Aleph 5 help

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.
 
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
 
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.
 
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.
 
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
 
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
 
jh6you said:
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


As designed this should only happen with a very large sine wave at the input. Maybe 12Vpk-pk or more.
 
You guys are never going to believe this. When I started the Aleph 5 project I had about half a dozen Bergquist K4 Silpad insulators in my parts bin. I knew I would need at least a dozen for the project, so I tried to purchase 36 TO-247AC KA10 insulators (High Performance) from Digikey. Unfortunately Digikey does not have them in stock, so I purchased them from another popular vendor on the web (name withheld to protect the guilty). When I recieved them, I saw that they were indeed insulators, or so I thought, and never looked at the part number on the bag. Well, I pulled the bag open none the wiser. and started to mount the boards to the heat sink. If you read this thread from the beginning you know what happened after that. While taking some measurements at the recommendation of blues I happened to drop one of the probes and it hit the heatsink. To my surprise I got a breif voltage reading. The heatsink should be isolated, so this is not possible I thought to myself. I removed the board from the heatsink to see if I had any FOD on the bottom that could have pierced the insulator. I found none but just in case I decided to change all the insulators. When I picked up the bag of insulators I noticed the label on the bag said Q Pad II. This is not what I ordered, so I looked at my copy of the online order, and I indeed ordered the KA10 material. I looked at the Bergquist site to find to my astonishment that Q PAD II is conductive. Well there you have it. In a chain of fopaws I was sent the wrong product and I never checked what I recieved. Well looks like the elevator didn't reach the top floor on both sides. The company I bought the pads from is sending replacements with an apology and I can only blame myself. DUH!!!!!!!!!!!!!!!!!
 
funny I ordered similar pads before and when looking closely at the specs it was indeed conductive. Too late as Digi-key has shipped my order...good thing I caught it before I mounting my beloved fets.

So is your A5 zooming...?

"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."

-I had all along thought you have Igor helping you around the lab:)
 
:) to hear that you did not ask her to measure the voltage...

Regarding safety, I trust nothing. I always do double check whether the drain is shorted or not to the heat sinks immediately after the mounting. I also check the earth line once more.

One day, my hairs exploded out of me one by one, piercing all different directions. It happened all at once in micro-second.

Safety! Safety! Safety!

Regards
jH