Question on which NIM amp for a 3He detector?

[Dennis P Brown]

Richard, that unit looks a lot like a modernized "Ultra" code machine.

[Dennis P Brown]

The TENNELEC TC 246 AMP/SCA COARSE GAIN MODULE arrived today and when tested in the NIM box, promptly blew the 3 amp fuse - that's a lot of power so it has a serious short in that black box - that is a definite no go for that unit. So, have to return it.

Likely its one of their special units that does not work with a std NIM box. I noticed it had about 25 pins on its back - that NIM box doesn't support that number of pins so likely a few of those extra pins caused the shorting to ground. Guessing it has so many pins on that module to only be used in their special NIM box.

Guess I'll look inside to see what caused the issue and see if the device (likely) connects the 120 AC to something it most certainly shouldn't.

Update - yes, 90% of those pins have no use in my unit; however, the 246 does have a pin for the 117 AC line and in the unit that has a green wire that goes directly to ground via the module case! They certainly made sure that if you use any box but their's, then that NIM box's fuse is toast! So, makes sense they have all those useless pins and then designed one so it guarantee's via the NIM std to kill the box - they had class (mis)management back then - no wonder they were bought out.

Worse part, this unit was tested and certified by the seller as working (which I don't doubt - they didn't use NIM boxes but connected via cables so they'd never see that fatal issue with the design) - I'll let them know; maybe if that 117 service was removed it might work but not gonna test that. Not my job.

Update: the company wants the unit returned and will refund (with shipping costs) the price. They were very helpful and intend to test their other units to see if the #33 pin also is shorted on those units. So they can flag them and not cause issues. They were rather interested in the NIM information you guys provided to me about Tennelec and their amps not always being std NIM compliant.

[Richard Hull]

Pin 33 to case ground!! That is unbelievable. What a dastardly thing to do. Are you positive it is pin #33??

All stock NIM bins only have 10 pins in their back plane buss wired. Period! Thus, any folks making a custom module loaded with other pins are free to use any free pins for any reason to talk to their other custom plug ins. Thus, if your module has 21 pins poking out the the back of it....No problem! It will not blow the fuse in the bin! Why? ...The power buss lines are sacrosanct if you want to sell your module for future general use.

Again, I could design a custom SCA NIM module that will talk to 4 other modules that I make as a grouped effort. I use a lot of those reserved pins to chat with my other modules. However being smart, I also place regular BNC in and out plugs so it will also stand alone for anyone using a stock NIM bin. Naturally, I would have to wire a custom bin with all the reserve pins running across the buss and sell the bin with my modules as a plug and play NIM system.

Later, if the bin I custom wired is surplused without my 4 custom modules, it is still a fully functional stock NIM bin! The same goes for all my 4 custom modules, they are good to go individually, but instead of them not having those reserved pins wired, they will still live a happy life in a stock bin doing their individual thing they were created for. However, they will have to be externally jacked in via BNC plugs just like any normal NIM module. All of my custom pins in the module now just slip into dead holes in a normal bin.

I would never tie 120 volts (Pin 33) of a my custom module to ground to deliberately blow a bin fuse in an effort thwart any of my custom modules having a future use.

Stupid, stupid, stupid...

Richard Hull

[Richard Hull]

Whoa there Nellie!! I owned a 246 module, tested it and used it in a normal bin!!! I have it on my inventory list. I also have the data sheets! (Attached) It is a normal module!!

Note: ON the second page cut sheet. They say this 9 pin preamp plug will work with any other manufacturer's preamp! this means this module is fully compatible with normal 9 pin wiring. They note and warn that earlier Tennelec preamps MUST have a special power module purchase to function.

If the real pin 33 is connected to the chassis that is an after market issue. Please look at the plug again. It can be easily misread as the pin numbering manner is stupid and odd ball and I have trouble with it to this day! I fear to are seeing the buss AC ground pin #42 which is always green within the modules going to ground. As this module doesn't even use 120 volts for power the other two pins near ground 41 neutral, and 33 hot are either not there or are not connected! They are all jammed together at the base of the NIM plug. I think you are misreading. Every NIM module has a green wire from #42 to the modules metal case.

Again I owned this very module and verified it in a normal bin!! I am sure that due to its great value to NIM people who have picked over and purchased from my many NIM modules at HEAS for the past 10 years that it sold to someone. I haven't heard anyone complain.

Check it over again.

I am positive you have a dead shorted tantalum cap on one of the DC power lines I could fix it in 5 minutes, but send it back.

Richard Hull

[Dennis P Brown]

I double and triple checked this issue and it was the 117 AC plug (#33) in the NIM connector and the module connector that mated with that specific pin went to ground in the unit. Again, I looked inside the case and that pin was affixed to a green wire and appeared factory installed. The green wire even had a proper "eye" connector soldered to the wire that was bolted to the case! There was no 'after market' appearances to that bundle or solder joint.

I agree the other pins make sense in light of your post - now I'm wondering if I had simply cut that green wire in the module then the unit would work fine, maybe? I will likely talk more to the company (they have three units besides this one.) As per an earlier post, I was warned some of these Tennelec amp units would blow other devices apparently in their misguided attempt to sell only their units bundled to their NIM system, maybe? But to ground a hot 117 AC line is beyond the pale, I'd think.

Maybe mine was modified by someone for a reason beyond my understanding. No matter, certainly some/most(?) 246's are safe with the common NIM standard but relative to my unit, all I can say is, "It's a trap!" (bad reference to a bad Star Wars movie.)

I do like the features on this unit and maybe it is worth trying again. They intend to ohm all the unit's for the 'pin #33' to see if they too are grounded. Again, maybe worth getting and just eliminating that ridiculous ground wire - if that is the case.

As always, thank you Richard for your well researched information and advice.

[Richard Hull]

Test

1. pull all modules out of the bin
2. replace the fuse
3. plug the bin in and turn it on. all power voltages 12 and 24 should there. If you have a pilot light it should glow..

Bin is in good working order

4. unplug the bin from the wall power outlet. Leave it unplugged for the rest of the tests.
5. put an ohm meter on the ohms range (low 1 ohm range.)
6. firmly connect one lead of the ohmmeter to the case of the power supply or somewhere on the unpainted area of the cage. leave this lead connected for all the rest of the tests.
7. connect the other lead to the AC plug ground lug.
8. this should read near a dead short ground via buss to case and AC plug
9. connecting the free lead of the ohmmeter to either AC plug neutral and hot blades should read open infinite ohms
10. now turn on the power switch
11. both blades should still read open.
12. leave power switch on.
13. put in the TC246 module only!
14. Now read the two blades if any one of them reads zero ohms.......You do indeed have a short within the module to ground.
15. If you read 10 ohms or 20 ohms, you have a bad capacitor or shorted component to ground in the electronics of the module.

Richard Hull

[Dennis P Brown]

Thanks Richard; I will check this out before I get the next amp unit! Then do as you suggest with the amp unit. Still, I'll need to pick up the required fuse - don't have any with the 3 amp value (only higher or much lower values)

A somewhat less expensive Tennelec 246 (that has just 5 pins and no pin at the post #33!) was available so I ordered it. There are some features on the unit that I simply don't follow but thanks for that excellent post on the unit! I now have a somewhat better understanding on some aspects of the unit; however, a lot is still Greek to me but I'm sure that once I start using the unit (if it works!) I will figure it out (I hope ; I did order a service/user manual and hopefully, that clears up any remaining issues (and I don't end up needing the 'service' aspect of the manual!))

So, guess having a pulse generator along with an oscilloscope is essential in order to enable one to test the unit. That will be my next 'learning' experience.

[Rex Allers]

Dennis,

For a better perspective toward us following in the peanut gallery, could you take a picture of the NIM connector on your TC 246 module and post it here.

My curiosity would like to see what pin positions are populated.

As to your fuse, for Richard's measurement suggestions, the unit will be unplugged drawing no current, so any fuse current rating should be fine for this test.

Also, if you remove all the modules, a fuse rated lower than 3 Amp (maybe 1, 1 1/2, 2 would be good) should allow powering up the Bin to measure voltages.

[Dennis P Brown]

Good points. I do not have the 246 module any more - I sent it back. I did open the unit and a green wire was connected to pin #33 as I posted.

For FYI here is the photo of my NIM box connectors for modules. In the lower, right hand side of the NIM connector are three connectors for module pins: two are for power AC pins - neutral (#41) and Hot (#33). #42 is a ground; possibly the return ground direct to the power cord. The number values printed on the connector are easy to read if you magnify the image.

The upper connectors are the various +/- DC voltage supply connectors and a ground.

The rather large lower pins/connectors are grounding points. I assume the upper two are as well.

I did measure all voltages from the connectors and confirmed that connector #33 is 120 AC; the +/- 6 volt connectors are null, and the +/- 12 & 24 volt DC are all hot and correct. I'll connect the plug and fuse and check the ohm readings on the grounds.

[Rex Allers]

Thanks for the pic. These connectors in the bin have all the asterisked pins in the connector listings and no more. (As one might expect.)

I actually wanted to see a pic of the connector on the back end of the TC 246, though. Assuming you still have it.

[Dennis P Brown]

In the previous photo there is a Tennelec 246 but this unit has the normal pin configuration; nothing like the unit I was supplied (still a 246) but that had a ground pin for #33. It had 25 pins on its back and all were wired to the internal circuit board!

[Rex Allers]

So you have already sent back the strange one? That's the one I was hoping to see a pic of its connector.

I believe Richard's testing procedure intended to have that one plugged back in in step 13.

Edit...
Sorry about my reading skills. I just realized you said,
"I do not have the 246 module any more - I sent it back. "

So, I just wanted to apologize for missing that key info.

[Richard Hull]

Rex is absolutely correct! Without the bad module 246, all my tests are moot.

I deem it insane that all 25 pins on the module's plug were wired to the internal 246 card inside. What the ---- were they doing there? I worry someone had heavily modified the old 246 as an aftermarket non-Tennelec item and that the 33 ground was a very deliberate act.

The big upper and lower pins are merely guide pins and serve no other real purpose. (prevent misalignment and breakage of the tiny pins)
I have seen these guide pins reversed on some modules designed for a custom NIM rack filled with custom modules. Such modules will not mate with a normal NIM rack as the males will jam together and not allow the module to proceed to the plug. Those original NIM spec designers seem to have thought of everything!

[Dennis P Brown]

The new TC 246 arrived today; it has just five pins on its connector and no pin located at position #33, either . I placed this unit in the NIM box and did a power check and, of course, nothing at all happened. Which, is for me right now, a good thing.

Also, the pulse generator arrived (later, power loss around here even hit the Post Office. I'm running on a house generator now for a second day.)

As an extreme novice in this area of knowledge, all I can say is I think (dangerous for a novice) I know enough basic stuff to perform the most simple of tests. Hope this isn't too painful for the pro's here.

I first tested the NIM Tennelec 525 rate counter (counts/sec) and it appeared to agree with the pulse generator so that unit seems to be working. Determined its threshold to trip is about 1.0 volt. Not sure if that is normal or an issue. The unit does follow (i.e. counted) 10 Hz to 10,000 Hz accurately on its display scale (using the various base 10 log settings.)

The 246 I am rather ignorant of how to use it. I did apply 50 mV to the input (@ 10000 Hz) and it amplified the output signals on the oscilloscope with each gain setting change. It managed to give 5 volts on its highest gain of 500. That value appears to be off but that might be me. Still, the 246, does appear to be roughly working.

Richard's two pages on the TC 246 that he posted certainly helps but I will need to google a number of terms. Guess when the service/operation manual arrives I can look into the 246's various features in greater detail. This was more a sanity check (of me) just as much as a test of whether these units operate at all.

At some point, I need to do a similar procedure with the pre-amp. That is more a back box to me and I will certainly be asking questions on that unit when I can get to it. This is the same unit Richard mentioned and I assembled (with 3 kV caps.) The board is commercial so definitely more a black box as far as how to properly adjust the amp - and that unit is extremely sensitive (and prone to failure if mis-handled.)

[Richard Hull]

I just did a FAQ on setting up an SCA as part of a NIM neutron detection system.

viewtopic.php?t=14550

Richard Hull

[Dennis P Brown]

That is a great addition - thank you Richard!

Of course, I never thought of a gamma source for initial testing. I would think my uranium ore could handle that aspect of the test - though, hardly 'hot'.

The diagram and instructions are very clear and useful. I made a printout as a reference for my 'lab' (along with the NIM pin maps.)

Providing the 'what and why' relative to the devices really does clarify the overall approch. The instructions on use of the oscilloscope is an excellent addition and extremely clear. Especially giving the threshold voltages/expectation values for the detector outputs and the oscilloscope settings is absolutely gold for novices (like me using this type of equipment.) Further, the discussion on the upper/lower 'window' control for the 426 and the principle of setting these controls is clear and explains its purpose and usefulness relative to the detectors.

This FAQ has answered a host of questions I either didn't think of or some I likely would have asked later as I struggled with these devices - really informative.

[Dennis P Brown]

A FYI; the Cremat company has a page on testing the touchy CR-110 pre-amp.

https://www.cremat.com/testing-a-csp/

This page assumes one has only the CR-110 'chip'. If one has their circuit board - that unit already has a test connector to interface with the function generator.

[Dennis P Brown]

A question on the TC 246 unit. On the side of the box are switches - do they only apply to their pre-amps or are there specific configurations these should be set for using any general pre-amp?

For instance, I am under the impression that all common coax connectors use 50 ohm. So, the out put should be set to 50 ohm? What use is 1 ohm, then?

Not at all sure what 'shaping' means.

I assume the input polarity is determined by the pre-amp's output signal type?

[Richard Hull]

It looks like the output impedance is good at 50 ohms.
You will need to seen what the output pulse polarity is on the preamp using the scope. perhaps you already know it from working with the CR-110
Is the preamp output pulse above or below the zero reference on the scope? The last user, looks as if they chose negative.
The other switch is basically letting you set the output shape and position relative to a baseline or not....
1. Bi-polar (positive and negative levels pass to output). 2. You can also choose unipolar, only outputs a pure positive pulse or negative pulse based on your setting in #1 3. within the unipolar selection you can choose not to have the BLR (Base Line Restorer) active (in) or to let the output pulse fall where it will with reference to your base line (out).

You should play with this module using the scope on the output and a pulse generator or pulser on the input to get a feel for what these switches do. Sadly, you have to slip the module in and out with each change of the switches.

As we are counting only here, I like to go for unipolar, BLR (in)

Have fun and discover much.

Richard Hull

[Dennis P Brown]

Thank you again; I realize that discovery is part of the learning experience but after reading both your supplied page (very useful) and the tech manual, those buzz words were still, well not fully clear. The BLR. was a complete mystery. I kinda deduced what pulse shape was (the oscilloscope makes that clear) but why impedance would be changed (kinda know why, now) was a rather strange switch to have but I didn't understand uni-polar or why one would want that feature - till your post.

I did read on the Cremat page that their CSA flip's the polarity of the input signal.

I do enjoy getting to use the oscilloscope but understanding the basics of the switches is extremely helpful.

[Richard Hull]

The use of the oscilloscope comes readily with use. The important thing is to get a presentation on the screen that gives the valuable information needed. The main things are to learn how to use the time base knob and channel amplitude knob to get the image contained on screen and to adjust the trigger control to stabilize or lock it in place. Single pulses or irregularly repeated pulses are flawlessly captured and frozen in time with a DSO scope. Virtually 100% of all modern "lunch box scopes" are DSOs. Back in the day a Good DSO Tek scope hit the $5000 price range. A dual channel Saelig of 60 mhz can be had new for under $400 all of these are a DSO. I have seen used ones at hamfests go for $200.

Richard Hull

[Dennis P Brown]

Decided to test the CSA since the TC 426 appeared fine. The CSA appears to work correctly (per the company's overall instructions - apply 1 KHz, about 1 volt pulse to the 'test' connector (not yet marked on my preamp.))

The dual scope mode does appear to match their image but I should have off-set the two signals a bit more. The signal from the pre-amp is from the CR-150's output connection (marked Signal.) The generator pulse input is to the left of that coax connector (unmarked on the preamp box.)

From that source (CSA output sig.) I'm getting the initial response rise from the pre-amp and then a slow decay on the scope. They show it going to zero but I'm no where close to that - maybe my cap is too large(?) I noticed that there really isn't exactly significant amplification (about a 2x factor) by the CSA but it does match up roughly to what the company's pic/value displays (again, minus the decay to zero.)

Guess any significant amplification isn't the primary purpose of a CSA? I was expecting ...well, something more significant, I guess. But then, what do I know about preamps using a CSA?

I 'grounded' each channel to establish a common baseline. No idea if that matters in any way. Playing with the oscilloscope I set the "DC" mode for each channel as well the "AC" mode but both look identical - so, no idea what the significance of those two settings have on the channel/scope relative to this type of test.

Aside: the pulse current that is shown in the company's pic is not displayed on a scope but apparently, they added that as a FYI to their display picture to let you know that the CSA provides a 'current' spike when it see's a signal.

The scope settings were according to the mfg. for testing the CSA. The CSA chip (110) is mounted on their board (CR-150 and enclosed in a metal box with appropriate connectors.) The pulse generator is on top of the oscilloscope and was rather cheap ($30 w/shipping.) It works fine for testing the TC 426, rate meter, and this CSA.

Since I'm still missing a second high voltage cable, using a real detector and with my barely hot ore sample source, may have to wait. Also, I guess I could use the fusor (and may have to) instead of the ore but it is rather short on gamma's but i'd think, could provide some neutrons (33 kV @ 30 ma and 5-8 microns D2).

I might be able to make the extra HV cable I need but that is a bit challenging relative to my skills unless I figure out a work around that doesn't require adding another connector end

[Dennis P Brown]

I decided to look at the TC426 amp and see what the various switches on the side do - only the Shaping appears to make a significant difference. The input polarity just reverses the spike direction; the impedance has no effect.

As can be seen in the oscilloscope traces, the shaping switch's two settings change the response pulse from a single spike (unipolar) to a dual response (Bipolar).

The TC426 is set to a gain of just ten. The input pulse is 0.1 volt p-p; again,1 kHz rate. Oscilloscope is 0.1 v/div.

I don't know how one gets along w/o a pulse generator when working with such equipment - and they are so cheap and easy to use now.

[Dennis P Brown]

After exploring these features on the TC 426, I tried the window mode (SCA input) but there is no output response when using that input/connection. Not sure if I should bother investigating that problem - I have a service manual and could trace some of the circuit but the unit does basically what I wanted it to do. The issue for me will be getting the detector, preamp and NIM system to work once I have enough HV cables. Whether I can detect neutrons is, apparently, still a remote issue at this point in time. I have no gamma source the fusor will be it when I get to that point.

Wondering if I should install lower cap (but not voltage rating) in the pre-amp? Maybe a faster decay is better?

[Richard Hull]

For working an SCA you need a very sharp pulse and not a nice square wave or even the classic NIM pulser.

You need a generator that can produce a sharp narrow pulse on the order of 1-2 microseconds width. 5us would do. To test you adjust its amplitude to 1 volt peak on a scope. Feed it to the SCA and set the lower limit to 000 and the upper limit to 000. As the upper limit is advanced you should watch the scope carefully and see the pulse come and go about the 100 point. (assumes 10 volt up/lower range. Some work on a 0-1 volt range.)

All detection pulses in radiation electronics are very fast short pulses. SCAs are built for this and tend not to do well or even work at all with wide fat pulses.

Some also allow for shaping time selection.

Richard Hull