Canberra Series 30 MCA

[Wilfried Heil]

I´ve been lucky enough to obtain a Canberra 30 MCA, that I hope to repair.
Type: 3100S, Serial No.: 179568
From the date codes on the TTL ICs in it, I would say summer of ´78.

The good news: it is easily switchable to 230V.
The bad news: it doesn´t do a thing.

Otherwise, the unit is very well build and seems ok. Power supply has some burn marks, and I´m checking the electrolytic and tantalum caps everywhere.

I have no documentation whatsoever, which would be rather helpful.

Does anyone here still have the manuals and schematics for this MCA and could possibly make copies of them? Or knows were I could get them?

[Richard Hester]

Try Don Orie at Oetech.com for documentation. Canberra does not support this MCA any more. Also, plan on replacing every tantalum capacitor in the instrument. The old "teardrop" tantalum capacitors used liberally for power supply bypassing did not age gracefully, and are shorts just waiting to happen. You can find a bad cap and replace it, and the instrument may function briefly until another capacitor takes its turn to pop ( it happened to me). It's better to get rid of them all. I'm replacing mine with 22uF, 35V high quality electrolytic caps, with an occasional 1uF film capacitor sprinkeled in to reduce the power supply rail impedance. I expect the electrolytics to last until I shuffle off this mortal coil.
You will most likely need to fix the power supply board as well, as I suspect it gets nailed each time a tantalum pops and shorts the supply. I had to replace a 741 opamp on the +12V supply to get mine running again. The power supply is on a board all its own, and there are test points right at the top edge of the board to make diagnosis a little easier. I suspect either the +12V or -12V supplies is defective.

[Wilfried Heil]

I´ll try to get the manuals first. From what I´ve seen, the power supply was apparently overloaded. All voltages are there, but more or less off nominal and oscillating when loaded.

Thanks for the tip on the caps and the link to Oetech.

[richnormand]

I have some (if not most) of the service diagrams for that unit if you are interested (not the user manual). makes a book about 2.5cm thick.

My e-bay unit was also dead when I got it and most of the issues were pass transistors, 714 op-amp and electrolytics in the power supply. In particular the start circuit.

First thing: power supply was dead, all of it. Removed all boards and CRT sub-assembly. Get a few volts. It was the start circuit electrolytic capacitors C1 and C2 that were dead, then a 2N3055 pass trans, then a Zeener for the 24V, then the -12V reads about +8V. That is the 2N3904 Q7 that is shorted and a bad 741 op-amp. Remove all electrolytics to check capacitance and replace another one… Supply board is now OK with no load. Adjust the 30V supply.

Second, plug the CRT back, no effect. No vertical, horizontal or video drive at the plug.

Three: go to board one. Boy what a mess! Several reworks and repair attempts. System clock is dead. No 5V on the rail but it is OK on the power supply board. Turns out to be the motherboard is missing a holding screw on the corner and the pressure flexing the motherboard shorted the 5V edge pin on the metal angle holding the plastic foot for the frame, it’s welded there, the connector insides are not making contact anymore! Happened before I guess, since the connector contact had sputter deposits on the side and that did not happened on my watch! Replace stuff and connector. Lots of work to remove the motherboard et al.

Four: now have +5V to board one. System clock still dead, solder bridge on IC A19 on a rework that looks more like a homemade repair with IC A8 around the Xtal. Still no horz drive but vertical signal is now there (from the 5V connector repair I assume)

Five: no output from IC A86. Bad IC replaced. Board OK.

Six: put CRT back on line as there are signals at the plug now. Well the CR406 is shorted. Looks like they save some money there with a duplicate transistor. All the controls need cleaner lube. Video is not normal.

Seven: Board 4, you guessed it. The tantalum caps replaced and two resistors obviously burned (but still OK)

Now has been working non-stop since Sept 05 with a 3" NaI and a 5" scintillator setup.

PM me should you be interested in some photocopies of the problem areas and such. You will need a decent oscilloscope to make some sense of the boards (which seem to be missing in your pix...!! Your unit also looks pretty clean, mine was a horrible dirty mess.) I also found a transistor curve tracer useful to identify bad power supply pass transistors that were not shorted but had been sufficiently strained by heat to be off spec.

Thanks to Kristian from this forum for the initial info.

Cheers

Rich

[Richard Hester]

Rich - It sounds like you got an exceptionally grubby example of the breed. My Series 30 MCA was clean inside, though a bit battered outside. BTW, I have also encountered tantalum capacitor problems in a Tracor plug-in MCA and a Tennelec SCA. If the power supply is properly current limited, the ensuing short doesn't cause any other damage. Unfortunately, this appears not to be the case for the Series 30 MCA.

[richnormand]

Looks like mine was from a undergrad university lab populated with all the dead boards they had and then stored outside. That is why I stressed the need of a good scope and curve tracer to fix stuff……

Quite a learning experience, I might ad. It was a cheap $50 on e-bay so I should not complain. On the other hand, it has been working non-stop since Sept/05 counting muons and such.

It is attached to a 5” diag by 2” thick plastic scintillator with a 5” Dumont PMT, a 3” by 2” thick NaI Harshaw detector and a Bicron 1702 X-ray detector.

[bwsparxz]

I too got a Model 30 off Ebay for $50. Of course it does not work. So I got the manual with the schematics. The power supply voltages are very low . All the electrolyics need replacing along with all the tantalum caps. All the logic chips seem to still be available but a few are odd ones. On board 4 is a COM8017/7923 SMC (AY3-1015) Is this a custom chip? any idea of what it could be?, I am still searching for this part. Also has anyone ever copied the PROM on board 4. I would think you could do this, correct me if I am wrong. I am worried that during repair I could damage it and it could still stop the unit from working. Any tips to troubleshooting the powr supply board. It appears that the 30V buss supplies the 24v buss from zeners and the 24v feeds the 741 opamps. So if either of these voltages are low then it stops the rest of the voltages. I could be wrong here.

[Richard Hester]

The chip you mentioned is an old standard UART serial communication chip. It might still be findable at a surplus place somewhere. You can minimize the danger to your boards if you yank everything but the power supply board and tget that part in working order first. While the rest of the boards are out, you can do stuff like replace shorted tantalums, etc. Then cautously ease them back in.

[richnormand]

I would first remove all loads including the CRT assembly. If you read my previous post, the start circuit was bad with dead electrolytics. Then follow the +30V supply and work your way through to all the others. Pass transistors, zeener diodes and 741 op-amps were most of the trouble. Pays to be thorough on the PS board as all the rest depends on it. After you have all the voltages OK then plug in the other boards one by one while keeping a lookout for the 30V supply at each step. I was lucky as I had to replace few of the tantalum caps on my unit and it has been running non-stop since last September.

I have the schematic if you are interested. See previous post.

I’ll look at the board 4 for the chip you describe. If it is a custom, it may be difficult to get one these days. If it is a prom it may be possible to get a copy from another unit. On the other hand, until you get the PS board working to specs it will not matter much.

[bwsparxz]

What does the prom do ? Is it possibly the video character generator . So the 30V buss is the starter voltage for the power supply? By looking at the schematic it seems to be.

[richnormand]

The start circuit activates the buss. After that all the others are referenced to the stabilised 30V, if I remember correctly. remove all loads and get it running.

In other words:

"Fix the PS board first to specs before anything else...."


Brian Willard wrote:
> What does the prom do ? Is it possibly the video character generator . So the 30V buss is the starter voltage for the power supply? By looking at the schematic it seems to be.