HeNe laser ballast resistor ID

[ChristofferB]

I've pulled an old jdsu helium neon laser head and psu from the scrap heap at my local uni. It sais defective, and the issue is clearly visible : a blown ballast resistor.

Not sure if there is also one in the potted psu, but this resistor goes from tube cathode to the aluminium enclosure as ground/ return.

It looks to be a carbon composite resistor, but I can't figure out either power rating or resistance - I know the code Is right there but the colours are slightly off.

I can't get it to make sense with the common ballast values for laser tubes (10-100 kohm)

I tried measuring the two halves, and one measured about 35k, the other 15k, but values kept jumping, and the accuracy is likely pretty low.

What do you think?

[TimFox]

I would guess 30 k, 5% tolerance, 2 W carbon composition.  However, the power of ten (orange?) is the least accurate guess, since it is badly discolored at the break.

[ChristofferB]

That would make sense and match the measurements, but that also means first and 3rd band originally were the same color, which is a bit of a stretch.

I will try 30k. I guess the worst case scenario is the tube wont lase.

I also wondered if it was 300 ohm, and just acted as a balance, not the main ballast but that doesnt make sense with the measurements unless it heated so much it failed high impedance.

[TimFox]

Do you have access to a Clarostat 240C or similar power decade resistance box for testing?  Mine goes up to 1 megohm with a 50 mA rating on the x10k decade and 1000V DC rating.   IET makes similar units in their HPRS series.  They are much handier than power resistors clipped together on the bench.

[ChristofferB]

Unfortunately I don't, but yeah that would be ideal.

[mikeselectricstuff]

Have you tried measuring between the lead and the broken face ?

[ChristofferB]

Yep, but the variations were large depending on probe contact area, numbers probably quite a bit off.

Big half measured something like 35K
small half 15K

[mikeselectricstuff]

I think 30K is the most likely. The value isn't too critical - its main purpose is to stabilise the supply against the negative resistance characteristic of the tube. 
If it's too low, you may get flickering, if too high, power will be slightly low.

[ChristofferB]

I'll go for 30K then, if nothing else to see if the tube works at all, since it looks like it took a bad hit by the resistor blowing.

Thanks!

[mikeselectricstuff]

These resistors often run pretty hot - It's very likely that the resistor is the only issue - the tubes are very  robust

[ChristofferB]

That makes sense. But the more worrying thing is: the laser head rattles. I can't figure out what it is, and the tube itself is potted on one end.

[mikeselectricstuff]

That makes sense. But the more worrying thing is: the laser head rattles. I can't figure out what it is, and the tube itself is potted on one end.

You can do a quick check for gas-tightness by zapping with a piezo gas-lighter

[ChristofferB]

That's brilliant! Is the light pulse really slow enough to see?