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PostPosted: Mon Aug 14, 2017 7:43 pm 
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Hey BYOCers,

I just completed a Crown Jewel build which has the drive channel working perfectly, but with the 18V boost engaged, the volume drops and can only be heard faintly when maxed out.

I'm assuming it's something to do with the module build or power supply, and I was wondering whether I needed an 18V power supply to run any of the 18V modules? The only mod I did to the circuit was use different coloured LEDs for the indicators - not sure if this affects anything. I'm using a 9V Pedaltrain Volto power supply as I thought the charge pump would take care of the voltage.

Any suggestions on what causes this problem would be much appreciated.

Cheers guys!


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PostPosted: Mon Aug 14, 2017 8:30 pm 
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Paperboy_1962 wrote:
....I was wondering whether I needed an 18V power supply to run any of the 18V modules?

No, the charge pump provides the +18V that the JFET boost needs, assuming of course that everything is built & working properly.

Paperboy_1962 wrote:
The only mod I did to the circuit was use different coloured LEDs for the indicators - not sure if this affects anything.

Nope, that wouldn't affect anything.

Did you dial in ~9.0V on the middle pin of each of the trimpots?

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PostPosted: Mon Aug 14, 2017 8:38 pm 
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My module board doesn't have any trimpots on it. I've noticed that it is different to the board shown in the build manual.

It has a few more resistors with no place on the board for the pots. I understand photos would help but I can't at the moment - I'll post photos when I have a chance today.

I'm sure I have the transistors oriented the correct way too, and that they're identical.


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PostPosted: Mon Aug 14, 2017 9:09 pm 
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Ah, you have one of the original batch of JFET boost boards that used fixed resistors to bias the two transistors rather than the trimpots. That change was made to compensate for normal JFET production variability. But unless you got a really bad one, I wouldn't expect it would make the kind of difference you're hearing.

Two ideas:

1) Please post photos--large, well-lit, well focused ones--of both sides of your JFET boost module.

2) Do you have access to a multimeter and know how to measure DC voltage with one?

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PostPosted: Mon Aug 14, 2017 10:13 pm 
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I see. I thought it might have been an old design. Thanks

I have a multimeter but I don't know which points to measure. Let me know which lugs and I can tell you the readings.

I'll upload photos after work tonight and get back to you!


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PostPosted: Tue Aug 15, 2017 9:30 am 
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Paperboy_1962 wrote:
I have a multimeter but I don't know which points to measure. Let me know which lugs and I can tell you the readings.

As marked:

Attachment:
trans_legs.jpg


BTW, I have a copy of the instructions for that earlier version, which is where I got the above diagram that I marked up. If you want them, PM me with an e-mail address and I'll send them to you.

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PostPosted: Tue Aug 15, 2017 6:01 pm 
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I measured the joints as indicated, and I'm reading 17.3mV. Is this correct?


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PostPosted: Tue Aug 15, 2017 6:54 pm 
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Paperboy_1962 wrote:
I measured the joints as indicated, and I'm reading 17.3mV. Is this correct?

Nope, not even close.

Are you sure that you're measuring correctly? Black probe to ground and red probe on the test point. While you're at it, measure the the +18V solder joint on the top of the module and see what you're getting there. Should be about 16V for the original release of the Crown Jewel.

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PostPosted: Tue Aug 15, 2017 7:02 pm 
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Ok no worries, thanks for that.

While I'm at it, can you please indicate which points will allow me to measure and check whether the internal charge pump is generating 18V to the drive channel? Just curious whether this is working as it should.

Cheers


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PostPosted: Tue Aug 15, 2017 7:12 pm 
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Paperboy_1962 wrote:
...can you please indicate which points will allow me to measure and check whether the internal charge pump is generating 18V to the drive channel?

That's exactly why I asked you to measure the DC voltage level on the +18 solder joint on top of the boost module while it's plugged into the pedal and powered. See marked up photo below:

Attachment:
18V_joint.jpg

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PostPosted: Wed Aug 16, 2017 5:21 am 
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I measured 15.46V on the 18V lug, 2.24V on Q1 and 2.22V on Q2. I guess the trannies are the culprits?

What should I do about them?


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PostPosted: Wed Aug 16, 2017 10:03 am 
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Paperboy_1962 wrote:
I guess the trannies are the culprits?

Not necessarily. It just means that they aren't getting enough voltage, but that could be due to a different cause. In fact, I would consider it very unlikely that BOTH transistors would be defective to a nearly identical degree.

Here's something that's quick & easy to try--make sure that all the component leads on the back of the boost module are trimmed down as close to the tops of their respective solder joints as possible. Then cover the back of the module with electrical tape, reinstall & test it.

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PostPosted: Thu Aug 17, 2017 5:39 am 
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Just tried trimming the solder joints as close as possible, electrical taping it and reinstalling it, to no avail. I measured the voltages when plugged in and working but still the same result, and the same quiet sound at max volume.

I also tried re flowing the joints and ensuring nothing was shorting out.

Do you think it could be something else? The drive section seems to be working perfectly though.


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PostPosted: Thu Aug 17, 2017 7:15 pm 
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What is the DC voltage reading at (as shown in my photo) the bottom of the leftmost 4.7K resistor and at the top of the rightmost 4.7K? They should read the same as that +18 solder joint.

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PostPosted: Fri Aug 18, 2017 8:37 pm 
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Yep they're reading the same 15.4V at those points


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PostPosted: Fri Aug 18, 2017 8:44 pm 
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How about the other lead of both of those resistors?

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PostPosted: Fri Aug 18, 2017 8:46 pm 
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They're both reading 2.1V on their other lugs


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PostPosted: Fri Aug 18, 2017 9:08 pm 
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How about ground, do you measure close to zero ohms from the ground pin on the module to an unpainted spot on the case of the CJ?

Also, how long are those component leads sticking out from the solder side of the module? They should be cut of flush or at least pretty short.

The FETs themselves are self-biased in this circuit. That means current should flow down from the +18 volt supply through the resistors connected to the FET drain pins, then on out through the source pins, and then through the source resistors to ground. The source resistors are fairly low value, about 600 ohms. That being case, they shouldn't have very much voltage compared to ground. You can see which pin is which, both physically and as shown on the schematic, by looking at the pictures on the first page of this datasheet: https://www.onsemi.com/pub/Collateral/2N5457-D.PDF


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PostPosted: Fri Aug 18, 2017 9:17 pm 
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Yep I'm not reading any voltage between the ground and CJ enclosure.

The solder joints are cut as flush as possible while still having as small amount of solder I can get to ensure good connection. I've checked the joints and it doesn't look like anything is shorting out, and I've tried covering the joints with electrical tape in case it was shorting out with the mother board. I've also reflowed the joints.


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PostPosted: Fri Aug 18, 2017 10:23 pm 
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Paperboy_1962 wrote:
Yep I'm not reading any voltage between the ground and CJ enclosure.


Okay, but assume you understand that not reading any voltage is not necessarily the same as reading near zero ohms of resistance.
Also, sometimes its good to measure something by more than one method to check for consistency. A broken meter lead, loss of power to the module, etc., could be various reasons that no voltage would be read, but a resistance check might fail. The reason I asked is because you seem to be getting an odd result, that both transistors may be open circuit. Unless they have been damaged somehow, it would be very unusual to have two bad new ones.


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PostPosted: Sun Aug 20, 2017 7:23 am 
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Thanks for all the help guys, I really appreciate it.

Are there any other checks that I can do that would help troubleshooting this issue? Really trying to get this to work and I can't think of anything else.

Cheers


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