DC on the speaker output

[rievax_60]

Re: DC on the speaker output

I have explained the reason for that.
Look at the voltages on R153. Current is flowing out of the amplifier's input stage.

[redwire]

Re: DC on the speaker output

OK I see things are latched up, Q101 E-B backfeeding through R154, not the problem.
It's a good puzzle.

But if Q101 C-E is then off, I'd expect no current flow through R157 but there is.
I also see Q106 on trying to turn on Q109 but it's not happening, Q111 staying off.

Sometimes I find a damaged or leaky transistor- not completely open/shorted causes this kind of thing.

[rievax_60]

Re: DC on the speaker output

Q101 could be leaky although the voltage readings indicate that Base current matches its Emitter current.
There is 0.4v across R166. This indicates that Q105 is passing 8.5ma. This would not be unusual if the amplifier was working properly. But under these circumstances with the high positive output offset voltage, the loop should be trying to correct the offset by turning off Q105.
I think the cause is either Q105 or something close. Look at its C-E voltage. If D101 was leaky, which I have seen happen when small diode leads corrode, there would be no current through Q105. Assuming that Q101 is really off.

[redwire]

Re: DC on the speaker output

Q105 and adjacent circuit are covered in something, flux?
Q105 could have C-B short, but Q106 has a lot of current flowing compared to R166.
Q101 C-B or C-E leakage, Q109 on vacation. That's as far as I get.

OP will have to check some transistors, voltage readings show trouble but not exactly where it is.

Bizarre the PCB is double-sided but not plated-through holes!
Select components have the top-layer soldered to their leads (to make a via). Am I right?
Have to be careful not to miss a component's soldering including both sides of the pcb.

[petehall347]

Re: DC on the speaker output

did i mention poor grounding can cause weird shit like this ?
i still think its just something with a bad connection .
need to start seeing those negative voltages come back ,

[rievax_60]

Re: DC on the speaker output

Q105 C-B voltage looks very suspicious, 0v.
Powering up the amplifier with Q105 Base pin floated would tell us a lot

[madan1]

Re: DC on the speaker output

Thank you guys for the replies!
I have not check them thoroughly, but will do it tomorrow.
Today I had some time to play around with the unit -
q101, q103, q102, q104, q106 and q107 are replaced with new ones
Also, I resoldered all components ( removing the old solder and applying new ).
Then I removed q108 and q109 and did some more tests ( see images )

test 1 - q108 and q109 removed
test 2 - q108, q109 and q105 removed
test 3 - + c105 removed
test 4 - + r166 removed

and still have ~+15V where it should be ~0 ( compared with the working channel )

By the way, in the spreadsheet in my previous post I have included the V values from the working channel ( https://www.badcaps.net/forum/attach...5&d=1498835892 )

[petehall347]

Re: DC on the speaker output

put the transistors back in .
then disconnect q105 base .if dc offset drops to zero the drive circuit is fine and the problem is in the differentials ...
this is always handy .. try page 31 ..i know its not exact same as your amp but pretty similar ....

[rievax_60]

Re: DC on the speaker output

I still suspect Q105 being the main problem.
The readings in test 2 indicate VA(voltage amplifier) current flowing through Q107, there shouldn't be. D101 is suspect leaky also.
VA current can be found from voltage readings across the 47 ohm resistors. They should be the same in a functional amplifier.
Never disable the circuitry involving Q107.
Watch that yellow glue. If it turns brown, it becomes conductive and corrosive.

[rievax_60]

Re: DC on the speaker output

Or PCB contamination causing leakage.

[petehall347]

Re: DC on the speaker output

just need to see what happens with q105 base lifted . this will tell a story and what to look at next .

[rievax_60]

Re: DC on the speaker output

just need to see what happens with q105 base lifted . this will tell a story and what to look at next .

I would like to see that also. Test 2 indicates an extra problem.

[rievax_60]

Re: DC on the speaker output

In test 2 there is 8ma flowing through R157. This current is not flowing through Q101 as it can only pass a maximum 1ma of tail current.
There must be a short between C-B pads of Q105 on the PCB.