KR-6400 mystery adjustment

[P.O.]

hello there fellows Kenwoodists...

I'm starting to do some service on my KR-6400, and I'm wondering what this trimpot on the preamp circuit is doing... According to the schematic, it seems to cater only to the right channel of the FM input. Some sort of multiplex carrier-nulling adjustment? How do I set it? It's the only trimpot on the preamp board.

Maybe someone with the real service manual would know? me I only have the schematic that's currently available on the 'net.

thanks

 

[chazix]

Maybe someone with the real service manual would know?

Until/unless that someone comes along, I will venture a high-confidence guess that it is an FM stereo separation trimmer, to be adjusted for best compromise of minimum L channel with a R-channel-only signal, and vice versa.

(If you don't have a stereo generator that can output one-channel-only signals, I would just leave the adjustment alone.)

HTH,

chazix

 

[P.O.]

Thanks for your educated guess!!
I was thinking along the same line. I could however not resist the urge to frob it some, but I returned it where it was set.

 

[Watthour]

Are you asking about VR1 on the preamp board?

 

[P.O.]

Yes: it's the only trimpot on the preamp board...

 

[Watthour]

The best guess I can offer is that this trimmer can attenuate the Right channel FM audio signal after it leaves the ceramic filters, and was intended to balance the channels' output before it hits the preamp stage. It is curious that the designers chose the right channel only, and I cannot determine why BOTH channels did not have an adjustment since both of the channels coming out of the demodulator/decoder (IC2) seem to be configured for the same gain/voltage/filtering. Unless they somehow found that the decoder IC had a more predominant Right channel output, it doesn't seem to make sense. The data sheet from TI doesn't appear to show any difference in output levels between the channnels.

Probably the best annyone could do would be to feed a signal through the FM section and force the decoder into mono mode, then observe the signal amplitude on both Left and Right channels at the preamp board, then adjust as necessary (if possible) to achieve the same P-P signal levels.

 

[P.O.]

Well, yours certainly goes along Chazix's interpretation, and I buy that... Until we get a definitive answer from the service manual (if there is one), that's the best we can do. Not a critical adjustment by any stretch anyways...
Thanks!!

 

[Watthour]

It isn't a true stereo separaton adjustment, since it is after the decoder. I suspect it may be a factory "band aid" for something found in the unit during production, but that's also just a guess.