Bottlehead Forum
Bottlehead Kits => Crack => Topic started by: StivVid on April 11, 2016, 02:31:43 PM
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Ok. I had a problem with the small board which I sorted out--had one of the transistors oriented incorrectly.
Now I've built the big board. Did my initial voltage checks and everything seemed ok. Plugged it in with some headphones and listened to a few minutes of music and it sounded glorious. Left it on to go downstairs to get something and when I got back, noticed light wisps of smoke coming from near the transformer. Unplugged the headphones and lifted the amp off the base to have a look while still plugged in. The 2 LEDs nearest the power supply glowed very dimly, and only the LED nearest the board's edge on the 12AU7 end was lit.
I've reflowed several joints and plugged it in to try again. Same result. Here are the voltage readings as of now. Any help would be greatly appreciated.
OA= 73
OB= 45.4
G= 0
B+= 86
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Reflowing joints is not usually the solution if there is smoke. Look for shorts and improperly mounted transistors.
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Thanks for the quick response, Doc.
I think the smoke was from wires that were laying across the 270 Ohm resistor attached at 13 and 15. That and the other 270 Ohm resistor got VERY hot.
I resoldered the center legs of both TIP50'S. What else should I be checking?
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The 270 ohm resistor will get hot normally in operation, but will get super hot very fast if there is a short in the circuit. I can't judge by your description which case this is. By improperly mounted transistors I meant look at the actual mounting of the transistor to the heat sink. The screw and heatsink need to be properly insulated from the back of the transistor. Putting the assembly together improperly is a common error.
http://bottlehead.com/smf/index.php?topic=8456.0 (http://bottlehead.com/smf/index.php?topic=8456.0)
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No. I've checked and double-checked the mounting of the resistors. They're correct.
Can a take a photo of any particular area of interest? Would that help?
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I've checked all the transistors on both boards. All are oriented correctly and heatsinks are mounted properly. I am noticing now that when I start the amp up, the 12AU7 LEDs light up first, then the LEDs on the small board, then only the 3 LEDs I described earlier on the big board. The LED nearest the edge of the board on the 12AU7 end glows really brightly--abnormally so, I'd say. As it gets bright, the LEDs on the small board dim down to almost nothing.
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Reflowing joints is not usually the solution if there is smoke. Look for shorts and improperly mounted transistors.
I can follow a set of instructions--especially when they're as clear and concise as those provided with these kits. Beyond that, I really don't have much knowledge or understanding of electronics. I'm learning... But it's a slow process.
If I have a short somewhere, where are the likely places one might occur? How do I check for shorts? Please forgive my ignorance. Every time I build one of these kits I learn a little more.
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I went back to the crack manual to do the resistance checks again.
Terminals 6 and 10 are giving correct readings, but terminals 7, 9, B3 and B6 are either giving no reading or are off the charts.
Does that make any sense?
Not sure if the resistance checks in the stock Crack manual are still valid after Speedball upgrade?
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Pictures of the big PC board from a couple of different angles might be helpful.
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B+= 86
This nearly always indicates that the TIP50 transistors are not appropriately mounted to the heatsinks.
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Here are some pics. I checked the TIP50 mounts again. They look good to me.
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This big board looks different to the one in my manual. This one also says V1.1. Are there 2 versions?
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@Deluk
Yes there is a newer version of the board. It uses the same components, just a little bit different layout.
@StivVid
You can't really judge continuity just by how a solder joint looks. That center pin on the TIP50 on the offending side actually looks a little dry in the photos. It costs nothing to reheat that joint and add a tiny bit more solder to see if it solves the problem.
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The other way to tell if you have issues with the TIP50 mounting is to run the Crack with no 6080 tube, then see if you have proper voltages on the small board (the "O" voltages will be correct, the "I" voltages will be a little higher).
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Okay...
I reheated all the legs of each TIP50 adding some more solder to the connections with the board.
Here are the voltage check results with both tubes installed:
Big Board:
OA= 28.7
OB= 13.9
G= 0
B+= 52
I also checked the voltage with the 6080 removed from the circuit, as suggested. Here are those results:
Small Board:
OA= 74.3
IA= 268
B-A/B= 0
IB= 265
OB= 75.9
Hope that sheds some light.
Thanks so much for all your help!
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Something very strange just happened...
I plugged the 6080 back in and fired the amp back up. LEDs lit up in sequence as they normally do, except that 3 lights now burned very brightly on the big board. The other lights on the 9 pin socket and small board gradually faded out until I don't think they were emitting any light at all. All that were lit were 3 lights on the big board burning very brightly.
I rapped my thumb against the side of the base while wondering what was going on, when suddenly all the LEDs lit at their normal brightness. They're still lit now.
Here are the new voltage readings:
Small board:
OA= 75.7
IA= 186.6
B-A/B= 0
IB= 186.6
OB=77.6
Big board:
OA= 110.7
OB= 109.2
G= 0
B+= 187.4
I'm going to leave it on for awhile to see what happens. What do you think?
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Hang on... Just lost another LED on the big board. Only 3 are lit. 2 on the end nearest the octal sockets, and 1 on the end nearest the 9 pin.
I have no idea what's going on.
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If the LEDs came on with a tap on the case I would guess that you have a loose connection somewhere, or possibly a short that opened when you tapped the case. Look for loose connections, unsoldered wires, loose bits of cut off wire ends rattling around, terminals touching, excess leads touching, etc.
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I've been all over this thing with a fine toothed comb. I can't see anything amiss. Visually, everything looks good.
Could there be a fried component on the board somewhere?
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Yeah, that basically confirms an issue with the big PC board, which is nearly always a TIP50 that isn't properly installed. Please double check the shoulder washers and the ordering of all the parts.
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Ok... I disassembled the entire big board. Dismounted both TIP50's from their heat sinks. (The transistors were mounted properly all along, I just disassembled and reassembled in the same order.) Replaced one thermal insulator pad that looked iffy. Reassembled everything including replacing all the hook up wires for the big board.
Just fired it up and everything seems to be working.
Here are the new voltage measurements:
Big Board:
OA= 111.4
OB= 109.0
G= 0
B+= 185.7
I hope this is the end of it. Gonna keep an eye on it for awhile. Can't thank you enough for your patience and understanding--not to mention your great wealth of knowledge. It's a pleasure to be a part of the Bottlehead community.