Elam 251E PSU question

[kags]

We have a vintage 251E with the original PSU. The mic had almost no output and seemed microphonic. I checked the voltage from the PSU (with the mic disconnected) and measured around 150 V (plate) and 0V (expected +4V for heater). I noticed a white wire that seemed to have broken free in the PSU. The wire connects to the bottom of the CT1104 power transistor and seems to have come off the terminal it is floating next to. I can't imagine why the lead came off unless it wasn't ever tacked on properly after some repair. I haven't found a schematic that resembles this PSU and before I tack this wire back on I wanted to see if anyone had any input on it. The mic was working fine during a session just last week.

thanks!

 

[RuudNL]

With the complete schematic it should not be a problem to locate the fault.
(The only difference is the filament voltage.)

 

[Khron]

Technically then, that's not an ELA M 251E, but a non-E - the 4V heater voltage points towards the initial AC701K-based mic, afaik. "E" stood for "export", with the field-replaceable and much more widespread 6072 tube.

Just nitpicking...

 

[Tubetec]

The white wire looks like it goes to the transistor collector , if its off then you wont see any heater voltage , definately check the transistor for shorts , maybe someone dissconnected it for a reason , or if your lucky it was a just the solder joint that failed .

 

[ZHOUJUN]

The white wire of my PSU is the connecting wire under the CT1104 and is connected to the upper end of the upright resistor on the upper half of the PSU，I think it may have fallen off due to vibration，This wire must be connected to the upright resistor to provide heater power for the microphone.

 

[MagnetoSound]

The white wire of my PSU is the connecting wire under the CT1104 and is connected to the upper end of the upright resistor on the upper half of the PSU，I think it may have fallen off due to vibration，This wire must be connected to the upright resistor to provide heater power for the microphone.

I think you’re probably right, but refer to the schematic and be absolutely sure.

You only get one chance with a 701.

 

[Khron]

I think you’re probably right, but refer to the schematic and be absolutely sure.

You only get one chance with a 701.

... Not that you couldn't / shouldn't test the PSU with a dummy-load first...

 

[MagnetoSound]

Yeah, but still …

 

[kags]

Thanks for everyone’s input - it’s back up and running. This is a 6072a version, not the 701, which makes it a 251E. I mentioned 4v, which would have been correct for the 701, but is wrong for this mic. I believe the mic was last serviced by Toby Foster. It is a very good sounding 251 and gets used a lot.

 

[tomas.borgstrom]

It's easy to raise the filament voltage with the variable resistors