bad NE570 compandor?

[Dimitree]

I built a Deluxe Memory Man clone, based on my own PCB layout,
it doesn't work as it should, since the delay signal is really low and there is no feedback (probably because the delay is too low).
So I used an audio probe and found that the signal is perfectly fine until the NE570 expander input (pin 14-15), but it gets higly attenuated on the expander output (pin 10-11).
If I short NE570 expander input to output, everything is fine (except of course that the expander is not working anymore).
Do you think I need a new NE570? or maybe there is a mistake on the surrounding circuitry?
I used a new NE570D (smd) by ONSemi..probably is not the same as the old NE570?

this is the schematic:
https://idoc.pub/documents/deluxe-memory-man-schematic-qn856vqrdkn1

many thanks for the help!

[Rob Strand]

Maybe the problem is the rectifier section.  If the rectifier input cap is bad or there's a bad connection or a short in that area the expander will be shut-off.   Possibly even check the rectifier filter cap, is it around the right way?

[Dimitree]

thanks, the rectifier cap (are you talking about C23 right?) is ok..I also swapped it (and measured) but same results.
When you say that the expander will be shut-off, that means no output on the NE570 out pin? because I actually have an output, but is really low compared to its input

[Rob Strand]

thanks, the rectifier cap (are you talking about C23 right?) is ok..I also swapped it (and measured) but same results.

Yes that's correct.

When you say that the expander will be shut-off, that means no output on the NE570 out pin? because I actually have an output, but is really low compared to its input

I mean the expander will produce a low output because it is stuck in the low gain state.

Given you are getting some signal the gain cell (pin 14) is probably working but it is stuck in low gain.    Normally a signal at the input of the rectifier (pin 15) will raise the voltage on the filter capacitor (C23) and that voltage rise will increase the "gain" of the gain cell.

Suppose at this point we assume the gain cell is working.   What you should see is the DC voltage on C23 increase when the signal is present.  The DC voltage should get higher as the input level gets higher.

If your R36 value is wrong, a low value, it might stop the voltage on the cap rising.

Perhaps check the DC voltages on the other NE570 pins as well.

[j_flanders]

the signal is perfectly fine until the NE570 expander input (pin 14-15), but it gets higly attenuated on the expander output (pin 10-11).

an output, but is really low compared to its input

That's what the expander does: make the output less loud than the input.

1) For normal (guitar) input levels the compressor makes the input signal louder, as loud as possible, preferably just below clipping the bbd's.
2) Then after that, the bbd's add their 'normal' level noise to the loud audio signal.
3) Then the expander makes everything less loud again. The 'normal' level noise will become low level and the high level audio will become normal level (the same level as went into the compressor)

The end result is noise (level) reduction.

So, it's perfectly normal to have lower signal level at the expander output compared to its input.

As a side note:
The compressor will also make a too loud input less loud. But under normal circumstances (guitar signal) you won't go over the limit.
But in those cases the expander will actually make the output level higher than its input, to bring it back up to the original level from before the compressor.

[Dimitree]

thank you for the support!

I checked every single part arount the IC, also swapped the IC, but it didn't make a difference..
I can hear a nice signal on pin 14,15 (even the feedback repeats) but then on pin 10/11, the signal is weak and with no repeats (and possibly a little slow attack)
Those are the voltages on the NE570 (when no audio signal is present)

pin    v
1.      -12V
2.3    -13V
4.      -14.9V
5.      -13.1V
6.      -13.1V
7.      -5V
8.      not connected
9.      -13.1V
10.    -7.1V
11.    -8.8V
12.     -13.1V
13.      0V
14.15   -13.1V       
16.      -13.9V

I applied a sine wave to the input, to check amplitude of the audio around the NE570,
this is what I got:

pin 2,3 = 3.7Vpp
pin 5 = no signal
pin 6 = 5Vpp
pin 7 = 4Vpp

pin 10 = 0.3Vpp
pin 11 = 0.3Vpp
pin 14,15 = 0.8Vpp

[j_flanders]

What test signal are you sending in?

No signal on pin 5 seems odd to me. Is the feedback loop from pin 7 through those 68k resistors connected to pin 5? Is C9 ok?
Still, that wouldn't explain your actual problem.

From the output of the compressor (pin 7) through the antialias filter, the bbd's, make up gain stage(s) and the reconstruction filter you seem to end up with very little voltage on the input of the expander.
You should see more or less the same kind of level there(pin 14, 15) as you have on pin 7.

Can you probe along the way from compressor output to expander input and see where you loose so much signal?

[Rob Strand]

pin 2,3 = 3.7Vpp
pin 5 = no signal
pin 6 = 5Vpp
pin 7 = 4Vpp

pin 10 = 0.3Vpp
pin 11 = 0.3Vpp
pin 14,15 = 0.8Vpp

Your AC voltage at the output of the compressor (= input of the delay-block) is 4Vp-p but the input to the expander (pins 14, 15) is only 0.8Vp-p.    So either something is clipping because your AC input level is too high, and/or the gain trimpot VR7 and VR9 have not been set-up correctly.   

You can also get low levels due to premature clipping of the Delay chips if the Bias trimpots are not set correctly.

I don't know if this is the problem but it doesn't help.  A low voltage at the input to the Expanding will cause an even lower output.

Do you have any notes on the set-up procedure?

One thing you missed is pins 1 and pin 16 are not supposed to have AC signal.   What you expect to see is the *DC* voltage *across* the caps C6 and C23 increase as you increase the AC input level.  The voltage on the cap is like an DC measurement of the signal level.  That DC measurement is what the chip uses to control the gain.  If that doesn't work the companding/expanding won't work.

Don't try to measure the voltage *across* C6 or C23 with your Oscilloscope, as moving the ground point could damage something, use a multimeter.

No signal on pins 5 and pins 12 is also OK because these points are virtual ground points of an inverting feedback amplifier.

BTW, the output of the expander is pin 10, not pin 11.  Pin 11 will show a low voltage.