Need help with calculation on repair Manley Ref mic PSU

[djowel]

Hi guys,

I have a Manley PSU for a Ref Mic Cardiod on my bench. The owner used a non-manley 7 pin XLR cable, which had pin 7 connected to ground.

Pin 7 supplies the 250 voltage for the 12ax7 tube inside the mic, so needles to say, the psu said: nope, I'm going on a smoke break.
Luckily only the resistor (R3) burned through, no other damages.

Although the resistor was seared well done, I managed to identify a R100J marking under the microscope.
It measured 14.5 x 4.5 mm (0.5 x 0.18 freedom units), so that would be a 100Ω 2w fusible wire wound resistor.

But can someone help me with the calculations to confirm the right values of this resistor?
A 12ax7 draws ca 1.5 mA at 250v. With a 100Ω 2w the resistor would give out at 2.5A.

Does that seem correct? Or did I misread the markings, that were severely burnt.

Inside the refmic there is another 350Ω 3w resistor in path with the 250v input as a secondary protection, which made
me also doubt my readings on R3 on the psu.

All input are welcome!



Input board with R3 removed. Resistor is connected to 250v input pin and plus of C1 (33uF 450v) to Pin 7 of XLR.
Burnt R3


secondary resistor inside mic board.

 

[Khron]

A 12ax7 draws ca 1.5 mA at 250v.

Is that what the power supply is supposed to be putting out (ie. 250V)?

 

[djowel]

Is that what the power supply is supposed to be putting out (ie. 250V)?

to my understandig, yes

The resistor would be a overcurrent protection

 

[Khron]

The resistor would be a overcurrent protection

Then 100 ohm's plenty fine. Even though I'm pretty sure there are some even higher-value resistors "upstream" of it, which would further limit the current, but i could be wrong...

(Later edit)
PS. https://groupdiy.com/threads/manley...chematic-and-some-analysis.84774/post-1104129

 

[djowel]

Then 100 ohm's plenty fine. Even though I'm pretty sure there are some even higher-value resistors "upstream" of it, which would further limit the current, but i could be wrong...

(Later edit)
PS. https://groupdiy.com/threads/manley...chematic-and-some-analysis.84774/post-1104129

@Khron I understand you question now about B+. It seems the Ref Mic Cardiod I'm working on differs in design from your link post.

I think they redesigned the mic pcb to work with their new manley PSU which outputs 250V? I havent made any measurements yet, since mic or psu havent been operational.

So 100ohm 2w is correct and should be safe as an intended current protection?