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MP573 Microphone preamplifier / Re: Bias Adjust

« on: July 15, 2024, 05:43:32 PM »

Did you get your issue solved Evan? I had a similar problem, my MP just kept droppong indefinitely. Turns out my Q6 (one with the 90 degree bent legs) wasnt contacting the PCB...a little more solder sorted it!

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Discussions about the products / Re: Preamp suggestion

« on: July 15, 2024, 05:30:42 PM »

Hey Jasper, I have the same Cranborne Setup you have. Was wondering if you experience the same "issue" I do with the Sound Skulptors I have running in the ADAT, I have to physically switch the link switch off and then on to link Pre-EQ-comp. Whereas, with the R8 the summing mixer seems to retain the setting when switched off and then back on. The ADAT seems to "turn off?

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MP573 Microphone preamplifier / Re: MP573 general power check and deviating DC voltages

« on: June 15, 2024, 11:52:41 AM »

In lesser company I might have argued that there was nothing wrong with my soldering, since I'd used Flux on every joint, perfected my technique...but there it was Q6 and its three pins, two attached to the board and one, like the Wright brothers, had lifted off the board. OR, I just soldered it to mid-air...

Soldered it correctly and sheepishly considered all the other joints in the area, and they are looked good.

Pulled out the DMM... and got near perfect results for the voltage test. Adjusted the P2 and got 3.000v after a good few turns with the blue tool JPK sent me.
Plugged in the Mic's and SOUND!Glorious SOUND!
Did all the other checks and all seems delightfully positive!

Thanks JPK

Appreciate the guidance - as always, you were spot on with where to look! I think it took me all of 30 seconds to see the problem once I knew where to look!

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MP573 Microphone preamplifier / Re: MP573 general power check and deviating DC voltages

« on: June 04, 2024, 11:44:26 AM »

OUTPUT TRANSFORMER RESISTANCE MEASUREMENTS:
Yellow - Orange: 7.8 ohms
Red - Brown: 6.8 ohms
Blue - Green: 20.8 ohms
Violet - Black: 20.8 ohms

INPUT TRANSFORMER RESISTANCE MEASUREMENTS:
2-4: 24.8 ohms
3-5: 24.7 ohms
7-9: 133 ohms 
8-10: 133 ohms

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MP573 Microphone preamplifier / Re: MP573 general power check and deviating DC voltages

« on: June 04, 2024, 11:34:34 AM »

Check the usual suspects:

• Check you didn't forget the jumper on CN1,
• Check you didn't leave the polarity switch on the center position (=mute),
• Forgotten solder joint,
• Bad solder joint (reflow suspicious joints),
• Reversed components (electrolytics, diodes, transistors, IC's). In case of IC's, probably destroyed, better replace,
• Flipped IC's or transistors,
• Trim all the component leads and test pins as short as possible. Nothing must touch the chassis plate.

ALL CHECKED[/list]
Check the DC voltages:
First check the supply voltages as described in the setup guide.
short circuit check  v- an OV  6.5M ohm
V+ and OV         6.9M ohm

Removed DI and unplugged CN2

Connect the (-) lead to TP2 and the (+) lead to V+. 24.39v
Connect Output transformer 24.37v

Measuring Q6... Started at about 3v and plummeted to o.6v before i turned off the device. Same issue as before.

Next, without any input signal, connect the voltmeter black probe to TP2 and note the DC voltages you get at the points shown on the attached picture. Compare with the expected results (more or less 10%).

A  2v6
b  4v1
c  22.5
d  13.5
e  2.01
f  12.85
g  0.1  !!
h  24v !!
I  4.1  !!
j  23.5  !!
K  24.34   !!
L  0.1  !!


@JPK - All goes pear shaped from G....

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MP573 Microphone preamplifier / Re: MP573 general power check and deviating DC voltages

« on: May 31, 2024, 09:53:24 AM »

I did read up on the relays and how if they dont work they get rather hot...so hopefully that is all that was wrong. Have ordered 3 in case...

Thanks mate! I'm hoping this will fix the issue. Otherwise @JPK help will be appreciated!

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MP573 Microphone preamplifier / Re: MP573 general power check and deviating DC voltages

« on: May 30, 2024, 01:00:32 PM »

Havin similar issues with my MP573 at Setup time.

Had no issues with the short circuit check that I could tell.

Power of 24.3 with the Output transformer connected and disconnected....

When looking to set up and calibrate the bias of Q6 my DMM started at around 20v odd and slowly dropped to around 1v but still wouldn't settle. Around 1.3 - 0.7v seemed the norm.

I've looked for obvious soldering issues on the PCB but see nothing. I found that RELAY 3 on the underside of the PCB looked like it may have overheated and suffered some heat damaged ...

I'm at a loss to understand what to do next?