Yamaha HS5 speaker crossover issues

[robertkirby]

Hello,
I obtained a HS5 monitor. The speaker is having some corrosion present on the amp PCB. On power up, the woofer pops once and sometimes oscillates back and forth for a few seconds, and pops on power off. The woofer does work, but it is significantly quieter than normal. The tweeter works perfectly fine. Multimeter analysis shows that the woofer is receiving some voltage even when no audio input is supplied, and when an input is supplied, the voltage is about half that measured in the tweeter. Also, both voltage regulators work fine. I think this means that the woofer amp is "leaking" voltage somewhere, which would explain the unwanted voltage reaching the woofer and its lower power.

If anyone with experience with amplifiers and audio could help me out or point me in the right direction in terms of which components to test that would be greatly appreciated.

 

[TLS Guy]

Hello,
I obtained a HS5 monitor. The speaker is having some corrosion present on the amp PCB. On power up, the woofer pops once and sometimes oscillates back and forth for a few seconds, and pops on power off. The woofer does work, but it is significantly quieter than normal. The tweeter works perfectly fine. Multimeter analysis shows that the woofer is receiving some voltage even when no audio input is supplied, and when an input is supplied, the voltage is about half that measured in the tweeter. Also, both voltage regulators work fine. I think this means that the woofer amp is "leaking" voltage somewhere, which would explain the unwanted voltage reaching the woofer and its lower power.

If anyone with experience with amplifiers and audio could help me out or point me in the right direction in terms of which components to test that would be greatly appreciated.

Unusually, there is a service manual issued.

It seems these units are intended to be serviceable by board change.

The power supply is reasonably conventional. So check voltages are are correct.

The power amps, are totally integrated circuits and are problematic. I suspect there is DC offset occurring indicating failure of an integrated circuit power amp.

There should be no DC voltage at all to the speakers. NONE.

I suspect these boards have surface mount components that are not intended to be serviceable.

So I suspect you will have to isolate the problem to the board or boards, then find replacement boards.

These look like pretty basic units intended to be of the "throw away" variety to add to the mountains of electronic waste.

To service a unit like this a FET high impedance meter is required, a signal generator and o-scope at the minimum.

 

[Eppie]

I couldn't get TLS Guy's link to work, so here is the manual on Elektrotanya. I think his analysis is correct. Those amps do not have discrete components. They are using an LM3886TF amplifier IC. If there is corrosion on the PCB, as you mentioned, the first thing to do is remove all of the corrosion and resolder all of the affected areas. Make sure that you have a +15V and -15V coming out of the regulator section. Then look closely at all of the components surrounding IC403 and IC404 to see if that is the cause the DC leak. Check the limiter circuit as well. If the surrounding components test ok then the problem may be with the power amp ICs.