AXT Systems

Dual 1176LN Troubleshooting
If you are having problems getting your newly-constructed 1176 clone calibrating or operating, here's a few things you might like to check to help find the problem.

Steady-state DC bias conditions
Assuming you have the power supply working (first thing to check), use a voltmeter to measure the DC voltages on each of the transistors with compression disabled and ratio set to 20:1.
These DC voltages may differ by a few percent, depending on your supply rail settings, transistor types and temperature. However if they are way off then you have a problem, possibly caused by a wrong value resistor or a transistor inserted incorrectly.

If you use the testing spreadsheet and fill in the values as you measure them it should indicate which ones are OK and which ones are not.
Test report showing DC Bias voltages
Example test spreadsheet shoiwing DC bias tests
Test report showing AC gain
Example test spreadsheet showing AC gain tests

Amplifier Stages - 1kHz AC signal trace
There are three amplifier stages to check.

1. The input amplifier
Unless you are using a transformer input stage, you will have an op-amp balanced input amp.
Connect a 1kHz 1Vp-p sine wave to the input connector and measure the output at the top of the Input pot.

Here's a scope snapshot of these waveforms.
Input test signal
Input: 1kHz 1Vp-p (200mV/div)
Signal at top of input pot
Top of input pot: 1Vp-p (500mV/div)

2. The signal pre-amp
This is the amplifier stage between the input pot and the output pot.

Using the same connection as used for the input amplifier test, measure the signal at the base of Q2, Q3 and Q4 and also the top of the Output pot.
The following scope images show typical results for this measurement. The gain of this stage is about 26dB.

If you use the testing spreadsheet and fill the measured values in it will calculate the gain of the stages and indicate whether there is a problem.
Signal at Q2 base
Signal at Q2-B: 80mVp-p (50mV/div)
Signal at Q3 base
Signal at Q3-B: 12mVp-p (10mV/div)
Signal at Q4 base
Signal at Q4-B: 1.9Vp-p (500mV/div)
Signal at top of output pot
Signal at top of Output pot: 1.9Vp-p (500mV/div)

3. Line driver amp
This amplifier stage drives the transformer at its output and is fed from the Output pot.

Continue the measurements on the base of Q5, Q6, Q7, Q8 and Q9 and also on the + side of C15 (ie the output).
The following scope images show typical waveforms you would expect to find at these points.

The gain of this stage is approximately 10dB.
Signal at Q5 base
Signal at Q5-B: 480mVp-p (200mV/div)
Signal at Q6 base
Signal at Q6-B: 18mVp-p (10mV/div)
Signal at Q4 base
Signal at Q7-B: 18mVp-p (10mV/div)
Signal at Q8 base
Signal at Q8-B: 2.1Vp-p (500mV/div)
Signal at Q9 base
Signal at Q9-B: 2.1Vp-p (500mV/div)
Signal at output
Signal at output: 2.1Vp-p (500mV/div)

4. The gain-reduction amplifier and rectifier
The signal at the to top the Ouput pot is fed through a resistor chain and tapped, according to the compression ratio selected, to feed the input of the GR amp stage.

Continue with the 1kHz input signal and enable compression. Select a ratio of 20:1, which takes its signal from the top of the resistor chain. Turn up the input pot until the signal at the base of Q12 is 0.13Vp-p.
Now measure the signal at the base of Q12, Q13, Q14 and Q15 and finally at the junction of D3 and D4 cathodes (the output of the rectifier).

The following scope images show typical waveforms at these points.
Signal at Q12 base
Signal at Q12-B: 130mVp-p (50mV/div)
Signal at Q13 base
Signal at Q13-B: 1.3Vp-p (500mV/div)
Signal at Q14 base
Signal at Q14-B: 150mVp-p (50mV/div)
Signal at Q15 base
Signal at Q15-B: 1.2Vp-p (500mV/div)
Signal at D3/D4 cathode junction
Signal at D3/D4 cathode junction: (20mV/div)