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Old 10th June 2014, 02:57 AM   #1
Bengali is offline Bengali  United States
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Default Mini Alpeh high dc offset

Hi,

Its been awhile, really rusty, decided to build another mini Aleph.

I checked the rail voltages about +/- 14VDC with a 12V 400VA antek.
I have CL60's on each primary.

No power shorts, nothing seems shorted to chassis or Gnd. Im using Brian's black boards, latest version. -Sig tied to ground.

When I power on I measure -14V at the speaker outputs for both channels, no load connected.

Im using star ground, Neg. Speaker terminals are going back to star ground.
The pos. Speaker terminal wired to pcb output.

The gnd and cgnd on the pcb are going to star ground.

I did install a 500ohm trimmer on R6 and preadjust to 250ohms.

DC offset should never be that high??? Looks like its just
Seeing the -14VDC power rail at the speaker output.

I will pull the pcb off and inspect again. I did not measure any shorts
At the irp240's.

Thanks for your help
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Old 10th June 2014, 07:02 AM   #2
Zen Mod is offline Zen Mod  Serbia
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post exact schm an I'll tell what to measure
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Old 10th June 2014, 07:23 AM   #3
Bengali is offline Bengali  United States
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Quote:
Originally Posted by Zen Mod View Post
post exact schm an I'll tell what to measure
Schematic attached. R6 has a 500ohm trimmer.
I went through older threads, Q3 might be bad?

All brand new parts and I did match the Q1&Q2.

Thanks ZM
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Old 10th June 2014, 09:04 AM   #4
Zen Mod is offline Zen Mod  Serbia
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voltages:
across R8
across R18
across Z5
across R14
across R27
across R28

both inputs grounded while testing , no load at output
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Old 11th June 2014, 01:29 AM   #5
Bengali is offline Bengali  United States
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R8 5.10 5.12
R18 9.08 9.08
Z5 8.85 8.86
R14 12.75 12.43
R27 0.34 0.35
R28 0.33 0.34

Q6 cold
Q7 warm

Voltage rails +\- 14.9V
Speaker outputs -14.4V

Inputs grounded No Load

Resistor values installed
R14 47.5K
R12 750
R17 100
R27 0.33
R28 0.33
R23 0.47
R24 0.47
R25,R26 DNI

Last edited by Bengali; 11th June 2014 at 01:33 AM. Reason: Update
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Old 11th June 2014, 05:25 AM   #6
Zen Mod is offline Zen Mod  Serbia
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double check continuity of everything and possible shorts

crucial voltages are ok , but there is something wrong

I believe these pcbs are tricky - easy to make mistake when using just one pair of outputs

if nothing else , give me pics of both bottom and top of pcb
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Old 11th June 2014, 05:38 AM   #7
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Quote:
Originally Posted by Bengali View Post
R8 5.10 5.12
R18 9.08 9.08 ---> to high that why at output -14,4V --> check circuit ,change new Q1&Q2.
Z5 8.85 8.86
R14 12.75 12.43
R27 0.34 0.35
R28 0.33 0.34

Q6 cold
Q7 warm

Voltage rails +\- 14.9V
Speaker outputs -14.4V

Inputs grounded No Load

Resistor values installed
R14 47.5K
R12 750
R17 100
R27 0.33
R28 0.33
R23 0.47
R24 0.47
R25,R26 DNI
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Old 11th June 2014, 10:25 AM   #8
Zen Mod is offline Zen Mod  Serbia
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Quote:
Originally Posted by dungdochi1978 View Post
to high that why at output -14,4V --> check circuit ,change new Q1&Q2.
not necessarily to change them

from some reason there is output offset , greater then inner LTP mosfet can rectify , that's why same one is closed - entire LTP current is going through outer mosfet

however - by memory , those pcbs are made as sort of universal , and one need to triple check how to connect things for different iterations (one or more output pairs -then placed on daughter boards)

not having them and not having any pcb file , I can't check proper position of parts

in every case - lower output mosfet is wide open , resulting in minimal voltage sag across it , and there is neg voltage at output .......

so - first complete check of parts installed and possible shorts/bad solder points and - if that is OK , desoldering input mosfets and checking them in matching jig
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Last edited by Zen Mod; 11th June 2014 at 10:29 AM.
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Old 11th June 2014, 02:52 PM   #9
Bengali is offline Bengali  United States
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Thanks

Before I took measurements I reflowed all
All pcb pads

I will pull Q1-Q3. I matched them prior
To installation. Q6-Q7 were matched from
Tech diy.

ZTX450 on Q6 No Good?
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Old 11th June 2014, 03:15 PM   #10
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Quote:
Originally Posted by Bengali View Post
....
ZTX450 on Q6 No Good?
in this case - ZTX is certainly not the culprit

I think it's time for some pics
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