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ART Pro Channel Mic Pre noise troubleshooting help.  Schematics, MP3 & Pics.
ART Pro Channel Mic Pre noise troubleshooting help.  Schematics, MP3 & Pics.
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Old 17th June 2020, 01:52 AM   #1
BassAmp is offline BassAmp  United States
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Location: CA
Default ART Pro Channel Mic Pre noise troubleshooting help. Schematics, MP3 & Pics.

Hello everyone.

I have an ART Pro Channel mic preamp that has developed an issue that I'd really like to fix.

I've tried to be as thorough as possible, sorry for the long post.

Here's the problem:
When the Preamp Output knob is all the way counterclockwise or off and the Gain and Master Output are set to to 0dB (12:00, these exact settings don't really matter, this is just for example), the unit is quiet. As soon as the Preamp Output knob is turned clockwise, serious hiss (or "sshhhhh") is heard through the output. As the Preamp Output knob is turned further clockwise, the hiss gets louder and louder until the knob gets to 0dB or 12:00. Then, as the knob is turned further clockwise the hiss decreases until it is fully clockwise (+10dB) at which point the hiss goes away completely.

Also, if a sound source is plugged into the input, you can't hear it over the hiss until the Preamp Output knob gets to about 9:00 or -8dB. As the Preamp Output knob is turned further clockwise, both the input and hiss get louder until about 12:00, then the hiss decreases as the input continues to increase. When the preamp knob is fully clockwise you can clearly hear the input with no hiss.

One other interesting observation is that when the VU meter is set to "Preamp" & "Internal" via the 2 front panel switches, the dial immediately pegs hard right and no knobs have any effect on this.

I have attached both of the schematics.

Here is what I have done for troubleshooting thus far:

1. Visual/smell test. Nothing looks or smells burnt. None of the caps appear to be bulging or leaking.

2. I have swapped out all of the tubes and this doesn't change anything.

3. Measured the +/- voltage of A7 on pins 8 and 4. These are stable +/- 15V

4. I have measured the values of the Q20 transistor (TIP112). A youtube video suggested this is a common source of problems.
Base: 48.43V
Emitter: 47.44V
Collector: 61.03V

3. I have measured the input and output of the U1 Voltage regulator (L7815CV).
Input: 20.7V, Output: +15V

I would have done more measuring, but to get to the bottom of the main board, I have to completely disassemble the thing and make some sort of test fixture. I will do this if necessary, but I haven't yet.

I have also attached a recording of what the noise sounds like. At the beginning of the recording the Preamp Output knob is fully CCW. I then turn the knob fully CW (:03 - :10). You can hear the hiss increase then fade away (the hum is single coil pickup hum). Then I turn it fully CCW (:12-:16). Next I turn the knob clockwise after playing the top 4 strings on the guitar. You can hear the noise increase and then disappear as the guitar gradually comes through.

Finally, I have attached a picture of the front and inside of the unit.

It seems to me that the source of the problem is right after the Preamp Output pot and before the Compressor, EQ and VU Meter. This leads me to think that the most likely culprits are either or all of the following: C43, C48 (22uF 25V) and/or A7 (NE5532). Because the problem disappears when the Preamp Output pot is cranked, I'm leaning towards C43 and/or C48, but this is purely a guess. The fact that the input is clean when the Preamp Output is cranked leads me to think it is not the 5532, but again, this is pure speculation.

Does this noise sound characteristic of a certain type of component failure? Does it sound like one or more of the opamps is faulty? Does it sound like a faulty electrolytic cap(s)?

Would anyone be willing to take a look and offer some insight and tell me if I'm on the right track or completely out to lunch? Does anyone agree that C43,C48 A7 are the likely offenders?

If not, could someone point me in the right direction?

Any and all input would be greatly appreciated.

Thanks for looking.
Attached Files
File Type: zip ProChannel-Schematics-MP3-Pics.zip (1.46 MB, 37 views)

Last edited by BassAmp; 17th June 2020 at 01:54 AM.
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Old 17th June 2020, 09:05 AM   #2
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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If your noise is gone with the pre amp output pot at zero, forget everything after the pot.
Firstly lift one leg of C48, if the noise has gone resolder and lift one leg of C43, if gone lift one leg of C10, if the noise is still there it is probably the valve. If the noise has gone, re solder C10 and keep going backwards to wards the input section.
Next is to lift one leg of C8 and C11. That will isolate the noise to one section.

Do you understand why there are three transistors per phase input?
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Old 17th June 2020, 08:08 PM   #3
BassAmp is offline BassAmp  United States
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First and foremost, thanks for taking the time to look into this and to respond. Greatly appreciated!

I thought I was on to something...apparently not. It really seemed like the problem would be downstream of the Preamp Out pot since the noise eventually disappears when the Preamp pot is cranked fully CW. In other words, it seemed like something downstream of the pot eventually behaved correctly once it received enough signal from the pot, but I am certainly not going to argue with you. I will make a fixture so I can desolder as you instructed and test immediately without having to put it all back together each time.

Could you clarify a couple things? You said to forget everything after the pot and to desolder one leg of C48 and then C43, however both of these are after the pot.

Also, wouldn't I want to desolder one leg of C13 before going to C10?

Quote:
Originally Posted by JonSnell Electronic View Post
Do you understand why there are three transistors per phase input?
Are you referring to Q1, Q3, Q4 and Q6, Q8, Q9? Unfortunately, that is way over my head at this point. If you would care to elaborate or point me in a direction, I am all ears.

Thanks again.
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Old 18th June 2020, 02:19 PM   #4
brig001 is offline brig001  United Kingdom
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Quote:
Originally Posted by BassAmp View Post
One other interesting observation is that when the VU meter is set to "Preamp" & "Internal" via the 2 front panel switches, the dial immediately pegs hard right and no knobs have any effect on this.
The VU meter is DC coupled from PreAMP_OUT (from 215Schem-A E5 to 215Schem-B C2), so if it goes hard right, A7A must have DC on its output. Measure pin 1 on this to check. If that is the case, either the NE5532 is dead (or its supply) or there is a problem with C43 or R119

Brian

Last edited by brig001; 18th June 2020 at 02:24 PM.
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Old 18th June 2020, 06:07 PM   #5
BassAmp is offline BassAmp  United States
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Brian,


Thanks for taking a look and for the input. I'll check these things out.
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Old 18th June 2020, 08:48 PM   #6
BassAmp is offline BassAmp  United States
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There is -12.3VDC between pin one of A7 and signal ground. I used the grounded side of R119.

R119 measures 7.99K so that's probably good.

Would you suggest replacing A7 and C43?

Thanks again
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Old 18th June 2020, 09:08 PM   #7
MarcelvdG is offline MarcelvdG  Netherlands
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I don't trust A7A from 215Schem-A.pdf. If for whatever reason there is excessive noise current flowing through its input, that will only cause noise voltage when the PreAMP Output pot is somewhere halfway rather than fully clockwise or counterclockwise, much like you observe. Besides, brig001 already found another reason to distrust A7A or its supplies.

Edit: I hadn't read post 6 yet.
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Old 19th June 2020, 11:43 AM   #8
brig001 is offline brig001  United Kingdom
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Quote:
Originally Posted by BassAmp View Post
There is -12.3VDC between pin one of A7 and signal ground. I used the grounded side of R119.

R119 measures 7.99K so that's probably good.

Would you suggest replacing A7 and C43?

Thanks again
I'm guessing that the capacitor is at fault here. If you disconnect C43, does pin 1 go to 0V or close?

If it does, swap the capacitor

If not, check the other voltages on A7. Pin 3 should be 0V and pin 2 should be the same as the output

Brian
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Old 20th June 2020, 12:05 AM   #9
BassAmp is offline BassAmp  United States
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Thanks again for the input.

I finally got the thing disassembled and somewhat ready for testing and surgery.

I'll report back with my findings.
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Old 20th June 2020, 10:24 PM   #10
BassAmp is offline BassAmp  United States
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Update:

Before proceeding, I found that pins 1, 2 and 3 of A7 all have -12.3VDC, not just pin 1.

I disconnected C43 from the circuit, and pins 1-3 still have -12.3 V, the noise is still present and the VU meter still pegs.

Also, the Preamp Out knob no longer has any effect on the noise, it's always at maximum. The noise and output are still controllable by the Compressor Output and Master Output.

Is it safe to assume that A7 is at fault? I'm guessing the -12.3 V is leaking from the -15VDC on pin 4. Is that a fair assessment?

Please let me know what you think.

Thanks again.
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