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Old 3rd January 2013, 01:31 AM   #1
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Default Tube mic preamp problem

I bought this unit as is. I was told that it receives a signal from channel 1 above 80% gain and channel 2 has to be 100%. Upon trying it out, the Hi-z inputs work fine. When I plugged microphones in the problem was confirmed for me. I thought dirty pots but the position in which they "engage" input is consistent.
This circuitry is far beyond me but I would like to learn about the way this functions, the proper way to test it and the best repair solution.
Thanks
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File Type: pdf ART-MPA-Gold-schematic.pdf (292.7 KB, 77 views)
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Old 3rd January 2013, 02:21 AM   #2
gootee is offline gootee  United States
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So it's possibly something that is used for both of the MIC circuits but is not used for the Hi-Z inputs.

Both of those use the tube section so that sholuld be OK.

Could be some power supply rail that's only used by the MICs. How's the phantom power voltage look? can you measure the voltage? Might as well check all of the power rail voltages you can, if the phantom power voltage is OK. BE SAFE.
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Old 3rd January 2013, 02:33 AM   #3
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This is a pretty complex preamplifier and something that should be serviced by an experienced technician since, by your own admission, is far above you. However, the place to start is the power supply. Be certain the mains input voltage switch is set to the proper voltage (115/230v) and the tubes light properly.

The supply has seven (7) different output voltages that must be confirmed before anything else is addressed. These are:

+5V (U100)
+15V (U101)
-15V (U102)
+12V (U105) (filaments)
-15V (U106) (meter lights)
+48V (Q100 from multiplier network)
+150V (from extended multiplier network)

If you are very lucky, a power supply voltage source will be down which is a pretty common failure mode. Anything more then this and you will need more up close and personal help.
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Old 3rd January 2013, 03:34 AM   #4
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Voltages all appear good within .03 VDC. U106 is -17.9 but lights are unplugged. I have not found extended multiplier network. Other question is, why don't I see a voltage at the input of regulators? I expected to see an unregulated voltage and an unregulated voltage at the 7x15, 7x12,7x05.
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Old 3rd January 2013, 08:41 AM   #5
Tesla88 is offline Tesla88  Italy
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Do u read correct voltages at the output of regultators? If so voltages at the input of regulators must be ok
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Old 3rd January 2013, 04:26 PM   #6
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It appears that the problem must be in the OPA2134 or something close to mic OPA213. That is the only circuit that differs from the instrument section.
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Old 3rd January 2013, 05:51 PM   #7
gootee is offline gootee  United States
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Quote:
Originally Posted by argonrepublic View Post
It appears that the problem must be in the OPA2134 or something close to mic OPA213. That is the only circuit that differs from the instrument section.
The four OPA2134 are only in the instrument section. Are the Instrument inputs the ones that you called "Hi-Z", before? You said that the Hi-Z inputs worked fine. I thought that the MIC inputs were the only ones that had a problem.

Can you check to see if you can measure 48V between the center pin and one of the other pins, at each MIC input jack (with no MIC plugged in)?

Everyone should note that the schematic can be SEARCHED, when using the PDF file. Just select Edit then Find and type in something, for example +15, and it will go to the first +15 that it finds, on the schematic, and will highlight it. Then selecting "next" (an arrow button, in Adobe Reader X v10.1.4) goes to the next one and highlights it, etc. It's VERY handy for tracing things around the schematic.
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Old 3rd January 2013, 06:06 PM   #8
gootee is offline gootee  United States
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When you measured the power supply voltages, under what conditions was that done? (Switch and knob settings? MIC(s) plugged in? etc.)

Except for the power supply voltages, there's not much (if anything) else that wouold affect BOTH mic channels. And if the instrument (Hi-Z?) inputs work ok, that seems to rule out most of the rest of the system. (But I could be wrong.)

If that's the case, then if it's not the 48V phantom power, then maybe it's not a single problem. It could be that someone plugged in a bad mic or cable (or an output instead of an input?) and when it didn't work they tried the other input jack, and did similar damage to that circuit.

Last edited by gootee; 3rd January 2013 at 06:18 PM.
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Old 3rd January 2013, 06:24 PM   #9
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My mistake U203B appeared to be labled mic pre. Now that I look again, It is feed from mic pre. THANKS
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Old 3rd January 2013, 06:25 PM   #10
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All voltages were checked without anything plugged in.
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