View Full Version : Synrad 48-2 Repair

Donn Watson
12-21-2012, 12:45 PM
Good Day,

This is my first post after reading your posts for several years. With all of the knowledgeable members here and the previous threads I've read regarding the subject of tube repair, I thought I'd seek some advice.

Laser: ULS-25PS, mfr 1996
Tube: Synrad G48-2 mfr. 1996
Power out: 13 watts at aperture
Power Supply: 28.2
RF Driver III, rev C

One of the driver boards in this tube was really cooked; read that LOTS of carbon, two lands burned and two caps no longer identifiable.

C7 1000pf mica coupling cap CARBON ONLY

C4 100 uf/50v VERY CHARRED

The lead from coax matching line to tube feed-tru VAPORIZED.

This older board uses a MRF-148 MOSFET driving a pair of MRF-174 devices in parallel. With the DVM I had handy, I found no shorts, but the meter did not develop enough voltage for me to test the devices. If they are bad, they appear to be available via RF Parts.

Synrad did supply me with two pdf manuals, one each for the F & G series 48-2 tubes in my two lasers. The gentleman I spoke to who helped me was in the marketing department. Considering the cost of replacement tubes, I would like to repair this RF driver board if I possibly can.

Tools on hand:
dual trace 60 mHz scope
thru line rf wattmeters
dummy loads
laser power meter
analog and digital voltmeters
rf driver III schematic

Unfortunately, this ULS-25PS also suffers from a faulty 'u-step' board in the 'stepper motor control box'. I am testing this laser now with a 'u-step' board from the second laser. This is, however, a topic for a later post and thread which might include "Chinese DSP controller". :-)

Does anyone with experience have suggestions that might aid me recovering this now no longer supported 'rf driver board' for a G series 48-2 Synrad tube?

Thanks in advance and it's nice to have an excuse to post and introduce myself.


South Kingstown, RI

Richard Rumancik
12-23-2012, 11:27 AM
A few people have successfully repaired RF boards but usually it was a single capacitor that was blown. If your board is extensively damaged maybe you could buy a used board from Synrad that would be a better candidate for refurbishment. But if you think it is salvageable it won't hurt to try.

Technically these RF boards require a "tuning" procedure to match the board output with the laser tube. This requires an oscilloscope and some technical know-how, plus some way to exercise the laser on the bench. (Like a Synrad controller.) The only person on this forum that I know that has done this is Rodne. Perhaps you have the skills and equipment to do this. Most people just replace the bad capacitor and hope for the best.

There are a few threads on the forum about repairing these boards so try to look those up first. The capacitor style for one of the problem capacitors was changed around 2000-2002 timeframe (not sure exactly when) so be certain you are using the latest Synrad-recommended part.

The voltage of the power supply is also important. The output of 28.2 is probably in the right ballpark - unfortunately Synrad marked many of these tubes with a misleading label (something like 28-30VDC) which made people think that the input voltage was not critical. But in fact the applied voltage is supposed to be maintained at the same value at which the laser was tuned. If you elect to replace caps and not tune, then there is probably no reason to tweak the voltage. Yours was probably tuned at 28V. (Some of the GCC Synrads were tuned at 30V.)

AL Ursich
12-23-2012, 3:44 PM
The GREAT people at http://photovaclaser.com/?p=201 might be able to help you with advice, even the actual repair or just parts. Worth knowing when your in the Business.

May not be related but do a search on the topic of "Bad Caps" on the web. About 15 years ago some guys working at I believe a Korean Capacitor Factory decided to move to Taiwan. They took with them the new formula for the Electrolyte that fills the gaps between the Foil layers.

The guys left before the final version of the Electrolyte was developed. They take up working with a Taiwan Capacitor Factory and all testing appeared to work out GREAT... What they did not know is that the as the electrolyte got older it started to break down and made a Explosive Gas... Caps worldwide started popping like popcorn.... Making for a lot of work for electronic techs... The bulging of the capacitor prior to popping is the sympthom...

I worked for Sony for 8 years starting in 1995 making 17 inch Computer Monitor Picture Tubes at first then ended up at a Service Center in Philly. At Philly we had a standing request for Overtime to replace known bad caps on Computer Monitors working through as many as we could do, lasted for a long time.

Good Luck with your Laser. I called Photo Vac last week for advice on the one I am selling on eBay this week.


Martin Reynolds
12-23-2012, 4:11 PM
Without looking at schematics, it is difficult to figure out what happened. Obviously, there's been a catastrophic short, and it is slightly possible that he vaporized cap was the problem. However, something shorted on the output side, and I wonder if it was the "tube" itself. I'd certainly start by checking out the electrolytics. The top of the can should be flat - if it is even slightly domed, the parts need to be replaced. Failing electrolytics can result in overvoltages in other parts of the circuit. Then, I'd look at semiconductor in the path and see if any of them took a hit. And this stuff is all dangerous in a nasty way to eyes and other internal organs, so working on these can get you maimed, blind or dead.

Donn Watson
12-24-2012, 9:25 AM
Rich, Al, Martin and All,

Thank you all for your useful and interesting feedback (no pun intended :)). When I discovered that the power output was approximately half of what it should be, I expected the issue to be with one exciter board, hopefully a faulty component. I was not prepared to see the destruction of numerous devices including the PC board. When you consider the amount of RF power required for actual laser power produced, there is a significant amount of energy developed by the pair of MRF-174 transistors, hence the potential for misdirected 'heat'.

The RF path from the transistors to the tube feed-through was open and enough heat was developed at the feed-through to render it physically 'wobbly' to the touch. Because it is also a gas port I don't want to cause any further damage or render the tube useless for refill in the future if, necessary. Instead of cement, epoxy or bedding compound, perhaps I should insert something softer and removable, like RTV or polysulfide bedding compound.

At one time I worked with RF daily, but I've been engraving for over twenty years now and have no experience with the inner workings of these tubes, probably because they were so well built, have lasted this long and have only required repair once. I have an eighteen year old F48-2 that produces 34 watts and that's after passing through three mirrors and a 'window'.

With other work going on, one laser and two routers functioning, I haven't been quicker to get this laser repaired, but I am learning a lot in process. I've gained a lot from reading related posts here, some written five years ago and more.

Thank you very much for your input. It is appreciated. Enoy the upcoming holidays. I'll report back with what I'm able to learn and hopefully repair.



Philip Tarnawskyj
02-02-2014, 12:17 PM
Long shot but does any one on this threat know of a cheap synrad 48-2 tybe ?

Dan Hintz
02-02-2014, 2:33 PM
Long shot but does any one on this threat know of a cheap synrad 48-2 tybe ?

eBay searches and going to the laser resellers are your best bet...