MI Audio Crunch Box - gated sounding, then dies.

[mikestahlme]

I just built this MI Audio Crunch Box today. Here's the vero



I think this is the correct schematic:
https://dl.dropbox.com/u/54382528/miaudiodistv2rx7.jpg

The layout is from IVIark's blog and is verified.

The problem I'm having is that when I turn it on the sound is kinda gated. After playing for a few minutes it just fuzzes out to nothing eventually.
After leaving it alone for a while it works again.

Here are my voltage readings:

My power adaptor is supposed to be 9v but it's putting out 11.2

IC
1. 4.93
2. 4.96
3. 5.30
4. 0
5. 5.30
6.constantly changing between 1.90-3.10
7. 11.02
8. 11.24

D1
+ 1.43
- 0

D2
+1.51
- 0

I'm gonna guess this is a capacitor problem but I really don't know.
Thanks for any help or pointers. If you've experienced this "gated, then quits, then works again" problem, please share what solutions you've found...

[jymaze]

I would say you have to recheck all the connections/solder joints around the right part of the opamp.

Also, take a multimeter and check (circuit being not powered) that resistance between pin 7 and ground/V+ is huge, then that resistance between pin 6 and ground/V+ is huge. Also check that you really have 1M of resistance between pin 6 and 7 and not infinite.

If the above is not true, then it may be the cause of the problem and you have to track were there can be a mistake/short-cut.

Also, I check that all your cuts are of perfect quality with a magnifier (especially for the cut under C3, 100n). Sometimes very small pieces of copper can go unnoticed.

[mikestahlme]

Thanks,
With my meter set to the 2000's range here's what I read.

Pin 7: 305
Pin 6: 354
Between 6 & 7: 143
I checked the track cuts again and they are all in the correct places and cut completely. The jumpers are also correct.

I'm wondering if a component is going out of spec as it receives power. I was hoping someone would be able to tell me what would have to be the wrong value in order to get a gated sound. I know jfets are biased with a resistor/trim pot, so by misbiasing it you can end up with a gated or splatty sound. I'm not sure what would cause that in this situation.

Also there's a lot of crackling and noise before it cuts out. It also sounds a lot more like a fuzz than a "marshall in a box"

[alexradium]

the wrong value is full voltage on pin 7,it should be around those on pin 5 and 6.
recheck every trace especially around pin 7 and 8.maybe under the ic socket?
i never liked stripboards,i always ended up with problems,i use them for little circuits with no more than 10 components

[mikestahlme]

"It should be around those on pin 5 and 6".

Do you mean pin 7 should get a reading similar to 5 and 6? Or that 5 and 6 should be around full voltage?

[alexradium]

"It should be around those on pin 5 and 6".

Do you mean pin 7 should get a reading similar to 5 and 6? Or that 5 and 6 should be around full voltage?

the only place where you have to read full voltage is pin 8.
on pins 5,6,7 you have to read similar values like pin 1,2,3.
the theory is that your AC signal rides on top of that half voltage and when it gets to  the top it fuzzes out.
Now you are costantly having a 5.3V on pin 5 and 11V on pin 7,there is a mismatch,hence the gated sound.

[jymaze]

I am not sure I understand the results of your measurements at pins 6 and 7:

1) Remove the IC to make these measurements

2) Resistance from pin 6 to ground and/or V+ should be infinite

3) Resistance from pin 7 to ground and/or V+  should be infinite

4) Resistance from pin 6 to pin 7 should be around 1 mega-ohms

Any other result says you have a vero mistake.

Okay, I have to clear that up before : Did your try another IC? I assume you did, any double opamp will do. Once I chased a problem for hours and the opamp was at fault, I was just assuming it was working because why would it be faulty after all (it was a brand new opamp, still wrapped) ? I was wrong... It happens.

In operation, pins 5 and 6 should read the same, just like pins 2 and 3. If an opamp has its 2 inputs being apart, there is only 2 solutions:

1) The opamp is faulty
2) It is not working in normal operation (problem in the feedback loop)

Let me know what you get.

[mikestahlme]

Removed IC
Pin 6 & V+: Infinite
Pin 7 & V+: Infinite
Pins 6 & 7: 118 when set to the 200k range, 151 when set to the 2000k range.

I haven't tried another IC yet, I made the same assumption. I have a TL072 I can put in.

[Pyr0]

With those readings between 6 & 7, it looks like you might have R5 wrong. Are you sure it's a 1M resistor ?

[anchovie]

Photos of the top and bottom of your board, please!

[jymaze]

For sure check that R5 is really 1M (mistake or blown up resistor)! It could also be a faulty C6!

With only 150 between pins 5 and 6 you have a negative gain: divided by 66. 6 which is the number of the beast, hence the devilish failure of your circuit!

More seriously, opamp don't really work as expected below unity gain, so on the top of killing your distorted sound, it will just behave erratically and may have a DC drift, which explains why it takes some time to fail. Basically I think the output starts in the middle and the DC drift slowly takes it to the V+ rail where it gets stuck.

[mikestahlme]

I had accidentally placed a 10k for R1, But R5 was a 1M. I replaced it anyway though.
I changed C6 from 100p to 47p to try and get more gain.

after making these changes it is still the same. Next I will try the TL072, when I'm home from work.
I'll try to have some pictures up as well.

[slacker]

If you've socketed the opamp, remove it and with the circuit powered measure the voltages on the socket pins 5 to 7, you should get about 5 volts on pin 5 and nothing on pins 6 and 7. If you get any voltage on pins 6 and 7 then you have a short or a misplaced component connecting them to one of the voltage rails.

[jymaze]

I think it is not a shortcut of pin 6 or 7 to the rails, but a shortcut between pins 6 and 7 since mikestahlme posted the following:

"Removed IC
Pin 6 & V+: Infinite
Pin 7 & V+: Infinite"

Once again:

If you really measure 120/150 ohms between pins 6 and 7 (while having the IC removed, R5 being 1M, and C6 being not being fried), there is a short cut somewhere! It is the only solution electrically speaking!

What about removing the IC and desoldering one leg of R5 and one leg of C6 and measuring the resistance between pins 6 and 7? It should be infinite then. If you have 150 (or anything else for that matter), then you have a to chase a shortcut, again.

By the way, changing C6 to 47p from 100p should just give you a little more sizzle, but not more perceptible gain. It is of course not the cause of your problem (unless you fried C6, which is pretty common with small ceramics because they are fragile). It is a matter of taste.

I really hope you will get that one to work, it is really good sounding!

[slacker]

True, but if the problem was only a short or partial short between pins 6 and 7 you would expect them to have the same voltage, and you would just get no or little output. Given his voltages and what you just said it's probably a bad opamp. I just always do the voltage check as a matter of course.

[jymaze]

Slacker,

As measured, 150 ohms would be a partial short, making a gain lesser than unity and an opamp that could behave randomly and get stuck to one rail. It is possible.

Of course a full short would have 6 and 7 identical.

[slacker]

Thanks for the explanation, I knew some opamps don't like a less than unity configuration, but I didn't know it could could cause them to get stuck like that.

[mikestahlme]

I haven't had the time yet to sit down and test this thing, hopefully tonight. But I've seen you mention the opamp "getting stuck" acouple of times. What do you mean by that?

[jymaze]

Hello,

By "get stuck", I mean that sometimes under non-standard conditions, opamps that are unstable can manifest that unstableness by having their output (pin 7 here) going to one power rail or the other. It would explain why pin 7 is almost at V+, but not quite because the LM833 is not a rail-to-rail opamp so a small voltage drop is expected at full positive excursion.

If there was a real short of pin 7 to V+, voltages would be mainly identical. This small voltage drop at pin 7 basically gives away that the higher than expected voltage comes probably from the opamp and not a shortcut. Electronics is all in subtleties... But I may be totally wrong on this one anyway, it is just a theory.

A good way of seeing that an opamp is not in standard conditions is to check if there is a difference between the 2 inputs (pins 5 and 6). Under standard linear conditions they HAVE TO BE identical in term of DC and AC.

It is part of the opamp Golden Rules, and the most important rule even:

I. The output attempts to do whatever is necessary to make the voltage difference between the inputs zero.
   
II. The inputs draw no current (or at least it is negligible enough).

So the 2 different voltages you reported for pins 5 and 6 tell me your opamp is not operating in standard conditions, hence the possibility of unstableness and erratic behavior. Obviously, another reason for erratic behavior is a defective unit.

[mikestahlme]

Thanks for the explanations guys! Sorry for the delay, this was kinda put on hold because of the holiday.

I pulled out one lead from R5 and C6 and measured infinite resistance between pins 6 and 7 like you said.

This opamp did come from China, but I ordered it through Tayda who I thought was pretty reputable. Maybe it's defective like you said?
I guess my next step would be to put in a TL072 (also from Tayda) and see what happens.