Amp troubleshooting: offset

[gruni]

Hi everybody!

I have an amp that I have to fix. The problem with the amp is 1,2V offset at the input and at the output.

The offset at the input appears at the junction between R322 and R310. On the other sides of both the resistor and the cap, its 0V, so the offset isnt produced by the offset-trimpot or the preamp. the strange thing is that the same offset appears on the output, at the same level, so its not amplified (gain is set to 23.5 by R343/R344)

what should i look at first?

shematic:
http://lorbas321.square7.ch/dlf/E-800.pdf

 

[bob91343]

I see no junction between those two resistors. R322 connects to a pot and R321.

 

[godfrey]

The capacitor (C318) in series with R344 means the gain is unity at DC, so it makes sense that the output offset is the same as the input offset.

Maybe the input offset is caused by a bit of leakage current from somewhere. Any sign of spilled coffee/beer/whatever on the PCB?

 

[gruni]

sorry, i meant C310, not R310!

there is nothing spilled on the pcb... but as the offset seems to be produced by the input, ill keep searching there.

 

[gruni]

i measured some voltages. hope this helps!

 

[godfrey]

From the voltages around the emitters of T301 and T302, it looks like they're not conducting any current. That suggests T303 or it's emitter resistor (R327) may be dead.

To confirm there's no current through T301 and T302, you could check the voltages across their collector resistors (R323 and R324).

btw: There's a trimpot to the right of R322. Does fiddling with that change the offset? It should allow about +- half a volt adjustment

 

[gruni]

fiddling with VR301 just changes the voltage right of R322, but this already is 0V.

ill look at T301/302 carefully

EDIT: T303 is ok in diode test, R327 is 15k. anyway, ill measure the voltages across R323/R324

 

[gruni]

voltages across the resistors is about 2V, so theres definitely current. the failure must be somewhere else

 

[AndrewT]

T304 & 305 have ~ 600mVbe
T301 & 302 have 0Vbe
No other T Vbe have been posted.

R322 has 1.2V across it.
Is R322 passing ~0.04mA or is it damaged?

What currents are passing R323, 324, 328, 329, 327, 332, 325, 326, 330, 331, 333, 336?

Show them on the post5 extract

 

[gruni]

i just unsoldered all those resistors on one leg and they all measure as they should. so i think they arent damaged.

im clueless. everything measures just as it should... perhaps i should ground the input and watch what happens...

 

[AndrewT]

T301 & 302 have 0Vbe
No other T Vbe have been posted.

R322 has 1.2V across it.

everything measures just as it should...

No.
What you have measured so far shows that roughly half your readings are wrong.

 

[gruni]

No.
What you have measured so far shows that roughly half your readings are wrong.

but when measuring the parts unsoldered, they measure as they should, dont they?

 

[AndrewT]

now let's see the currents added to your schematic.