Help repair Yahmaha HS80M Low Cut

[injector]

Hi,

I found this forum through a search engine and noticed that there are some experts here
I'm a mechanical engineer but I do not know much about electronics.

I had a Yamaha HS80M Studio Monitor, in which the subwoofer has made no sound.
The high tones worked perfectly.

I found the service manual on the internet as well as a DIY manual for measurement and repair.
How I repaired my defective Yamaha HS80M studio monitor - Galleri
YAMAHA HS80M HS50M Service Manual download, schematics, eeprom, repair info for electronics experts

I then dismantled and measured the monitor:
U1 and U2 have spent +15 and -15V, but when measuring from the -15V I accidentally caused a short circuit and immediately the woofer is now working again : )
All switch positions, however, no longer work in the low cut, so my problem was probably due to a broken lowcut, which I have shot through the short circuit.
Now the lowcut is broken and leaves sound through it, that´s what I thought from logical thinking why the sub is working back again.

Therefore, I would like to buy a subwoofer to my monitors but I would need the lowcut again and now try to repair this.
According to the wiring diagram, I hiked the -15Volt and come to the conclusion that it should be the IC105 or similar in the area.

Can someone help me where to measure?
How to identify the broken part?

Thank you in advance for your help

 

[nigelwright7557]

1/ Check all DC voltages for sensible values.
2/ Put in a signal and follow it through and see where it disappears.

 

[sgrossklass]

The low cut is almost entirely passive. If that does not work but woofer level seems normal otherwise, check whether the ground connection at pin 7 of S4 is present.

That the original problem fixed itself when a large transient on -15 V occurred may indicate a bad solder joint in the signal path or at one of the opamp power pins.

So really, all of this points towards soldering issues, or maybe a cracked board. Look for things like that.

It can't hurt to measure IC105 output DC offset at pins 1, 7. There should be a small positive offset of about 40 mV. Offset between the input pins (2/3, 5/6) should be single-digit mV or lower.

 

[injector]

The low cut is almost entirely passive. If that does not work but woofer level seems normal otherwise, check whether the ground connection at pin 7 of S4 is present.

That the original problem fixed itself when a large transient on -15 V occurred may indicate a bad solder joint in the signal path or at one of the opamp power pins.

So really, all of this points towards soldering issues, or maybe a cracked board. Look for things like that.

It can't hurt to measure IC105 output DC offset at pins 1, 7. There should be a small positive offset of about 40 mV. Offset between the input pins (2/3, 5/6) should be single-digit mV or lower.

Thank you
Danke werde ich mal testen morgen