Mutron III - filter envelope detection troubleshooting

[bandus]

Hi everyone!

I'm working on a Mutron III, which works partially, but it seems that there is no envelope detection happening. I'd like to make sure that the original LDR (labelled: 0805) is working properly. If I measure it, in the diode mode (red to red dot, black to the other lead) it says 1.5V - shouldn't it be around 0.6V as any regular LED?

If I'm injecting a regular 1K sine wave, I get a 2V positive pulse wave like signal on the output of the A5 op amp. After D2 it drops around 0.2V and never reaches the next opamp A6. One difference compared to the original unit is that after A6 it has an NPN transistor connected as per below and strangely I'm reading a -7.25VDC signal on the DMM at the output on leg 1. Could that opamp go wrong and cause an error in that envelope detection circuit?

Can you guys head me in the right direction on moving forward, I'm a bit stuck at this point.
Cheers!
   

[PRR]

LDR ...If I measure it, in the diode mode (red to red dot, black to the other lead) it says 1.5V - shouldn't it be around 0.6V as any regular LED?
......strangely I'm reading a -7.25VDC signal on the DMM at the output on leg 1. Could that opamp go wrong

A "regular" red LED threshold is near 1.6V, not 0.6V (that's a plain diode). 1.5V may be the spec for low current or for IR LEDs.

-7V might be a sick opamp (can you swap it?) but really sounds like wrong input voltages telling the output to go past where it can go. How about all voltages on that opamp, in both modes?

[idy]

Yes, voltages for the opamps, all pins.
What about the transistor? The schematic has one, does your unit?
Is it a mutron or a clone?
What kind of opamps? 4558? Something else?
What power source? Two batteries like the schematic? What do those batteries measure out of circuit?
These are bipolar, so you you have -9v +9v and 0v ground. or -7.5 and +7.5 if the batteries are dying.

[bandus]

It's an original Mutron III - has that extra transistor in it as pictured but, an 2N5172 (NPN). 

Collector: A6 pin8: 9,36VDC 
Base: A5 pin1: -7,25VDC
Emitter: 0805 red dot

Opamps are 4558s.
A5/A6 voltages are below
1:-7,25VDC
2: -
3: -
4: -8,67VDC
5: -
6: -
7: -
8: 9,35VDC

Power sources are a battery reading about 9V out of circuit and a battery clip to boss style socket converter to PSU. About 9V on that too.

[duck_arse]

2: -
3: -

5: -
6: -
7: -

these are not valid readings. we always want, need the numbers. please.

[bandus]

Drive up position:
1: -7,25 VDC
2: -0,012 VDC
3: -0,042 VDC
4: -8,67 VDC
5: 0,237 VDC
6: 0,259 VDC
7: 0,560 VDC
8: 9,35 VDC

Drive down position:
1: 8,72 VDC
2: 3,19 VDC
3: 3,71 VDC
4: -8,35 VDC
5: 0,237 VDC
6: 0,259 VDC
7: 0,560 VDC
8: 9,35 VDC

Cheers!

[idy]

It would help if we knew which IC was which. The schematic numbers the opamps A1, A2...we need to know which opamp is in which package.

And there is a third dual opamp. We love these numbers. Can we see them?

So the transistor is the same type (NPN) as the schematic, just a different brand.

Interesting choice, one battery and one power supply. Have not seen that.

[bandus]

It would help if we knew which IC was which. The schematic numbers the opamps A1, A2...we need to know which opamp is in which package.

And there is a third dual opamp. We love these numbers. Can we see them?

So the transistor is the same type (NPN) as the schematic, just a different brand.

Interesting choice, one battery and one power supply. Have not seen that.

These voltages are for the A5 and A6 opamp stages. Mine has a different NPN transistor right after A6, but before the LDR, 2N5172.
About the PSU, this one I had in hand but might get a proper PSU for it.

[bandus]

Finally had some time to spend on this Mu-tron, but I'm still scratching my head on moving forward...

I've measured the resistor side of the LDR and found that for the first part (between A2 and A3) I'm measuring 8,1K when no audio signal is there - if there is signal then 0,8M (low range, drive down) and for the second part (between A3 and A4) 11K when no signal is there, if there is signal then 1,7M (low range, drive down).

In BP and HP mode the signal is pretty weak, barely any is going through, in LP mode the sound is full, but there is no filter movement. Not sure that this is the nature of the beast, but it seems to me that at least the filter part is doing its thing. Any idea on how to make sure that the LDR is correct or moving forward?

Cheers,

[bandus]

I've managed to have some level of envelope-following action going on. There was a tantalum cap (C8 - 4,7uf) in the detection circuit that went wrong - of course, what else... Still sounds odd in the High Range setting, especially in High Pass mode like if the filter has no place to go beyond. So I have to dig through, but finally some progress!

For further reference: always be suspicious of the tantalums!

[idy]

Filter that won't go any further, there's a couple of things to think about:
Isn't there a trimmer in there somewhere? It went between R19 and 20.

Turning the sensitivity ("Gain") knob down may keep the filter from topping out.

there are other things you would do if you are experimenting, but with an original unit you would hope to be close and not have to do too much. But I hear there is a cutoff where early units were tuned correctly and later were hit or miss and needed... a little trimming.