Avalon VT 747 SP calibration

[globule_655]

Hi there !

I have an Avalon VT 747 SP which has a little balance problem. One of the side is 6 to 9dB lower than the other. The tubes needed to be changed also and I replaced them... When doing a little research on the subject I read here and there the unit needs to be calibrated after changing the tubes. There are indeed a few trimmers inside the unit but no indication about what they're for.
Anyone knows about the calibration procedure for that beast ? Avalon won't provide it...

Thanks !
Cheers !

 

[fazer]

Its in the manual I believe.  Contact Avalon for a manual.

 

[globule_655]

Hey fazer
I already contacted Avalon but they said I had to send the unit to them for calibration but I find the price a bit steep for a few trimmers to adjust. There is nothing in the manual I have and I read somewhere that the alignment/calibration procedure was out there on the net at some point. Maybe someone still has it and can help me...

 

[fazer]

Sorry I don't have the info on cal.  It was a lot of years ago at a facility thats not in Bus anymore.

 

[jwhmca]

This it?

 

[mjrippe]

Looks like it, but page 1 of 2!?!

 

[globule_655]

Yes jwhmca that's it ! 
Do you have page 2 also ?
Thank you very much !!
Cheers !

 

[jwhmca]

Page 2

 

[squib]

6 to 9 db lower ... that's more than a calibration

is that with or without any compression?
is it with the input and output level controls at 12 o'clock ... ie unity gain?

have you checked that the issue isn't outside of the unit?

 

[globule_655]

6 to 9 db lower ... that's more than a calibration

is that with or without any compression?
is it with the input and output level controls at 12 o'clock ... ie unity gain?

have you checked that the issue isn't outside of the unit?

Hey squib
The imbalance was due to bad tubes, which I changed. The cal was needed after changing them but there was no more any "problem" per se (only a 0.6dB difference between input and output, compressor threshold a little higher than before, little things like that)

 

[ericyoshino]

Does anyone know how to calibrate the L-R channels? I did a tube set replacement and calibrate the compressor circuit per the document attached, but the unit itself is -.5 unmatched between both sides without any modules engaged (EQ, compressor, TSP).

Thank you

 

[pucho812]

Good things come to those who wait. Patience young Padawan.
Ap references an audio precision machine. If you do not have an AP you can use a tone generator of your choice as long as it meets the following criteria able to generate a sine wave of 1 kHz at 0 dBu.
You can also ignore noise floor measurements when doing field calibration and FFT stuff in the field.

Check and confirm 747 is set to work at mains voltage for your country.
Plug 747 into Variac set to 0V
Turn on Variac and raise the variac until you have it at mains voltage Push all switches in and confirm all the leds light up
Turn all knobs and make sure they are lined up and that there is no scratches Make sure the knobs have clearance and are not scratching the face plate Make sure the zip tie for the wiring to the output pot is in place
Check Voltages
T1 Should be 180VDC
T2 should be 6.2VDC
T5 and T7 should be 75VDC +/-5VDC
Check middle pin of every amp board should all be mv
Check positive pin on all opts should read between 1.36 to 1.42V
Check voltages on u7 pins 4 and 11 should be +/-17VDC
Connect Ap to both channels in and out and set AP to generate 1K @0dBu Check level with Ap.input and output at center detent no eq no compression Level should be even on both sides @ 0dBu.
Press high gain in
AP should read +10dBu
Press tsp in
AP should increase.2dB
Turn off high gain and TSP Turn Output Pot full right Ap should read +5dBu Turn Output pot far left
Ap should read -27dBu +/-1dBu
Turn output pot to 9 o'clock position
AP should read .5dBu difference between channels
Turn output to center detent
Turn input pot to far right
AP should read 8dBu
Turn Input pot far left
AP should read -21dBu
Turn input pot to center detent
Adjust vr2 on Main Board to adjust channel 1 level to 0dBu as reflected on AP
Adjust vr3 on Main board to adjust Chanel 2 level to 0dBu as reflected on AP
Adjust VU meter to 0VU by turning trimpot accessed through the faceplate
Change level out of AP and watch the LED metering to make sure both channel track
together To adjust this metering at all Adjust vr4 on main board for Ch 1 and vr5 for CH2. Set AP back to generate 1K @ 0dBu
Press compressor IN
Turn Compressor knob far left
Turn Make up gain knob far right AP should read 10.45dBu
Press TSP IN
AP Should jump 1.3dB
Turn off TSP
Turn Make up Gain Far left Turn Compressor pot far right
Turn Threshold far Left
Turn Attack and Release pots to fast
Adjust VR4 on card 5600-7474 so that ch1 is -10.10 on AP
Adjust VR6 on card 5600-7474 so that ch4 is -10.10 on AP
Adjust VR2 on card 5600-7474 So that the VU meter shows -10VU
Turn Compression pot to 10 O’clock Position
Adjust VR5 on card 5600-7474 so that ch1 is -3.10 on AP
Adjust VR6 on card 5600-7474 so that ch2 is -3.10 on AP
Adjust VR3 on card 5600-7474 So that the VU meter shows -3VU
Turn Compressor pot far right
Observe measurement
Repeat steps 51 - 57 until both channels match
Turn Compression pot far left and slowly increase it till AP reads -.4dBu
Adjust vr1 on Opto Card So that the Blue GR LED starts Blinking
Turn Compression pot far right
Monitor Noise with AP
AP Should show -88dBu noise floor
Press TSU in again
AP should show -88dBu Noise Floor
Press EQ IN
AP Should Show around -80dBu
Press EQ out
AP Should Show -88dBu again
Connect 747 Ch 1 to FFT for following adjustments/measurement
Set FFT to read 2dB increments
Press EQ in
Plus up and Down on all eq sliders and watch that they go to correct levels.
Variable amplitude LF +/-24dB 15Hz shelf response Variable amplitude MF1 +/-8dB 125Hz selected Q response Variable amplitude MF2 +/-4dB 500Hz selected Q response Variable amplitude MF3 +/-4dB 2kHz selected Q response Variable amplitude MF4 +/-10dB 5kHz shelf response Variable amplitude HF +/-20dB 32kHz shelf response
Push Compressor IN
Turn Compressor pot far right
Press SC-Listen IN
Turn Both sidechain threshold pots far left
Turn HF sidechain frequency Pot far right
Turn LF sidechain frequency pot to 12 o’clock
FFT should show two lumps on the line
Press sidechain IN
FFT level should drop to the bottom of the screen 85. Press sidechain Listen out
Repeats FFT measurements for ch 2

 

[fragletrollet]

I don't have an Avalon but just wanted to encourage such a detailed, helpful response. Long live GDIY