unless thats not the board in the photo that you are talking about. Seven are used for an Aleph 1.2 though. why the 7 resistors for a 2?
It pretty much looks to me like it is now working ok though by the voltage readings you've provided. I'd inch it up a bit more...say to 75% voltage and let it sit for a while. See what happens. Check the 1 ohms first though...later on you can match up the fets better if need be.
Mark
P.S. With rails reversed it goes up in smoke almost immediately!
Sounds like there is some mismatch between devices. were these all fomr the same batch of 240's? Also double check and make sure the 1 ohm resistors have not gone up in value slightly...they should be exctly 1 ohm, plus or minus .1 ohm!! When I fired up my forst 2 I had the rails reversed(how stipid am I) and I blew one output and one one ohm resistor. Replacing them and setting the rails correctly straightened it out on the next attempt. The second Aleph worked perfectky from first turn on. Both have hardly been powered down since I got them both up and running last december.
How exactly did you go about matching devices? Also I noticed that you have seven resistors stuffed into the board at R-22 to 27. The drawing only calls for 6 to be in circuit for Aleph 2...unless thats not the board in the photo that you are talking about. Seven are used for an Aleph 1.2 though. why the 7 resistors for a 2?
It pretty much looks to me like it is now working ok though by the voltage readings you've provided. I'd inch it up a bit more...say to 75% voltage and let it sit for a while. See what happens. Check the 1 ohms first though...later on you can match up the fets better if need be.
Mark
P.S. With rails reversed it goes up in smoke almost immediately!
Hi Mark,
I should have responded sooner. Yes, I have gotten the amp up to full voltage for at least one hour with no problems except for the mis-matched currents.
In fact, I may have actually had my variable transformer above 100%. I forgot that it can go to 140VAC. Don't know if this is what caused my problems. My resistors are 1%, but I did not "match" them.
The fets were matched for Vgs at .5A/14.7V for 30 seconds
For the 6/7 parallel resistors, the difference is quite small.
.5/7 = .071
.47/6 = .078
Now, to try putting some sine waves through the amp.
Thanks,
Dale
I should have responded sooner. Yes, I have gotten the amp up to full voltage for at least one hour with no problems except for the mis-matched currents.
In fact, I may have actually had my variable transformer above 100%. I forgot that it can go to 140VAC. Don't know if this is what caused my problems. My resistors are 1%, but I did not "match" them.
The fets were matched for Vgs at .5A/14.7V for 30 seconds
For the 6/7 parallel resistors, the difference is quite small.
.5/7 = .071
.47/6 = .078
Now, to try putting some sine waves through the amp.
Thanks,
Dale
Member
Joined 2002
You guys are talkig about aleph 2's since im in this form i might want to ask where are the points of source to mesusure when powering up a aleph 2 i have. i dont see in the manual any where. Help i dont want to see smoke eheh
Jason.
Jason.
Hi Jason,
Actually, if you look at the schematic in the service manual, there are several operating points shown. Voltage across R11 and R14 ~4-5V.
Voltage at Q1/Q2 Source at ~4V
Q3 Gate at ~9V below rails.
As indicated by several people, make sure that the power supply voltages are correct. Also, make sure that the output boards (or PTP) are correct. The connections are not identical!
If you dont have a variable transformer, use a lightbulb.
Also, be careful not to short anything while measuring. Remember that those power supply caps hold lots of energy.
Actually, if you look at the schematic in the service manual, there are several operating points shown. Voltage across R11 and R14 ~4-5V.
Voltage at Q1/Q2 Source at ~4V
Q3 Gate at ~9V below rails.
As indicated by several people, make sure that the power supply voltages are correct. Also, make sure that the output boards (or PTP) are correct. The connections are not identical!
If you dont have a variable transformer, use a lightbulb.
Also, be careful not to short anything while measuring. Remember that those power supply caps hold lots of energy.
Dale, Am curious as to what the reccommended resistance is there.... .7 , .8, and what effect the differences might have on how the amp sounds, or works?
Thanks!
Mark
Thanks!
Mark
Mark,
From my limited knowledge in this area, the resistors are used to sense the current at the output of the amp. The voltage is used in the negative "Aleph" feedback. A slightly higher value would result in a little more feedback voltage. Does this mean more feedback? I really don't know.
Sound differences? Doubtful.
The inductance of the resistors may be a problem since it would add some reactance and slow down the feedback.
Dale
From my limited knowledge in this area, the resistors are used to sense the current at the output of the amp. The voltage is used in the negative "Aleph" feedback. A slightly higher value would result in a little more feedback voltage. Does this mean more feedback? I really don't know.
Sound differences? Doubtful.
The inductance of the resistors may be a problem since it would add some reactance and slow down the feedback.
Dale
Member
Joined 2002
what i ment was if i see no smoke at a power up i useualy power things up to half there rail voltages. now with that in mind i want to start mesureing points to tell me that things are all well. now if i power it all up to full rail voltages. i should start cheacking right. i dont have money to just turn up the variac and see if it works.
If you're going to blindly run it up without checking anything, save the cost of a variac, plug it in and cross your fingers. I don't think you will blow the heat sinks, and that's were the money is.
Member
Joined 2002
no i have a variac though. i just wanted to know what voltage it is safe to power it up at ( per rail. ) so i can do voltage checks.
Dale,
Its funny...when you calculate either six .47 ohm resistors you get what it should be which is .0783 ohms. If you use five - .5 ohm resistors you get.083, and if you use seven .5 ohmers you get .0714 ohms...about the same amount in the opposite direction of the prescribed value. Seems odd..... Again, I don't know how this will effect the current source end of things but I'm don't think it'll matter too much. On my 2's I'm running the bias at about .6 volts across the source resistirs and am getting a bit smoother sound fomr them than with them set at .5 volts. I have since installed fixed resistors in place of the pots I put in them to be able to tweek things. I could go higher, but there isn't much range left with thepots in there. Some have said just leave R-18 out of the circuit if you have alot of heat sinking. Hs anyone reading this thread ever done this?
Mark
Its funny...when you calculate either six .47 ohm resistors you get what it should be which is .0783 ohms. If you use five - .5 ohm resistors you get.083, and if you use seven .5 ohmers you get .0714 ohms...about the same amount in the opposite direction of the prescribed value. Seems odd..... Again, I don't know how this will effect the current source end of things but I'm don't think it'll matter too much. On my 2's I'm running the bias at about .6 volts across the source resistirs and am getting a bit smoother sound fomr them than with them set at .5 volts. I have since installed fixed resistors in place of the pots I put in them to be able to tweek things. I could go higher, but there isn't much range left with thepots in there. Some have said just leave R-18 out of the circuit if you have alot of heat sinking. Hs anyone reading this thread ever done this?
Mark
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