21

MP573 Microphone preamplifier / Re: 300 Ohm not working + suddenly change in output volune

« Last post by Dolbah on October 21, 2024, 05:34:49 PM »

Can you plug the other output transformer to this unit to check if the transformer is the culprit?

This has been done several times, it follows the main board. I'll look into the areas you suggest

22

MU524 Vari-Mu Tube Compressor / Re: Fried Resistors

« Last post by JPK on October 21, 2024, 05:03:30 PM »

1 Ohm 0.6W 1%
MBB02070C1008FCT00

23

MP573 Microphone preamplifier / Re: 300 Ohm not working + suddenly change in output volune

« Last post by JPK on October 21, 2024, 04:58:40 PM »

Unit 1: output transformer yellow wire: 8.4VAC (Gain=40, TRIM=Min).
unit 2: output transformer yellow wire: 8.4VAC (Gain=40, TRIM=Min).

Both units have the same level on the output transformer primary.
The issue must be the transformer or post transformer: R25/C9, R26, the phase switch or the finger connector.

Check the solder joints of R26 and the phase switch.
Can you plug the other output transformer to this unit to check if the transformer is the culprit?

24

MU524 Vari-Mu Tube Compressor / Fried Resistors

« Last post by AbYdIy-28 on October 18, 2024, 10:49:24 PM »

Hello, While I was setting up my Varimu with a voltmeter i accidently fried two resistors, R35 & R36. I was wondering if you could send me a part number for those resistors so I can replace them. 
Thank you! 

25

EQ573 Equaliser / Re: High pass not working EQ573-II

« Last post by Dolbah on October 17, 2024, 08:29:52 PM »

Check the values and solder joints of R26, R27, R9, J6a, J6b, C13...C22, L1, RN1, RN2.

thank you! it was somewhere on that route  All ok now

26

MP573 Microphone preamplifier / Re: 300 Ohm not working + suddenly change in output volune

« Last post by Dolbah on October 17, 2024, 07:56:12 PM »

With the impedance set to 1200, the gain should be just a little under unity, around -0.7dB.

Ok, so the issue is still:

If i unplug the Transformer on unit 2 (the one with new Relay) for a while, the volume is normal, after a while (5-15 minutes) The volume on this unit slowly rises and ends up higher then the other.


I did some tests:
----------------

Unit 1 (The one without any apparent problem):
Output pot = 0
Gain knob: 10
Impedance 1200
Input level: -12dB
Output level: -9,79 dB

That's 2,21 dB difference

Unit 2 (The one with replaced Rly3 and gain issues):

Output pot = 0

Gain knob: 10

Impedance 1200

Input level: -12dB

Output level: -6,58 dB



That's 5,42dB difference

----------------

General powercheck:

Unit 1:
V+ and TP2
 without Transformer: 24,24 VDC
With Transformer: 24.15 VDC

Unit 2:

V+ and TP2

without Transformer: 24,35 VDC

With Transformer: 24.25 VDC




Try to follow the signal

Send a good constant sine signal on input, like 0.5VAC measured between pins 2&3 of the input XLR. Check the level with the preamp connected because the input impedance of the preamp may pull the signal down.



Connect the voltmeter black probe to TP2 and trace the signal on:


Unit 1:

• Input transformer pin 7: 0.9VAC,
• TP1: 4.669VAC (Gain=40, TRIM=Min),
• output transformer yellow wire: 8.4VAC (Gain=40, TRIM=Min).

unit 2:

• Input transformer pin 7: 0.9VAC,
• TP1: 4.68VAC (Gain=40, TRIM=Min)
• output transformer yellow wire: 8.4VAC (Gain=40, TRIM=Min).

To verify the transformers DC resistances


unit 1:

CARNHILL VTB1847 (unplugged) :

• Yellow-Orange : 7.6 ohms
• Red-Brown : 6.4 ohms
• Blue-Green : 21 ohms
• Violet-Black : 21 ohms

CARNHILL VTB9045 :

• 2-4 : 24 ohms
• 3-5 : 24 ohms
• 7-9 : 144 ohms
• 8-10 : 142 ohms

unit 2:

CARNHILL VTB1847 (unplugged) :

• Yellow-Orange : 7.6 ohms
• Red-Brown : 6.4 ohms
• Blue-Green : 21 ohms
• Violet-Black : 21 ohms

CARNHILL VTB9045 :

• 2-4 : 24 ohms
• 3-5 : 24 ohms
• 7-9 : 133 ohms
• 8-10 : 132,6 ohms

Update:

I have noe switched input tranformers between the units. The issue does not follow the transformer, so it must be somewhere on the board.

27

MU524 Vari-Mu Tube Compressor / Re: MU524 - P1 Pot Part Number (10k LogB)

« Last post by JPK on October 16, 2024, 04:31:21 PM »

P1 is a 10k log pot (Omeg LT16BU - 10KB - BUSH C - 6.0MM - SPINDLE F1 - L=32MM)

When audio is input, the levels go to max (12dB) on the meter
Sorry, which meter?

It seems that other than Pins 1 & 2 being joined on the PCB
This is normal. The center pin is connected to the + pin.

The pot must be measured with the front panel disconnected or you will be measuring the input transformer coil. If the measured value of the pot goes from 0 to 10k, varying with the knob position 0 to 10, then it is OK.

If you confirm the pot is damaged, contact me here for a new one.

28

MP573 Microphone preamplifier / Re: 300 Ohm not working + suddenly change in output volune

« Last post by JPK on October 16, 2024, 04:16:46 PM »

With the impedance set to 1200, the gain should be just a little under unity, around -0.7dB.

29

EQ573 Equaliser / Re: High pass not working EQ573-II

« Last post by JPK on October 16, 2024, 03:59:42 PM »

Check the values and solder joints of R26, R27, R9, J6a, J6b, C13...C22, L1, RN1, RN2.

30

MU524 Vari-Mu Tube Compressor / Re: Light Hiss/Noise

« Last post by egelmett on October 16, 2024, 04:00:37 AM »

I only have the Midas L6 and two Empirical Labs EL500s - one is worse than the other...it may be them? Hmmmm...

But, strong line levels may be the issue: recording soft vocals and finger-picked acoustic guitar leaves a lot of space for noise to be apparent.

They're a lot better than just after the build. With your advice, like I wrote previously, it helped to go over solder joints and make sure C12 wasn't impacted.

Now, I'm dealing with P1 having failed on one of the MU524 - asking you about the part number or if I can buy a couple from you, in another post.