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Ice Cube Clock - Low HV
Moderators: adafruit_support_bill, adafruit

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Ice Cube Clock - Low HV

by Barry914 on Sun May 11, 2014 11:47 pm

I assembled the clock, and checked the HV at the appropriate step. It was about 60V. I completed the assembly but when I got to the part where I checked the VF display, only a few random segments lit up, and they were very dim.. The HV is about 14V with the VF unplugged.. The 5V supply is dead on.
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Re: Ice Cube Clock - Low HV

by adafruit_support_rick on Mon May 12, 2014 7:38 am

Difficult to see, but make sure you've got Q3 installed the right way.

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Re: Ice Cube Clock - Low HV

by Barry914 on Mon May 12, 2014 8:54 am

The rounded edges match the silkscreen. I played with the buttons but I can't see what mode it's in. The voltage is about 18V, not 13. I removed Q3, no change in voltage, so I reinstalled it. I'm thinking if it's not the VF tube it's got to be the driver. I have no idea how one removes the chip from the socket with the tools I have (circular saw, maybe?).

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Re: Ice Cube Clock - Low HV

by adafruit_support_bill on Mon May 12, 2014 9:16 am

Before you pull out the circular saw, carefully short the two outer pins of Q3 and see if that makes a difference.

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Re: Ice Cube Clock - Low HV

by Barry914 on Mon May 12, 2014 10:03 am

Nope. I imagine if I wish hard enough it won't turn out to be the VF tube. It would be a bear to replace. I spent quite a bit of time going over the parts placements and I'm pretty sure I didn't make any mistakes, but I would bet everyone says that. I will say with confidence there are no solder bridges or cold solder joints (NASA certified tech back in the day). I probably should have stopped the assembly when the VF tube test failed, but it's a little late for that now.

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Re: Ice Cube Clock - Low HV

by adafruit_support_bill on Mon May 12, 2014 10:26 am

If we need to replace the tube, we'll send a new side-board too. We wouldn't be so cruel as to make you desolder the tube.

But let's rule out the other possibilities first. The symptoms you describe are consistent with low voltage to the tube heater.
What voltage do you measure on R3 - on the end closest to Q3?

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Re: Ice Cube Clock - Low HV

by Russell 27 on Mon May 12, 2014 1:36 pm

ICE TUBE circuit.PNG
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If this helps, these are measurements I took using a BS250 MOSFET, display looked good with these readings. Sometimes with the kit MOSFET Vr was closer to 2.5 volts and tube did not function well. At default minimum brightness, ANODE (+UB) @ 14 volts isn't too far off.
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Re: Ice Cube Clock - Low HV

by jarchie on Tue May 13, 2014 5:54 pm

adafruit_support_bill wrote:Before you pull out the circular saw, carefully short the two outer pins of Q3 and see if that makes a difference.

I'm surprised that shorting these pins didn't restore the display; it has for other users with similar problems.

Just to rule out any possibility of ambiguity... With respect to Q3, the pins to short are the outer pins of Q3, as Bill wrote. But with respect to the board, the pins to short are the outer and inner pins of Q3, that is the pin closest to the edge of the board and the pin closest to the center of the board.

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Re: Ice Cube Clock - Low HV

by Barry914 on Tue May 13, 2014 6:43 pm

Bill, the voltage at the drain of Q3 is about 3.17v, and about 2.07v at the load end of R3. I'm a bit embarrassed about what I said about shorting Q3's D & S not making a difference. If I use a clip lead that has actual continuity it does indeed make a difference - I get a dim but properly operating display. So, what's the next thing to do?

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Re: Ice Cube Clock - Low HV

by adafruit_support_bill on Tue May 13, 2014 6:55 pm

If I use a clip lead that has actual continuity it does indeed make a difference - I get a dim but properly operating display. So, what's the next thing to do?

Good! That is what I suspected - you have a marginally functional Q3. If you contact support@adafruit.com with a link to this thread, we will get another one out to you. Jarchie and Russel27 have done some good work identifying alternatives to the stock Q3 and will probably propose those to you as well. If you choose to go that route, please let us know how it works for you.

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Re: Ice Cube Clock - Low HV

by Barry914 on Tue May 13, 2014 7:37 pm

Thanks Bill and jarchie. Where can I find info on the alternatives?

As an aside, this is a really nice product. The case is very well designed and executed, and the VF display looks so cool. The most challenging part of building this kit was (so far) separating the 22 twisted VF leads and threading them through the holes in the PCB without breaking anything. Cutting them in a spiral is definitely a great idea. The kit was a birthday present, the best one I've gotten in a long time. I can't wait to get it all buttoned up.

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Re: Ice Cube Clock - Low HV

by jarchie on Tue May 13, 2014 7:45 pm

Barry914 wrote:Thanks Bill and jarchie. Where can I find info on the alternatives?

Russell did a wonderful comparison of Q3 substitutes a while back, but here's my usual Q3 spiel:

My personal suggestion is to replace Q3 with a ZVP2110A, available from Digi-Key (ZVP2110A-ND) or Mouser (522-ZVP2110A). Russell mentioned the BS250, which he found to perform slightly better than the ZVP2110A, but more users have tried the ZVP2110A with good success. Another excellent and frequently used alternative is the is the PN2907A/resistor combination described by Russell.

Replacing Q3 with a ZVP2110A, BS250, or PN2907A/resistor will provide more voltage to the VFD filament and prevent two other problems that occasionally effect Ice Tube Clocks: First, the dim digit problem is where the initial and/or final digit appears significantly dimmer than the others and is caused by the low filament voltage in the Adafruit design. Second, low filament voltage also accelerates cathode poisoning, which can shorten tube life and make the 3rd and 6th digits appear dimmer than the others. A clock with both issues is pictured in the initial post of this thread.

By the way, if you're interested in what I believe to be the underlying reason for the problem you describe, it might be worth checking out this post.

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Re: Ice Cube Clock - Low HV

by Barry914 on Wed May 14, 2014 12:23 am

I am tempted to replace the MOSFET with a bipolar transistor a la Russell. I see Radio Shack lists 2N3906's which should be OK - Vceo -40V and Ic -200mA. I'm wondering how many times I can replace Q3 before I destroy the board so I'm going to throw this out there before I do it to see if anyone thinks that this is not a good idea. Adafruit is sending out a replacement MOSFET, but I hear a little voice telling me that part is marginal in this application. I guess transistor sockets are about as common as passenger pigeons and CK722's :( I'm kind of into instant gratification, so mail order is not my favorite thing.

I totally get why Adafruit doesn't have a sales counter, but I kind of long for the days when you could walk into Lafayette Radio when you needed a part.

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Re: Ice Cube Clock - Low HV

by phild13 on Wed May 14, 2014 1:43 am

long for the days when you could walk into Lafayette Radio when you needed a part.

me too....
The ZVP3306A is rather marginal for the clock and I would agree that an alternative would work much better.
I like using the ZVP2110A or the PN2907A/resistor combination mentioned (I have tried the PN2907A/1K ohm) , either work just about as well as the other.

I have not tried this particular combination (and I don't have one at the moment) but since Q3 is just acting as a switch, I would think that you could substitute the Radio Shack 2N3906 for the PN2907 and either a 470 ohm, 1K base resistor or whatever Radio Shack has in stock for a base resistor within that range.

If your going with an alternative Q3, then something else to consider is to replace the 22 ohm R3 resistor with an 11 ohm or 10 ohm (I think Radio Shack has those). This will provide a bit more current to the filament without hurting anything.

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Re: Ice Cube Clock - Low HV

by Barry914 on Wed May 14, 2014 10:30 am

Got to love Radio Shack. Their site lists a bag of 15 2N3906's for $3.49, but what I actually got was a mixed bag of 5 each 2N2907's,. 2N3906's and 2N4403's. I guess that's not such a bad thing. In any case, I managed to install a 2N2907 with a 1K base resistor without destroying the PCB and lo! nicely lit digits. I didn't do much fooling around once I saw it was working for fear of jinxing (or more likely destroying) something. I'm going to put it in the case and go from there. BTW, I think the 1K resistor is a bit low. 4.7K to 10K should be fine here but I'm in no mood to experiment. Back to the workbench, and thanks to everyone who contributed to this resolution. Those posts were interesting reading, too.

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Please be positive and constructive with your questions and comments.