I messed up the timing cap of the bbd clock in my DMM

[j_flanders]

I have an XO version Deluxe Memory Man with a rather short maximum delay time of around 420ms.
After adding an extra 67pF cap in parallel to the 220pF timing cap of the cd4047 bbd clock the delay time increased to 770ms which is way more than I want.
After removing the extra cap, the maximum delay time is still 770ms!
How is this possible? What did I break?

At pins 10 and 11 I measure 5300Hz.

Calculating the frequency, given the R and C: 1 / (8,8 *97500 Ohm * 0,000000000220 F)) = 5297Hz
The 100k pot and 5k1 resistor in series measure 97500 Ohm.

Is the above formula correct?
If so I don't understand how it could ever have been 420ms.
Because for that I'd need less than 220pF capacitance in the formula, and there's only additional capacitance: 1nF caps and the varactor diode/ in parallel.

Any help to explain this mystery or suggestions about what to measure?

Here's that part of the circuit:


Here's a photo of that part of the pcb:

[Mark Hammer]

This is why I rarely attempt a mod with a surface-mount board.  The thing is that it's too damn easy to sweat something out of place.

[j_flanders]

I have modded smt boards before, pedals and amps and other stuff. I recently completely modded my Behringer vintage time machine for example. Even up to lifting the tiny pins of the compander chip to add series resistance and stuff. So, it's not my first attempt at this and I have never had any problems so far.
It's just that I cannot explain what is happening.

The R is still the same so my C must have increased. But according to the formula I'm getting the correct frequency for a 220pF cap, which contradicts with the frequency I was getting prior to adding the extra cap.
Any chance you could explain the behaviour I'm experiencing? Or comment on that frequency formula? Or have suggestions on what could have gone wrong?

Why were those C and R values choosen in the first place btw? Why 100k + 240pF and not 50k + 480pF.

I know Howard Davis and Analogman mod these pedals for more (correct) delay time and they say to just turn down the delay time knob if you want the original behaviour back.
Is that really the case? Isn't the varactor diode in parallel to the 240pF cap and wouldn't that change the modulation depth when you have a larger timing cap?
If the LFO varies the capacitance of the varactor diode in a 20pF range or so, it'd have more effect on a 240pF cap than on a 480pF cap.

The pedal still works, it's just that my old max delay time of 420ms is now at 11 'o clock on the delay time knob instead of at 5 'o clock (max).

[Mark Hammer]

Well first, you are a more courageous and meticulous person than I am!  I've gone so far as to do one mod on a Behringer P90 clone and that was enough to make me swear off doing so again.  Such things are for the brave and well-equipped. 

Why those component values?  No idea.  Once upon a time, EHX pedals were made from whatever bargains Mike Matthews could score; hence all those different issues of Big Muff Pi.  But that era has long since passed and EHX has enough purchase power to use whatever it wants for as long as those components are in production.  In other words, it doesn't use 100k and 240pf because 50k were "hard to find".

As someone clearly less familiar with SMT manipulations, does heat risk changing the value of such components?

[anotherjim]

Yeh, heat and other things. With such a high input impedance to CMOS chips, then solder mask, flux residue, cleaner residue, etc; there's scope for changing things enough in designs where pF matter. To put it another way, if it works according to how fast capacitors charge & discharge and the voltage thresholds, then changes in leakage current or insulation properties can matter. Rework can even remove a quality problem from manufacturing that nobody saw. Ever removed an SMD chip and found a load of residue/crud was hiding below it?

[Prehistoricman]

Yeh, heat and other things. With such a high input impedance to CMOS chips, then solder mask, flux residue, cleaner residue, etc; there's scope for changing things enough in designs where pF matter. To put it another way, if it works according to how fast capacitors charge & discharge and the voltage thresholds, then changes in leakage current or insulation properties can matter. Rework can even remove a quality problem from manufacturing that nobody saw. Ever removed an SMD chip and found a load of residue/crud was hiding below it?

It's quite common for reflow processes to make loose solder balls and these can cause all sorts of trouble.

[PRR]

> the 220pF timing cap

I don't see a 220, but a 240.

The CD4047 oscillator F is not just RC. The datasheet should explain. I see factors of 4.40 and 2.48 but am a bit confused.

I have no idea how you could put in a cap, be too slow, take it out, and no change. Except it sounds like yesterday's shopping: we put the bags and coupons out, and they didn't make it to the store. Lost something else the trip before that. So "Logic" says the cap you 'took back out' is still there. But I have no footing to question your own observations....
_____________
EDIT... the old NatSemi sheet is better than TI's hand-me-down. And it gives worked examples.....

10 kHz 220k 100 pF
1 kHz 220k 1000 pF

So 100K 220pFd should also be 10KHz, 100K 240pFd is 9KHz, and 95K makes ~~8.5KHz. Rate is inverse of C so another 67pFd puts you down to 0.8 or near 6.66KHz. On paper....

[j_flanders]

I don't see a 220, but a 240.

The schematic says 240pF but the pcb has a 220pF cap (C42) along with an empty spot (C59) to add 22pF. See the photo of the pcb in my first post.

I have no idea how you could put in a cap, be too slow, take it out, and no change.
So "Logic" says the cap you 'took back out' is still there.

I added the 67pF in the empty spot (C59), and as you can see in the picture, it's no longer there.
I'm completely baffled as to where that additional capacitance suddenly came from.

The obvious place would be a damaged C42 22OpF cap but after removing it, and checking, it actually was 220pF, so I have absolutely no idea.
Any way, I replaced the 220pF cap with a 120pF cap and now I have a max delay of around 550mS, which was my original goal. The modulation also sounds back to normal and the minimum delay is back around 35ms.
I'm happy again.

The CD4047 oscillator F is not just RC. The datasheet should explain. I see factors of 4.40 and 2.48 but am a bit confused.

It's a bit confusing (at first) But there's the formula for the monostable config and for the astable config and for astable there's a difference between pins 10, 11 and pin 13.

The formula for the frequency in the astable configuration is:

1/(4,4 * R(Ohm) * C (Farad)) for pin 10 and pin 11. These go to the bbd clock pins.
1/(8,8 * R(Ohm) * C (Farad)) for pin 13. Unused in the DMM

Calculating for DMM (pcb) values :

Max Delay : 220pF and 100k pot + 5,1 kOhm resistor:
1/(4,4 * 105100 * 0,000000000220) = 9829Hz
Or in seconds period: 1/9782 = 101µS
Multiplied by 4096 delay stages = 4096 * 0,000102 = 413ms delay

Min Delay: 220pF and 5,1kOhm:
1/(4,4 * 5100 * 0,000000000220) = 202560Hz
Or in seconds period: 1/202560 = 5µS
Multiplied by 4096 delay stages = 4096 * 0,000005 = 20ms delay

That max delay of 413ms is what I measured originally. It's also what most people report as their max delay.
The schematic also says: " 100uS(P4@CW) "
So how or why would EHX advertise this pedal as: "Up to 550 mS of vibrant echo"?
No amount of 'normal' component tolerances/deviations would get it up there.