9090 VU meter adjustment

AndyPrice44

Active Member
Hello,

I have recently purchased a Mcintosh MC7300 amp with VU meters. I purchased it from audio classics and they have tested everything and it all meets spec. I am using a decibel meter to see how many watts it takes to get my JBL L-112 to play at 85db level at my listening position.

I would think that when using the same speakers and test track that it would take the same amount of wattage from both the Sansui and the Mcintosh to achieve the 85db reading at the listening position. This is not the case though.

I am assuming that the reading on the Mcintosh meters is correct since it has just been tested and meets spec 100%

What I am wondering is why I am not getting the same wattage reading from both units to achieve the same decibel reading. It is reading a higher wattage on the Mcintosh unit.

The sansui unit is right under 1 watt to get 85db at my listening chair. The Mcintosh is right about 2 - 2.5 watts. I feel like the sansui is making more power than what is showing on the meters.

Is there a way to test and calibrate the meters on a Sansui 9090? How accurate are the meters on a 9090? Have any of you guys ever seen this?
 
I believe there is an adjustment, it is in the Service Manual. The 9090 meter is calibrated at a particular voltage into 8 Ohm load, not Watts.
 
my copy of the service manual does show provision of an adjustment in the block diagram but does not provide a procedure and a quick look at the schematic diagram doesn't reveal where the adjustment is...

However, as dr*audio says, you could inject a 1KHz sine wave at various amplitudes into the amp and measure the output across the speaker terminals or a dummy 8 ohm load.

If you don't have the equipment/know how to do this you're kind of stumped.

For a spot check, if you have an RMS reading multi-meter, you could measure the voltage across the speaker terminals at your chosen volume/listening level on both amps and compare them.

And no, the meters are not very accurate, mostly for decoration and a rough guide.

Here's a table of voltages you can use for reference

Vout Iout Power in 8 ohms
0.632 0.079 0.05
0.896 0.112 0.1
1.264 0.158 0.2
2.000 0.250 0.5
2.832 0.354 1
4.000 0.500 2
6.320 0.790 5
8.944 1.118 10
12.648 1.581 20
20.000 2.500 50
26.077 3.260 85
28.288 3.536 100
30.264 3.873 120
31.623 3.953 125
34.640 4.330 150
40.000 5.000 200
 
Someone had fiddled with the meter adjustments on my 9090. I have a pair of sansui speakers with LED power meters built in and am planning on calibrating my meters to match their readings..

SR
 
The meter adjustment pots are on the protection circuit board; the one with the speaker relay on it. I think you may have to get to them from the bottom, can't remember.
The calibration procedure is given in the 9090dB Service Manual. Connect 8 Ohm dummy loads to the speaker jacks. Connect an oscillator to the Aux input, set for 1KHz sine wave, 440mV RMS, and adjust the volume control for 20V RMS at both speaker terminals. You need a meter with flat frequency response out to at least 4KHz, and capable of reading True RMS. Or you can use an oscilloscope and set the output to read 56V peak to peak. Then adjust VR1 and VR2 so the meters read 50W.
 
With my 9090, if you look behind the FM signal meter, you should see a square hole in the chassis, if you shine a light down that hole, you should be able to see both meter adjustment pots, thats how i tuned mine... :)

SR
 
sounds like this is going to be more involved than I thought. I don't have all the tools necessary to do this. I guess I will have to take it to a tech who does. It is just annoying that my music will already be to a loud level before the needles even begin to move...
 
If memory serves, you connect 8 ohm dummy loads. Inject a 1k Hz sine wave. Measure output voltage with a true RMS A/C voltmeter. Use the volume control to bring the output to 20.0v and set the meters to 50 Watts.

When all that is done, you have meters that are more eye candy than meaningful displays of power.

- Pete
 
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