51X-F76 test procedure.Important.
1). When running on +/-16V, in 500 format, install jumpers JP1, JP2
and cut pins 17/18 of card edge connector.
2). When running at +/-16 and +/-24V, !DO NOT! install these jumpers,
they can cause damage to power supply.
3). After running many audio tests, the best sounding FET's here, IMHO, are 2n5457,
BUT the PCB was designed for BF245.
Stereo kits were supplied with matched within 0.5dB in gain reduction range 2N5457 FET's.
IF using 2n5457, they have pinout GSD instead of DSG like BF245.
They won't be damaged if installed wrong way, but compressor will not work properly.
The right way to install 2N5457 in sockets:
After PCB's stuffed and everything checked, every component in place-http://i251.photobucket.com/albums/gg291/diy33609/51F_X76/51xf76_assembly/P1050847.jpgStep 1. Audio (sound path).
Do not install audio and gain reduction FETS (2N5457 or BF245, Q1 and Q10).
Do not connect RATIO/ATTACK/RELEASE board.Current draw.
Do not install input/output amps and chips in sockets; check if no smoke and current draw;
check -10V at LM337 out.
On FP (10 pin, dual raw) connector on motherboard,
short pins 9 and 10 to switch relays from BYPASS state to COMP IN.
(PC jumper will work fine).
Install input/output amps. Check if current draw is less than 70mA each +/-16 and +/-24V.
If don't have lab power supply, measure voltage on R91, 92, 93, 94 resistors (10R),
it should be maximum 0.7V at any resistor.
Now, we can check the amplifiers
. Input amp.
Close input and output pots, and, if my version of input opamp is used,
put DC voltmeter between amp's out and ground adjust amplifier's output
for +0.3...1V with trimpot located on amplifier's PCB.
Note 1: let amp heat up 5-10 min.
Note 2: to get VERY tasty sound from this amp, connect 1...2k, 1/4W resistor between its out and gnd.
Scope shows here transformer's, FET's and amp's residue on hot signal, COMP=OFF.
(For those who knows what is real cool harmonic signature)))http://i251.photobucket.com/albums/gg291/diy33609/51F_X76/51xf76_assembly/P1050930.jpg
If "original" amp used at 24V power supply, check its output is in range +12V +/-10% ref. to gnd.Output amp.
Check DC offset is in range few millivolts ref. gnd for DOA or is +12V +/-10% for "original" amp.
Same way as with input amp.
Let's check input/output pots, maximum gain +37db and distortion.
Feed -20dBV (0.1V RMS) to input of 51XF76, input pot=max, output pot=max,
with 2:1 input transformer or balanced line receiver, and 1:2 output transformer,
maximum gain should be 36...37dB (output shows +16...17dB).
Feed 1V RMS to input, put input/out pots somewhere in the middle, check the distortion is less than 0.02%.
Check max. undistorted output is +24db at least.
Let's adjust 0=+4dB on meter
Put GR/OUT switch in OUT position;
Connect AC voltmeter to output, adjust input/output pots for 1.23VRMS output and turn MTR_OUT pot untill yellow led
(0db) lits on; than, turn it back a bit to let LED go OFF, again, slightly turn to lit the yellow led ON.
This way, we can have more precision in multiple channels (the led meter is not linear thing, but we want
to catch the same threshold to light on the leds).Step 2. Compression (sidechain, GR metering, BIAS, etc).
Connect RATIO/ATTACK/RELEASE board (and remove jumper between pins 9 - 10).
SLAM is OFF, LINK=OFF, RATIO at pos. COMP OFF, ATTACK=MIN, RELEASE=MAX.
Feed -20db to input, INPUT=MAX, adjust output for 0db.
Put FET's in sockets (Q1, Q10; mind, silkscreen is for BF245, 2N5457 are reversed).Adjust BIAS for -1db.
If building stereo pair, adjust second chan. for -1dB BIAS as well.
Than, LINK=ON. Both should remain in same level. If no, adjust a little bit BIAS of one of channels.
+/-0.2db is fine.BTW, don't forget to connect wire at back side of PCB if use LINK.
Back to Channel 1.Distortion cancellation circuit.
Adjust DIST (R16) to get minimum of distortion on sinus wave.
Or, misadjust a bit for color, but mind, too much will not sound good.
0.01....0.04% THD is fine.
This step requires audio analyzer or PC soundcard.GR meter.
Remove input signal, put METER to GR, and adjust 0 with 0GR trimpot.
Yellow LED should light ON. Light ON the +1 RED LED, than, turn the pot back, just to leave yellow LED.
Repeat this step 1-2 times.
Apply input signal 0db, and put INPUT approx. at 14.00 pos.
Set RATIO to 4:1 and check the output.
Find input potentiometer's position which has output change
between COMP.OFF and 4 ratio setting of 10dB gain reduction.
Adjust GR_SCALE trimpot just to turn ON -10 db LED, turn it back,
and find the threshold of -10 LED.
Repeat from !AA to !BB till you get nice meter reading
with and without 10db of gain reduction.Apply audio and listen. Have a fun!