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

« Last post by JPK on June 07, 2024, 05:32:53 PM »

The transformers are OK.
There is something wrong around Q4, Q5, Q6.
Check all the solder joints around there.

62

AB528 Microphone preamplifier / Preampli AB528

« Last post by Mateo_K on June 05, 2024, 03:56:47 PM »

Bonjour, tout d'abord merci de proposer ces kits en DIY. 

Je viens de finir le montage de mon premier AB528, la led rouge à l'interieur est bien allumée et la led de la face avant s'allume aussi lorsqu'il y a du signal néanmoins je n'ai quasiment pas de signal en sortie. 

Lorsque les deux potentiomètres sont au maximum j'ai un signal faible, distordu et aigu comme s'il y avait un coupe bas à 6000 hz. 

Le symptôme est présent aussi bien sur l'entrée mic/line que sur l'entrée DI.

Que puis-je faire pour remédier au problème ?

Grand merci d'avance,

Matéo

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

« Last post by Gavin1393 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

« Last post by Gavin1393 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....

65

First Aid / Re: Need help with 573EQ, solder on gold connectors

« Last post by JPK on June 01, 2024, 06:09:56 PM »

Just remove as much solder as you can with a desoldering pump or solder braid.
You won't be able to restore the gold layer but it should still work.

66

First Aid / Need help with 573EQ, solder on gold connectors

« Last post by speirs on June 01, 2024, 03:40:08 AM »

Hello, I was working on my 73EQ soldering some leads and when I lifted my soldering iron, a drop fell on one of the gold connectors.

How should I remove this? Should I use solder braid to try to remove it?

Is the the connector permanently damaged now? Will I need a new pcb now?

Thanks for any help and advice. I don't want to keep working on soldering in parts if I ruined the pcb board.

67

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

« Last post by JPK on May 31, 2024, 11:30:39 AM »

Can you do the measurements shown here : DC voltages, and output transformer check?
We will add a new relay to your new order.

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

« Last post by Gavin1393 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!

69

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

« Last post by slothproductions on May 31, 2024, 08:44:17 AM »

If I was you, I would try replace the suspected relay. At least thats what fixed my problem

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

« Last post by Gavin1393 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?