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October 06, 2022, 07:41:07 PM

Author Topic: New Crackatwoa build. Voltage issues [resolved]  (Read 1285 times)

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Offline Thermioniclife

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Re: New Crackatwoa build. Voltage issues
« Reply #15 on: July 31, 2021, 09:28:51 AM »
Yeah, 78vdc is not good, do not plug your phones. You should have 0 vdc as you had before. As for AC volts you should have just a few milivolts. It's as if the coupling caps are not present or they are shorted, allowing Dc volts to pass
« Last Edit: July 31, 2021, 09:31:26 AM by Thermioniclife »
Lee R.

Offline plasticon47

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Re: New Crackatwoa build. Voltage issues
« Reply #16 on: July 31, 2021, 10:22:10 AM »
Hey Thermioniclife:

Well, I am really glad I asked my question before plugging in my phones.  Thanks VERY much for your response.  But, this is perplexing.  I literally changed nothing other than the jack from TRS to XLR.  I wired WHITE (formerly TIP) to pin 1, BLACK to pin 2 (with a jumper to pin 4), and RED to pin 3.  I didn't touch the coupling caps and the unit worked fine with the TRS jack last night.  All voltages pass with the XLR jack in there with the exception of checking pins 1 and 3 of the XLR jack (78 VDC on each).

There are resistors from TIP and RING on the TRS jack to ground (on the jack).  Does there need to be resistors on the XLR jack as well?  From looking at the Mainline, there are no resistors on either the TS or the XLR jacks.  I am rather confused.

Offline Thermioniclife

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Re: New Crackatwoa build. Voltage issues
« Reply #17 on: July 31, 2021, 11:30:11 AM »
Stand by while look at the schematic
Lee R.

Offline Thermioniclife

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Re: New Crackatwoa build. Voltage issues
« Reply #18 on: July 31, 2021, 11:35:31 AM »
Ok, there are 2k2 ohm resistors from tip and ring to ground you should install them. Having said that I'm not sure if that is the problem. So if you still have them put them in and recheck voltage. Are you sure you measured 78 volts Dc?
« Last Edit: July 31, 2021, 11:45:27 AM by Thermioniclife »
Lee R.

Offline plasticon47

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Re: New Crackatwoa build. Voltage issues
« Reply #19 on: July 31, 2021, 01:12:53 PM »
Thermioniclife:
Thanks for the advice.  And I am positive 78VDC was observed on the WHITE and RED wires.  I was kind of wondering about those resistors.  I went to take them off the TRS jack to put on the XLR one and one broke in half (must be too much heat...rats!).  So, the closest resistor value I have in my "scrap parts box" is 2.49Kohm.  Can I use two of those instead?

And I'm really hoping Paul can respond.  I see his name on the schematic, so his confirmation of your (Thermioniclife's) advice would make me feel MUCH better.  Is the lack of resistors on the XLR jack what is causing my problem?

And once again, Thermioniclife, I really appreciate your response and assistance.   

Offline Thermioniclife

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Re: New Crackatwoa build. Voltage issues
« Reply #20 on: July 31, 2021, 01:32:56 PM »
As I stated earlier I do not feel that will solve the problem, but I would install the 2.49k resistors and check the voltage again. But having said that those resistors would not cure your problem. I think they are there to let a little current flow to ground at startup so when you plug in your phones you will not get a pop in the phones. Of course when PB responds I'm sure he will have the answers you require and dispel my whacky advice.
Lee R.

Online Paul Birkeland

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Re: New Crackatwoa build. Voltage issues
« Reply #21 on: July 31, 2021, 02:35:29 PM »
Did you put the resistors back on the new jack? They are not optional.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline plasticon47

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Re: New Crackatwoa build. Voltage issues
« Reply #22 on: July 31, 2021, 02:47:55 PM »
Hey Paul and Thermioniclife:
I mentioned I only have 2.49K resistors to use.  I went ahead and installed them on the XLR jack and checked voltages.  Now, all voltages are good, including 0VDC on the RED and WHITE wires (pins 1 and 3 on the XLR jack).

I saw Paul's post just now about the resistors not being optional (so I have my answer!)

Three questions:

1.  Is is okay to use the 2.49K resistors I have in lieu of 2.2K resistors?  Am I losing anything (power/sonically/other) by doing this?
2.  If I don't use the resistors (what I was doing previously), is that what caused the readings at the jack of 78VDC for the WHITE and RED wires (pins 1 and 3 respectively).  I want to make sure I don't have some other unidentified issue?
3.  Am I good to go? (i.e. - can I power up and connect my headphones now?)

Thanks to both you guys for your kind help.

Online Paul Birkeland

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Re: New Crackatwoa build. Voltage issues
« Reply #23 on: July 31, 2021, 03:01:58 PM »
Yes, if you don't have the resistors installed, you'll get a lot of DC voltage appearing at the output of the amp and you'll risk damaging whatever headphones you plug into the amp.

The value of the resistors on the headphone jack isn't super critical on the Crack-a-two-a because we soft start the operation of the 6080, so the voltage appearing on the amp side of the coupling caps rises very, very slowly.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline plasticon47

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Re: New Crackatwoa build. Voltage issues
« Reply #24 on: August 05, 2021, 08:43:29 AM »
I just wanted to put a pin in this thread.  I have a working (and working REALLY well) Crack-a-Two-a.  I also just wanted to thank both Paul and Thermioniclife for their help with my voltage woes.  It is nice to know you can just post a problem to an online forum and get such fast help. 

I have been listening to music the last couple nights and this thing sounds great.  All the good things from a Crack, times two.

I do have a question about tubes, but i will post that in a different thread.  Thanks again.