Adjusting Analog VU Meter

[DreadPirate]

I have a Kenwood KA-501 amp whose output meters are not indicating identically, one falls a bit short of the other. I have not verified yet, I intend to do this with an SPL meter, but I am assuming that the sound is of equal power from both channels and that the problem is in the meter. In a typical amp like this, what is the method of adjusting the needle sweep? My primary concern is to get them reading identically when an equal signal is applied to both channels and the balance knob is centered, regardless as to whether the reading really means anything.

 

[acenovelty]

When you remove the face plate cover of the meter, there is a small set screw attached to the movement. Here you can change its relative setting to match the other side.

 

[anatech]

Hi DreadPirate,
The set screw inside the meter is for setting zero.

There will be a PC board inside with some trim pots that will allow you to set the span.

You will need a 400~1KHz constant tone (I use a signal generator) and an accurate meter good at those frequencies. Your garden variety DVM is very little use past 120 Hz. A VTVM (calibrated) would be a suitable secondary instrument. Do not attempt to use a sound pressure meter. There are too many variables at work and you could easily get the same reading from each speaker and have the VU meter out by 3dB.

Get the service manual or schematic for this. There are other controls inside that can bring you grief if misadjusted. The smoky kind of grief.

-Chris

 

[DreadPirate]

I was suspecting there was some kind of onboard pot for this. I have posted the Kenwood KA-501 schematic here if anyone is interested in having it:

http://depositfiles.com/en/files/35694/KENWOOD-KA501.pdf.html

 

[DreadPirate]

VR3 and VR4, looks like.

 

[anatech]

Hi DreadPirate,
Yes, those are the ones. Set them without a load (they just measure voltage output) to keep your amp cool while setting these. Set at "0" while running a signal in via a "Y" cord. Sine wave no higher than 1KHz or you may burn out the zobel network.

-Chris

 

[DreadPirate]

I'm not understanding the Y cord part, I think you are overestimating my digital prowess, I'm a rank beginner.

I do have a Dick Smith ESR meter that apparently has a usable 1KHz output, but I wouldn't begin to know how to utilize it.

 

[Nigel Goodwin]

As already suggested VR3 and VR4 adjust the two meters, they don't actually read in any particular units (and aren't really of much use anyway, just a visual gimmick).

No need to get carried away with audio generators and such (although it does make it easier and more accurate). Just play a mono signal through the amplifier and gently adjust ONE of the pots until the needles move the same.

The suggestion of the Y cord was just to give a mono input.

If you can't find a mono source?, how about radio?, or use a stereo source and adjust on that - but that will take a LOT longer until you're happy, as it's basically a question of making a slight adjustment and then playing a number of tracks to see what you think.

 

[anatech]

Hi Nigel,
Thanks. I post how I would normally do things on the bench. Most people interpret the "0" as full power when in fact it's past clipping!

-Chris

 

[DreadPirate]

Done. I used a "white noise" file and set them even at about mid sweep. Oddly enough, they diverged a bit towards the home position, I'm guessing there is something wrong with the movement on one of them, but all in all worked fine. "Good enough for Government work," as they say.

Attached is a link to a nudie. There are two circuit boards, this one and one vertically attached to the jacks. I am guessing the one in the rear is the pre-amp? This board looks real easy to remove, would it be worth the trouble to re-cap the 15 or so 'lytic caps with polypropylenes? I've never done this before, but doesn't look too difficult...

http://depositfiles.com/en/files/38643/KA501_Nudie.JPG.html

Thanks for the help!