Echo Blue Delay thinks it's an octaver

[FootSolder]

Hi Sages,

I'm stumped by the behavior of a Fuzzdog Echo Blue delay (Deep Blue clone) that will not delay but creates an ugly octave-down instead. This version includes the dirt mod (which adds a drive control for the repeats), when this and/or the Feedback control is raised beyond about 30% it goes straight into self-oscillation. But no actual repeats.

I've built about 6 of these without issue; I checked all the parts values and orientation, switched out the TL072, the 78L05 and the PT2399 from a working delay. Reflowed all the pads. Grateful for any suggestions!







Voltages I get with a OneSpot supply:

78L05 I 9.62, G 0.00, O 5.74

TL072
4.84   9.75
0.00   4.86
4.56   4.86
0.00   4.84

PT2399
5.73   2.77
2.77   2.77
0.00   5.70
0.00   2.40 (drops slowly to 0.95)
3.02   2.78
2.77   2.77
0.74   2.77
0.77   2.78

[duck_arse]

is pin 2 of the TL0 really 0V? it shouldn't be. is pn 14 of the PT really 5V7? a 5V reg should be outputting 5V, not 5V74. things to check.

also, oddly, your regulator has/shows/is reported as "9.62" at its input, but the TL0 shows having "9.75", but connecting to the same point.

[FootSolder]

Let it sit with power for a bit then retested. TL072 Pin 2 is actually 4.94, PT Pin 14 measured 5.72 same as 78L05 Out 5.72

The Reg In was 9.91, same as TL pin 8, also 9.91. The readings were also very similar with the original Reg before I swapped it out, thought it seemed too high as well.

I have had no delay with a bad PT chip in this circuit but never the octave down and huge oscillation

[Kevin Mitchell]

THE DUCK HAS SPOKEN
It shouldn't be any more than 5% - so anything over 5.25V is suspect.

Also, the schematic has the pin numbers reversed as if it was supposed to be a 7805 instead of a 78L05 (reversed pins).

[FootSolder]

The BOM calls for a 78L05 and I have checked images of the other working Echo Blues I built and they all have the 78L05 in that orientation. If I've already replaced the Reg, what is causing the issue with the too-high voltage and would you expect that to mess up the PT2399?

[FootSolder]

FWIW Running it on a depleted battery (around 7v) stops the oscillation but still produces the octave down instead of delay, REG Out being 4.5v

I'm assuming it's an issue with the PT2399 but don't know what else to check, any suggestions? TIA

Revised voltage readings:

78L05 I 9.62, G 0.00, O 5.72

TL072
4.84   9.75
4.94   4.86
4.56   4.86
0.00   4.84

PT2399
5.73   2.77
2.77   2.77
0.00   5.72
0.00   2.40 (drops slowly to 0.95)
3.02   2.78
2.77   2.77
0.74   2.77
0.77   2.78

[ElectricDruid]

While 5.7V is within the "absolute maximum" (AKA the smokepoint!) of 6.5V for the PT2399, it's *outside* the recommended operating conditions (max 5.5V). I don't know whether that could cause the problem but it certainly won't help.

I think I'd check the 5V supply. 5.7V is out by 14%, which seems like a lot of error for a 78L05. Nat Semi's datasheet says "+/-5%" which seems more like it.

[Kevin Mitchell]

These regulators have a dropout voltage of 2V. It needs at least 7V in for 5V out and to meet it's suggested current rating.
I think your regulator(s?) are out of spec and causing current related issues. The PT2399 demands more current at higher delay speeds. What you're seeing is;
1: out of spec regulator
2: PT2399 being forced to function at an inconsistent low frequency causing an "octave down" effect along with other issues.

I'll note that I'm speculating and only did a bit of research. Other than that, your solder work is clean and values look right.

[duck_arse]

the shaggy dog has spoken, too.


whatever is connected to pins 13 and 14 is causing problems. both those voltages should be sitting at half the 5-ish volts supply. none of the pins on that side should be slowly anything, rock steady at half about 5V only allowed.

pull the IC, check for shorts between pins 1 and 14, maybe.

[FootSolder]

Many thanks for the suggestions.

I took out the replacement REG and added a socket, tried numerous 78L05s from different manufacturers, on three different power supplies and a battery - all gave a very consistent output of 5.72

I did notice that all the power supplies (including the 9v battery) measured over 10v, I'm wondering if my multimeter is reading properly?

Pulled the PT2399 and checked for shorts, tried a bunch more PT2399s. There is a 2n2 cap between pins 13 and 14, so I replaced that. I found that if I shorted pins 13 and 14 it kicked off the low oscillation, if I tapped 13 with a probe it stopped. No idea if that means anything.

If I were to assume that my meter is off and the REG out is correct, what else might be stopping the delay?

[ElectricDruid]

If the solution is to assume the measurements are wrong, we're in trouble!! 

However, they may be. Change the battery in the meter first?

[FootSolder]

I am an idiot. One more time, with a fresh battery in the meter...

78L05 I 8.50, G 0.00, O 5.00

TL072
4.24   8.48
4.23   4.24
4.24   4.24
0.00   4.24

PT2399
5.00   2.42
2.43   2.42
0.00   4.98
0.00   1.60  (drops slowly to 0.85)
2.74   2.43
2.42   2.43
0.60   2.42
0.63   2.43

So I think everything looks healthy apart from pin 13, correct?

The attached image is of the 'dirt mod' that adds a gain control that includes C23 (4u7) at pin 13, so I replaced that cap. Pin 13 still slowly drops voltage, still sounds like a bad octave.

[duck_arse]

PT2399
5.00  2.42
2.43  2.42
0.00  4.98
0.00  1.60  (drops slowly to 0.85)
2.74  2.43
2.42  2.43
0.60  2.42
0.63  2.43

So I think everything looks healthy apart from pin 13, correct?

nice work on the battery. but it's pins 13 and 14 is bad/wrong.


have you tried pulling out the PT2399, and then measuring the voltages on the empty IC socket? if not, then, goto. please.

[FootSolder]

Socket:

5.00   DROPS
0.04   DROPS
0.00   DROPS
0.00   0.00
0.00   DROPS
0.00   0.00
0.00   0.00
0.00   0.00

Pins marked as DROPS seem to start around 1.00 then fall to around 0.15, dropping quickly at first then increasingly slowly

[duck_arse]

you are showing pin 13 as 0.00. so that doesn't drop, is fixed steady? it looks like a DC path to 0V somewhere, but there isn't one showing on the circuit diagram. and your first circuit dia didn't show "a" mod, connected to the pin we are now looking at. tsk tsk tsk. try disconnecting that pot/mod, see if anything happens.

[FootSolder]

Yes, Pin 13 was steady at 0.00, and apologies for the schematic confusion I didn't realize that option wasn't included on the original.

Removed the mod, the octave effect and oscillation disappeared but still no delay. Pin 13 steady 0.00 and Pin 14 4.97

[duck_arse]

perhaps again pull the PT, and measure the resistance between ground and pin 13 and ground and pin 14.

[FootSolder]

Pin 14 and Pin 13 have no measurable resistance

[ElectricDruid]

Pin 14 and Pin 13 have no measurable resistance

Is that "zero" or "infinity"?

[FootSolder]

Infinity, my meter just reads "1  ."