T-Rex Comp Nova troubleshooting

[DIY Bass]

I am pretty sure I know what is stopping this working, but I am not sure why.

Schematic:  see attached - it was online, but may have disappeared




Symptoms:  The signal will go all the way through the pedal, but for most control settings there is a significant volume drop - almost inaudible at times.  The Comp control and Level control both need to be full up to get about equal output volume with the bypass signal.  Comp control seems to work as a direct gain control, rather a compression level control.

What I have found out so far:  Power to the ICs seems to be about correct - The Op amp has 0v on pin 4, 8.6 on pin 8 and 4.2 on most other pins.  Pin 3 is at 3.6V

At the junction of Ca12, Ta4 and Ta5, I get 8.4V.  My understanding is that this should be a DC voltage that varies with the amplitude of the signal at Ta2.  regardless of signal level from about 20mV peak to peak (440Hz sine wave) up to 300mV peak to peak, this voltage stays rock steady on 8.4V.  The emitters of Ta4 and Ta5 are both at 0V, as expected.  The bases of Ta4 and Ta5 are also both at 0V, which is not what I would have expected.  Ta2 measures B - 4.2V, C - 5V, E - 3.6V. 

I am assuming that there is a problem with the Bases of the transistor pair, but I am not clear about what fault would bring both on them to 0V.  Any ideas (or am I looking in completely the wrong place?  Always possible :-)

Thank you

[PRR]

> The bases of Ta4 and Ta5 are also both at 0V, which is not what I would have expected.

This is correct as far as it goes. In addition you should have signal. When signal exceeds the 0.6V turn-on of a BJT, the transistor should pull Ca12 ground-ward. So do you have signal?

[DIY Bass]

These voltages were all measured with no signal.  I'll have to set up a signal and see.  I do know that with a signal I never get anything other than 8.4V at Ca12.

[DIY Bass]

OK, I am getting signal at the bases of Ta4 and Ta5.  With my signal generator (phone :-) ) set to maximum output I can get an input signal of 0.7V peak to peak (sorry about p-p voltages - that's the easiest to measure off my o-scope).  With the comp control set to maximum that will give 1V p-p at pin 5 of the OTA, and 0.9V p-p at the bases of the trannie pair.  That will actually drop the voltage at Ca12 to 3.5V, so something is happening there.  Voltage there only alters in the last few degrees of rotation of that pot and only with the input signal at the max I can provide.  Sounds like that bit of the circuit might be working as it should be.  Bummer.

For reference, if i input 200mV p-p (which I think is about the signal strength of a single guitar note?)  I get 20mV output with the level control set to full and the comp control set to min.  With comp set half way i get 35mV, and with comp set to full I get 125mV.  That just doesn't seem right to me that there is only one usable setting and even that has a significant drop in signal strength.  I can provide some p-p voltage reading at different parts of the circuit if needed.

[DIY Bass]

OK, as near as I can tell, this is what's going on.  With the level control set to max.  The signal out of the OTA (pin 5) is consistently about 1/3 in amplitude by the time it reaches Ta6.  Pretty much all this drop is in Ra16.  This measures pretty close to the schematic value so I assume that this is supposed to be the function of this component.    Amplitude is halved again by the output of the device, pretty much in between the emitter of Ta8 and the output, so I assume by Ra26/Ca17.  Again I assume these components are designed to have this function.  If the output amplitude is by design at least 1/6 of the output from the OTA, then for it to be impossible to get unity gain through the pedal, then the gain through the OTA must be too low?  The OTA inputs measure the same amplitude pretty much as the input signal, So the buffering up until that point is pretty much unity gain.  Any ideas as to what to look at?

[PRR]

What is voltage at Ta3 E?

Are Ra11 Pa2 really and truly the values indicated?

Very-very-very carefully measure the voltage at OP1 pin 1. (A short here will kill the chip.) I forget if it should be 0.6V or 1.2V, whatever. If pin 1 has been blown, it will be higher than that.

[DIY Bass]

I will be away from said comp pedal now for just over a week. I will make those checks when I get back and see what it can tell

[DIY Bass]

OK, I am back with this one.

Ta3 E measures 7.7V with no signal applied.

Ra11 and Pa2 are really those values as measured in circuit.  Pa11 varies from 0 to 225k ohms and Ra11 measures as 21.9k

OP1 pin 1 measures 1.1V

[DIY Bass]

OK, I am now very puzzled.  Pleased, but puzzled.  I ended up very lost looking for the problem, so I decided to model the circuit in LTSpice and see what it was supposed to be doing at a theoretical level.  Got that done and ran a few virtual sine waves through it.  Thing is that the theoretical result that I was getting matched up with the actual physical measurements except that there was a distinct voltage drop across Ca17 in real life that was not there in the model.  It made no sense that a component that is in the signal chain for both effect and bypass conditions could alter the signal volume of the affect vs bypass though.  I measured it every way I could and decided that the voltage drop across that capacitor should not be there, and that I would at least see what happened with a new one.  I tacked in a through hole cap in place of the surface mount original and lo and behold it is now possible to even get a bit of gain when the effect is on.  The manual syas it should get up to 3dB gain, so that is about right.  I have no idea why changing that cap could influence the affect volume and not the bypass, but I'll take it.  Now just hoping it works when I box it back up :-)