McGregor - CrunchPlus

FeVeR2112 · Post by **FeVeR2112** » 26 Nov 2020, 01:07

The CrunchPlus has been on the YouTube demo circuit recently and I have one on the bench for tracing.
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As with any pedal that hides it's parts, it becomes a mystery which needs to be solved....

In this pedal, the quad op-amp and two transistors were sanded down and four diodes had its labels removed. The parts were then painted gold. At least it was not gooped.

With removing the paint, the diodes were revealed and the testing/readings of the transistors also gave them up.

There is no ground plane and all grounding is point to point.

My thanks to Bugg for his assistance in getting the schematic to this point.

Your comments / analysis are most welcome.

Here is the schematic so far:

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and the gut shots:

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and the traces:
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roseblood11 · Post by **roseblood11** » 26 Nov 2020, 01:56

R19 to gnd, VREF is between R7 and R19

FeVeR2112 · Post by **FeVeR2112** » 26 Nov 2020, 03:58

roseblood11 wrote: ↑26 Nov 2020, 01:56 R19 to gnd, VREF is between R7 and R19

The schematic above has been updated. Much Thanks.

bajaman · Post by **bajaman** » 26 Nov 2020, 10:58

something is wrong with the values for R2, R4 and C2 - I suspect R2 is a much larger value or R4 is much smaller and C2 is definitely suspect at 100n !
with those values the input gain is only approximately 2.7 times and has a low pass 3db at 20Hz - perhaps R2 is 750k and C2 10n ?

FeVeR2112 · Post by **FeVeR2112** » 26 Nov 2020, 16:02

bajaman wrote: ↑26 Nov 2020, 10:58 something is wrong with the values for R2, R4 and C2 - I suspect R2 is a much larger value or R4 is much smaller and C2 is definitely suspect at 100n !
with those values the input gain is only approximately 2.7 times and has a low pass 3db at 20Hz - perhaps R2 is 750k and C2 10n ?

Rechecked:

R2 is 6K8 (Blue,Grey, Black, Brown, Brown)
R4 is 43K (yellow, orange, black, red, brown)
C2 is 100p (Wima, 100/100)
C6 is 1n (Wima 1000/100)

Schematic above is updated. (keep it corrected rather than reposting changes). I will recheck resistor color codes and post if any were incorrect.

Thanks for the catches!

FeVeR2112 · Post by **FeVeR2112** » 26 Nov 2020, 21:39

Observations:

Don't see many pedals that use (WIMA) film boxes for pF values.

Footswitch is one of the Demont ones that cost a mint ($15USD)

A nice feature for having a trim pot for the LED brightness

Use of Anti-log gain pot (C500K)

Clipping using two transistors with diodes reminds me of Zendrive

Heavier gauge wires for connection off board connections

Ground connections are point to point without a ground plane on either side of the PCB

bajaman · Post by **bajaman** » 26 Nov 2020, 22:03

I think R2 should be 68k and R4 4k3

cheers
bajaman

FeVeR2112 · Post by **FeVeR2112** » 26 Nov 2020, 22:11

bajaman wrote: ↑26 Nov 2020, 22:03 I think R2 should be 68k and R4 4k3
cheers,
bajaman

My old eyes.... I used a magnifier visor this time.

R2 is Gray, black, blue, red, brown = 80K6
R4 is yellow, orange, black, red, brown = 43K

Not saying its right design but the values are the values

I have updated the schem/trace above.

bajaman · Post by **bajaman** » 26 Nov 2020, 23:58

can you measure R4 with a multimeter please
?
cheers
bajaman

FeVeR2112 · Post by **FeVeR2112** » 27 Nov 2020, 00:39

bajaman wrote: ↑26 Nov 2020, 23:58 can you measure R4 with a multimeter please
?
cheers
bajaman

Measured: 43K5

I have also measured all the resistors and these values are correct (within the 1% tolerance).

Manfred · Post by **Manfred** » 27 Nov 2020, 00:49

Hi FeVeR2112.
Are you sure with the types of clipping components, I ask because they are painted over?

FeVeR2112 · Post by **FeVeR2112** » 27 Nov 2020, 01:02

Manfred wrote: ↑27 Nov 2020, 00:49 Hi FeVeR2112.
Are you sure with the types of clipping components, I ask because they are painted over?

I took the picture before clearing the paint off. The transistors labeling was sanded off and the diodes had 1N4 V but was missing the 3 characters. The V indicates the Vishay brand and the missing numbers would be right between the 1N4 and the V. The forward voltage is 720mV and that Is the same as 4148 so I am confident these are the diodes. The other diode was painted but the label was still there as BAT43.

The transistors are likely 2N7000 as this is a typical make for a diode setup - Thank you to Bugg for that.

The IC is connected the same as a TL074 with V+ to pin4 and ground to pin11.

FeVeR2112 · Post by **FeVeR2112** » 27 Nov 2020, 01:47

...also.... a TL074 ($0.70USD) could be the OPA4134, OPA4228 or a LM837..... Given the top shelf components I would not be shocked it it was the OPA4228 ($8.80USD) or similar high quality quad-opamp.

Manfred · Post by **Manfred** » 27 Nov 2020, 02:37

I took the picture before clearing the paint off. The transistors labeling was sanded off and the diodes had 1N4 V but was missing the 3 characters. The V indicates the Vishay brand and the missing numbers would be right between the 1N4 and the V. The forward voltage is 720mV and that Is the same as 4148 so I am confident these are the diodes. The other diode was painted but the label was still there as BAT43.

Thanks!
Which diode of these is the BAT43?
I redraw the clipping circuit so that the circuit structure is better visible.

: McGregorCrunchClippingCircuit.JPG (16.32 KiB) Viewed 2897 times

Manfred · Post by **Manfred** » 27 Nov 2020, 03:17

Which diode of these is the BAT43?

Sorry, stupid question, the diode is in the other clipping circuit.
What color is LED1 in this clipping circuit when it is activated?

FeVeR2112 · Post by **FeVeR2112** » 27 Nov 2020, 05:43

The clipping LED is a 5mm Clear Red. vF=1.86

FeVeR2112 · Post by **FeVeR2112** » 27 Nov 2020, 07:00

Thanks for the redraw. I have redrawn the schematic above with your layout. I also corrected the D2 polarity...

FeVeR2112 · Post by **FeVeR2112** » 17 Jan 2021, 05:42

I spent some time playing the pedal today and found the tone control to have a very limited range. In checking the schematic I note the Tone pot (B10K) has a 10K resistor across the lugs that effective reduced it 5K.