Hi Michael,
Okay. If you arm yourself with the schematic from Jakobs site:
Here!
...we can start with the "divide and conquer" method of troubleshooting.
First thing- PSU. Those voltages look spot on. That's good!
If you look across the schematic from left to right you can easily follow the audio path. Forget that this is a compressor for now- we just want to pass an audio signal. The first part of the schematic centred around the NE5532 can be ignored- you've got the input transformer. So the signal comes in to the LL1540. The output is taken from the pad "1" on the PCB next to the two 10k resistors by the input transformer, not pad "1" by the NE5532!
The first thing the input signal hits is the Input Level pot- this feeds the "compressor" part of the circuit- the FET attenuator. This is buffered by the "Signal Pre-Amp" formed around Q2, Q3 and Q4. The sidechain is fed from the output of this preamp. The preamp buffers the FET and ensures it sees a constant impedance. The output from Q4 feeds the Output Level pot. The wiper of the output level pot feeds the "Signal Line Amp", this buffers and boosts the output signal to feed the output transformer and the outside world.
So here we have some very simple signal stages which we can break down and examine to find where the signal is getting lost.
Right, have you got a small amp such as a guitar practice amp? If not, a hi-fi amp will do, feeding a small speaker. A guitar amp is better because it has a higher input impedance of ~1M. The line-in of a hi-fi amp is often 47k or so, which is a bit low, but can make a good enough job.
What you need to do is connect the input of the 1176 to a constant signal source- sine or music is fine- we're just trying to trace it. Then, make up a lead to go from the input of the monitor amp (jack, phono etc) with screened cable, to bare ends. Strip and tin the ends, and connect anything from a 0.01u to 1uF non-electrolytic capacitor to the centre of the screened cable. Insulate the connection, and sleeve with tape or sleeving the free end of the capacitor lead, leaving enough of the capacitor lead showing to use as a "probe".
Cut the screen of the cable back to 1cm or so, and solder a 30cm length of standard cable to it. Insulate this connection so no screen wire is showing, and then connect a crocodile clip to the other end.
What you've got now is a rough 'n ready signal tracing probe. Make sure the voltage rating of the cap is at least 50V. You'll probably be using a poly cap, so their ratings are usually more than sufficient.
Connect the crocodile clip ground lead to the 0V of the 1176 (if no croc clip, solder this ground lead from the test cable to the 0V trace on the PCB for now) Make sure that you connect to 0V and not a power rail, because it will cause a short circuit through the mains ground in the hi-fi/guitar amp. (I use a battery powered amp for this because it guarantees no earth loops or short circuits!)
Carefully power up the 1176 and keep the volume down on the monitor amp.
Don't use headphones because the "clicks" from the cap charging when you hit a DC voltage
will hurt your ears!
With the input signal connected, and the input and output controls up midway, and with nothing connected at the output, touch the leg of the "probe" capacitor at pad "1" on the board next to the input transformer. Carefully bring up the volume on the amp and listen for signa. You should hear your input signal clearly, and not distorted.
Next, touch the probe to the wiper (middle terminal) of the Input pot. Again check for signal, and adjust the input level pot- the monitored signal should vary. If no signal at all, check input wiring, transformer wiring, and pot wiring.
Next, move to the output of the Signal Preamp. Touch the probe to the terminal at the clockwise-rotation end-of-travel point on the Output Level pot. You should get signal. Then try the wiper- again it should vary. Don't forget to keep the Input Level pot turned up midway here! And watch the level of the monitor amp too.
If no level at all, then either the sidechain is faulty- causing the FET to be biased fully "on" and attenuating the signal, or their is another fault in this amp. To check the FET isn't causiing the problem, switch off and desolder it. Without the FET there is no compressor action at all, it just acts as a line amp.
If this section passes signal, then the last suspect is the Signal Line Amp. Probe at the -Ve side of C15 (that feeds the output transformer) and check for signal. If there is nothing, then the fault lies in this section.
Once you've found where in the chain the signal is disappearing, you can start checking the signal flow within each amp stage to find out where the fault is. This simple probe won't work as well for "inside" the transistor amp stages, because the possible lower input impedance of the probe/amp combination can upset the bias for the stages and/or load the audio signal at higher impedance points too much to make sense. The DC-blocking cap in the probe helps prevent bias problems, but if the input impedance of the monitor amp is low, you can get funny results w.r.t signal levels.
Here's a link to my
Turbo Signal Tracing Probe :wink:
Hope this helps- once you've narrowed it down to a stage, it can get a bit easier. Let us know how it goes.
:thumb:
Mark
