original 1176 rev D troubleshooting question

[skeptic12]

Hi

I am looking at a Rev D for a friend.  The issue is that the unit clips before any gain reduction happens.  Basically I have to turn the input to 3 o'clock before GR kicks in and the signal is clipped at that point (asymmetrically)

I have replaceded the electrolytics and replaced Q2 Q3 and Q4 hoping it was an issue in the input
I have adjusted Qbias successfully, as well as Null offset and GR meters as per original manual and MNATS and HB

the fact that I am able to set Qbias makes me think that GR is happening but it is not kicking in early enough, but I am not sure where to look next

I am trying to wrap my head around the circuit and would be thrilled if anyone has any pointers

Thanks in advance

 

[skeptic12]

Bump anyone?

 

[Ilya]

Does it clip if you remove the FET in the signal chain?
Do you have a scope?

 

[skeptic12]

hi
thanks for writing!

I will try removing the FET tomorrow and see what happens.

yes I have a scope- the signal starts to clip asymmetrically when I turn the input to around 1 o'clock.  when I get to approximately 4 o'clock the gain reduction kicks in

 

[Ilya]

After you try removing the FET (which I think won't help, but you need to do it to exclude FET from faulty parts) you'll need to find where the signal goes sour. With a scope check the signal at the output pot and see if the signal is fine there. If you see clipping, it means that the input section is faulty. If not, it's the output section that gives you problems. After you isolated the section, check the signal at transistors stages inside the faulty section and see where the signal corrupts.
This is a very coarse roadmap, but hopefully it will lead you in the right direction.

 

[skeptic12]

The FET didn't change anything when removed.  I am able to calibrate the Qbias so I assume the limiting is happening (?!?)

My voltages around Q2, Q3 and Q14 are on the whole rather low compared to voltages on the MNATS and hairball schematics, so I checked back to 30V supply and was only getting 28V.  I replaced the zener and the diodes but still stuck at 28V.  I replaced the 2200 ufd filter caps already.
-10V is spot on

I am seeing 56 VAC on the secondary of the mains transformer, that is rectifying to 33VDC which drops to 28VDC after the 75ohm resistor before the zener.

Any thoughts on how to get it back up to 30V?

 

[David Kulka]

Something may be drawing excess current and causing your 30v to be slightly low. Or maybe you happened to get two out of spec zeners.

In any case the slightly low voltage would not cause the problem you're describing, though whatever's causing the distortion problem may also be pulling the power supply down. I'd start by looking for a fault in the output amp stage.

 

[Roangibs]

Hi

I am looking at a Rev D for a friend.  The issue is that the unit clips before any gain reduction happens.  Basically I have to turn the input to 3 o'clock before GR kicks in and the signal is clipped at that point (asymmetrically)

I have replaceded the electrolytics and replaced Q2 Q3 and Q4 hoping it was an issue in the input
I have adjusted Qbias successfully, as well as Null offset and GR meters as per original manual and MNATS and HB

the fact that I am able to set Qbias makes me think that GR is happening but it is not kicking in early enough, but I am not sure where to look next

I am trying to wrap my head around the circuit and would be thrilled if anyone has any pointers

Thanks in advance

I'm having this problem, but in reverse for my Rev D. GR is happening and calibrating correctly, but I have to crank the Input stage almost to 80% before I get any Gain Reduction.

Did you have any luck solving this issue?