HEADROOM Headphone Amp repair help :)

[solderdude]

It certainly is less than good practice for sure and the person doing this mod could certainly have done a better (less hasty) job for sure.

 

[JazKazman]

New photo of the voltage regulator part number as requested (info sheet link below).

https://www.ti.com/lit/ds/symlink/l...ttps%3A%2F%2Fwww.ti.com%2Fproduct%2FLM336-2.5

Added underside pics of the tiny amplifier PCB board just for fun . Still working on my first measurement readings (once I figure out exactly how to do them ;-)

 

[JazKazman]

Measure the voltages (opposite signal ground which is the RCA shields for instance) on points A, B, C and D and post the results here... for starters.
View attachment 323226
Then we know what the power supply does and does not do and where to measure next.

Okay - so don't laugh but my first attempt at getting these readings was a joke. Not sure if these numbers even make sense as I wasn't even sure what multimeter setting to use and whether to use the red or black plug on the RCA shields but hey I did get some readings at least ;-) I guess I better google me some beginner lessons. Cheers!

 

[solderdude]

black lead on the RCA shield.
red lead on each measurement points.
Set the multimeter on 200 V DC setting (arrow points to the correct setting).

Check the operation of the voltmeter first on a battery (1.5V or 9V) to see if what the meter shows is a correct value.
Also check the 20V DC setting on those batteries.
When that test goes well (gives expected values)

I would expect to see around + 21 to +25V on A (measure on 200V DC setting)
I would expect to see around - 21 to -25V on B (measure on 200V DC setting)
I would expect to see around - 15 to -17V on C (measure on 20V DC setting)
I would expect to see around + 15 to +17V on D (measure on 20V DC setting)

The numbers you got make no sense.

The voltage regulators are LM317 (positive regulator) and LM337 (negative regulator)
Opamps were used to set the output voltages on the regulators instead of the usual 2 resistor design and most likely to check if both the power rail voltages are present to ensure a correct on/off operation and PS fault detection (as there is no output relay).

Given the fact that this is already a difficult thing to do I would suggest to not work on the amplifier PCB's.
This requires expert knowledge when working without schematics as there is also a crossfeed circuit on those amplifier PCBs.

But before anything see if the DC voltages check out.

 

[JazKazman]

Thank you very much. That helps a lot. I did notice the DMM 9V battery was flashing low so perhaps that was why it was acting strange. Armed now with your info and a new battery should do the trick. Will report back Sunday night. Cheers

 

[JazKazman]

black lead on the RCA shield.
red lead on each measurement points.
Set the multimeter on 200 V DC setting (arrow points to the correct setting).

Check the operation of the voltmeter first on a battery (1.5V or 9V) to see if what the meter shows is a correct value.
Also check the 20V DC setting on those batteries.
When that test goes well (gives expected values)

I would expect to see around + 21 to +25V on A (measure on 200V DC setting)
I would expect to see around - 21 to -25V on B (measure on 200V DC setting)
I would expect to see around - 15 to -17V on C (measure on 20V DC setting)
I would expect to see around + 15 to +17V on D (measure on 20V DC setting)

The numbers you got make no sense.

The voltage regulators are LM317 (positive regulator) and LM337 (negative regulator)
Opamps were used to set the output voltages on the regulators instead of the usual 2 resistor design and most likely to check if both the power rail voltages are present to ensure a correct on/off operation and PS fault detection (as there is no output relay).

Given the fact that this is already a difficult thing to do I would suggest to not work on the amplifier PCB's.
This requires expert knowledge when working without schematics as there is also a crossfeed circuit on those amplifier PCBs.

But before anything see if the DC voltages check out.

Okay so I've now tested a battery as suggested and it reads as 9 volt (so we know the DMM is working Unfortunately the PCB readings don't look as promising as this is what they are : A=00.1 (200V)
B=00.3 (200V)
C=00.0 (20V)
D=00.0 (20V)

I tried two different DMMs with the same result.

 

[JazKazman]

I'm guessing something is fried somewhere...

 

[solderdude]

Weird... you mentioned the LED did light up.
That LED is fed by the regulated power supply.
Also there should be at least a voltage across the rectifier caps (test point A and B).

If the LED does not light up check the resistance of the wires of the transformer windings (NOT connected to mains !!!!)
Measure each primary and secondary winding.
If you don't know how to do that stop working on it.
Also check the mains fuse in the power inlet if the LED does not light up.

 

[JazKazman]

Okay I'll give it all a go and will google the instructions first. If those measurements require power though I may just call it quits as you suggest as I wouldn't want to risk injury ;-) BTW the first pic in this thread shows the LED lit up just before I removed the PCB. Maybe I'll try it again with the board back in the case just to make sure.

 

[JazKazman]

Also as noted, when it powered up a few days back I could hear an electrical contact/static sound when initially inserting the headphones but no audio. Thanks again for your patience btw. Cheers

 

[bothu]

New photo of the voltage regulator part number as requested (info sheet link below).

https://www.ti.com/lit/ds/symlink/lm336-2.5.pdf?ts=1698953511441&ref_url=https%3A%2F%2Fwww.ti.com%2Fproduct%2FLM336-2.5

Added underside pics of the tiny amplifier PCB board just for fun . Still working on my first measurement readings (once I figure out exactly how to do them ;-)

Strange locking output transistor.
It looks like some dark protruding crater on top of it.

 

[JazKazman]

I had a closer look and yes that dark spot is there but it looks more like a pyramid sitting on top as opposed to a crater (?)

 

[JazKazman]

Hello all.

I have now retried powering up the board after a light cleaning of everything (with one of my softer watch repair brushes and ISP 99% and it sure looks prettier ;-) but...does it power up or not? Well yes (see the red light on in photo) and having the headphones on while doing it produced that nice audiophile 'thump' sound as it did. I was excited thinking it was working again but alas...still no audio coming through. So knowing now that all is well as far as that goes what would you recommend my next step be?

Thanks again for all advise offered.

Cheers

 

[solderdude]

What are the measured DC voltages ?

Such a 'bump' on the IC usually means it is fried, which probably means at least the IC and possibly the output transistors might need to be replaced.
This is not going to be easy as you would need to remove the module from the main board without damaging either of them... if it turns out this is the problem.
The DC voltages can give some clue. Do the LM317 and LM337 regulator get hot ?

 

[bothu]

I had a closer look and yes that dark spot is there but it looks more like a pyramid sitting on top as opposed to a crater (?)

I guess this is a sign of a broken output transistor, the silicon chips inside the transistor have "evaporated" through the encapsulation.
This can generate more broken parts and maybe the power supply goes into a safe mod and shuts down.

 

[JayGilb]

Please measure the DC voltages and you might want to purchase a signal injector pen. They can be purchased for less than $40 and are very helpful in repairing audio devices.