drillz
Mechanical
- Sep 15, 2007
- 21
I would be very grateful for help on the test procedure for a video amplifier that my company makes.
I am sure my company will not be harmed by me anonymously posting a link to a brief-ish form of the schematic here, as its only simple inverting stages and an analog multiplier IC (AD835R).….which gives an offset to the signal as well as multiplying the signal by the “Gain/Contrast” voltage.
Here is the Video amplifier under test:- (see diag 1 Schem)
DIAGRAM 1:
This amplifier circuit has two biasing voltages….
1. The brightness (-5V to +5V) voltage
2. The “gain/contrast” (0 to 5V) voltage
-These are shown on the above (Diagram 1).
“Brightness” basically appears to affect the DC level
“Gain/Contrast” appears to affect the signal amplitude.
The “Gain/Contrast” bias voltage is fed to an AD835R multiplier whose action is as in the above (diagram 1).
I am required to test these amplifiers but I am concerned that the test procedure is sub-optimal. In particular, I am sceptical about the fact that the multiplier output is first zeroed with a pot when there is no input signal to the amplifier chain, and when the “Gain/contrast” input voltage to the multiplier is zero volts.……then later, this SAME pot is adjusted to provide a DC offset to the output signal…..
…..this seems strange, and I believe that a different pot should be used to set this DC level….
Here is the
TEST PROCEDURE:
1. Centre all pots.
2. Connect 100R load to output.
3. Switch on (power to IC’s)
4. Set “Brightness” input to 0V
5. Set “Gain/Contrast” input to 0V
6. Allow 30 mins for circuit to stabilise.
7. Adjust POT3 to give 0V at “W” output of AD835R
8. Adjust POT2 to give 0V at pin 6, IC4
9. Set “Gain/Contrast” input to 5V
10. Set “Brightness” input to 500mV
11. Apply pulse train to “SIGNAL IN”…….(see Diag 2)
DIAGRAM 2:
12. Use POT3 as coarse adjustment and POT2 as fine adjustment to set output pulse train (across 100R load) to the following diagram….(i.e. to get the signal “base-line” at 500mV)……………… (see diag 3)
DIAGRAM 3:
13. Adjust POT1 to set peak-to-peak amplitude of output pulse train to 200mV
14. Repeat 12 to 14 till output is set as follows:- (see diag 4)
DIAGRAM 4:
- - - - - - - - - - - - - - - - - - - - - -
-My problem is with step 12 in the above test procedure……I believe that ONLY POT2 should be adjusted to set the “base-line” (see diagram 3) level of the output signal to 500mV....
....Not POT3 and POT2 as required by step 12.
POT3 is first adjusted in step 7, -I feel that to re-adjust POT3 in step 12 is ruining the initial zeroing of the multiplier’s output.
I would be most grateful for any thoughts on this.
I am sure my company will not be harmed by me anonymously posting a link to a brief-ish form of the schematic here, as its only simple inverting stages and an analog multiplier IC (AD835R).….which gives an offset to the signal as well as multiplying the signal by the “Gain/Contrast” voltage.
Here is the Video amplifier under test:- (see diag 1 Schem)
DIAGRAM 1:

This amplifier circuit has two biasing voltages….
1. The brightness (-5V to +5V) voltage
2. The “gain/contrast” (0 to 5V) voltage
-These are shown on the above (Diagram 1).
“Brightness” basically appears to affect the DC level
“Gain/Contrast” appears to affect the signal amplitude.
The “Gain/Contrast” bias voltage is fed to an AD835R multiplier whose action is as in the above (diagram 1).
I am required to test these amplifiers but I am concerned that the test procedure is sub-optimal. In particular, I am sceptical about the fact that the multiplier output is first zeroed with a pot when there is no input signal to the amplifier chain, and when the “Gain/contrast” input voltage to the multiplier is zero volts.……then later, this SAME pot is adjusted to provide a DC offset to the output signal…..
…..this seems strange, and I believe that a different pot should be used to set this DC level….
Here is the
TEST PROCEDURE:
1. Centre all pots.
2. Connect 100R load to output.
3. Switch on (power to IC’s)
4. Set “Brightness” input to 0V
5. Set “Gain/Contrast” input to 0V
6. Allow 30 mins for circuit to stabilise.
7. Adjust POT3 to give 0V at “W” output of AD835R
8. Adjust POT2 to give 0V at pin 6, IC4
9. Set “Gain/Contrast” input to 5V
10. Set “Brightness” input to 500mV
11. Apply pulse train to “SIGNAL IN”…….(see Diag 2)
DIAGRAM 2:

12. Use POT3 as coarse adjustment and POT2 as fine adjustment to set output pulse train (across 100R load) to the following diagram….(i.e. to get the signal “base-line” at 500mV)……………… (see diag 3)
DIAGRAM 3:

13. Adjust POT1 to set peak-to-peak amplitude of output pulse train to 200mV
14. Repeat 12 to 14 till output is set as follows:- (see diag 4)
DIAGRAM 4:

- - - - - - - - - - - - - - - - - - - - - -
-My problem is with step 12 in the above test procedure……I believe that ONLY POT2 should be adjusted to set the “base-line” (see diagram 3) level of the output signal to 500mV....
....Not POT3 and POT2 as required by step 12.
POT3 is first adjusted in step 7, -I feel that to re-adjust POT3 in step 12 is ruining the initial zeroing of the multiplier’s output.
I would be most grateful for any thoughts on this.