Seventh Circle N72: odd distortion on scope

[soapfoot]

does it adjust to correct nominal voltage?

JR

It does, in fact, and the voltage at R40 is bang-on when I do so as well... so that mystery is solved. No change to the behavior, though.

 

[soapfoot]

One other thing to throw out there--

the mounting of T1 required a bit of rework on this unit, as I'd made a wiring error (which I caught before ever applying power). The rework seemed to go off without a hitch, and all transformer resistances/continuity match a known-good unit, so there seems to be no issue.

But could an issue with T1 conceivably cause what I'm seeing here?

 

[moamps]

Measuring probe was connected to the output XLR, CONN2, pins 2-3

Correct, you can bridge also pin 3 to PGND to get clearer picture.

When you say to measure AC voltage on potentiometer, are you referring to R19 or R39? And do you want this measurement with signal applied or no?

R19, pin 3
Apply the signal to the input to get there a sine signal about 0,5V peak. Set your scope input channel to cal,
do you use 10x probe?

 

[soapfoot]

R19, pin 3
Apply the signal to the input to get there a sine signal about 0,5V peak. Set your scope input channel to cal,
do you use 10x probe?

R19 pin 3 shows 2.4 VAC with nominally 5Vpp of 1kHz applied to input (CONN1)

Connecting scope to R19 shows similarly-distorted output. Probe is indeed set to x10 (it has also been for calibrating other normal units)

 

[moamps]

Is signal on R19 distorted?

 

[soapfoot]

Is signal on R19 distorted?

It is indeed

 

[moamps]

So your input is overloaded, or something is faulty in the first stage. Try to lower input signal first.

 

[Ike Zimbel]

A couple of other thoughts: I know you've looked over the board several times, but have you compared it to a working board at the same time? If you do a nice, neat job soldering, you'd be surprised how "right" a solder bridge can look (don't ask me how I know that ), so a nice, neat solder joint that is joining two pads that aren't meant to be joined could be throwing you off.
As well, did the rework of the transformer mounting involve un-soldering anything? If so, you might have removed a via that is supposed to be connecting the circuit from one side of the board to the other. If that's the case, you need to compare with a working board and ohm out each set of pads on the transformer.

 

[soapfoot]

A couple of other thoughts: I know you've looked over the board several times, but have you compared it to a working board at the same time? If you do a nice, neat job soldering, you'd be surprised how "right" a solder bridge can look (don't ask me how I know that ), so a nice, neat solder joint that is joining two pads that aren't meant to be joined could be throwing you off.
As well, did the rework of the transformer mounting involve un-soldering anything? If so, you might have removed a via that is supposed to be connecting the circuit from one side of the board to the other. If that's the case, you need to compare with a working board and ohm out each set of pads on the transformer.

Indeed, I've got two units side-by-side... this one, and a working one.

But I'm probably going to pore over it one last time (and reflow all of SW1 once more) before sending it off.

 

[swpaskett]

Still thinking. You said you had to do some work on the input tranny. If there is a possibility it is damaged, you could disconnect it and apply your signal directly to the S1a deck. That would be simple if it has wire leads, harder if it has PC pins, but doable. OTH, it is hard to imagine any damage to a tranny that would lead to a distorted waveform that looks so much like serious overdrive.

Distortion is present at all gain settings, both on the stepped gain switch and all positions of R19. Stepped gain switch behaves normally at each position (in terms of adjusting level). All joints on this switch (SW1) have been reflowed twice.

I missed this earlier. Do not get the wrong idea here -- I believe you -- but the part about the switch adjusting gain properly and the distortion being present at all settings and reducing the input signal makes the distortion go away (from an different post) is incongruous. The only conclusion I can draw is that something is wrong somewhere where it is not affected by the gain setting, exactly the opposite of what I was convinced had to be the case earlier. IOW the problem is independent of the gain setting but dependent on input signal level. (Excuse the italics; I want to emphasize the logical interconnection of these statements.) That said, if you scope the collectors of Q1 and Q2 and the emitter of Q3 and maybe the collector of Q4, you might be able to determine the stage where the distortion starts. My thinking here is that one stage has a lot more gain than it should, and that points to a problem in the part of that gain setting feedback network that does not include S1. That network would probably require some serious simulation to understand, but I would be suspicious of all those Rs and Cs wrapped around Q4 and Q5. because I cannot wrap what is left of my brain around it.

If the suggestions above can locate the fault to the neighborhood of one transistor you can maybe hone in on one small area, though with global feedback that may not happen; e.g. you may see the same distortion through the entire signal chain (as has been my experience), but it's worth a shot.
Your checking thus far has ruled out wrong part placement and the PCB construction makes miswires nigh on impossible, but you have not ruled out defective parts, which is your most likely problem.

I also noticed, from the same post quoted above, you verified the values of all resistors, I presume by checking the color code. The time will come you have to desolder some of those parts and verify by measurement they are the correct value. I tried to say this earlier -- resistors and caps are jellybean parts and that means they are likely suspects.
Manufacturers have honed the process to the point that resistors only get spot-checked on the way out the door, probably caps, too. How many do they sample? 1 in a hundred, or a thousand, or ten-thousand? I dunno, but I do know that I have pulled enough open and off-value resistors out of stock over the years that I ohm out each one before it goes in a board. If the manufacturer finds a defective resistor in a lot they have a choice: burn the lot or check them all and see how many they can salvage. In the US, back when we made resistors, they just tossed the lot; the labor was too expensive to test 100%, but that was the 80s and this is now and China makes our resistors and testing can be automated so maybe I'm just blowing smoke. YMMV. Those with more recent manufacturing experience may have more valid opinions than I can offer on the subject of commodity parts quality bought on the spot market, where most hobbyist parts come from.

 

[soapfoot]

Still thinking. You said you had to do some work on the input tranny. If there is a possibility it is damaged, you could disconnect it and apply your signal directly to the S1a deck.

Now this is interesting--

I didn't disconnect T1, but I did try applying signal (still 0.5Vpp) across C1 (essentially T1 secondary), and I got a perfect, clean, undistorted sine wave at output. I had to increase generator output to 1.4Vpp to show anything resembling the type of distortion I am seeing when applying signal to CONN1 input.

Re-applying 0.5Vpp signal to input and scoping the output of Q1 showed the distortion, however! (albeit at a lower amplitude than on output, obviously).

Does this seem to indicate that I have a faulty T1? It's almost as though there's an error in which I'm getting too much "free" gain from T1, maybe? I wonder if the wrong taps are connected somehow? (seems unlikely since it's PCB mount, but I did have to do some rework to this PCB, so maybe there's damage?)

 

[swpaskett]

You're definitely getting some gain from T1, but how much and is it the right amount? You can compare the transformer gain on this board to one of your known good ones. I suspect T1 is ok, but comparing to known good would prove the point.

 

[soapfoot]

Interesting progress:

Removed transformer from main PCB, and from its adapter card/daughter board (a laborious process!)

Applied 1kHz, 0.5Vpp signal to the transformer primaries in series, as configured in the circuit. Scoped the secondaries in series (as configured in the circuit).

The below picture is what I saw--it looks similar to what I was seeing when looking at the output of the completed unit (perhaps with a bit of noise and/or ringing from the unloaded transformer superimposed). Again, this is only the unloaded transformer, removed from the circuit--no adapter card or anything else.

Seems a pretty fair bet that I've got a damaged transformer then--the biggest question is how?

As I mentioned above, I had to do a bit of rework (desoldering/resoldering). Could my soldering iron have magnetized the core? Is there an easily-imaginable way that mechanical or heat-related stress could cause a transformer to distort?

At any rate it looks like I've found the problem (and it's the single-most-expensive component in the whole circuit, ugh)

 

[moamps]

You don’t have to complicate things that much, just take the DMM in the ohms position and measure the primary and secondary resistance. If one is infinite, then that winding is broken or you didn't connect the primaries or secondaries well together.

 

[swpaskett]

It would take a lot more heat than a soldering iron could produce to do any permanent damage to the core. You could possibly damage a winding though.
Steel and iron both lose their ferromagnetic properties when heated up to around 500 degrees or so. (Weller used this feature in their WTCP line of soldering irons years ago -- the ones where you had to change the tip to change the temperature. I still have the one I bought 50 years ago.) But that change is not permanent; soon as it cools below the critical temperature it becomes ferromagnetic again.
This is strange, though. Could the core be saturating? (Need a magnetics guy to weigh in). If that is the issue I don't think this behavior could be caused by anything you could have done -- it would have to be a defect in the core material (permeability issue, maybe). I would be looking for a warranty replacement before shelling out any cash.

 

[swpaskett]

A bit on core saturation: Core Saturation in Transformers - Come4Concepts

 

[soapfoot]

You don’t have to complicate things that much, just take the DMM in the ohms position and measure the primary and secondary resistance. If one is infinite, then that winding is broken or you didn't connect the primaries or secondaries well together.

I already measured resistance on the windings. They look okay, and yet the transformer appears to distort with signal applied.

 

[soapfoot]

For completeness: each half of the primary (pins 2,4 and pins 3, 5) shows 24.9 ohms

Each half of the secondary shows 135 ohms (pins 8, 10) and 141 ohms (pins 7, 9).

This all appears normal, which I'm tempted to speculate might indicate some kind of issue with the core(?)

 

[swpaskett]

If the core is saturating, you should be able to reduce the input to a point where the distortion goes away. Saturation is just overdrive, like driving an amp into clipping -- the core reaches a point where the magnetic flux (not sure that is the right term) no longer increases with increasing input voltage.
That is the limit of what I remember from magnetics class some 47 years ago.

 

[swpaskett]

Ooh, had another thought - how does the primary resistance compare with the primary of one of your good boards?