HH MOSFET M900 DC offset 267mV when pot twisted to max?

[bonesbiggis]

i have set the bias on both Channels to 600mv
and set dc offset on Channel 1 to 0 mv

problem when offset stays at 267 mv when the trim pot is turned to maximum on Channel 2 ?

 

[rayma]

Could be the input transistors Q1/Q2 for channel 2 are degraded.

 

[bonesbiggis]

have checked some transistors and q8 seems out of reding q6 little out q10 out of reading

 

[rayma]

The input pair must be closely matched, or the offset will rise. They often degrade with time or hot swapping.
But also you could try replacing C3 first, in case it is leaky.

 

[BSST]

rayma may be spot on.

I presume you are adjusting VR2. Does output respond to VR2 adjustment, i.e. can you make output voltage worse, but not better? To confirm viability of the pot, would monitor the wiper as you adjust--- it should adjust between about +/- 2.7V

Assuming the pot is OK, set it back to max as before and please measure voltages at bases of Q6 and Q7 and reconfirm output at +267mV. Also measure voltage across R30. Ideally, it will be 0mV, but any voltage across it is evidence of leakage through C15 and might be the cause. If the input pair was perfect, 4.7mV of leakage showing on R30 would give 267mV offset. i.e. 4.7uA leakage.

 

[bonesbiggis]

should start changing all electrolytics?

 

[BSST]

No.

Please measure as suggested to help determine the source of the problem. The problem might be an input transistor. We are trying to find the defective part.

 

[bonesbiggis]

Okey i can take a measure

 

[bonesbiggis]

 

[bonesbiggis]

Also did put X2 310v ac caps on the on off button. Is this okey ?

 

[bonesbiggis]

rayma may be spot on.

I presume you are adjusting VR2. Does output respond to VR2 adjustment, i.e. can you make output voltage worse, but not better? To confirm viability of the pot, would monitor the wiper as you adjust--- it should adjust between about +/- 2.7V

Assuming the pot is OK, set it back to max as before and please measure voltages at bases of Q6 and Q7 and reconfirm output at +267mV. Also measure voltage across R30. Ideally, it will be 0mV, but any voltage across it is evidence of leakage through C15 and might be the cause. If the input pair was perfect, 4.7mV of leakage showing on R30 would give 267mV offset. i.e. 4.7uA leakage.

Pot is fine , Shall I measure between ground and base off Q6 Q7 ?

 

[BSST]

Yes, please measure as described in second paragraph of post #5.

Thanks!

 

[bonesbiggis]

okey some new info i have change Q6 Q7 Q8. replaced all shorted and leaky capacitors. still same problem ......????

 

[bonesbiggis]

Yes, please measure as described in second paragraph of post #5.

Thanks!

thanks for providing me.

Base of Q6= 1.024 V

Base of Q7 = 1.027V

DC offset Cannel 2 = 618mV

How do i measure across R30? one probe on cathode and the other probe on anode of the resistor ?

 

[BSST]

Yes, directly across the leads of R30 is preferable. Grabber clips or tacked in hookup wire is an approach. You want to avoid probing accidents that can wreak havoc.

Make certain the Mode switch is in Stereo position. That ensures against leakage current through C35.

 

[bonesbiggis]

R30 = 0.005V

hehe its in stereo tnx

 

[BSST]

5mV across R30 suggests C15 is leaking current. 5mV across 1k implies 5uA current which flows through R31. 5uA*56k = 280mV.

 

[bonesbiggis]

C15 is changed with a low esr cap and cant be leaky

 

[BSST]

ESR is not the same as leaky. If not through C15, where else can the 5uA of current be flowing?

Confirm that the "3G" terminal really is at ground voltage. I still suspect C15, but perhaps some other path of contamination. That 5uA current is flowing somewhere.

 

[bonesbiggis]

what if i change the trim pot to 25k instead of 22k ?