[habasud]

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

[habasud]

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

I'll be back soon.

[habasud]

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

[coostor]

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.

[JPK]

Can you give more details like the voltage checks here?

[coostor]

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

[JPK]

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.

[coostor]

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.
 

[JPK]

Measure the resistance of the tube heater between pins 4 & 5 (tube removed). You should get around 3-4 ohms.

[coostor]

Resistance is 8.9 kohm...

[JPK]

Dead tube. Contact us here for a replacement.

[coostor]

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.

[JPK]

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.

[coostor]

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...

[JPK]

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.

[coostor]

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.

[JPK]