Bottlehead Forum
Bottlehead Kits => Legacy Kit Products => Crack-a-two-a => Topic started by: Skipperrik on January 11, 2025, 06:33:50 AM
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Out of the blue my Crackatwoa has developed a buzzing sound in both channels. I looked at this post, https://forum.bottlehead.com/index.php?topic=14712.0 and saw where his problem was a dirty 12AU7 pin. I have changed all of the tubes multiple times with no resolution so it must be something else.
The voltage tests results are:
Low Voltage C4S
IA 193
OA 72.9
KregA 8.29
bRegA 163.7
IB 150.2
OB 72.8
KregB 5.42
bRegB 150.2
High Voltage C4S (A/B)
IA 192.9 / 190.5
OA 163.2 / 150.2
bA 0.3 / 0.6
IB 0.3 / 0.6
OB 100.2 / 93.1
bB 163.9 / 150.2
I also tested it with only the 12AU7 tube in place:
IA 216
OA 64.3
KregA 9.0
bRegA 217
IB 217
OB 64
KregB 1.4
bRegB 217
These values seem high with the exception of OA and OB which are normal and KregB which is low.
Do any of these values give a clue as to where the problem could be?
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Low Voltage C4S
IB 150.2
bRegB 150.2
High Voltage C4S (A/B)
OA 150.2
This is one channel's worth of regulated high voltage connections. They are all wired together and they all have the same voltage.
Low Voltage C4S
IA 193
bRegA 163.7
High Voltage C4S
OA 163.2
This is the other side. You have 163.2V at the output of the high voltage C4S, but somehow 193V at the input of the center C4S board. This is a bit of a contradiction, as these connections are wired together. On the A side of the center C4S board (low voltage), +reg, Breg, and IA are all wired together, so they can't have different voltages unless one of those connections is no longer present or you have a very flaky ground connection in the amp disturbing the reliability of your voltage measurements.
My guess is that you have a broken wire that isn't permitting the regulator to work properly. A buzz in both channels would make me wonder if there's a broken or loose ground wire in the amp, and this is both causing the buzz you're experiencing as well as the regulator malfunction.
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I believe I found an assembly error that could account for the voltag imbalance. On one of the high current boards I swapped the locations of the 22KΩ 5W and the 47KΩ 2W resistors. I don't think that would cause the buzzing though since it has been dead quiet up until yesterday. I've run a continuity check on every wire in the amp and have found no breaks. I'll reverse the positions of the two resistors and follow up when I get it completed.
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I found that one leg of a TIP-50 resistor at position B (see photo) has ground continuity. The corresponding leg on the TIP-50 resistor on the opposite board does not. Which is correct?
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Sorry, the photo didn't get uploaded. Here it is.
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The TIP50 portion of the circuit would not be very likely to cause the problem you're experiencing. Is it the base, emitter, or collector that's showing a ground reference? What is the DC resistance of this measurement? Is it bidirectional?
Since your OB voltages on the high current C4S boards are correct, I'd be inclined to ignore this for now and focus on how three terminals that are wired together are able to have different voltages on them.
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Sorry, the photo didn't get uploaded. Here it is.
It looks like your probe might also be touching IB, which is grounded.
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That was just for the photo. I was careful not to touch any surrounding surfaces and still got a ground signal. I'll go over all of the resistors, capacitors, etc. and make sure they are all correct. I don't believe there is a faulty ground wire since I have checked and re-checked every wire and connection. I'll keep looking.
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I'll go over all of the resistors, capacitors, etc.
You have three terminals that are supposed to be wired together that are showing different voltages. This is not a bad capacitor, resistor, or transistor. This is a flaky solder joint or broken wire in the amplifier.
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I found an fixed an errant wire on one of the regulators but I'm still getting high output voltages.
Low Voltage C4S
IA 192.4
bRegA 192.1
High Voltage C4S
OA 192.3
At least they are all pretty close but what could be causing the high output? I don't think it's a ground issue. I've checked the wiring and continuity of every wire multiple times and re-flowed every connection.
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What are the Kreg voltages now?
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KregA 8.87
KregB 5.8
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There's an X pad on each side of the center board where you measured those Kreg voltages, what DC voltages do you see at each X pad?
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A side
x=125.7
B side
x=2.51
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A side
x=125.7
Wow that is crazy, I'd be looking at the 2.49K resistor and the 431 regulator where that voltage is present.
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I checked the resistors. The B side measures 2.44KΩ but the A side measures 7.4KΩ. That's bizarre!
How do you check a regulator?
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I think I'd start by replacing the regulator resistor on that funky side.
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Okay. I’ll order one and let you know the results.
Thanks for your help, Paul. I really appreciate it.
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I received a couple of new 2.49KΩ 1/4w resistors. I measured one of the new ones prior to installation and it measured 2.48KΩ. After removing the original one and installing the new one it measured 7.5KΩ, just like the original one did.
What could be causing this?
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Are you measuring the resistor with your meter across the resistor itself while it's soldered into the circuit? Did you measure the ones you took out?
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Yes, I measured it before inserting it (2.48KΩ) and in place (7.5KΩ). The one I pulled out measures 2.4KΩ.
The resistor on the B side measures 2.44KΩ in place. Something is different on the A side.
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Pull the 431 regulator on that side and measure again.
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It's still 7.5KΩ.
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I'd post some pictures of your board, something is definitely not right...
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The regulator is still out. I may have to replace it. On the underside, the red wire on the left broke while removing it so it will have to be replaced.
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Just to clarify something, in the photo of the underside of the board the black wire appears to be connected to the Kreg terminal. I had to disconnect it from the -reg terminal to remove the capacitor and it only appears to be connected at Kreg. It was connected correctly before I removed the capacitor.
Just for the heck of it I measured the resistance across the capacitors. I don’t know if this helps but here are the values:
A side (the problem side)
0.1uf 400V = 146
2.2uf 250V = 145.8
B side
0.1uf 400V = 34.2
2.2uf 250V = 35.4
That’s a pretty big difference so it should be an indication of something.
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I have no idea how the measurement of the 2.49K resistor is increased by installing it in the circuit. Something is definitely amiss, but I have no idea what that is. Did you end up replacing the 431 regulators?
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I don’t have a replacement but the value is the same whether it’s in the circuit or out. I could swap the one from the good side if you think it would help but that’s a lot of trouble.
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I have a very embarrassed look on my face. I pulled the transistor out previously, not the regulator. I just pulled the regulator out and the wayward resistor now reads 2.48K ohms. I guess the regulator was allowing current to be sucked out of the leads of my ohm meter.
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I need to replace the transistor. Mouser has two, an MJE350 and an MJE350G. The G looks like the one that is used but I want to make sure it's the correct one since the specs differ. Should I order the 350 or the 350G?
https://www.mouser.com/c/?q=MJE350
Is this the correct regulator?
https://www.mouser.com/ProductDetail/926-LM431ACZ-LFT3
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I don't immediately see the difference between the G and the non-G, so I wouldn't sweat that. Yes, that is an appropriate replacement for the regulator.
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Okay.
I just wanted to say again that I appreciate your help and your patience. I'm sure it can be frustrating to deal with newbies and I'm a great example of that. Thanks!
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I'm happy to report that the new parts are installed and the voltages are all within spec. The hum is still present however. I noticed that it isn't present when I turn the amp on, only after the circuits kick in. Also, immediately after switching it off the sound gets significantly louder until it fades away. Could this help in locating the cause?
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I'd be poking around with a wooden chopstick to see if you can disturb whatever connection is causing the problem, then you'll know where to focus your resoldering efforts. A cheap-o pair of headphones plugged into the amp is a good idea for this kind of testing, as it can produce loud noises if the poked joint is particularly loose.
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The hum is still present however. I noticed that it isn't present when I turn the amp on, only after the circuits kick in.
I had a similar problem in C2A..in my case, there is a noticeable hum/distortion in both channels.. But turns out to be a problem with the selector switch in the C2A.. I need to push it hard to the current selection and then the static disappears.. I am suspecting a loose wire or a bad solder joint ...you *might* be having a similar issue perhaps. worth a shot..
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Thanks for offering the suggestion, Happy Ghost. I have a DAC that has a volume control and I only use one input so I bypassed the selector, balance and volume controls. So far, poking and proding hasn't helped to locate the problem. I"m still searching.
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Hi Paul. I believe I've narrowed the problem down to the headphone jack and would like to get a replacement.
Awhile back I sent an email to Bottlehead asking if longer lengths of your power cord could be ordered. I got no response. Last week I sent an email asking for someone to call me so I could order parts. I haven't gotten a response.
Do you guys sell parts other than what's listed on your parts page?
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A tree fell on Doc B's house and they have been a bit occupied at the moment but should be getting back to the office shortly.
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I’m sorry to hear that. I hope there wasn’t a lot of damage.
I ordered a new locking jack so hopefully that will solve the problem.
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Yes, we have had quite a ride the past few days getting power back to our house. After a couple of false starts it looks like we have it fixed just in time for tonight's snowfall. I am back at the office for a short time today to get through as much correspondence as I can.
I'm afraid we only offer the power cord in the 6 foot length. The gauge of the wire used puts some limitation on the current capacity and we feel that a longer run might possibly degrade the performance.
Re parts - we only sell replacement parts for our customers who may need them. If you have broken or lost a part we can help you out. Best way to handle this is an email to [email protected]
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Thanks for chiming in, Doc. I'm sorry you have experienced the wrath of nature. My heart goes out to you!
I've been having an ongoing problem with wires breaking in my Crackatwoa. Apparently the wire stripper I used put a ding in them and over time and movement from tracking down a ground loop hum, it causes them to break. I'd like to order more wire so at your convenience, please have someone give me a call so I can give you a credit card number.
Thanks again and I hope things smooth out for you!
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I've been over every solder joint in the amp and checked each wire's continuity and haven't found the source of the hum. This morning I noticed a burned spot on one of the 270Ω wirewound resistors. Could this be the cause, and what would cause it to burn like this?
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See page 39 in the manual, those are not properly installed.
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PB beats me to the punch once again!
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New resistors are installed correctly along with new wires from 25L to 33U and from 25L to 31L. I’m afraid the hum is still there. I’ve reflowed every joint in the amp again and replaced several wires. I’ve even run a direct wire to ground at every ground termination point to check for broken wires but the hum is still there. It’s a little more melodious now (a slow pulsating sound) but it’s very much present. I’ve run out of options with the solder joints so I have a couple of questions:
1.) Is it possible that a component on one of the boards is bad? I’ve prodded all of them with a chopstick but there’s no indication of trouble.
2.) Could it be the transformer? I’ve reflowed all of the taps.
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If you had a whacky voltage, then maybe there could be a dead part on one of the PC boards. Likewise, a failing transformer is going to throw out some odd voltages.
Loose hardware can also create the problem you're experiencing.
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I just checked the resistors on the low voltage board and the 2.47Kohm at the RC position measures 28.9 ohms. This is the opposite side from the one that had the bad regulator. That side measures fine.
What could cause the regulators to go bad?
Here are the voltage figures from the low current board:
IA 150
OA. 64.0
KregA. 4.3
bRegA. 150.1
IB. 169
OB. 63.4
KregB. 1.54
bRegB 168.2
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Those are not working voltages. The 1.54V at KregB is no good, and bregB is also not correct.
This can happen if the A side high current C4S feeding that side is shorted and dumping excessive current into the regulator, from a miswire, or potentially from a failed 431 regulator.
The 2.49K resistor does not go in the RC position.
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Sorry, I mistyped. It’s the 2.49Kohm resister at R3.
I’ll replace the regulator and see if the voltages straighten out.
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I swapped sides with the low current board to see if the hum followed (it didn't) and now a new problem has cropped up. The two LEDs next to the "A" on one of the high current boards do not light. The following are the voltages on that board:
IA 187.1
OA 172.5
bA 0
OB 0
OB 98.8
bB 172.5
Opposite high current board:
IA 195.4
OA 148.8
bA 0
OB 0
OB 85.4
bB 148.8
Low current board:
IA 172.7
OA 67.1
KregA 1.943
bRegA 172.5
IB 148.9
OB 62.6
KregB 5.56
bRegB 148.9
Do you have any suggestions about how to solve the LED issue?
Originally I thought the ground hum was in both channels but the right channel is faint so I think it's probably bleed over and the real problem is with the left channel. I've gone through every step in the manual to make sure the wiring is correct, I've checked the continuity of every wire, examined every connection, checked the value of every resistor, and swapped all of the tubes in an effort to find the ground fault. Everything checks out. I'm about to throw in the towel. Do you have any other suggestions?
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The LED issue is the regulator not working, and this could be for a very, very wide variety of reasons. The most important thing to do is to not assume that the LEDs are the problem and go about resoldering them, as that will damage them and make more problems.
Issues that would cause the problem you're seeing would be as follows:
1. A 6AQ5 might no longer be glowing or isn't present in the socket, causing the shunt regulator not to pull the B+ down to 150V.
2. The high current C4S board is improperly installed on the faulty side. The regulator might be having issues from excessive available current which can happen from mixing up the wires connected to that board.
3. There's an actual short on the high current C4S board causing the current to not be regulated.
4. There's a broken jumper on the center C4S board where the 431 regulators live, which is causing the regulator not to work appropriately.
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Thanks. This is helpful. I’ll check each item and get back to you.