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Possible Problem With Pre-Amp Channel One
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u2bassace
Prenoob

Joined: 08/06/2014 04:12:29
Messages: 5
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I picked up a 24.4.2 on Ebay that was used. Said near mint and though it's not in bad condition probably used more than what I would call mint. Checked the scene presets and this board had some top Country bands played through it. Outfit I bought it from is in Nashville.

Anyway go to function check it today and run some music into channel one. I start to bring up the gain and as I get about 1/3 turn the meter pops from 1/4 to clipping out. Almost like BAM! Checked all the other channels and they are fine.

Here are a few videos I shot of the problem.

https://www.youtube.com/watch?v=ktYdBtTJaGw

https://www.youtube.com/watch?v=bLDdoXkJegg

https://www.youtube.com/watch?v=1ArWvOBWPG8

Hope Youtube links work this is the first time doign this on this forum.

This message was edited 2 times. Last update was at 08/06/2014 04:25:50

u2bassace
Prenoob

Joined: 08/06/2014 04:12:29
Messages: 5
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BTW this was after I updated the firmwear and reset it back to factory settings.
fredy2
Presonic

Joined: 10/04/2013 06:15:47
Messages: 207
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Having traced out the preamp section of one of my boards during repair I can give you a couple tests to do. I want you to do this to compare the same two channels as your video. You will need a set of headphones with 1/4 inch plug. plug the headphones into the LINE jack right below the XLR for each channel. Plug it in ONLY part way, lightly, slowly and listen to the level comparing the two channels. Note you will hear the audio ONLY in one side of the headphones and that is normal. Do this with the trim set at say 12 o'clock for both.
This test is to determine if the input stage of the preamp is where the problem is or the following stage. If you find this test shows very close results between the two channels, then we will do the same test with the headphones in the INSERT jack for each channel.
Here are the probable causes: Bad input trim pot... this is a two section linear pot in which one section varies the input preamp section and the second varies the gain of the second gain stage. Another possibility is a bad pin contact berween the back connector board and the board with the preamps. Unfortunately, the signals to and from the trim pot run back a forth between boards several times and if one of these connections goes bad, the symptoms can be like yours. Also try running the signal into the LINE jack and compare the levels between channels... If the line inputs behave the same bad way, then one of the connection on the second gain stage has opened up or the pot is bad, We can do a lot of troubleshooting from those back jacks now that I have the schematic. In my board, the preamp board was being pushed down causing an interboard connector pin to not make contact. I had to grind a little off the edge of the preamp board as in my case, the channel was dead instead of high gain,
u2bassace
Prenoob

Joined: 08/06/2014 04:12:29
Messages: 5
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Thank you, back to the studio tomorrow, I will try your suggestions. I really appreciate it. I own a 16.4.2 and a 16.0.2. I need this for a bigger gig in a couple of weeks and beyond. Channel one being down won't kill me for this gig. But obviously I want it right even though I paid a discount as it was used, but still good money.
fredy2
Presonic

Joined: 10/04/2013 06:15:47
Messages: 207
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One other problem I had was the input capacitors on the A-D board. They are 100/6.3 surface mounted electrolytics. It is possible that one of yours is failing and when you get the level up high enough, the failure breaks down and signals get= restored.

Doing the tests I outlined will isolate the problem. On a board I refurbished, I replaced all these caps... about 40 of them on the A-D board as they started failing one at a time. You need some skill at surface mount repair to do the work.
u2bassace
Prenoob

Joined: 08/06/2014 04:12:29
Messages: 5
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Had to keep the board for a gig last week. (needed more channels than my 16.4.2 could handle and the reason I picked up the 24.4.2) Got by without channel one and the rest of the board functioned perfectily. Have a short Youtube video of it in action I will post.

http://youtu.be/FSjxWMplsHA

This message was edited 2 times. Last update was at 24/06/2014 23:26:50

fredy2
Presonic

Joined: 10/04/2013 06:15:47
Messages: 207
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A quick in place test can be done at the venue by putting a set of phones into the INSERT jack of channel one. you will ONLY hear sound on one side of the phones, but try the gain trim control and see if the headphone volume jumps when you move the trim... If it is smooth there, then the problem is likely that input cap for channel one at the input of the A-D. If it jumps in the headphones the same, then the trim pot or connections are likely bad.
 
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