Aleph3 clone offset problem

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Hello friends
I am a rookie, none electronic background. I am building Aleph3 clone based on an ebay PCB, All fet devices are matched. I changed R106 to 500R VR and R113 to 100K VR for adjusting DC offset and bias. A power supplies is about +/-23Vdc.
I face on Output offset problem. Right channel seems okay. It start at 70mv then I can easily adjust to 7 mV. But on left Channel it start at 3.7V. I try to adjust but not much effect. Moreover a LED will dim when turn too much.
I have checked many time on installed parts and quite sure they are correct as schematic. I measured voltage on both side to compare please see in picture. Please give an advice what I should do next. Thank you
 

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Official Court Jester
Joined 2003
Paid Member
leave R106 fixed - 220R

replace R108 with series string of 220R fixed value and 500R trimpot

it happened due to difference in mosfet Vgs

as long as values are in 10% bracket , everything is fine

offset of +3V7 means that you need to open lower half mosfets just tiny bit more
 
I tried by parallel 390R with 5K POT, set at max first. At first offset be decrease when decreasing POT until about -2.6V it would be start to swing to 17V and LED dim down.

At that condition I tried to turn R106 (now 500R POT) to increase offset more minus like -3.0V. It would be back to stable again.

Then I back to decreasing 5K POT more but again when offset be around -2.6V it would start to swing again. Then I repeated step by tuning R106 many time but seem no improvement , I stopped.

It will good idea to ask you before burning it. Now R108 is about 77R and voltage at Q103 drain leg is about 7.4V,the best offset I can reached -2.3V.
 
What is the thing over the '610s? (see subtle arrow)?

Is it shorting the tabs together?

Otherwise, I'd go through the ebay board trace by trace to check for errors, and maybe make certain ALL the fets are matched. I've built countless Alephs, and never had an offset issue if ALL the fets were matched VGS. edit - never needed a pot to adjust
 

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Hi Zen mod , The pot was set at 220 at starting point then trying to decreases R108 lower than 390R as you suggested.

mpmarino :they have insulator pad, do not touch each other. The good channel has be good since first SW on , without any VR to adjust. I just decided to add when fixing the bad one.


I believe heat sink is big enough.
 

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:eek:
These input fet is second set actually. At First set, I got about 1.2V, cannot remember plus or minus, then reload second magazine and installed two VRs and get -3.7V. …do not know what to do next ......

I may take some parts out for check and re solder all joints again. Do you think where be a possibly bug area? I really hate to take part out /in through hole pcb.
 
Official Court Jester
Joined 2003
Paid Member
when all parts are by prescription , with just two trimpots you must be able to set everything

positions are R108 , R113 , while everything else as shown on schematic , fixed values parts

leave it for few days , unburden your head , then be back with fresh energy

for starters just compare visually all parts between channels
 
I changed the parts as you told on both channels. The situation are still same.
When turn the new POT 500R at R108 the DC offset can not be lower than -2.5V then fluctuate , LED dim down.

I reckon it might be have some malfunction or dead part there.

All compared voltage seem be okay except at the Q103 drain leg . It is about 6.7V but the work channel is only 3.5V.

What I should do next? thank you
 

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Official Court Jester
Joined 2003
Paid Member
-if input mosfets are OK,
-if there is no some wrong resistor value,

you must be able to set DC outout offset varying R108 ; in this case you need to decrease it's value (lower bank of mosfets opened too much)

with inputs shorted and no load ........ use multiturn trimpot for R108 , if you don't have steady hand , needed for single turn trimpots
 
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