[Benmar]

Hello,
I just build two MU524. The first one is working and I had no problems with calibrating the unit . 
But the second one has over 26Vac at the Output in Step 6, were it only should have 3-4Vac. In Bypass it is the normal 1Vac at the Output. As soon as I engage the the bypass to „In“ its 26V. I checked the pots and could‘t find and mistakes. Do you have and thoughts? 

V+ and V- are +- 15-16V 
B+ is 46V
TP1 = 30V
TP2 = 27V but maybe its because I didn’t end the calibration. 

Best regards! 
Benjamin 

[JPK]

Reversed input transformer? The label must face towards the back.

[Benmar]

Thank you for the response! 

The input transformer label is facing to the back and the Torx screws are on the correct spot upwards. 
I resolder every joint and checked the components (except the small ceramic capacitors, they are to small to read). Everything seems fine.

When I check the voltages I get: 

Initial conditions:
IN pot @ max
OUT pot @ min
REC pot @ max
Bypass toggle @IN
All Push buttons @UP
Input signal: Sinus 1kHz, 1VAC
Jumper JMP1: removed

Results:
TP1 = TP2 = 29.3VDC (black probe on 0V)
TP1 = TP2 = 2.4VAC
TP3 = TP4 = 13.8VAC
Output: 26.5VAC (between XLR pins 2&3)

Changed conditions:
Jumper JMP1: on 1-2
Input signal: Sinus 1kHz, 0.3VAC
OUT pot @ max

Results:
TP3: 0.21VAC (black probe on 0V)
TP4: 8.24AC
TP5: 2.32VAC
TP6: -2.77VDC
Output: 16.4VAC (between XLR pins 2&3)

[JPK]

Can you check TP3 and TP4 for DC volts. They should be at 0V.

[Benmar]

TP3 to 0V = 0Vdc
TP4 to 0V = 8,2mVdc

[Benmar]

Could be something wrong with pot 1 (Input pot). When I measure the output voltage it jumps between 2-26V when I press the input pot a little to the side. Do you have an idea what could be the problem there?

[JPK]

Please do the test again (Initial conditions/Results) with the front PCB removed.

when I press the input pot a little to the side
It could be a defective pot or a bad solder joint on the pot pins or the connectors between front and main PCB.

[Benmar]

The initial condition measures (without the front PCB) are:

TP1-0V = 29,6Vdc
TP2-0V = 29,4Vdc

TP1=TP2 = 3,4Vac

TP3=TP4 = 0,01Vac

Output = 1Vac

I resolder every pot joint and the connectors between the PCB's. The output voltage still changes, wen I press against the front PCB. I have ordered new input and output pots. maybe this will fix it. 
I have checked all the 5x2 and 10x2 connections between the PCB's with an DMM and most of the connection pairs on the 10x2 connector are linked. Is that right? 

[JPK]

TP1=TP2 = 3,4Vac
TP3=TP4 = 0,01Vac
This is not related to the pots.You need to solve this first.
Remove all the IC's.
Without IC's and front PCB TP3 should give 1.8VAC.
Make sure transformer T2 doesn't touch the PCB.


Is that right?
Yes right, except at 3 ends.

[Benmar]

Without front PCB and IC's TP3 = 2,3Vac

[JPK]

So it must be the pot.
Contact me here for a new part.

[Benmar]

Yes, it was the pot! It's working just fine now...thanks you very much for the great help!!!

[MakingSomeWaves]

Hey I'm having a similar issue with my MU524: getting an unusually high output for step 6 of the setup guide (about 5.6 VAC from the output with initial conditions). When I turn the output knob up, it goes up to 22VAC.

Input transformer label is facing the back, and I feel as though I've done a good job double checking component placement and values.

With no front PCB and with all ICs removed, I get exactly 3VAC across TP3 and 0V.

[JPK]

Check values of R1, R2 and P1.
In step 3 initial conditions,
Do you hear the pure input tone at the output or is it hum or noise?
What are the AC and DC voltages you get on TP1 & TP2?

[MakingSomeWaves]

Check values of R1, R2 and P1.
In step 3 initial conditions,
Do you hear the pure input tone at the output or is it hum or noise?
What are the AC and DC voltages you get on TP1 & TP2?

- Values of R1, R2 and P1 are correct

- I hear pure tone at step 3

With no front PCB and no ICs, 1VAC sine wave applied I get:

TP1: 1.66VAC, 30.42VDC
TP2: 1.75 VAC, 30.41VDC

[JPK]

These values are good.
With power off and still no IC's, measure the minimum resistance value of P1 between pins 6 and 7 of U1. It should go from 0 to 2k2.

[MakingSomeWaves]

These values are good.
With power off and still no IC's, measure the minimum resistance value of P1 between pins 6 and 7 of U1. It should go from 0 to 2k2.

Did you mean to say P2? P2 measures from 2.075k ohms to 300m ohms across pins 6 and 7 of U1

[JPK]

Sorry, yes P2.
It seem P2 does not go down to 0 Ohm.
Can you check directly on the pot, between the center pin and each side pin, after disconnecting the front PCB?

[MakingSomeWaves]

Sorry, yes P2.
It seem P2 does not go down to 0 Ohm.
Can you check directly on the pot, between the center pin and each side pin, after disconnecting the front PCB?

- I think my nomenclature was a little unclear in my last post. It went down to 300 milliohms, or .3 ohms, so I think it was functioning okay.

- I built by second of my stereo pair and it passed all tests perfectly. After some more troubleshooting swapping out ICs and the tubes between the two units, the first unit started operating normally, so I guess something may have needed to be re-seated. So everything is great now! Thanks so much for your time in helping me troubleshoot - and thanks for making great products =)

[JPK]

[Jakov]

Very similar problem like the previous user, initial checks are OK,, V+, V-, B+ voltages OK. When I apply the 1VAC 1kHZ sine and engage the the voltage measures far to high approx 10,85 VAC and increase P2 it grows up 17,49 20 VAC.
I already checked R1, R2, they should be OK and are not touching the out- transformer pins. When I remove the ICs and measure between pins 6-7 of U1 I measure 0,6 Ohm to 2k2.

Disassembled everything mechanically and put again together (tube, ICs, PCBs..) several times with the fault is persistent.

help appreciated...

[JPK]

Do the output AC voltage falls to 0VAC when you remove the input signal?
Did you check the usual suspects?
Please report the voltages in the initial conditions.

[Jakov]

The usual suspects look OK.
The measured voltages:

Test step 4: DC voltage between two side pins: 6,0629 VDC

Test step 5:
V+=15,378
V-=-15,212
B+=46,012

Test step 6:
Sine 1 KhZ
in bypass voltage on output pins 2-3 =1.0007 VAC
comp on voltage on output pins 2-3 =10,517 VAC

When I remove the input signal, the voltage drops to 6,204 VAC and the multimeter shows 0,230 kHz

 

[Jakov]

nitial conditions:
IN pot @ max
OUT pot @ min
REC pot @ max
Bypass toggle @IN
All Push buttons @UP
Input signal: Sinus 1kHz, 1VAC
Jumper JMP1: removed

Results:
TP1 = 36,94 VDC
TP2= 38,20 VDC 

TP1 = 12,58 VAC
TP2 = 6,46VAC
TP3 = 5,66 VAC
TP4 = 5,47 VAC
Output: 10,51 VAC (between XLR pins 2&3)

[JPK]

When I remove the input signal, the voltage drops to 6,204 VAC and the multimeter shows 0,230 kHz
It seems you have a local oscillation.

Are you sure that C12 doesn't touch the tube or transformer PCB pins (assembly guide step 16)?
If not sure:
- check the AC voltage on TP4 with no input,
- remove C12
- check again the AC voltage on TP4 with no input.