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.

1176 Test Report Spreadsheet

 

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.

Q

B

C

E

2

1.04

1.81

0.50

3

1.81

11.8

1.17

4

11.8

30

11.2

5

4.88

28.3

4.40

6

28.3

14.4

28.9

7

28.9

14.4

29.5

8

14.4

30

13.7

9

13.1

0

13.7

12

4.47

15.1

3.89

13

15.1

30

14.5

14

3.45

16.6

2.86

15

16.6

30

16.0

 

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.

Amplifier Stages - 1kHz AC signal trace

There are three amplifier stages to check.

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 signal waveform

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The input signal waveform

 

Signal at top of input pot

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Waveform at top of input pot

 

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. 

 

Q2-B signal

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Signal at base of Q2

 

Q3-b signal

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Signal at base of Q3

 

Q4-b signal

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Signal at base of Q4

 

Signal at output pot

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Signal at top of output pot

 

The 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.

 

Q5-b waveform

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 Signal at base of Q5

 

Q6-b waveform

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Signal at base of Q6

 

Q7-b waveform

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Signal at base of Q7

 

Q8-b signal

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Signal at base of Q8

 

Q9-b signal

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Signal at base of Q9

 

Signal at C15+ output

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Signal at + side of C15

 

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.

Q12-b signal

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 Signal at base of Q12

 

Q13-b signal

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 Signal at Q13 base


Signal at Q14 base

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Signal at Q14 base

 

Signal at Q15 base

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Signal at Q15 base

 

Signal at D3/D4 cathode

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Signal at D3/D4 cathode junction