MixBuzz500 build/support tHread

[Bobby Baird]

Finished soldering this project up last week. Voltages are good. Passes audio, makeup gain works, but no compression. Followed Igor's troubleshooting advice from page 12 everthing seem good there. Kind of stumped where I should be looking. I have 2181A's installed and I have done the offset voltage adjustment for 0mV on pins 1 & 7 of 5532. Passing 1 khz sine i'm not seeing AC voltage at the output pin 8 of the 2181 on the vca replacement boards. I can see it on the input pin 1. Any clue what could be the error. I have voltage on both legs of the threshold pot.

 

[moamps]

....Voltages are good. Passes audio, makeup gain works, but no compression....

That's a good start. The output from the VCA is the current that enters the virtual grounded input from an opamp, so you can't see the voltage at that point. The signal should be present at the output of the opamp connected to the VCA.
Do you have a scope?

 

[Bobby Baird]

Yes, I have a scope.

The signal should be present at the output of the opamp connected to the VCA.

So, I should see AC voltage on outputs of the IC11 NE5532 if there is compression occurring? Pins 1 & 7 were I measured the offset?

 

[Bobby Baird]

Here's troubleshooting and mods I collected in this thread to post in one spot.

Issue: No makeup gain.

Check the voltage on wiper of makeup pot changes from 0 to about 8V; check R53, R59;
of course, at pin 6 IC7 there's 0V, it is inverting ground-referenced amp;
if compressor works, i.e. you get compression when changing threshold, the problem is not IC7B.

Gain reduction signal, or control voltage, comes to IC7B, thru R54.

Makeup voltage comes to IC7B thru R59.

Issue: Audio passes through in standby. Audio passes in 4000 and gssl mode.
I can use make up to increase level but no compression. I hear the relays when switching from bypass to 4k and gssl.
Meter is not moving. Can someone give me a starting point for troubleshooting?

First, let's check the voltage beween gnd and wiper of threshold pot,
it should change from approx. +6.5 to -6.5 Volt when rotating the THRESHOLD pot.

Then, let's check if audio passing thru sidechain: feed about 1V RMS sinus 1Khz to input, set 4000 mode,
sidechain filter OFF, check with scope at IC3d (14), IC3A (1), there should be sinus 0.5V RMS;
than, check if there's a signal on CTL pin 5 (10 pin IDC connector);
than, check the signal on IC11A (1), sinus, it's amplitude changes with position of threshold pot;
than, check if rectifier works-on IC10A, (1) should be rectified full-wave signal when threshold set to low (CCW);
than, check if there's a voltage CTL-10, should be saw-looking signal which changes depending on amount of compression and time constants;
than, check the voltage on IC7D (14), this is the control voltage (should be meter deflection), etc.....

Before all this,
Check jumpers;
Check if CTL and SC cables going to their channels respectively,
i.e., CTL1, SC1 to left and CTL2, SC2 to right channel;
Check all connections etc...

Issue: The meter is showing gain reduction as I turn the threshold knob.
However the audio doesn't seem to be effected at all, and the make up gain doesn't change the output either.

When turning MAKEUP and THRESHOLD pots, at the pin 7 and 8 of IC7b the voltage should change within range of couple of volts.

If yes, check resistors R60=100r, R75=49r next.
Than, go to audio board and check if there's voltage change when rotating the trimpots at CTL connector, pins 1 and 3
(or, check at R100, R45). Check R100=909R. Maybe, bad contact at pin 1 of CTL connector.
If this does not helps, check on VCA adaptor PCB: R101=732R, R102=100r, check if IC9 (NE5534) installed correct as well.

MODS

Regarding the release switch, you easy can have 6th position, just install
R||C, between A6 or C6 point of release switch to gnd...Try 220K and 0.1uf,
for very fast aggressive setting, or you can mix two RC for another auto mode.

Add a 1k trim pot before the VCA at R6 in order to match the gain on the left and right channels.

Meter calibration: R50 = 2k can be changed to get correct reading.

 

[moamps]

So, I should see AC voltage on outputs of the IC11 NE5532 if there is compression occurring?

Correct. You need to measure some AC voltage on pin 1 of IC11. Then you need to measure some DC voltage on pin 1 of IC10A. This DC voltage changes depending on the AC voltage level on pin 1 of IC11. IC10 and surrounding elements is a fullwave rectifier. Please check that.

 

[Bobby Baird]

Pin 1 IC10A

 

[Bobby Baird]

Pin 1 IC11A

 

[Bobby Baird]

Correct. You need to measure some AC voltage on pin 1 of IC11. Then you need to measure some DC voltage on pin 1 of IC10A. This DC voltage changes depending on the AC voltage level on pin 1 of IC11. IC10 and surrounding elements is a fullwave rectifier. Please check that.

This checks out as I move threshold pot voltages change.

 

[Bobby Baird]

I checked for continuity on all ribbon cables and control board headers. Everthing is making solid connections. Diodes are orientated correctly.

 

[moamps]

Pin 1 IC10A

This waveform doesn't look good, IMO. Could you measure IC10 pin 7?

 

[Bobby Baird]

This waveform doesn't look good, IMO. Could you measure IC10 pin 7?

pin 7 waveform All of these reading are with the threshold full CCW.

 

[moamps]

This is ok, please check out wireing and soldering around IC10 first opamp and ratio switch.

 

[Bobby Baird]

I have continuity on all soldered joints for IC10 to pin 1 of SC1 and SC2 through to the 4x3 ratio switch.

 

[moamps]

On pin 1 of IC10 you need to get fullwave rectified input voltage but inverted. Negative. Without the AC input voltage, the DC potential of that point should change to some negative values depending on the position of the ratio switch. If you do not get a negative voltage at that point, signal compression cannot occur. Set the scope input to DC and measure the signal at that point again.

 

[Bobby Baird]

Alright changed scope to DC. IC10 Pin 1 looks like this Ratio 2.

 

[Bobby Baird]

Ratio 4

 

[Bobby Baird]

On pin 1 of IC10 you need to get fullwave rectified input voltage but inverted. Negative. Without the AC input voltage, the DC potential of that point should change to some negative values depending on the position of the ratio switch. If you do not get a negative voltage at that point, signal compression cannot occur. Set the scope input to DC and measure the signal at that point again.

Looks inverted to me with scope set to DC. Its amplitude changes depending on threshold level, and the inverted waveform gets larger when you increase the ratio.

 

[moamps]

Perfect. Now go to IC7 pins 12 and 14. There should be seen that negative voltage but smoothed with capacitors in the release part. The negative DC voltage on these pins should depend on the input voltage and the set threshold pot.

 

[Bobby Baird]

Perfect. Now go to IC7 pins 12 and 14. There should be seen that negative voltage but smoothed with capacitors in the release part. The negative DC voltage on these pins should depend on the input voltage and the set threshold pot.

Not seeing anything on those pins. Only on pins 1,2,3.

 

[moamps]

Check the change in negative DC voltage at these points.