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Sparks! (Read 8637 times)
Fergstuff
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Sparks!
03/10/15 at 21:53:07
 
I have just built two ZKit1's in the same box. I powered both up and once the output tubes were warm noticed an internal arc on pin 7 on both output tubes on the same pc board. I also noticed some arcing between pin 6 and 7. The wiring look good ( same as amp one). Turned it on again and no sparks. Turned off went back later and more sparks. but this time blew both 100 ohm resistors? I have pictures if interested. Any input would be greatly appreciated  :-?
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Core32
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Re: Sparks!
Reply #1 - 03/10/15 at 22:44:16
 
Post a picture. Especially of the burned resistors.
I may be looking at an old schematic but I do not see a 100 ohm called for.
I have not moved my build to a permanent chassis but the temporary one is still playing nicely. So maybe I have enough experience to help out.

Edit: I suppose these are the virtual heater center tap resistors?
If so, post what transformer you are using. Model # and all.
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Fergstuff
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Re: Sparks!
Reply #2 - 03/11/15 at 04:36:24
 
I will work on the pictures but the transformers are the Edcor models that are recommended on the supplied parts list. As far as I can see the two 100 ohm resistors are not on the old boards or really shown on any of the build picks. They are located next to the brown wires from the Edcor transformer.
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Core32
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Re: Sparks!
Reply #3 - 03/11/15 at 11:55:35
 
They have to be the virtual ground components, and no, they are not on any schematics I have seen from Decware.
This was a later addition to help with hum since the Edcor transformer recommended (that replaced the old Decware supplied transformer) does not have a wired center-tap on the heater secondary.
Double check the transformer model # you received from Edcor because the simplest answer for your burned 100 ohm resistors is that your heater secondary voltage is much too high or that your 100 ohm resistors are the wrong value (much too low).
I also believe that the wire colors from the Edcor transformer do not necessarily match the color code on the board. There are a couple of threads on here discussing the color difference from the old transformer I think.
Is either board working or are you just in the early stages of testing?
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Fergstuff
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Re: Sparks!
Reply #4 - 03/11/15 at 14:43:04
 
I will recheck the voltages but they did look right when compared to the other amp which does not have this problem. I made this amp with a friend. We both made two. Three out of four are ok.  Although voltages seem a little high with them all. I will send pics and voltages later today.
Thanks again
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Fergstuff
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Re: Sparks!
Reply #5 - 03/11/15 at 17:24:11
 
here are a couple of images. I do have hi res versions but would need to email directly due to file size. The first is the complete amp.
amp on right seems ok. I have checked all solder joints and there is no bridging or crossed wiring.
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Amp.jpg
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Fergstuff
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Re: Sparks!
Reply #6 - 03/11/15 at 17:32:12
 
Here is a closer look at the blown 100 ohm resistor's. I do believe there may be a relationship between these blowing and the arcing on pin 7.??
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Problem_child.jpg
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Core32
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Re: Sparks!
Reply #7 - 03/11/15 at 18:24:53
 
I can't tell what the color code on those resistors "was" before they burned.
Do they appear to have the same color band sequence as the "working" side amp?
Can you measure the AC volts between brown wires on the good and bad amp? Should be around 6.3V AC.
And I would take out the tubes that are arcing while doing this testing.
And please be careful with your hands in there while powered on.
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Core32
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Re: Sparks!
Reply #8 - 03/11/15 at 18:42:18
 
Hold on. What are all those wires doing on the socket pins?
Are your sockets not soldered to the boards?
If not then I would look carefully at the wiring between the actual socket and the board socket-pin locations.
It's highly likely that you have one of the pins miswired such that the heater pins, 4 & 5, have a high voltage on them by accident.
That would cause the resistors to burn for sure even if the transformer voltage is correct.
I couldn't imagine how the board could be "mis-wired" internally but adding the wires between the board and socket adds another level of possible assembly mistakes.
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Fergstuff
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Re: Sparks!
Reply #9 - 03/11/15 at 23:14:53
 
Why make it easy? Roll Eyes I wanted the tube layout to be a little different than on the board. On hindsight......
The wires have all been triple checked and they are correct with no bridging  or continuity between them. The board has been gone over by several people and look identical. Voltages are close except on the brown trtansformer wires  6.6 v 4.7 v. but this may be due to the floating ground due to the shot resistors? The other amp reads 3.3 and 3.7
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Core32
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Re: Sparks!
Reply #10 - 03/12/15 at 00:16:23
 
Well obviously something is wrong.  :o
I don't exactly understand your voltage readings.
If you measure AC volts, putting the meter probes one each at the end of the transformer brown wires, not on ground, you should see about 6.3V.
You can do this easier by pulling tubes and placing one probe on pin 4 and the other on pin 5 on each socket.
Based on your readings I assume you are reading from ground/chassis to one brown wire end and on the good amp get 3.3V on one wire and 3.7V on the other.
If the AC on each pair of heater pins is at about 6.3V AC, then I would measure the following pattern (all the following are now DC measurements):
With all tubes out of the sockets, measure DC volts, on pin 4 to ground and then pin 5 to ground and do this for all three sockets, six measurements total.
If any of these measurements is any more than a volt DC there is a problem.
If all 6 measurements are in the millivolt range (zero volts DC is good too) then put any one tube back in.
Again measure DC on pin 4 to ground and pin 5 to ground of the remaining two empty sockets.
Any significant DC voltage shows a problem.
If all is good then add a second tube back in.
Repeat the measurements for pins 4 and 5 of the remaining empty socket.
If all is still good, put the third tube back in and pull either of the other tubes back out and measure pins 4 and 5 again of that newly empty socket.
This should be a pretty good test of the heater wiring and the tubes themselves for internal shorts.
HTH.
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