Ka-6004

[LeafLike]

Hi,

I have an old kenwood (KA-6004) .
When I turn the power ON, I have to wait at least 5min to finnaly hear some music.
During the 5min you can hear a relay switching, but no music!

I opened up the Amp and after a quick search I found the relay.
It's the Black one that sais SIRB
Pictured below

http://www.imagebam.com/image/3cf36177439453

I gues the relay is broke.
Any1 knows a component that could replace this relay?

Thanks

 

[ecluser]

Welcome to AK!

The SIRB is a bridge rectifier. You will find the schematic here:

http://www.fotoalbumet.com/u/00/24/40/files/Manuals/KA-6004.PNG

The problem may be in the relay driver circuit or a malfunction in the power amplifier section. The relay's job is to protect the speakers if a malfunction occurs in one of the amplifier section. Before blaming the relay, you must check if some abnormal DC voltage is present at the output of each power amplifier. Measure the DC voltage before the relay, at pins 16 and 20 on the power supply board.

 

[Bob91343]

I doubt if the relay is bad. I think the amplifier may have some blown parts or an excessive offset. Get the service manual and don't work on it unless you know what you are doing. (I see no relay in that picture.)

 

[24HACKER]

Contact Echowars - he can help you

 

[LeafLike]

There doesn't seem to be a hight Dc Voltage on Pin 16 and 20 . Both around 30 mV.

Next step ?

 

[ecluser]

With this low offset voltage, the relay should close.

Work from the relay, going upstream on the schematic.

Do you have the correct voltage on either sides of Re49 (by the way, I can't find the value for Re49, what is the nominal resistance of this resistor?) and Qe18

When the power is applied, Ce16 starts to charge and there is a high voltage at the base of Qe17, this transistor is driven into saturation. As a consequence, the base voltage on Qe18 is low and Qe18 doesn't conduct. Relay is off.

Ce16 takes a few seconds to charge through Re43. The time constant of this circuit (Ce16Re43) is ~ 4 seconds. After a few seconds the voltage across Re43 becomes too low to sustain conduction of Qe17. If Ce16 is bad (leakage) it is possible that the base voltage on Qe17 is always high enough to prevent Qe17 from turning off. Connect your voltmeter at the collector of Qe17 and see if this part of the circuit works as it should. As soon as the power is applied, the collector voltage on Qe17 should be very low, probably less than 0.5V. But after a few seconds it should go as high as 34V according to the schematic.