Fixing a Green HeNe (Lasos)...

[aryntha]

Trying to figure out what's going on with this Lasos head, a 77x series, Green HeNe -- the tube lights fine, i'm just trying to figure out whats going on with diodes and resistance -- I've never seen so many passives in a laser tube before; the MG and JDSU tubes i've seen have had 1 or 2 resistors and that's it.

In this head,

There’s 2x22k resistors in the rear, (and some 10k varistor or something that jumpers to ground); in the front there’s 2x22k+1x12k resistors.

Taking the broken diode out of the series , out of the rear potting, and jumpering the Alden (+) right to the resistor series (22k, 22k ---length of tube, wire--- 22k, 22k, 12k--front terminal of tube/OC) the tube sputters. If I reduce the front side resistance to 44k (take the 12k out of line) it works; (88kOhm total) but I’d like to get it up to ‘as was, factory spec ’, working; with 56k resistance in the front and 44k in the back (100kOhm total…)

... It could also be that the PS isn't doing enough current for this tube with 100kOhm, but is fine at 88kOhm. (and is likely operating underpowered...)

Or, I could assume (probably wrong!) that just ‘taking this diode out of the equation’ screws that all up somehow.

The diode that I took out of series “to make it work” was a “VM1-M100S-9134”. Anyone have any idea what sort of diode this is? Or what purpose it served? I can't find any references to it anywhere. If anything i'd like to get this back up to spec and get it working as was originally intended; re-pot all the stuff in RTV, and get it going.

 

[Cyparagon]

I read on the FAQ a while ago that this is actually a better way of implementing the ballast resistance - some in front, some in back, and some to ground (don't quote me). The one near the anode we're familiar with is just a bare minimum.

 

[LSRFAQ]

Sounds like you have a "bridged T".

Let me give you a analogy. You need a 25 Megohm resistor in series with a load. That load has one end tied to ground. 25M is not a stock part. You could use 5 pieces of 4.7 Meg in series. Or use three resistors in a "T" circuit, with the center leg of the T tied to ground, one of the top branches going to the PSU, and the other to the load. So one of the resistors shunts to ground, but the load still sees a effective 25 megs between it and the psu. And you end up needing smaller resistors, like 2 pieces of 2.2 megs and one 1 meg or something like that to make a 25 Meg. It lets cheaper resistors handle more voltage and current.

So you have some in the anode and some in the cathode. The ones direct to case ground in the cathode end help kill RF oscillations in the plasma that lead to noise on the output beam as well as act as a ballast. Plus if the tube has a 1000V drop across it, you can use a resistor with less of a standoff voltage specification.. HV resistors are expensive.

What the diode was doing there, I don't have a clue, unless your varistor looking thingie is actually a cap..

Try emailing that diode number to these guys and see what they say www.voltagemultipliers.com/ I mean its a long shot that LASOS could not get such a diode from their old pals at Siemens, but your never know.. (Lasos was Siemens' laser division, but was split off)

To quote a friend on another laser forum "This thread is useless without pictures!". Or at least a traced out schematic..

Steve