Problem with new Aleph J build

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I have set about making another Aleph J with different approach to heat sinking.
Based on a build posted by another member on DIYA, I am using two back to heat sinks made into a tunnel and forcing air through the fins with a ultra quite fan.

I am using a 500VA, 2 X 18V toroid feeding into standard Pass design PS with 60K uF on each rail , in a CRC config. The Aleph J boards are from Peter Daniel. The matched MOSFETs are from Tech-DIY.

Build is completed. Tested initially with a bulb, no problem. Removed the bulb, tested with full power. One channel seems to have biased perfectly with a DC offset of 35mv. The other channel is showing a DC output of nearly 17.35V. I have put a resistor and a trimpot in place of R8 to trim the DC offset, but is of no avail. No change.

Do not know where to look for to trouble shoot. Q2 & Q4? Have checked for loose solders, joints, wrong components etc. Cannot find anything. Seems to be a major problem on one channel though.

Need help please....

Cheers.
 
Can you only turn on the problem channel?

You should be OK turning on the amp long enough to measure. Check R18 and R19 first. If they are significantly higher than .5V or so then you're in trouble, but I suspect that you'll find that it is close to 0V.

My initial guess is that Q7/8 aren't being turned on. Either Q1a or Q1b is bad or your CCS isn't working.

Also measure the voltage across Z1
 
Is the output offset positive or negative relative to ground? At the voltages reported I would expect negative.

Are the rails both at the proper voltage?

Q4 CE is a bit high. Did you use ZTX450? Check the R24-27 values

Also check voltage across R16 and R17

Your matching doesn't seem very good for Q7 and Q8. R18 and R19 should be rather close, within millivolts.
 
Bob,

The supply rails are measuring +22.96 and -22.96 volts. The output voltage is not negative, but positive 17.75V Other voltages measured are:

R24 .. 0.26
R25 .. 0.101
R26 .. 0.237
R27 .. 4.71

R16 .. 0.362
R17 .. 0.392

Cannot make anything out of this. The output MOSFETs are perfectly matched ones. They can be either blown or working, but could not be mismatched. I have used the same outputs in an earlier build and working well.

Cheers.
 
I meant check the resistor values, but that may give us a clue.

R24 should have no DC voltage. Is C3 reversed?

R25 and R26 current are about the same, as expected (within likely measurement and resistance tolerance, but very low. R27 current is also about the same. That tells me you have (virtually) no current in Q4. Replace it.

You may have damaged one of your output devices when reusing them. Your active current source devices show good matching. Perhaps you mismeasured R19?

Are the inputs grounded for this test?
 
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Initially c3 was reversed and I corrected it before taking these measurements. No I have not shorted the inputs, but IN- is grounded. I will check the resistor values and post. I will also replace q4 and check. If one of the outputs is damaged, i will have a tough time, in which case, i might have to order a set of 4 again.

Thanks.
 
Changed PCB - Faulty channel works

Bob et al,

Thanks for all the help. My problem seem to have got resolved.

I tried changing the suspect Q4 ZTX450 on PDs PCB. I think I messed up and damaged the board. Had no option but to wire a spare PCB but with same output MOSFETs. Straight away works and DC offset is 40mV.

So both the channel work now. Playing music. Wonderful reproduction. Heat sinks get hot (5 secs touch) and the fan seems to be doing the job very nicely and quietly.

The PS PCB I had used in the earlier Aleph J build and discarded is now doing service in this new build and is demonstrating exactly the same problem that it did last time - slight hum into the system. The transformer also seems to buzz a little bit with this particular PS board. What could be the problem? It has 8 x 15000 uF caps in Pass CRC design. Would a faulty cap create this racket? I bought these Rubycon caps on ebay.

TIA.
 
Glad you're up and running. Lowish ACS current and high gain device current didn't make sense.

Just curious, did the jfets make the move to the new board or did they get replaced?

A very leaky cap could cause hum - look at the AC ripple on each rail. Check the current to the PSU with no load connected. Compare to another PSU board. The leak current could cause a load high enough to make the transformer hum as well.
 
Glad you're up and running. Lowish ACS current and high gain device current didn't make sense.

Just curious, did the jfets make the move to the new board or did they get replaced?

A very leaky cap could cause hum - look at the AC ripple on each rail. Check the current to the PSU with no load connected. Compare to another PSU board. The leak current could cause a load high enough to make the transformer hum as well.

Bob,

Thanks. New board has a new set of JFETs, but i believe the the earlier JFETs should also be good since they were new as well, unless blown during power up, which is unlikely.

Yes, the PSU board seems to be drawing higher than usual current, due to some leakage. I do not have a scope to check the ripple on the rails. Need to start desoldering each one and figure out.

Cheers.
 
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