Studio Monitor Amp Circuit Issues - Klark Acoustic (Klark Teknik spkrs 1989)

[JamieOxford]

Looking for some help fixing a pair or Klark Teknik Jade 1 Mk2 speakers.

They're from around 1989-1994 and look like this https://cdn.gearsource.com/catalog/stockitem/normal/02c24531-aa68-4165-b79d-d34c41bb7f8d.jpg

The amps in the back have 2 boards - one for the PreDriver circuit and one for the power amp circuit.
I've had them repaired by Klark Teknik before and they replaced a custom IC which has been mentioned in another thread on here before - IC5 (a KTR008) - aparently a  double LM4741 + 8 resistors, although the replacement has some capacitors also.

The  issue I have is one of the tweeters isn't working correctly - it produces no (or very little) sound and there is a buzz coming from it. I’ve tested the drivers and power amp with the pre driver board from the other speaker and everything works fine - so it’s definitely a problem with the PreDriver circuit.

It could be the same problem as before - that IC5 has stopped working.

When testing I’ve found a difference in voltage between speaker ‘A’ and speaker ‘B’ on the pre driver boards.

The output at IC2B pin 7 is the same on both speakers, but is different at IC1D pin 14 - the damaged speaker (Speaker B) measures around half the voltage of Speaker A at this point.
The point at which C5, C6 and R17 join is then around a quarter of the voltage of Speaker A, and this discrepancy flows through all the way to test point 8B just before the power amp section, where speaker B is a quarter the voltage of speaker A.

I’ve tried swapping IC1 between speaker A and B and I still get the same result.
I’ve also tested capacitors 5 & 6 and measured R17 and R18 and they all measure the same as the working speaker, so I’m a little stuck for where to go next. I've e-mailed KT but expect they'll ask me to send them in for repair again..

My only thought was if IC5 was pulling more voltage to ground than it should be, it may be affecting things further up the circuit.

Please see other measurements from my speakers attached. Schematic to follow…

 

[JamieOxford]

Schematic Attached - the voltage mismatch is around IC1D, above the IC list at the top/centre

 

[moamps]

First check HF trimmer P4.
Which meter and signal frequency you are using for measuring?

 

[Bo Deadly]

Are you measuring AC or DC voltages?

If DC, differences of millivolts is not significant. LM4741 I assume is a quad 741 which is an old part that can have significant / different offset currents. So 40mV vs 20mV is totally meaningless.

The best way to debug this would be with an oscilloscope. But you can also put a sine wave into it and measure the AC amplitude with a meter. That might show you some meaningful differences.

 

[JamieOxford]

Thanks for the replies.

I should have said - I've been measuring the waveform voltage on an oscilloscope (Hantek DSO5102P) and was using an electronic drum track as reference. Also tested with noise and sine waves but the drum track is consistent in level and less annoying to listen to!

The mv measurements are roughly the peak to peak values of the waveform measured.

I don't think P4 is an issue. I tested the resistances of P4 and P5 and they were the same - they're also the same pot.

It's odd that between P4 and C5 the voltage is the same between the two speakers and after C5 it's vastly different.

Any suggestions where to go next?

 

[Bo Deadly]

It could be a bad solder joint. Look carefully around the pins of those parts around C5 and IC1D. Even if you don't see any cracks, you could just re-flow and add a touch of solder to each of the pins in that area. Are the ICs socketed? If "yes", you could try to just remove and re-seating them (careful not to stab yourself and bend the pins!). Try swapping IC1D with one of the other LM4741s.