Bottlehead Forum
Bottlehead Kits => Crack => Topic started by: timindq on June 01, 2015, 03:35:40 AM
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I finished building my Crack yesterday with only minor issues during construction. After what was a fairly successful resistance check I went back and retouched a few solder joints, soldered some unsoldered wires and actually put in a missing wire. At that point the resistance check went fine.
When I turned it on to check voltages I ran into some real problems. There were some serious under voltage readings associated with the left channel. Also, the led connected to pin 3 on the 12AU7 wasn't glowing. I'll list the voltages which are incorrect. First listed is the connection point as labeled followed by the voltage reading.
5 - 10v
9 - 63v
A1 - 8v
A6 - 80v
B1 - 80v
B6 - 65v
The voltage on the hot side of the unlit LED is 1.2v while the voltage on the lit LED is 1.5v.
After looking everything over again and again I plugged in a source and a pair of headphones. The result was clean audio in the right channel and only static type noise in the left channel. The noise doesn't vary when the volume pot is turned.
Any advice is appreciated here. Thanks.
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What voltage do you have on T4?
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My meter reads 5.6VAC and 0VDC.
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Let me clarify, by T4 I'm assuming you meant the transformer's #4 which is 5.6VAC. If you meant #4 on the terminal strip up front, the measurement is 181VDC. In my original post the only readings listed are the readings which are out of tolerance.
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181V on T4 and 0V on T5 indicates either that the 22K resistor isn't well connected on one end, or you have a short to ground on the T5 end, maybe at the 9 pin socket.
-PB
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Yes, it was shorted to ground. I could see no reason for the short so I removed the black lead from the transformer and the tube sockets are no longer shorted. The tab #4 on the transformer is shorted to ground with nothing attached to it. Please advise.
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So sorry but I just got out my mag glass and checked the tab on the transformer. The shorted tab is #5. Tab #4 is also shorted of course as it has a black wire tied to the chassis as directed.
The transformer is wired correctly, it's just my missreading of the tab number earlier.
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After further checking, I'm certain the windings in the transformer are shorted between pins 4 and 5. No idea how to proceed from here.
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Please leave the power transformer terminal connections as they were when you first built the kit.
The terminals I was referring to are terminals 4 and 5, which are on the front 5 lug terminal strip.
-PB
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I reattached the black wire to the transformer tab #5. Before doing this I took resistance measurements with the following results:
A1 to A6 read 44.2K ohms - I measured at these points to ensure I wasn't disturbing the resistor connections at the terminal strip as they look ok.
A9 to ground read open
B8 to ground read open
After reattaching the the wire at the transformer, A9 and B7 read short to ground.
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I reattached the black wire to the transformer tab #5. Before doing this I took resistance measurements with the following results:
A1 to A6 read 44.2K ohms - I measured at these points to ensure I wasn't disturbing the resistor connections at the terminal strip as they look ok.
A9 to ground read open
B8 to ground read open
After reattaching the the wire at the transformer, A9 and B7 read short to ground.
Appreciate the help so far.
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Perhaps you could post some photos of your build?
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Will do. Thanks.
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Attached (hopefully) are the pics I took. There are some irregularities in the wire color because I used some Audioquest wire stripped out of a piece of excess cable. I used them in places where audio signal is passed as best I could figure. The pieces are 20 gauge solid copper.
Also, the black wire running from the transformer to the 6080 has been unwound from the red wire after I disconnected it. I will replace both wires after we sort this out.
I hope these pics tell you something. Thanks.
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Can you pull the 6080 but leave the 12AU7 in, then remeasure terminals 1, 2, 4, and 5. (Not power transformer terminals, terminal strip terminals)
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Thanks for the reply.
Do you mean measure voltage or resistance? I want to check before applying power without the tube.
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Measure voltage with no 6080 installed.
-PB
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Thanks,
Voltages are:
1 - 95 VDC
2 - 224 VDC
4 - 224 VDC
5 - 8 VDC
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OK, now repeat the test with both tubes pulled.
Do note that the power supply will take several minutes to drain down after you turn the amplifier off when run with no tubes.
-PB
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With both tubes pulled
1 - 225 VDC
2 - 230 VDC
4 - 225 VDC
5 - 9 VDC
Thanks
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What resistance do you get between A1 and A2?
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With or without the tube the measurement was the same. Resistance value varied like a capacitor was in circuit but averaged between 30K - 40K. Resistance shot down when I reversed the leads but would eventually begin to rise.
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I do appreciate you sticking with me here.
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This sure is an odd one. Can you measure the resistance of the 22.1K resistor across terminals 4 and 5? You should be able to do this with it soldered into the circuit (with the amp off of course).
Either the 22.1K resistor is open, or there's some bizarre short in the circuit that I can't see that's pulling the voltage at T5 down.
-PB
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Good morning,
The resistance across t4 and 5 measures 22.1k. I also measured the resistance from t5 to ground with the 12AU7 in place; this measured similarly to the A1 to A2, as if a cap is in circuit.
After work today I will recheck all wiring and see if I made a mistake somewhere that I missed the first time I checked.
Thanks again.
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You should be able to measure from T5 to ground without too much fuss, is it the same as T1 to ground?
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I borrowed a better meter and performed resistance checks. T1 to ground and t5 to ground are basically the same starting at 30k and going up until I remove the leads. If I reverse the leads the circuit reads short. If I leave the reversed leads on points the the resistance will eventually climb as before leads were reversed.
Man I wish I had an analog meter.
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Last night I did a thorough visual check of the wiring on my Crack. I see no discrepancies. Have to admit it is a easier now that I'm familiar with the thing.
After checking and measuring voltages again I decided to resolder the 22.1K resistor at t4 and t5. Voltages remained the same after touch up. T4 224VDC and T5 8VDC.
Wish there was one of those head scratching emoticons.
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Try putting the resistor back in. I believe the resistor may not have been properly installed, and wasn't allowing half the 12AU7 to conduct.
-PB
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Reinstalled the resistor and measured 22.2k ohms from B5 to A1. I used these points to ensure I wasn't moving any part of the resistor's connection. I then measured voltage at T5 and it was 9VDC. This measurement was with the 12AU7 in place and without the 6080 installed.
Voltage at T1 is 90VDC. The other side of tube seems to be working fine.
Tim
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Just to be thorough, try swapping the 22K resistors.
Also, with no tubes installed, what DC voltages do you get at:
Terminal 21
Terminal 2
-PB
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With no tubes installed I get 236VDC at T21 and 230.6VDC at T2. I'm heating up the soldering iron now and will get back in a minute or two.
Thanks for the assist.
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Hmm, OK, those voltages indicate 11mA of current draw with no tubes installed. I bet if you remove the 22.1K resistor between 4 and 5, you'll discover that your voltage at T2 will pop up a bit.
I'm still suspecting a short around the socket. It may be debris in there, look for something touching A1 that shouldn't.
-PB
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Swapping the resistors did not help. But I read your last response and went back and checked the tube sockets. I saw no issues but did wiggle them about and blew some air at them.
I then checked voltages and no longer had the drain at T5!!!
I plugged in the 12AU7 and both LEDs lit!!!
I'm going to give the tube sockets a thorough cleaning and try it out.
Another plus: with all this turning on and off and measuring voltage and resistance, I have yet to shock myself!
Thanks for the help and please let me know if there is something I should do before proceeding.
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Yeah, we may never know what was stuck in there, but I'm glad the drain is gone. The sockets are two pieces of ceramic that are riveted together, and it's possible that something worked its way in there.
In any event, if all your voltages look good, go ahead and give it a listen!
-PB
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IT SOUNDS WONDERFUL!!!
Seriously, no hiss, no hum, k.d. lang never sounded so good. With the music on pause I maxed the volume and still heard nothing but silence.
Thanks again for your help Paul. You can relax for a little while as I'm going to listen like this for a couple weeks before building the Speedball...We'll see how patient I am. ;)