SOLVED - Genelec 1029A Repair

[chilidawg]

With the Focal out of the way, I thought about checking my own monitors that I put in the storage for the past 3 years.

Turned them on, and ouch, both are having a major issue with the low frequency amplifier.
MONITOR A: loud buzzing noise that's independent from the volume control.
MONITOR B: no output.

DC offset at the output.
MONITOR A: LF 350mV to 580mV, HF from 240mV to 290mV
MONITOR B: LF 0mV, HF 130mV to 155mV

 

[chilidawg]

No luck in getting a schematic for an obsolete product from Genelec, but I went ahead in attempting to repair them anyway.

MONITOR B's problem turned out to be just a loose quick connector to the woofer's positive tab. Apparently, at some point before I put the monitors in storage, a house gecko entered the enclosure from the open ports and did quite a mess in there.

And because MONITOR B is fine, I took a lot of measurements and documented it.

With MONITOR A, I found a faulty 1K chip resistor at the non-inverting input pin (VIN+, pin #10) of the LM3886 that's driving the woofer.




Meter read the resistor in circuit at 1.6M ohm and outside the circuit at 293K ohm.



I replaced it.




EDIT: Added a video that I recorded after the repair.

 

[chilidawg]

With the loud buzzing noise gone, I can now hear a crackling-scratching noise that randomly come and go. Suspected it was the volume control pot then confirmed it is so, because when I hooked up the board to the pot of the other 1029A, it's very quiet.

Not sure how to replace the pot yet. Should I pry out the knob first? Is the pot secured with a nut, or is it like the power switch, held in the hole by a piece of memory foam? If someone here has done it before, please do tell me

Anyway, at this point, I'm almost satisfied.

 

[Khron]

Not sure how to replace the pot yet. Should I pry out the knob first? Is the pot secured with a nut, or is it like the power switch, held in the hole by a piece of memory foam? If someone here has done it before, please do tell me

It most likely IS nutted to the panel, otherwise its pins would need to withstand all the forces and movement that come from just operating the pot. And most often, pots don't come with knobs straight from the factory, precisely for such mechanical (fastening) reasons.

 

[chilidawg]

It most likely IS nutted to the panel, otherwise its pins would need to withstand all the forces and movement that come from just operating the pot. And most often, pots don't come with knobs straight from the factory, precisely for such mechanical (fastening) reasons.

Looks like a combination of being nutted to the panel as well as held in place in the tube hole of the enclosure by a large piece of round thick memory foam, just like the power switch. When I was going to rebuild the broken power switch plastic shaft so that it can toggle up and down again, I had to push the foam from behind the tube hole and it was a bit of a challenge in getting it out completely (as well as pushing it back into the hole)

Anyway, doesn't seem like I need to replace the pot. After I have turned it a few times while testing the speaker a few hours ago, it has returned to normal. It's now quiet, no more crackling-scratching noise coming and going randomly.

I have been repeatedly turning them on, listening to some music for about 10-15 minutes and then switching them off. So far so good.

EDIT: Looks like the pot is not nutted to the panel, but really just held in place by the foam. In the attached video you can see the knob wiggling around easily, but I just can't seem to be able to pull it out from the shaft.

 

[ccaudle]

I found a faulty 1K chip resistor at the non-inverting input pin

That is a weird failure. Resistors on an input pin should not be dissipating any appreciable power, I'm not sure what the failure mode for that would even be. Perhaps it was close enough to the power amp chip that it got hot, but those aren't SMD, so most of the heat should be going into the heatsink and not the PCB.

Anyone else have any experience on how a chip resistor could fail like that? Not something I would usually even think to check until well into eliminating other common failures.

 

[chilidawg]

That is a weird failure. Resistors on an input pin should not be dissipating any appreciable power, I'm not sure what the failure mode for that would even be. Perhaps it was close enough to the power amp chip that it got hot, but those aren't SMD, so most of the heat should be going into the heatsink and not the PCB.

Anyone else have any experience on how a chip resistor could fail like that? Not something I would usually even think to check until well into eliminating other common failures.

That's what I thought first too when the meter read out 1.6M ohm. What a weird failure. I would have missed it if I didn't check every component.

I don't think it got destroyed by the heat. During the entire 15 years I used the monitors in the studio, the volume control pot never moved beyond 8 o'clock, so power wattage would have been very minimal, and with the air con blasting right behind the monitors, they never got hot.

Genelec also designed the die cast aluminum enclosure to act as a heatsink for the power amp chips. A piece of thick stainless wire was made into a spring, and it pushes the chips against the enclosure. The "spring" is held in place by two screws. To remove the board for inspection and service, you only have to remove one screw to release the spring. It's literally quite a genius design, imho.

My only guess the failure is probably because a house gecko peed on the resistor, because for both monitors, inside I found dried gecko poops and a couple of rubber bands that have turned extra crispy. I always find rubber bands under my shelves and my fridge. I have a stack of rubber bands hanging on my kitchen wall, and the geckos often stole one or two repeatedly (mistaken for food, I believe)