pucho812
Well-known member
I was referring to doing the babyface mod. That’s where c5 comes into play.
Sorry for the confusion on my part there.
Avalon vt-737sp
- Thread starter Jdm5149
- Start date
Help Support GroupDIY Audio Forum:
Sorry for the confusion on my part there.
pucho812
Well-known member
Compressor calibration. All trim pots with the exception of gain reduction meter zero are internal.
In ye ad days, we ran a 1k sine at 0dBu into the line input with in and out pots at center detent. This should be line level 0dBu input identical 0dBu output with no compressor, eq, or high gain engaged. If not depending on how off it is, could be a trim pot or a bad component or worse faulty trace. If is a few tenths a db, adjust vr2 on main board until it’s 0dBu at measured output.
Adjust vr3 so that the meter reflects it.
Vr2 and 3 are on the motherboard. One is to the left of the vu meter while looking right at it, the other is behind the vu meter.
Pressed meter button to read gain reduction and adjust front trim pot to read 0 on the vu meter. It’s the trim pot you adjust from the front panel.
insert the compressor.
you should be measuring the output at all times.
There are 4 trim pots on the 7374 card that get adjusted. They are labeled below. Not to confuse them when trim pots on the main board as they share same part identifying numbers. It is a separate card.
Threshold all the way down
Attack and release at fastest it can be(far left)
Increase compression knob.
when the knob is at like 12 o clock l( between 5-6 on the silkscreen of the faceplate).
Adjust vr4 so that your output is -10dBu
Adjust vr3 (labeled -10vu) so that the meter should -10 on the vu in gain reduction mode.
From there move the compression knob to the 9 o’clock position.
Adjust vr1 on the same card so that you you see -3dBu at output.
Adjust vr2 so that the meter shows -3 in gain reduction mode.
Repeat -10 cal and -3 cal a few times as they are opto’s.
Last edited:
pucho812
Well-known member
Never tried it. At worst it won’t work.
If the meter is pegged in gr mode, then check the opamp rails. See what they measure. There are some chips on the 7374 card.
TheseGoToEleven
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The bulbs make no difference but you can purchase new ones at most auto parts stores if you don't want to order them somewhere else cheaper.
Edit: I assume the bulbs are just burnt out. You might want to double check that they are getting power supplied to them.
TheseGoToEleven
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Sometimes you don't state the test conditions and we have to assume things, so just in case this applies and you aren't aware... with meter button out, and using mic or instrument input, the VU meter won't show anything unless the unit has been turned on about 40 secs or so. After that, the meter should show the output level and be adjustable.
yes. Feeding signal thru line. 0db 1k. Unit on for more than 10 minute. I always wait for relay to kick in. I could adjust output with V2 but V3 does nothing. With meter switch in. Meter goes to zero. But no compression with any of those trim pot on 7374.
pucho812
Well-known member
so when you run line in and feed level you can measure level at the output?
When you meter output level it pegs? Can you lower the level so it doesn’t peg?
There is an op07 opamp 8 pin dip that drives the meter to look at output level.
It’s under the I/O card.
When you meter output level it pegs? Can you lower the level so it doesn’t peg?
There is an op07 opamp 8 pin dip that drives the meter to look at output level.
It’s under the I/O card.
When you say “pegs hard left” do you mean it’s resting or it’s being pushed left?
pucho812
Well-known member
Oh?
Let’s do this, when you test anything, put conditions.
So for example line input, 0dBu 1k sine wave. No eq, no comp, measured output is 0dBu 1k sine. Input and output pots at center detent.
meter button switches between gain reduction and output level. If you are looking at output level and
If you have no signal present and the meter is pegging hard to one side, it’s highly possible your opamp is not getting its bi-polar supply.
There is a neat thing going on on the mother board where it takes the bipolar 32vdc from the psu and regulates it to bipolar 15vdc. This is to run the opamps for the meter and the dc servo’s.
I would confirm at the op07 opamp you have bipolar voltage in reverence to ground.
Pin4 will be v- pin7 will be v+
Becareful not to short anything.
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