Lexel 88 problems - Help please

[smogthemog]

Lexel 88 problems?

I have a Lexel 88A PSU and associated head and I have a slight problem with it.

This is the Lexel that Lexat8 sold to Mr WaxStan (Rob) and Rob has VERY kindly donated it to me for research and development ('bugger about with') purposes

OK, here goes - When I hook it all up and after setting all the buck/boost - filament tap switch settings and attempt to fire the laser, it strikes and lases for a second but makes a horrible 'osscilation' type noise then goes out until you re-press the fire button. It then does the same thing again. 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 noise. The noise emminates from the HV strike coil in the head. I haven't run it for any longer than 5 seconds before switching off.
When it lases it is doing about 1.2W and the current/power output is totally unadjustable from the PSU, Light or current controlled, and is pulling about 35A, according to the PSU meter. The Pass bank voltage is very low, around 10V, according to the PSU meter.

I can completely remove the current and light control boards from the PSU and it still does as I have described so this to my mind points towards a pass bank problem.
I have read as much as I can about it from Sam's pages and from here at PL but just need to clear it in my head before proceeding.

I did a small amount of testing of the PB following Steve's instructions and I have found one of the PB transistors, Q7, to be short from base to emmiter. They may be others too but I found this one almost instantly once I started checking but then had to go and do something else and haven't got onto it again since.
I have purchased a full new set of PB transistors, 2N6259's and a 2N3773 which is one of the PB driver transitors. The other driver is a smaller cased 2N3738 which I am having difficulty finding??? anyone got one??
I also have a 5W 60v Zener to fit as well.
I thought I would change the whole lot and start from there but wanted some advice as to what it 'could' be that is causing this problem.
I intend on checking all the resistors on the PB too but just haven't got around to it as of yet.
I have transistor HFE testers and Cap testers, AC & DC clamp meters and a lab PSU if I need any of them in my quest
I am aware that the tube is possibly shot but I just want to get the PSU working before I go any further.

Does anyone have a tube for one of these? I may be interested if you have?

Any help appreciated.
Thanks
JIm

[mixedgas]

[QUOTE=smogthemog;142205]Lexel 88 problems?

I have a Lexel 88A PSU and associated head and I have a slight problem with it.

This is the Lexel that Lexat8 sold to Mr WaxStan (Rob) and Rob has VERY kindly donated it to me for research and development ('bugger about with') purposes

OK, here goes - When I hook it all up and after setting all the buck/boost - filament tap switch settings and attempt to fire the laser, it strikes and lases for a second but makes a horrible 'osscilation' type noise then goes out until you re-press the fire button. It then does the same thing again. 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 noise. The noise emminates from the HV strike coil in the head. I haven't run it for any longer than 5 seconds before switching off.
When it lases it is doing about 1.2W and the current/power output is totally unadjustable from the PSU, Light or current controlled, and is pulling about 35A, according to the PSU meter. The Pass bank voltage is very low, around 10V, according to the PSU meter.

I can completely remove the current and light control boards from the PSU and it still does as I have described so this to my mind points towards a pass bank problem.
I have read as much as I can about it from Sam's pages and from here at PL but just need to clear it in my head before proceeding.

I did a small amount of testing of the PB following Steve's instructions and I have found one of the PB transistors, Q7, to be short from base to emmiter. They may be others too but I found this one almost instantly once I started checking but then had to go and do something else and haven't got onto it again since.
I have purchased a full new set of PB transistors, 2N6259's and a 2N3773 which is one of the PB driver transitors. The other driver is a smaller cased 2N3738 which I am having difficulty finding??? anyone got one?

Snip the leads of the bad power transistor and try to start again. There are more then enough passbank transistors to run with one out of circuit. See how it does at that point and get back to me.

I have 3738s.
I have a source of a semiconductor inspection tube that can go into that resonator after moving the mounts with a allen wrench and hammer and reworking the stem covers a bit.

Steve

[mixedgas]

If you get CW tube current with no cards installed the zener is bad, is shorted or breaking down and driving the passbank early. The zener is there to turn the passbank on a bit to pass the ignite pulses and protect the transistors. But if your getting lasing with no cards and no shorted transistors, its the zener turning on, or in rare cases one of the two drivers is bad. Its OK to lift one leg of the zener for a few starts with the cards out.

Also if you have a bench supply with current limiting, or some 9V batteries and a big 470 ohm resistor, you can feed in a current limited 32 volts or so to the current card and see if it generates the +20, +/-15V and other voltages.


Steve

[smogthemog]

HI Steve.
Thanks for the reply.

I have found a scource for the 2N3738 and it should be with me next week. Thanks
I tried removing the bad TX and trying it again, it was still the same.
I have now removed ALL the transistors and will be replacing them all with new ones.
I have no idea what the PB TX's are that are in there at the moment? They are IR430's? Certainly not 2N6259's. I have new 6259's to replace in there.
I have checked all resistors and they all seem fine. I have a 5W 60v Zener but it's not in the same casing as the one that is bolted to the PB cooling plate, it's a large axial leaded type. I may replace the one that is in there and thermal paste the new one to the plate with a small clip holding it in place.

I will give the card the once over tomorow with my PSU but first I am going to solder all the little transistors in properly as at the moment they are just pushed into some very dodgy 3 leg transistor sockets - NOT good!!
I will replace all the TX's, diodes etc and then get back to you mate.
I also want to check the HV igniter caps before I do too much more to check for shorts.

When it did run, the tube/PSU would be at full current, around 35A (according to the PSU meter), and I could not turn it down. The PB voltage would be very low, certainly no where near the 70V.


What is the 'semiconductor inspection tube' used for??

Jim

[Diachi]

What is the 'semiconductor inspection tube' used for??

Jim

Semiconductor inspection...

It was probably from an argon laser used for semiconductor inspection ... I'll let Steve answer though.

[mixedgas]

The dodgey little sockets work quite well, I've never had problems in 20 some years of working on these. I'd leave them in, it makes troubleshooting easier, and btw, I've rarely had a part come unseated in one, and I've worked on dozens of these.

Do you have the tap charts ??? Without them your asking for trouble..

IR430s are the root of your problem. Its a power fet that is designed to be the drop in replacement for DTS430 and 2N6259 transistors. Get rid of the 430s and get matched transistors. If you can't get 6259s I have loads of them.

Here is what I do when I have a totally screwed up pass bank like that.

We'll use these drawings until I can scan you the real ones:

http://www.repairfaq.org/sam/88csch.gif

http://www.repairfaq.org/sam/88psch.gif

First of all, On the CPC board (current control) Pop it out of the laser.

Remove the transistors from their sockets.

Check all non zener diodes with a ohm meter that has a diode check function. They are slicon so you should see .62 to .75 drop depending on your diode test on your voltmeter. check them both forwards and backwards.

Test all transistors with a good tester that actually rams some current through them. If your using something with less then say 1 mA during test, the leaky MPSAs dont show up that well. Usually what happens is bad control card transistors become leaky. Or they develop reverse leakage.. Ie test NPNs with the PNP setting and look for leakage, minor reverse leakage is OK, but forward leakage is intollerable.


Find the anode of Zener Z1, a 5 watt zener diode. Apply 32 volts (bunch of pp3 nine volt cells. in series) or so current limited with a 200-300 ohm series resistor or use a current regulating bench supply set to 100 mA max. Z1 should be clearly clamping.

You should see the zener Z1 clamp to 27-30 volts depending on which part is installed. It doesn't matter which one, as long as its 30V or so. At z2 you should see 20V give or take .2V and at Z3 you should see +15 V give or take. Zeners are not that accurate. Power down your test jig and install Q2,Q3, and Q4 and power back up, you should see the -20 and -30 at the respective test points.

Install Q1 and make sure you have 6.3V there.

Leave The LM301 alone for now, they rarely fail unless one of the zeners are bad.. You would have to build a test jig for it, anyways.

install the tested overcurrent trip transistors and set the card aside. Pull the light card if installed.

Make sure the laser is disconnected from wall power.

Move to the passbank. Make a diagram of where the leads go to the small and large driver transistors. Unsolder both transistors so you can test them with your transistor checker. YOu can leave aLL the transistors soldered in the passbank while testing. Just pull the leads. I cheat and use a curve tracer. If both of them do not have good gain, or if they have ANY reverse leakage or are shorted, replace the offending part.

resolder the drivers if they pass.

Next up, get a current limited 12V or so power supply, or some sort of low current automotive light bulb that draws a few hundred mA and a non limited power supply. Hook the ground of the 12V power supply to -HV unregulated and the plus side to the center of the cathode transformer secondary. Using a potentiometer or batteries, apply a few mA of base current to the small driver, and see if the passbank will LOAD UP, the current limited power supply, or light a small light bulb in series with the passbank... In other words we're testing the passbank for gain and shorts by running it at 12V or so and a low current.

If it does not pass these tests, there is a short some where in the wiring, the opamp is bad, the clamp sener might be bad or something else is wrong.

One thing to do is desolder the ends of the 4 power diodes on the passbank and make sure they still act like diodes. You can NOT test them while they are wired as a bridge. Then resolder them. A blown power bridge tends to take part or all of the passbank with it...

Install the schmidt trigger parts on the control card, make sure the current pot is still good, and install the cpc card. make sure the cathode and buck boost transformers are tapped correctly and try to start it. Whatch out for badly adjusted overcurrent trips. If those tests do not find the fault, PM me. The manual has about 60 pages. If I have to, I can scan it..

You should know that the transistor insulators are beryllia, do not lick them or grind them into dust and breath it. As they are, their harmless, even if cracked.

When you have the passbank apart, check the 2 or 3 ohm emitter resistors for shorts to case, and for proper resistance. check the 150 ohm summing resistors on the passbank for proper resistance.

Whe you start it, turn the knob all the way clockwise and then back off by 1/3rd. If the control loop is marginal, you want the higher current so the transistors are in the green range. Lower currents cause more stress for the transistors if the taps are mis- adjusted.

Don't exceed 18 amps cw for long periods. The psu can be capable as much as 35 depending on how the factory set up the feedback but the tube is happy and lives long at 15-18 amps. I usually set the trip pot to 21-22.

90% of all faults on this PSU can be found with a ohm meter and within 20-40 minutes, as those who watched me at SELEM last year can attest. But if something is really buggered, it can take a while. Note that all my troubleshooting is off line. There are some things that can be done hot and prestart. Cathode voltage. AC test voltage of about 116 to 130V depending on taps. delta T for tube health, and DC across the tube before start etc.

Pull the bloody autostart relay when troubleshooting. That thing can get you into trouble. But leave the cathode warm up delay relay.

Steve

[mixedgas]

Well, it drove a two color fiber interferometer that looked for dust , bad spots, and other crap on silicon wafers. They wanted 450 mW of tem00 488 and 450 mW of 514 for 10,000 hours. Suffice it to say they have the best Brewster windows I've ever seen on a Lexel.

SVG makes inspection equipment for semiconductors, so the part is a 85S or 85SVG. Don't let the 85 part number fool you, these have much more gain per unit length then a normal LEXEL 85. The difference between a 88 and a 85 is bore diameter and optics.

Steve

[smogthemog]

WOW

Thanks for your time in helping me with this Steve, it's in excellent detail from you too!!! Makes it nice and easy for a chump like me to follow

OK, I have removed the 7x transistors from the CC and have found one of them to be U/S (C/B short) Unfortunatly I don't know which one it was that was damaged only that it was one of the MPS 6534's as I took them all out together , after marking the CC board as to which one's went where.
I have ordered a couple of each of the different types and hopefully they should be here this week.
I am a bit annoyed I don't know which one it was, I should have done each one individually but it's too late now

Just checking the board over

EDIT - found D2 S/C and D3 O/C

[mixedgas]

Ok, if those two Diodes are blown check R2 on the passbank, it current limits the 250-300 VDC high side rail to power the control card and check the plus rail zeners. It sounds like something shorted the negitive rail on the control card, perhaps the opamp working into a shorted passbank...

Your getting there, and it sounds like it will live again.

I'll scan the true schematics soon.

Steve

[smogthemog]

By R2, do you mean the 2K 25W resistor?

I have also found Z4, Z5 and Z6 all to be faulty. All O/C

All have been now been ordered.

It's taking time but as you say, I am getting there. I LOVE fault finding!

Thanks again for your help Steve.