Bottlehead Forum
Bottlehead Kits => Crack => Topic started by: M42 on January 05, 2017, 01:40:20 PM
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I am installing the small board for the Speedball upgrade. All the led's are illuminated, but I am getting a low voltage reading at OB. The indicated voltage is 11.12 volts instead of the anticipated 60 to 90 volts. The other readings are as follows:
OA = 69.5 v
1A = 232 v
B-A/B = 0 v
I have checked all my connections/solder joints and reheated what I thought might be suspect. I noticed that one of the led's on the nine pin socket was no longer illuminated (the one connected to A3 and the center terminal). Prior to the upgrade the amp always run dead silent, but during my last listening session with the amp I notice a low level hiss I'd not heard before. I'd forgotten about that before starting this project!
Would the failure of the socket led cause the voltage problems indicated at OB? I have done the chopstick test around the nine pin socket...the led does not light even intermittently. I know this issue should have been identified and dealt with in the first instance...my bad! Any advice would be appreciated.
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You need to reheat all the solder joints on the MJE350 on the "B" side of that PCB, as the solder has not been allowed to flow properly.
Having used the chopstick test (intended to find noise issues usually), if you've poked the LED enough times, you can certainly break it. If the LED is broken, OB voltage will be much higher than expected, and the voltage at A3 will settle to around 12V (and the LED will not be lit). Only if all three of these conditions are met should you consider the LED to be the issue.
(See page 20 in the manual for similar information)
-PB
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Thanks for your reply Paul. I have reheated the MJE350 on the B side, the results are the same...I'm still getting 11 volts and the led on the tube socket does not light. The solder has wicked up around all three legs of the MJE350 from the bottom of the pcb.
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Pop each wire out of OA and OB, then twist the board 180 degrees and reattach the wire from 5 to OA and the wire from 1 to OB, then recheck your voltages.
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The readings are now as follows:
OA 69.5v
1A 232v
B-A/B 0v
OB 69.9v
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I read the wrong column...OA reads 11v
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Is the white wire on the right side in the earlier picture properly attached to the tag strip? Insulation looks to just disappear into the hole.
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I'm certain I have a good connection there. I'll reflow the joint again this evening. I failed to mention in the previous post that the led on the nine pin socket (at A3) still does not illuminate.
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That's good information. When you flipped the board around and the low voltage changed from OB to OA, that tells us that the small PC board is working.
As to why the issue is occurring in the first place, some potential causes are debris in or around the tube socket causing a short between pin A6 and A7 (or A6 and A5), what DC resistance do you get from terminal 5 to ground?
Also, to reiterate again, this is a problem that has nothing to do with the LED that isn't lighting. If the LED was shorted out, terminal 5 would read something like 50V, and if the LED was open it would read around 170V.
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Measuring the resistance from the ground lug and terminal 5 the meter reads O.L the scale indicates M ohms. You will have to bear with me as the only reason I get these kits together are the outstanding build manuals. I know which end of the soldering iron gets hot....but beyond that............?
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How about the resistance between terminal 5 and A2?
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With power off -2949 ohms
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Ground lug to T5 is 2966 ohms
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Ground lug to T5 is 2966 ohms
Yeah, now we are getting somewhere. There are two 3,000 Ohm resistors in the amp, they connect to the 6080 back at the octal socket. If, somehow, there is a miswire or a short back there, then sending a few mA of current through a 3,000 Ohm resistor will get you the weird voltage you have.
Did this Crack work before you put the Speedball in?
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The amp has run without issue since I completed it about three years ago...it always ran dead silent. I had a chance to listen to it recently after it not being used for some time. It sounded great as usual, but as I was about to shut it off I did notice a low level hiss. I had not heard that before. I did not listen to it again before the upgrade kit arrived and forgot about it. As a result I didn't discover the led issue at the tube socket until then.
I have checked around the tube socket and find no debris which might be contributing to this problem.
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I would pull the Speedball and put the resistors back. There's an issue with the underlying build that needs to be resolved first.
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I reinstalled the two resistors. I could not find the identical value (22.1 kohm), I've used NTE 1/2 watt 22 k ohm, 2% in their place.
Resistance: Voltages:
1 .378 m ohms 91.5
2 .342 m ohms 229
3 0 0
4 .329 m ohms 228
5 1.06 26
6 2500 0
7 3050 26
8 0 26
9 2.95 0
10 2508 0
11 n/a 0
12 0 0
13 .635 m ohms 0
14 0 0
15 n/a 233
20 0 0
21 n/a 238
22 01.5 a1 26 b1 90.6
b3 2950 a2 0 b2 230
b6 2953 a3 1.45 b3 26
rca ground jacks: a4 0 b4 26
ground lug 0 a5 0 b5 230
center pin 2502 a6 90.7 b6 0
a7 0 b7 0
a8 1.56 b8 0
a9 0
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5 1.06
The amp should not be powered on again until we can determine what went wrong here. Your last resistance reading was 3,000 Ohms, now it's 1 Ohm. Neither resistance reading is correct.
I would be interested in seeing more photos of the build, as I might be able to spot the issue.
-PB
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Thanks Paul...I'll put some up this evening.
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A few more photos of the amp...
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Pull the 6080 and power up the amp with just the 12AU7, then measure the voltages at 1 and 5 again.
-PB
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Measured once more before pulling the 6080 tube:
led still out on 9 pin socket
terminal 1 92.4v
terminal 5 26.8v
6080 removed:
both led's lit on socket
t1 92v
t5 94.6
replaced 6080:
both led's lit
t1 78v
t5 80.5v
removed 6080:
both led's lit
t1 92.6v
t5 95.4v
6080 in:
both led's lit
t1 78v
t5 81v
I had the big tube out of the amp prior to this issue, but am certain it was correctly seated/oriented at the time of the voltage checks for the small board. It would appear I can reinstall the small board?
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If your keyway has broken off the 6080, and the tube was inserted off by just one pin's worth of rotation, that would explain 100% of the issues that you're having.
-PB
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I do have an index mark on the tube, but despite that I must have reinstalled the tube incorrectly??? This appears to be the only logical explanation...a humbling lesson to be sure. I installed the large board without incident. Voltages measure:
OA 103.9V
OB 101.6V
G OV
B+ 174.5V
The 6080 tube now appears to be microphonic...I have ordered replacement tubes. I've had little time to listen to the amp, but I'm very happy with what I hear so far! Thanks very much for your help with this.
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Leave Crack on for a while (a day continuous), might get rid of the microphonics.
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Thanks Tom, I'll give that a try.