Aleph 5 help

[kilowattski]

I never thought I would be asking for help but I have a problem with my Aleph 5 boards that is driving me nuts. They both behave the same way. When I power them up it appears as if the gain stage is pushing alot of current because when measuring the voltage drop across the 1 ohm resistors (R64 thru R66) I am reading about 2.5 volts. When I look across the 1 ohm resistors in the CCS (R40 thru R42) I read 0.5 volts as normal. I read the voltage across Q5 and it is indeed what I would expect at 5 volts. When I read the voltage across Q4 I read 6.5 volts which is much too high. I had a good second look at the boards and can see nothing obviously wrong but then again I may not be able to see the forrest for the trees. This just does not make any sense to me. I would also expect to see increased current draw from the CSS? It makes no sense to me.

[MikeW]

Are the other voltages of the front end correct? What is the dc offset at the output?

[wuffwaff]

Hi,

do you have a load connected? Any input signal?

Since current can´t appear out of nothing it must come from somewhere if not from the current source. It is possible that you sink a lot of current if the output is at a quite high negative dc offset and a load connected.

Or maybe you made the same mistake I made when first connecting my Aleph5 and changed the drain and source connections on the gain side..........

William

[kilowattski]

William,

Thanks for the help. The output sits at about +23 volts. I just took a measurement and noticed that there is only 0.5v across R11. I also noticed that the zener does have 9 volts across it. That means that Q3 is not conducting. If that is true how can I see 6.5 volts across R14? The measurements are made without a load or input. I must admit that I am very confused. I checked and the FETs on the gain side are installed correctly. They can only be installed one way because I am using Kristijans boards and they can only go one way and make thermal contact with the heatsink. I think I will take the night off and have a fresh look tommorrow.

[mastertech]

Hi kilowattoski
i am sorry i cant help you without a schematic my friend
cheers

[kilowattski]

Mastertech,

Thanks anyway. The schematic is on the Pass Labs website in the Aleph 5 service manual. Nelson does not permit his IP to be posted without his permission and will I honor that. I can provide a link though.

http://passlabs.com/downloads/old%20product/a5serv0.pdf

[Blues]

kWski,

Check Q3 by measuring across D-S, it should be about 25V at +/-34V rails. And its G-S at about 4V. Make sure it is mounted correctly.

Also check D-S of Q18-20 at 33V or so. G-S should also be 4V. While you're at it check for correct mounting of Q4. Make sure all resistor values are correct.

[jh6you]

Hi kilowattski

The measured voltage across R64-66 is 2.5V. It tells me that Q4 is there to restrict the maximum voltage to 2.5V. This again restricts the maximum input to the gate of Q18-20 to 6.5V. And, at this reach, all are frozen, leaving 6.5V across R14.

Anyhow, I believe that you already know where is the open you need to fill in. Good luck!

Regards
JH

[BobEllis]

could R11 be 22R1 instead of 221R?

[kilowattski]

Firstly, I want to thank you all for your offer of help. It will certainly keep me from pulling my hair out. I am writing this quickly before I go to work so if I left someone out I am truely sorry.

William: At this time I do not have a load or input connected.

Blues: I will make those measurements this evening. when I get home from work.

JH: It still doesn't make sense to me. I have been known to have a brain fart sometimes.

Bob: Mrs. Kilowattski and I verified that we had all the correct parts in the correct places. She read the reference designations and the color codes of the parts on the board to me and I verified it on the schematic. I also downloaded the datasheets for the FETS and had her verify that they were inserted correctly. I figured a second set of eyes may help.

Thanks all,

Guy