Kenwood KA-400

[anatech]

Hi gecko,
What voltage drop do you have across R31 and R32?

-Chris

 

[gecko]

Hi anatech! across R31 I have 0.396 V and across R32 0.39 V

 

[anatech]

Hi gecko,
Okay, that's about 8 mA, proving those stages are active (working). Therefore I am suspicious that VR5 (bias pot) is open. Power down and discharge the caps. Measure on ohms across R33(68 ohms) and turn the bias control VR5. The resistance reading should change to almost a short at one end. Either that, or there will be no change and then an abrupt change. There may be a feeling of roughness in the pot at that point.

-Chris

 

[gecko]

well, in one channel (R34) I have from 0 to around 100 ohms turning the bias control, and in the other channel (R33) I have from 0 to 55 ohms

 

[anatech]

Hi gecko,
R34 is open. It's either 100ohms or 68 ohms in parallel with your pot. It should be the same as the other channel, the schematic shows them as different values.

-Chris

 

[gecko]

I just checked R34 and 33 and both are okay, both are 100 ohms resistors

 

[anatech]

Hi gecko,
Did you lift one leg of the resistor? The pot and the resistor are in parallel, so one is open.

-Chris

 

[gecko]

yes, I lift the leg of the resistor

 

[anatech]

Did you measure both the pot and resistor? One of them must be open.

-Chris

 

[gecko]

okay, forget about it, I found what was the problem and it was all my fault, now I'm feeling like an idiot I have changed Q13 and Q14 for a 2SC1627 but I didn't realized that it has the legs inverted, 1-E 2-C 3-B, I just got the datasheet and noticed that
Well, now it's working perfectly, thanks for all your help, and sorry for the waste of time, I learned a lot of thinks though, like think before touch anything 😀

 

[anatech]

Hi gecko,
No problem. You learned something, so it's all good. If it makes you feel any better, I've done the same thing. That's one reason I take digital pictures beforeI take things apart. Sometimes the silkscreening on the PCB's are wrong too. I don't trust anything these days. Did I say how much I love digital camera's?

-Chris

 

[gecko]

Yeah, that's a good idea to take photos before doing anything 🙂

I still have a little problem, in one channel I have alround 256 mV in the output, and the output transistors gets a little hot in a while, in the other channel I have 50 mV, and it's working well, I'm not sure what is the correct current for this amp

 

[anatech]

Hi gecko,
Are you measuring across the emitter resistor(s) or DC offset from output to ground?

I will guess across the emitters. Adjust the bias control for 50mV across both resistors (25 mV across one).

-Chris

 

[gecko]

Yes, it's across the emitter resistors

 

[gecko]

and it's not stable, it changes slowly from 170 mv to almost 200 mv without move the pot

 

[anatech]

Hi gecko,
That might be because it's going into thermal runaway since the bias is so high.

Can you turn it down?

-Chris

 

[gecko]

Turn what down?

 

[anatech]

The bias current. What happens when you turn the 100R pot?

-Chris

 

[gecko]

oh, I got it 😀 that bias pot was a little dirty I think

 

[anatech]

Use a little contact cleaner on teh metal contacts if you can get to them. If possible, try to get a new control in case that one fails and blows the channel up.

-Chris