Bottlehead Forum
Bottlehead Kits => Crack => Topic started by: Beranga on August 28, 2022, 02:26:15 PM
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I have a problem with OA voltages: it is too low on small board and too high on large board. Before installing Speedball, I checked the amp resistances and voltages, all were in range. Then I installed the small board and checked its voltages and LED's, everything fine. Finally I installed the large board and got the following voltages:
Big Board:
OA 208
OB 94
G 0
B+ 215
Seeing the OA was too high, I rechecked voltages on the Small Board, which now had OA too low:
Small Board:
OA 34
1A 213
B-A/B 0
1B 213
OB 76
Then I checked the Crack voltages and found terminal 1's too low:
1 33
2 215
3 0
4 215
5 76
6 0
7 94
8 0
9 211
10 0
A pair of LED's on the large board remains unlit, and on the small board the led close to 1A is off now (before the big board it was fine).
I've gone throgh the boards reheating and re soldering some of the joints, but the problem remains the same. Please help!!!
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If you remove the 6080 and run the amp with just the 12AU7, do the small board voltages check out?
What does the bottom of the small board look like?
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Hello Paul,
Thank you for answering on a Sunday! No, without the 6080 the voltages are still out of range:
OA 34
1A 238
B-AB 0
1B 238
OB 76
I attach a photo of the SB bottom.
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Are you using lead free solder by chance? How hot of an iron?
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No, rosin core 63/37 solder. The iron goes up to 800°, I use it at max temperature.
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I would turn it up to max temp. Your solder isn't getting hot enough. You probably don't need to reflow the joints for the LEDs, but I would reflow the resistor and transistor solder joints at max temp.
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Hi, Paul. I went through every joint on both boards, reflowing them at max temperature. Still get the same voltages on OA (34 on SB and 210 on LB). The strange thing is that when the small board was first installed, all voltages checked out. It was only after I connected the large board that the problem appeared.
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With the small SB board removed and tilted up, what's the DC voltage at A8? Also if you have reflowed the LED solder joints several times, the LEDs themselves will melt and stop working.
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With the small board removed (I'm not sure what do you mean by titled up) voltage at A9 is 0.
I checked the LEDs with the diode setting on the multimeter. Some of them show a small voltage and bip, others show no voltage but light up when tested. Infinite resistance when tested backwards. I attach a picture of them on the powered circuit.
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The DC voltage at A9 will be zero, but what's the DC voltage at A8?
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Sorry, at A8 is 800mV
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Can you rotate the small Speedball board 180 degrees so that OA/OB/IA/IB are flipped to the other side?
0.8V of bias and 30V on the plate suggests the C4S on that side is not allowing sufficient current to pass. Flipping the board will either move the problem to the other side of the amp or it will stay put if there's an issue in the original build.
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I flipped the small board and voltages are now worse! But the OA voltage did not change to the other side.
OA 27
1A 105
B-A 0
1B 105
OB 73
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I would e-mail replacementparts(at)bottlehead(dot)com and get yourself a fresh small board. It will help if you have your iron turned up as high as possible so you can get the solder to flow out in a shorter amount of time.
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Hello Paul,
As you suggested, I installed a new small board, soldered with the iron at max temperature (pics. attached). I'm now getting low voltages on IA and IB:
OA 67
IA 97
B-A 0
IB 97
OB 63
The large board voltages are:
OA 61
OB 75
G 0
B+ 95
Thanks for your help!
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You now have a short dragging down the high voltage rail. Typically we used to see this a lot when the TIP50 transistors weren't properly installed on the heatsinks.
It it probably a good idea at this point to remove the Speedball (you can leave the little wires, just let them poke straight up) and replace it with the original resistors, as it's likely that you have an issue with the original build that needs to be dealt with.
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I removed the Speedball boards and reinstalled the resistors. Resistance check passed and the voltages seem fine:
1 76
2 177
3 0
4 177
5 75
6 0
7 105
8 0
9 105
10 0
I might add that I installed the diodes mod on the IEC for computer noise (getting a 13K ohms reading from chasis ground to IEC ground), don't know if it affects anything.
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The PN2907 on the A side of the small C4S board looks to be cracked, can you confirm that?
If it's not, you can try reinstalling just that board, then retest the amp.
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The PJ2907 is not cracked, it doesn't look like in the picture. I installed just the small board, and voltages are all right:
OA 68
IA 200
B-A 0
IB 200
OB 68
I imagine that the large board must be the problem. On that board, an alligator clip scratched one of the ground to LED vias, so I soldered a jumper in and checked for 0 ohm resistance from ground to the LED and ground to the next resistor. Pics. attached.
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Can you measure the DC resistance on the big PC board between the center leg of each TIP50 transistor and one of the solder points for the heatsink? If these touch, your high voltage rail will get dragged down and you'll end up with the issues you had.
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They don't touch, resistance is out of range between center leg of transistors to solder points of heatsinks..
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Don't know if it would help, but when the big board was installed, one of the LED shined overly bright. Pic. attached.
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I think your best bet would be to contact replacementparts(at)bottlehead(dot)com to see about obtaining a replacement large Speedball board and parts.
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Can I listen to the Crack with only the small board installed while the new board arrives?
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Sure, you can use the amp with the small board and the 3K resistors.
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Before ordering a new large board, I measured everything and found the short!!! I was a faulty 2N2222A, once replaced all voltages measurements seem fine:
Small board
OA 70
IA 191
B-A 0
IB 191
OB 70
Large Board
OA 108
OB 108
G 0
B+ 190
Paul, thanks for you patience and all the help!!!