DC on the speaker output

[madan1]

Here I have a jensen xa2150 car amp.
One of the channels works fine, but on the other there is ~20V DC. When a speaker is connected to that channel the protection kicks in and does not allow the amp to start. If there is no load, the amp turns on and the good channel can be used.
The strangest thing is that all resistors measure OK ( according the the schematics ) and all transistors also show good ohm values and no shortages.
Here are the schematics and what Vs are measured ( +/- few mV )

[budm]

Re: DC on the speaker output

If you look at the Voltage you show for the Emitter and Collector of Q105, BOTH LEGS have the same +21.4V readings. So are you sure that Q105 is good?
Also Q105 you show +22V at the BASE, it is PNP Transistor, and +21.4V at the Emitter which means that that transistor is not even biased on because the BASE is more positive than the Emitter which made it impossible to have the exact same Voltage at the Emitter and Collector of Q105

I did not look at ALL the Voltages yet but that Q105 Voltages stood out.
Q101, 104, 108 B-E also do not look right.
Also look at the Q107, Q109: You show +19.9V at the Emitter of Q107 and +19.4 at the BASE of Q109 which is impossible since the Emitter of Q107 is DIRECTLY connected to the BASE of Q109, so how can you have TWO different readings?
You should look at all the B-E Voltages of all the Transistors if they are properly BIAS or not.
You better check ALL your measurement again, to many errors.

[Longbow]

Re: DC on the speaker output

I agree. Q105 appears to have the same voltage on all 3 pins. Since this transistor is not forward biased, you do not expect any conduction. Q105 shorted, perhaps D101 shorted. Let us know.

[madan1]

Re: DC on the speaker output

Hmm...my bad - I got the readings like pin 1- XX, pin 2 - XX, pin 3 - XX, checked the pinouts in google images and.... got some of them wrong..

Here is the corrected readings image + table with cross reference with the working channel ( again in pin1, pin 2 and pin3 order ).
I checked q105 and the rest of the mentioned TR and again - no shortages.
I'm starting to think that the pcb is damaged and somewhere is injecting +V .. I'll make some HD images and upload em here.


EDIT - I think I found the problem - bended leg of Q108 .. I'll update shortly.

[budm]

Re: DC on the speaker output

I do not go by pin1, 2, or 3. I go by BASE, EMITTER, COLLECTOR since NOT ALL Transistor have same pin assignment, especially the Japanese Transistor.
When you see Voltages that do not make any sense then you need to recheck the works.

[madan1]

Re: DC on the speaker output

I do not go by pin1, 2, or 3. I go by BASE, EMITTER, COLLECTOR since NOT ALL Transistor have same pin assignment, especially the Japanese Transistor.
When you see Voltages that do not make any sense then you need to recheck the works.

You are totally right... but this is still kind of a new hobby to me and I still have to discover the good repair/diagnostic practices .

So removed the suspicious leg and discovered a bad pad. Removed the pad, soldered a bridge.. and still the same.

[dick_barton]

Re: DC on the speaker output

You should be able to compare the voltages on the components/transistors etc on the working side and do a direct comparison on the faulty side to see where the difference is since both left-hand and right-hand channels look identical.

[rievax_60]

Re: DC on the speaker output

All voltages will appear to be abnormal because of the negative feedback path from the output back to the input differential transistor pairs via R170. Voltages that might appear to be wrong could be due to stages reacting to an offset being caused by a fault elsewhere.
I deal with this sort of situation by tracing around the loop, looking for an area where the offset isn't being corrected in the expected direction.

[petehall347]

Re: DC on the speaker output

did you test the diodes ? start with d102

[R_J]

Re: DC on the speaker output

You could have an open Q106, or (like above post) open d102, you should have negative voltage at Q106 collector

[rievax_60]

Re: DC on the speaker output

The voltages from post #4 indicate that Q105 is passing current. It shouldn't be. Q101 should also be suspected.

[petehall347]

Re: DC on the speaker output

i think q106 is working ..could be wrong though .
most times i have this sort of thing is because of fuse resistors high/open . or bad solder or cracked trace . its never easy to find the fault when voltages go like this . will be a quick fix when found ...
also seen backwards transistors cause this by previous tech . i get all the best jobs .

[petehall347]

Re: DC on the speaker output

The voltages from post #4 indicate that Q105 is passing current. It shouldn't be. Q101 should also be suspected.

or the ceramic is shorted .

[petehall347]

Re: DC on the speaker output

The voltages from post #4 indicate that Q105 is passing current. It shouldn't be. Q101 should also be suspected.

or b to c shorted .. i missed that .. think you have it there .

[R_J]

Re: DC on the speaker output

From post #1 "The strangest thing is that all resistors measure OK ( according the the schematics ) and all transistors also show good ohm values and no shortages."
The voltages on that half of the amp should be negative with respect to ground, they are not so output Q111 is turned hard off (open) so there is no negative voltage being applied to to Q111 emitter to ballance the circuit so the result is only positive voltage through Q110 showing up at its emitter and the speaker
Another thing that does'nt look quite right is the voltage on R160, The one end has -15 volts but the other end (after the 2 100 ohms) is positive, I would think it should be closer to negative similar to the +15 on one side of R161 and then around +9 after the 2 100 ohm resistors, so maybe open or high value for R160

[petehall347]

Re: DC on the speaker output

the history of the unit always helps . like did it work then stop ? did someone else play with it first ? was it dropped or subject to power surge ? etc etc .
dying a natural death is easier to find ...

[rievax_60]

Re: DC on the speaker output

The 19.8v being applied to the Base of Q103 is causing the 19.2v on its Emitter due to Emitter Follower action. This high positive voltage is being applied to the Emitter of Q101, reverse biasing its B-E junction. It should be totally turned off.
From the voltage readings, the calculated Base and Emitter currents of Q101 are very close at about 9ma. This indicates that there is next to no Collector current.
If the voltage readings are accurate.

[rievax_60]

Re: DC on the speaker output

Btw, the current flow from Emitter to Base of Q101 would be due to zenering of its B-E junction because of the large applied reverse voltage.

[redwire]

Re: DC on the speaker output

The amp's input at Q102/Q101 base is +8.4VDC so the amp is doing what it should - stuck at the +ve rail. I would look at the input stage, that should be 0V.

[petehall347]

Re: DC on the speaker output

The amp's input at Q102/Q101 base is +8.4VDC so the amp is doing what it should - stuck at the +ve rail. I would look at the input stage, that should be 0V.

that voltage i suspect is coming through q101 . see it drop when going towards the pre amp