Lexel 88 problems - Help please

[dsli_jon]

Hey Jim! -

Cheers, m8! Sorry to hear of your troubles...

There is a short some place. You have not found it yet.

Well, I certainly won't pretend that I am anywhere's-NEAR the massive GRAIN-SILO of info that our Dear Professor-is (hi, Steve... , but based on my experiences, 'in the trenches', I'd keep ferreting-out his line of thinking, above... if I might offer a couple-quick observations / ideas:

1. Double-check any/all insulated-wires in the PB, especially wire-chases / holes / areas-where wires can (if-have damaged-insulation) touch the PB-plate, and/or connx they should-not be touching...ie: check areas such-as highlighted, here: (among-others)



... I suggest this, from, one-time, I was ferreting-out a PB-problem in a Coherent - same-deal - bloody-replaced nearly-everything, and it KEPT blowing... turned-out, the 'culprit' was one of the 'jumpers', running along-side one of the buss-lines - with a *tiny*, unseen-from-the-front 'nick' in the insulation - just-enough to allow the 'gremlin-arc' to ensue havoc... I cussed alot...then laughed alot... but fixed-it...

The noise emanates from the HV strike coil in the head.

2. I'd try taking-out the 'ignitor', and if-possible, use another one (do you HAVE another laser-head, even not gas-intact, but with a known-good starter?) to 'test-fire' w/O this ignitor, 'electrically-involved in the equation'... Got any more 'explicit' shots of the ignitor-area-wiring, etc? I'd recheck all that area, for any-type of short / bad-insul as-well...

When it lases the current/power output is totally unadjustable from the PSU, current controlled, and it's pulling about 25A, according to the PSU meter. The Pass bank voltage is still very low, around 14V, according to the PSU meter.

Steve - your expert-thoughts on results of any-shorts, here / bad-pots? Could-cause some PB-mayhem, no?

It strikes and lases for a second or so but makes a horrible 'oscillation' type noise then goes out until you re-press the fire button. Once you have done this a few times it lases for longer periods each time until eventually it stays on. It is still making the horrible noise.

Boy, I'd guess ultra-low tube-pressure, too, but not if you're POSITIVE the 'noise' is coming-from the ignitor-coil Does the noise sound-like an 'angry groundhog'? aeaeaeaeaeae!!!! ...wondering, too: have you checked the magnet, when it is 'running', to be sure it is actually working, and not being pulled-down by some un-seen short? Just thinking out-loud (and I'm reeealy-tired...

Lastly - and this is more of a 'PS / BTW', if you have not-already replaced these:



...there's a 'future-failure', waiting-to happen...

Hope this is of some help... I gotta go s l e e e e p....
cheers...
J

[smogthemog]

Hi Jon.
Thanks muchlyness for your reply mate.

1. Double-check any/all insulated-wires in the PB, especially wire-chases / holes / areas-where wires can (if-have damaged-insulation) touch the PB-plate, and/or connx they should-not be touching...ie: check areas such-as highlighted, here: (among-others)



I have done this once but will deffo do it again using my magnifying eye glass, I don't want to miss anything.

... I suggest this, from, one-time, I was ferreting-out a PB-problem in a Coherent - same-deal - bloody-replaced nearly-everything, and it KEPT blowing... turned-out, the 'culprit' was one of the 'jumpers', running along-side one of the buss-lines - with a *tiny*, unseen-from-the-front 'nick' in the insulation - just-enough to allow the 'gremlin-arc' to ensue havoc... I cussed alot...then laughed alot... but fixed-it...

There looks to be a few arcing spots on the baseplate but now I have moved everything I am unable to see where they have possibly come from.

2. I'd try taking-out the 'ignitor', and if-possible, use another one (do you HAVE another laser-head, even not gas-intact, but with a known-good starter?) to 'test-fire' w/O this igniter, 'electrically-involved in the equation'... Got any more 'explicit' shots of the igniter-area-wiring, etc? I'd recheck all that area, for any-type of short / bad-insul as-well...

See end of this post

Steve - your expert-thoughts on results of any-shorts, here / bad-pots? Could-cause some PB-mayhem, no?





Boy, I'd guess ultra-low tube-pressure, too, but not if you're POSITIVE the 'noise' is coming-from the igniter-coil Does the noise sound-like an 'angry groundhog'? aeaeaeaeaeae!!!! ...wondering, too: have you checked the magnet, when it is 'running', to be sure it is actually working, and not being pulled-down by some un-seen short? Just thinking out-loud (and I'm reeealy-tired...

Lastly - and this is more of a 'PS / BTW', if you have not-already replaced these:



...there's a 'future-failure', waiting-to happen...

All seen and re-crimped for the time being while I get some satiable cable from work to renew them completely. They where badly crimped when I got it hence the heat build up

Hope this is of some help... It CERTAINLY is Sir All info from anyone at all is all helpful!!!

cheers...
J

I completely stripped the head baseplate today and found something rather interesting.......... I had mentioned further back in the thread that there was bits of water ingress corrosion seen on the baseplate, before I started to remove things, and upon moving the filament connector block that was bolted down to the base I found the below:

[dsli_jon]

Hey Jim -

Hi Jon.
Thanks muchlyness for your reply mate.

Right-on...

I completely stripped the head baseplate today and found something rather interesting.......... I had mentioned further back in the thread that there was bits of water ingress corrosion seen on the baseplate, before I started to remove things, and upon moving the filament connector block that was bolted down to the base I found the below:

Hmm, well unless you had an active-leak, this would not 'likely' cause an issue...and if this thing somehow did-give the cath-leads continuity to the baseplate, would-likely-be more of a 'one-time PSU-fireworks / breaker-popping festival' HOWEVER, IF, somehow, you had a small water-leak in the magnet-jacket...???...(as-might happen in the case of an in-shipping freeze) - again, you're sure the mag was actually working, under-load? Steve - thoughts? Also, thoughts on the meter-cal board pots / cnnx?

Glad to help, where-possible... I gotta jet, so I'll let the P'fessor take-back over...

cheers...
j

[smogthemog]

Where would you consider a 'short' to be in the circuit, most likely to be between what? with the knowledge of what you have heard so far? Or is it a case of 'it could be anywhere'

I still dont fully understand the basics of how this PSU works? I get the 200vDC stuff but where does the 10-60V derive from?

Does the HV DC voltage present on the top of the main smoothing cap basically go to the Anode of the tube via the HV start coil?

Then, does the negative side of it all go to the center tap of the filiment transformer thus connected to the Cathode side of the tube?

Where is it connected and what does the magnet do in all this?

On the head drawing, what is C1 and R1 doing? What is their purpose?

There is something just not 'clicking' in my pea sized brain
Please please please someone explain this to Mr fikO here I just don't get it

Sorry about all these questions

[mixedgas]

Tube voltage plus passbank voltage = rectified line voltage , this is a constant and can only get changed a little bit by changing taps. Since a good 88 tube runs at 165 V +- say 7 V, changing slowly with amperage, your minimum rectified line (when loaded and running, open circuit needs to be higher for starting) needs to be about 175 V (165+10) and maximum should be 215 or so (165+60), this is because of the stress on the passbank. The passbank transistors are rated for 250 watts (infinite heatsink, heatsink at 25'C) each, so if you get more then 60V across the passbank, your stressing them. Hence the high end of the green range. If your lower then about 3V across the passbank, you can loose current control as the C-E voltage starts to be less then the B-E voltage the opamp can source and things could go open loop. Hence the low end of the green range.

Passbank voltage will be higher at low tube currents, as the passbank needs to "eat" more energy by becoming a higher effective series resistance to limit current in the tube. At 4-5 amps of tube, you should be approaching 40-50V of passbank. At 18-20 amps, you should have say 4-5V of passbank or less.

C1 in the head returns the ignite pulse to the ignitor, keeping it out of the passbank.

Anode voltage comes off the cap, flows through the ignitor secondary coil, into the tube, down the tube when lit, and to the passbank via the center tap on the cathode secondary, it flows through the transistors and returns to the caps other side.

The reason for the center tap is to balance the arc on the electrical center of the whole cathode, preventing one end from being eaten by tungsten migration and hotspots from the concentrated arc. The 3.2Vac on the cathode actually pushes the plasma back and forth a bit, making the emission even.

The tube has a slightly inverted I-V curve compared to a resistor, and will run away without current limiting. You are running a constant current through the tube, not maintaining a constant voltage.

The tube has one or two (at least) nonlinear regions in its operating range. Lowing tube pressure with age (gas gets buried in the tube walls) brings the low current instability closer to the point where the tube operates, and can cause oscillations. That may be what you are seeing. This is why you need to run the psu into a dummy load and see if the PSU is good.

Often times you get a insulator to fail or a misswire caused by accident that will NOT totally fail, and just causes sparks. This is what Jon and I are trying to rule out before we suspect the tube. The standard way to test a unstable laser head is to switch to a known good laser head, or on a system with fill, bump the tube pressure up. Since you do not have a fill system, but have a huge gas tank, fill is in theory, eliminated on your laser till its end of life. And usually on a Lexel, something else dies first before the gas pressure.

Also condensation on the cold passbank, that migrates elsewhere, has a habit of causing insulator failures in weird places, resulting in odd little failures.

Some times on three phase Lexels, and rarely on single phase Lexels, it can be as inane as a worn breaker contact, arcing slightly. Until you get a stable load on it, we cant tell.

Only rarely does a Lexel go totally bonkers like this, I've been working on them since 1992 or so, and this is only the 6th one I can remember that was not a easy fix. Once it was one of mine caused by condensation (needed total rewire after arcing) , once on the Universities big one (protection zener diode intermittant) Once on one I was given by a drug addict laserist who tried to bypass the interlocks by sniping and jumping any wire that offended him, Once at Selem with another bad zener, and once in a club where the owner thought the step down transformer I speced was too expensive and bought a constant voltage 3 Phase Sola transformer from a surplus guy, which wrecked havoc with the control loop.


I've seen ultra low pressure tubes maybe twice in Lexels, but your quickly pushing to #3. Gotta load the supply down with a resistive load, even if its just lightbulbs.

Steve

[smogthemog]

Wow Steve, thanks for that mate - TRULY appreciated is your time and help through all of this! It's unprecedented!

A lot of food for thaught there for me to chew on and YES - I now get it

Pretty obvious when it's explained.

I am gearing up for the LEM at the mo but will still be tinkering when time allows.

A lot to do in such little time as usual.

After the head is all checked and back together I will endeavour to get a load accross the PSU - I have still to re-do the PB before that too and perform and indepth check over of everything up close and personal

I have the tube you have for it in mind Steve and if the worst happens then I will get saving my pennies!


Thanks again for everyones help on this.

[smogthemog]

Hi Steve.

I am currently at the UKLEM and have the PSU working with the load connected
I desperatly want to get this running, If indeed it will run, before tonight and could do with your help again Sir

I am now rebuilding the head, replacing all the wiring fdrom where it came from uising the many photographs i took during the strip down.
I was wondering whilst doing this if at some stage someone had possibly 'wired' it up wrong and thus it now dosn't work and is blowing the PSU!!

Can you please advise me of how critical it is to wire it up exactly as is shown on the drawings that Sam has replicated? I.e. does the red and black wires that come from the magnet require to be connected the correct way around - the red wire is the forward connection to the magnet windings, towards the Anode end of the tube and the black cable from the magnet coil is the one that connects to the coil at the Cathode end of the tube?
On the diagram, the +HV (P) wire also connects to one side of the Magnet coil - Which side does this connect to - Red or Black?
The cable that comes from R1 (33K) in the HV Ignitor goes to one of the Mag coil connections, which one?

Also, is it critical which side of the 2x thick filament wires does the wires to the C1 & C2 combo connect too?

Thanks Steve

Jim

[mixedgas]

No issue with the cathode connections. Magnet can be backwards, but Anode "north" is preferred. Lexel tested the magnet for polarity at first incoming inspection and final checkout.

Steve

[smogthemog]

IT RUNS

870mW @ 20A tube current, 150V tube voltage.

The output power is rising as it is left to run.

I am running it at 12A to bed it in.

I believe it to have been wired wrong AND/OR the corrosion under the connector block was possibly to blame.

Thank you again Steve & Jon for all your help through this!!

I am now a VERY Very happy man

[mophead]

Congratulations!!!

It's always nice when they fire up.