Sound Skulptor forums

Build help and troubleshooting => MU524 Vari-Mu Tube Compressor => Topic started by: habasud on May 28, 2023, 11:40:57 AM

Title: No AC volage from TP1 on
Post by: habasud on May 28, 2023, 11:40:57 AM

Hi,

my MU524 fails from setup guide #6:

- DC voltages are present:
+/-16V, 48V B+, 33V @TP1+2

- Heaters are on 6.2V

- AC 1V/1kHz is at the in-/output when in bypass

- At "In"-pos. no AC voltage at all regarding TPs and output

I measure a short to ground from both grids and also across C4/R5 I think, this should measure at least some kOhms. Soldering seams clean.

Any ideas? Regards
Claus

Title: Re: No AC volage from TP1 on
Post by: habasud on May 28, 2023, 09:18:15 PM

...think, I found out: Input transformer needs to be turned :-[

I'll be back soon.

Title: Re: No AC volage from TP1 on
Post by: habasud on May 29, 2023, 12:35:27 PM

...after turning the input transformer in the right position, everything works as expected. Carefully reading the instuctions is helpful :)

Title: Re: No AC volage from TP1 on
Post by: coostor on March 27, 2025, 07:57:24 PM

Hi,

I measured and experienced the same things, even though the transformer is in the correct position.
What else could be causing this?

Thanks,
C.

Title: Re: No AC volage from TP1 on
Post by: JPK on March 28, 2025, 11:02:15 AM

Can you give more details like the voltage checks here (https://www.soundskulptor.com/forum/index.php?topic=347.msg1907#msg1907)?

Title: Re: No AC volage from TP1 on
Post by: coostor on March 28, 2025, 01:50:56 PM

Dear J-P,

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 = 45.2 VDC
TP1 = TP2 = 0 VAC
TP3 = TP4 = 0.003 VAC
Output: 0 VAC 

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

Results:
TP3: 0 VAC 
TP5: 0 VAC
TP6: 0.345 VDC
Output:0 VAC

Thanks

Title: Re: No AC volage from TP1 on
Post by: JPK on March 28, 2025, 05:41:50 PM

Is the tube heating? Does it glow in the dark?
If not, check value, direction and solder joints of L1, L2, U6, C30 to C33, D9, R38, R39, Q4.

Title: Re: No AC volage from TP1 on
Post by: coostor on March 29, 2025, 12:01:29 PM

Tube doesn't glow. Power up the lunchbox: 5,5 V then more to 6.2 V after 10 seconds.
Value, direction and solder joints of L1, L2, U6, C30 to C33, D9, R38, R39, Q4 are correct.
 

Title: Re: No AC volage from TP1 on
Post by: JPK on March 31, 2025, 07:43:52 PM

Measure the resistance of the tube heater between pins 4 & 5 (tube removed). You should get around 3-4 ohms.
(https://i.imgur.com/XfEHxQR.png)

Title: Re: No AC volage from TP1 on
Post by: coostor on March 31, 2025, 08:13:21 PM

Resistance is 8.9 kohm...

Title: Re: No AC volage from TP1 on
Post by: JPK on April 01, 2025, 11:03:00 AM

Dead tube. Contact us here (https://www.soundskulptor.com/en/content/6-contact) for a replacement.

Title: Re: No AC volage from TP1 on
Post by: coostor on April 02, 2025, 04:14:38 PM

I measured it wrong, the resistance is 3.4 ohms.
I tried it with a telefunken ecc189 tube, there is no difference in the result.
It doesn't glow and VAC values are zeros.

Title: Re: No AC volage from TP1 on
Post by: JPK on April 04, 2025, 10:05:48 AM

Check value, dirction and solder joints of L1, L2, C30...C33, R38, R39, D9, U6, Q4.
In particular, make sure U6 and Q4 are in the correct direction:
- Pin 1 of U6 towards L2, coil facing T1.
- Writings on Q4 towards R39.

Title: Re: No AC volage from TP1 on
Post by: coostor on April 06, 2025, 08:59:19 PM

The values, directions and soldering of L1, L2, U6, C30-C33, D9, R38, R39, Q4 are correct.
I have no idea what the problem could be...

Title: Re: No AC volage from TP1 on
Post by: JPK on April 07, 2025, 05:41:23 PM

Did you check the glowing in the dark? Because it is not very visible in daylight.

Power up the lunchbox: 5,5 V then more to 6.2 V after 10 seconds.
Do you confirm this? Check the solder joints of the tube socket and the header connecting the tube PCB to the main PCB.

Title: Re: No AC volage from TP1 on
Post by: coostor on April 10, 2025, 10:57:38 PM

U6 had to be replaced with a new one. No voltage came out at the output. 
After replacement everything works perfectly, all measured values are correct.


Thank you for everything,
C.

Title: Re: No AC volage from TP1 on
Post by: JPK on April 11, 2025, 09:54:00 AM

(https://fonts.gstatic.com/s/e/notoemoji/16.0/1f44d/32.png)