[Dolbah]

I have 2 units. They outputed uneven volume for a while, so i disassembled them, reflowed some joints and measured TP2 and TP4 to check the bias of Q6.

The Q6 bias was not at 3.0 VDC, so i readjusted them and reassembled them. Tested, and i now have equal output gain from both unit.



The next day i was continuing recording with them and realized that the volume was uneven again. I forgot to pay attention to which unit it was so i am not sure if the volume was higher or lower than the other.



Both these units have been connected to EQ573-II erlier. I stopped using the cable link because one of the MP73s sometimes had some kind of internal feedback problem, making a small pulse making a small "swosh swosh" sound and the led lighting up.  This was without any audio coming in to the unit.

This doen not happen without the EQs (also have EQs affected by this problem for a god while, but i have switched the polarity. Not ure if this can help cause this?) Maybe its the same MP573 unit.





I also had LM337T instead of an LM317T for U1, and you sent replacements.
Not sure if having the LM337T can have contributet to any of the symptoms i now experience?



I will disassemble again and try to get them equal once more. But if disassembling them resetts the problem, it makes me feel like there are some electrical components needing to discharge or something to make the unit functional.



One unit has non functional 300 Ohm too, not sure if this is the same untit.



Do you have any idea where to start looking for the cause of these issues?

[Dolbah]

UPDATE:

Today i disconnected the transformers from the mainboard for a few minutes, after reconnection them, the volume was even again.


300 ohm is still a issue.

Waiting to see which unit is changing levels.

[Dolbah]

UPDATE:

Today i disconnected the transformers from the mainboard for a few minutes, after reconnection them, the volume was even again.


300 ohm is still a issue.

Waiting to see which unit is changing levels.

Update:

It is the same unit that has the 300 ohm issue that also changes volume level. After 1 hour max. the volum of this unit is much higher than before.

[JPK]

The volume change may be caused by a faulty RLY3 relay. This relay is the one that switches the 300 ohms impedance.
You could try to replace it (TQ2-24V). To do this you will need to remove the input transformer. Follow the instructions here.


I stopped using the cable link because one of the MP73s sometimes had some kind of internal feedback problem, making a small pulse making a small "swosh swosh" sound and the led lighting up.
This issue is caused by the inrush current limiter on the MP573. You can disable it (if your lunchbox can take the high inrush current) by shorting the pins 1&3 on U1 (LM317). You can leave the IC in place and short the 3 pins together.

[Dolbah]

The volume change may be caused by a faulty RLY3 relay. This relay is the one that switches the 300 ohms impedance.
You could try to replace it (TQ2-24V). To do this you will need to remove the input transformer. Follow the instructions here.


I stopped using the cable link because one of the MP73s sometimes had some kind of internal feedback problem, making a small pulse making a small "swosh swosh" sound and the led lighting up.
This issue is caused by the inrush current limiter on the MP573. You can disable it (if your lunchbox can take the high inrush current) by shorting the pins 1&3 on U1 (LM317). You can leave the IC in place and short the 3 pins together.

Hi JP
Thanks! I desoldered the input transformer and reflowed all the relays just in case, but that did not help. Have ordered a small batch of new relays.
Are there any easy way to measure if the relay is ok or not?

Also, on a more superficial note,. I dig the new grey knobs on the MP73, could you give direction to where i get those?

About the EQ and the swosh sounds. It sounds like you are describing this as normal behaviour. If so, why am i only experiencing this on one unit?

Cheers!

[JPK]

Are there any easy way to measure if the relay is ok or not?
The easy way is to replace the relay.

I dig the new grey knobs on the MP73, could you give direction to where i get those?
Send me an email here.

It sounds like you are describing this as normal behaviour.
Not normal but quite common. It depends on which module will power up first and it depends on components value tolerances.

[Dolbah]

Got new relays today. switched RLY3 and now 300 ihm works again. The volume on the other hand is still wrong and seem to increase several dB after a while. Should i have unity gain with the small knob on 0 and the big one on 10? If not, what should the difference be on the output? 

[JPK]

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

[Dolbah]

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.

[JPK]

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?

[Dolbah]

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

[Dolbah]

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?

I desoldered all joints on R25/C9, R26, the phase switch and I inspected the the finger connector.
I then cleaned all surfaces with isopropyl ans desoldered with fresh tin.

Just to be sure I switched output transformers between the units.

Still same issue.

One thing I have noticed. At sometime during my search for this issue, unit 2 has stopped "resetting" itself, like it did earlier when the the output slowly rised after the output transformer was disconnected for a while. This seem to have stopped and now the volum is higher regardless.



But none og the units have the unity gain that you told me about though.

[Dolbah]

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?

I desoldered all joints on R25/C9, R26, the phase switch and I inspected the the finger connector.
I then cleaned all surfaces with isopropyl ans desoldered with fresh tin.

Just to be sure I switched output transformers between the units.

Still same issue.

One thing I have noticed. At sometime during my search for this issue, unit 2 has stopped "resetting" itself, like it did earlier when the the output slowly rised after the output transformer was disconnected for a while. This seem to have stopped and now the volum is higher regardless.



But none og the units have the unity gain that you told me about though.

Update: This issue might have been 2-sided. After testing yesterday, i turned some switches on both unit, to see if anything weird happened. And on unit 1, when selecting 300 ohm and then back to 1200 ohm, i suddenly gpt a volume difference.there too, but on this, the volume was lower. So i reflowed all suspects around the switch and i now have stable sound on both.  So it might seem like i hade a increasing issue on unit 2 and a decreasing issue on unit 1.

Tested a while yesterday, and i have a steady signal going into both now for 15 minutes with no isuues 🎉 Will leave the signal on while working today, and check in now and then.

[Dolbah]

This issue is caused by the inrush current limiter on the MP573. You can disable it (if your lunchbox can take the high inrush current) by shorting the pins 1&3 on U1 (LM317). You can leave the IC in place and short the 3 pins together.

Not normal but quite common. It depends on which module will power up first and it depends on components value tolerances.

I forgot about this one, and I have some questions:

• "shorting the pins 1&3 on U1 (LM317). You can leave the IC in place and short the 3 pins together." PIN2&3, or all 3?
• How do I know if my lunchbox can take it?
• What will happen if I accidentally put this in a lunchbox that can't take it?
• Since this is a common thing and not a building error, it sound like a design flaw that "might" happen if you're unlucky with your components(?). Is there not any other ways to fix this?

Just to be clear, this happens with one of the EQs, not both, and it follows the EQ if I switch slots. It does not follow the Preamp.

[JPK]

PIN2&3, or all 3?
All 3.

How do I know if my lunchbox can take it?
It simply won't start if it cannot take it. The PSU will not power on, believing there is a short circuit on the rails.

Is there not any other ways to fix this?
The previous version of the MP573 did not have the slow start. We added it as a work around for poorly designed PSU's. If you have a good lunchbox, it is better to remove it.

[Dolbah]

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?

I desoldered all joints on R25/C9, R26, the phase switch and I inspected the the finger connector.
I then cleaned all surfaces with isopropyl ans desoldered with fresh tin.

Just to be sure I switched output transformers between the units.

Still same issue.

One thing I have noticed. At sometime during my search for this issue, unit 2 has stopped "resetting" itself, like it did earlier when the the output slowly rised after the output transformer was disconnected for a while. This seem to have stopped and now the volum is higher regardless.



But none og the units have the unity gain that you told me about though.

Update: This issue might have been 2-sided. After testing yesterday, i turned some switches on both unit, to see if anything weird happened. And on unit 1, when selecting 300 ohm and then back to 1200 ohm, i suddenly gpt a volume difference.there too, but on this, the volume was lower. So i reflowed all suspects around the switch and i now have stable sound on both.  So it might seem like i hade a increasing issue on unit 2 and a decreasing issue on unit 1.

Tested a while yesterday, and i have a steady signal going into both now for 15 minutes with no isuues 🎉 Will leave the signal on while working today, and check in now and then.

I shorted the U1 pins today. When i reasembled the modules, the uneven volume came back. I have now found a way to recerate the issue. It seem that the P1 gain knob might be the issue, but its still a bit weird.

Unit 1
Step 1. With the module powered, i can unplug the Output transformer and plug it back in, and this wil reset the issue. Volume goes back to high/normal
Step 2. If i wiggle the P1 gain pot, the volume goes down a lot, in steps. For each wiggle i loose a little. This happens every time. I just wiggle back and forth and i loose volume. The thing is that i can not wiggle the volume back again,  to get volume i need to redo step 1.

I have desoldered and resoldered P1, it did not help.

UPDATE: Desoldere P1, and sprayed som cleaner into the pot. It made it a tad looser on the twiddle, but seem to have solved the problem.

[Dolbah]

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?

I desoldered all joints on R25/C9, R26, the phase switch and I inspected the the finger connector.
I then cleaned all surfaces with isopropyl ans desoldered with fresh tin.

Just to be sure I switched output transformers between the units.

Still same issue.

One thing I have noticed. At sometime during my search for this issue, unit 2 has stopped "resetting" itself, like it did earlier when the the output slowly rised after the output transformer was disconnected for a while. This seem to have stopped and now the volum is higher regardless.



But none og the units have the unity gain that you told me about though.

Update: This issue might have been 2-sided. After testing yesterday, i turned some switches on both unit, to see if anything weird happened. And on unit 1, when selecting 300 ohm and then back to 1200 ohm, i suddenly gpt a volume difference.there too, but on this, the volume was lower. So i reflowed all suspects around the switch and i now have stable sound on both.  So it might seem like i hade a increasing issue on unit 2 and a decreasing issue on unit 1.

Tested a while yesterday, and i have a steady signal going into both now for 15 minutes with no isuues 🎉 Will leave the signal on while working today, and check in now and then.

I shorted the U1 pins today. When i reasembled the modules, the uneven volume came back. I have now found a way to recerate the issue. It seem that the P1 gain knob might be the issue, but its still a bit weird.

Unit 1
Step 1. With the module powered, i can unplug the Output transformer and plug it back in, and this wil reset the issue. Volume goes back to high/normal
Step 2. If i wiggle the P1 gain pot, the volume goes down a lot, in steps. For each wiggle i loose a little. This happens every time. I just wiggle back and forth and i loose volume. The thing is that i can not wiggle the volume back again,  to get volume i need to redo step 1.

I have desoldered and resoldered P1, it did not help.

UPDATE: Desoldere P1, and sprayed som cleaner into the pot. It made it a tad looser on the twiddle, but seem to have solved the problem.

No, it didnt. Problem back today.

Unplugging output transformer still resets the volume, but it slowly goes down after a while. This time the P1 knob doeas not provoke the issue anymore.

Suggestions?

[JPK]

How about you send it to us for verification? I'm a bit out of ideas...

[Dolbah]

How about you send it to us for verification? I'm a bit out of ideas...

I'll concider it, but it's not tempting to use more € on this. Buying replacement for broken relay alone cost 50€ inkl. freight and import.

The "new" way of resetting the issue is now to fip the Ohm switch back and forth a couple of times.Could this also be relay issue?

[JPK]

Yes, possible. This switch activates 2 different relays: RLY2 and RLY3.

[Dolbah]

Yes, possible. This switch activates 2 different relays: RLY2 and RLY3.

I switch both Relays now, same issue afterwards.

[JPK]

You need first to find out in which section the level change takes place:
- input section: you can measure the signal level on tranformer T1 pin7 (against TP2),
- preamp section: measure the signal level on TP1 (against TP2),
- ouput amp section: measure the signal level on tranformer T2 (between yellow and brown wires).
This should narrow the search scope.