SA 571 expander problem

[michauorin]

I have a problem with getting the ad3208 to work.
Using signal scoping i was able to trace that the signal gets nearly completely lost when getting back to the sa571 to get expanded.
The expander circuit seems so simple that i don't even know where to look for potential troubles.

Shall the 22k resistor be matched as close to 22k as possible? Cause i used a 5% tolerance one...
I believe the caps should be OK, for the 470 nF one i used MKSE and ceramic for the 100pF.

I'm not sure what to look for, i may try swapping the chip, as i have a spare one, tho i don't believe it would help tbh.

May it be the input signal is too weak to pass the expander threshold? I don't have a proper oscilloscope so i can't check exact levels, but my DMM tells me there's around 40 mV AC entering with a strum. Measured at minimal delay. At max the signal gets pretty weak, falling down to 7-10 mV, so maybe tis' a problem with the mn3208 chips? Although i can imagine why the signal gets weaker at longer delays in BBD's i'm also not sure it it's normal operation, after all the delay levels should remain constant regardless of delay length?

Or should i seek the problem in the gain of the NPNs at bbd output?

There's some oscillation topping at 3 kHz, with a total bandwidth around 0-5 kHz (tho may be something much higher, i don't have an oscilloscope, and with audio scoping i can measure only up to 22 kHz) going on after the bbd's before the expander. And before the first bbd input too. Funnily enough it doesn't pas through the bbd, between the bbd's there's only the guitar signal.

[armdnrdy]

A couple of questions.

Where did you source the MN3208s?

Have you checked and double checked that you have the correct value resistors in place?

Now...if you did...check again!

Build an audio probe so that you can see where the signal is disappearing.

http://www.diystompboxes.com/wiki/index.php?title=Debugging

[Marcvv]

Hi,
Post your dc voltages. It will be easier to respond when they are listed.

Succes

[michauorin]

Ok, so:
-The MN's are from china. It might be the problem, but then i would expect them to not work at all, whereas they do.
The only problem with the MN's themselves were the pain of biasing (i expect the 5% resistors didn't help either) and that the signal gets weaker the longer the delay is.

I did audio scope the signal, as i said in the first post, but here You have a listed scope log : (in order of signal chain)
   [Element]               [Result]
-TL072 pin 5,6,7           Clean signal
-SA571 first half
    pin 10                      Strong clean signal + noise
    pin 11                      Weaker clean signal + noise
    pin 12                      Nothing (strangely, why there's signal on r3 in, and no signal on -in)
    pin 14                      Weaker clean signal + noise
    pin 15                      Weaker clean signal + noise
    pin 16                      Weaker distorted signal + noise, low freq oscillation bandwidth up to 3 kHz, but tops around 150-200 Hz, 450-500 Hz and somewhere further
-Repeats send               Nothing, i guess i'll have to check it later
-First NPN
   base                          Nice and clean signal again
   emitter                      Clean signal + noise again
-First BBD
   input (pin7)                Clean signal + noise, wider bandwidth, goes higher than 5 kHz
-Second BBD
   input (pin7)                Slightly distorted signal after the first BBD, no noise again
-Second NPN
   base                          Same as at BBD output, still no noise
   emitter                      Amplified but noise appears again
-Third NPN
   base                          Same signal, no noise again, i can see a pattern here
   emitter                      Once again amplified, but noise reappears
At this point the signal level is much lower than before the bbd's, the lower notes are quite heavily distorted, the distortion happens in the BBD's

Signal before bbd's measures -6 dB VU (after amplification for measurements)
Signal after bbd's measures -29 dB VU, 23 dB loss is a lot... but i believe that's what the output npn's are for

-Second half of SA571
   pin2       
   pin3        for either of those, same signal as at the third NPN output + noise
   pin6
   pin7        for either of those noise only

I don't see any reason to check further, as the signal path ends here, at the expander.
(The second half of tl072 is dead quiet, except for minimal noise).



I will measure DC voltages and post them later, but apart from pin1 sa571 they seemed normal. Pin 1 started around 0.9 V and was gradually floating down while measurement.

I will also check the elements value again later, i might consider swapping some of the important resistors to 1% as some are 5% at the moment.

[UPDATE]

DC's:
SA571:
pin1 = floats from 0.9 v down
pin2,3 = 1.82 V
pin4 = 0 unsurprisingly
pin5 = 1.83 v (? internal vref tolerance?)
pin6,7 = 4.82 v
pin8 = 1.82 v
pin9 = 1.82 v
pin10 = 4.26 v
pin11 = 1.82 v
pin12 = 1.83 v (again?)
pin13 = 9.17 v (supplied from a 9v zenner+bd139 stabilised psu, that tends to float +-15 mV depending on room temp)
pin14,15 = 1.82 v
pin 16 = 1.06 v

I'll measure the rest later.

[armdnrdy]

I did audio scope the signal, as i said in the first post

You said "signal scope" which is generally associated with an oscilloscope.

I did see that you mentioned that you do not have an oscilloscope so...it was not clear what you were referring to.

One thing that you did not mention...are you getting any delayed signal after the BBDs?
If BBDs are "bad" they generally do not pass any signal.

Does turning the bias trimmers change anything? (the 10K trimmers at the BBD inputs)

[michauorin]

Oh, that part, my bad indeed (i tend to formulate my thoughts not as clear as i should). Well, i used pc as signal analyser, so that's a kind of signal scoping after all. I'm used to referring to most things i analyse as signals, a habit from EE studies.

Yes, the BBD's pass the signal with delay, i may run some tests by recording output of BBD lines in parallel to their input to measure and see if the delay corresponds the clock frequency.

Yes, the biasing works correctly, i have biased the BBDs the best i could using only my ears and screwdriver, they wouldn't pass signal at all if i didn't.
Small deviation from the bias point introduce major distortion, larger causes the chips to stop passing any signal at all.
I'm not sure how tight the bias normally is in these, but my chips needed pretty precise biasing.

Oh, the rest of DC's, i forgot to measure it.

[armdnrdy]

A couple more questions.

Did you etch a board from an existing layout?

What type of BBDs are you using? Panasonic/Mitsubishi, Shanghai Belling, Cool Audio?

[michauorin]

The chips [14 pin version] are marked with an M on them so i'll guess mitsubishi, but i wouldn't trust the markings too much [i did order them from china after all], i bet they are restamped.

Oh, right, that's a hint, since it's easy to spot which ones have the markings sanded off (the pin 1 mark is sanded off as well) i just noticed that one of the chips plugged is sanded, and the other seems more legit. As i have 3 spare, and two of them seem legit, i'll try swapping in a momemt.

I etched the GGG board, came out more less ok, i had to fix a few paths broken by an electromagnetic stirrer ('twas a dumb idea to take a magnet prone to uneven contact with the board i should expect it to damage the traces right underneath it's edges, for the next time i'll use it i'll separate the board from the magnet using a mesh or sth), but nothing serious. Except it took some effort to fix, but i checked the continuity afterwards and it was ok.

Remaining voltages:
MN3102
1. 8.5 V
2. 4.43 V
3. 0.63 V
4. 4.45 V
5. 4.55 V
6. 4.53 V
7. 4.54 V
8. 7.98 V

First MN3208 (right one)
1. 0
2. 4.41 V
3. 3.03 V
4. 3.03 V
5. 8.51 V (my 8.2V zenner gives this voltage, supposedly a lower voltage one might work better, as these chips like to run lower, right?)
6. 4.43 V
7. 5.92 V (that's a pretty high bias, isn't it?)
8. 1.03 V

Second MN3208
1. 0
2. 4.45 V
3. 5.85 V
4. 5.86 V (yes, there is around 10 mV difference in outputs :| )
5. 8.51 V
6. 4.43 V
7. 4.92 V (this is the one with markings not sanded of, or rather not as prominently)
8. 7.98 V

TL 072

1. 4.59 V
2. 4.59 V
3. 4.59 V
4. 0
5. 4.19 V
6. 4.59 V
7. 4.59 V
8. 9.18 V

NPN's [BC 550c] [as they appear in signal path]

Before BBDs
C 9.18 V
B 4.24 V
E 3.65 V

After BBDs
C 9.18 V
B 5.77 V
E 5.17 V

C 9.18 V
B 5.14 V
E 4.54 V

The SA 571 voltages are a few posts before.

[EDIT]
Oh great, i think i fried the MN's by swapping them, (input signal is ok, no output at all).

[armdnrdy]

Look at pin 8 (VGG) of the first BBD.

Double check that voltage. If the BBDs are socketed...check the socket pin with the BBD removed as well.

[Fender3D]

You can easily test 571 by lifting it and placing 2 jumpers one from pin2 or 3 and pin 6 or 7, and one from pin11 and pin14 or 15

[michauorin]

Look at pin 8 (VGG) of the first BBD.

Whoops. Bingo. 
A perfect cut in the BBD1 vgg supply trace, small enough to remain unnnoticed at a first glance, large enough to not create a solid contact by pcb tinning.
Probably by removing the bbd from the socket i created enough tension to break loose anything that partially shorted it, and that's why the first socket stopped working at all.

Heck, i could have cut it by accident with my razor sharp DMM leads during pcb testing.

I'll fix that and see the results, then i may or may not try shorting 571 out of the circuit depending on if it'll work