Possible replacement for my now partly failed ST-360 Counter

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Dennis P Brown
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Possible replacement for my now partly failed ST-360 Counter

Post by Dennis P Brown » Sun Feb 26, 2017 7:49 pm

Well, I know I was pushing the scintillation input port on the ST-360 beyond its max voltage and as a result that circuit is now inoperative. Digital electrons like that isn't forgiving and repair is highly unlikely. I should have taken the time to add a voltage divider but I was in too much of a rush to get my first neutron detection; then since it worked, too concerned with other matters to stop and go back and make the critical fix ... and now, I've learned another important lesson ...learning the hard way is something one faces when they know better but ignore that. Aside: I will try to look into getting repair manuals or just a circuit diagram and see if I can trace out any issues that don't require specialize chips. Not hopeful, however. (at least the Geiger side of the ST-360 still works.)

That all said, I have repaired my old, surplus Ludlum Geiger counter (the unit has few micro-chips(!) and it was just a few bad voltage regulators to replace and readjust the voltage output.) Since that device is also a counter (using an analog scale, of course) then it could be modified to other tubes. Of course, this device also reads at a lower voltage (compared to my neutron tube of 1.99 kV) so this time I will make a divider (I will use a 50 M-ohm and 15 M-ohm set of resisters to keep current draw to a min) to protect the unit (so it only see's 600 volts (it can handle over 700 volts.) Then I just need to connect the neutron detector's output co-axial and I'd think the Ludlum should follow the neutron tube's detection (these tubes work like Geiger tubes but have a higher voltage.) I do realize the two tube types will not cross calibrate.

That all said, will this work or am I trying to do something the unit can't handle? I could use the Geiger side of the ST-360 but like the features the Ludlum offers.

Another aside: tried silver activation and while I had possible results, they were too much 'in the noise' to believe. Even at 32 kV and 50 ma, my system in three minutes does not produce enough neutrons. My chamber gets far too hot to make that aggressive of a run any longer (even with an aux. fan I added to the fusor chamber) and I think I need to water cool the unit. Since that will require my removing the fusor from the support case, I will also paint the support case, too. Might as well do some cosmetic chores as I do practical ones ... .

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Dennis P Brown
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Re: Possible replacement for my now partly failed ST-360 Cou

Post by Dennis P Brown » Mon Feb 27, 2017 6:18 pm

Well, first off, I see an issue with using the Ludlum for what I just considered after thinking just a little (lol) about my approach - inputting a signal at the same voltage scale (i.e. the unit produces an excitation voltage!) to the Ludlum is asking for issues so that is a method best abandoned ... . Instead, I will just use the ST-360 with the still working Geiger counter input connector - that can be zeroed out (voltage wise. ) Then I can try and see if that can be used as a counter. The divider will allow a safe voltage level signal.

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Re: Possible replacement for my now partly failed ST-360 Cou

Post by dario_tortato » Mon Feb 27, 2017 10:17 pm

I insert the manual with schematic if you can serve:
ST360_Operations_Manual.pdf
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Re: Possible replacement for my now partly failed ST-360 Cou

Post by Rex Allers » Mon Feb 27, 2017 10:53 pm

Dario,

Yesterday, I was going to suggest that manual for schematics but then I looked at it. The main schematic in that particular (ST360) manual is rather low resolution and has all the labels shown as blank squares. Compared to trying to work out the circuit by just reverse engineering the board, I guess it has some advantages in describing the basic connections. But when I took a quick look at the "censored" schematic in the manual, I couldn't even spot where the input section was.

There are many other ST manuals available on their site (www.spectrumtechniques.com) that have complete schematics. Several years back I got some ideas about how things might be done by looking at the schematics of some of their devices. I applaud the rare companies, these days, that share schematics of their products. I knew that this company was one of them, so I was surprised when I looked at the ST360 and found that particular schematic was only a low-res, blanked-out version.
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Re: Possible replacement for my now partly failed ST-360 Cou

Post by Richard Hull » Tue Feb 28, 2017 1:20 pm

I, too, once had an issue with an Ortec, NIM, digital rate meter/timer/counter. I picked it up in warranted, not-working condition. Alas, even armed with the schematic, the issue led to one of those classic, early 8035 pre-programmed 40 pin IC controllers so common in the 70's and 80's. Fortunately, Ortec still had a supply of these factory programmed controllers and for 40 bucks I slipped in the new chip and this module now serves as my main counter/timer in the fusor IV rack of electronics. Typical older NIM modules and GM counters are easily repaired as a lot of micro controller useage had not come to the fore and common components will fix the bulk of them.

Micro controllers offer tremendous complexity reduction and vastly expanded capabilities that once required hundreds of discrete components to achieve. However, this gleaming sword of wonderment is double edged and cuts both ways. An obsolete, no longer manufactured, microcontroller from an IC company, with its secondary application, OEM manufacturer's, custom software can forever leave an entire, expensive device, worthless. Even with a correct NOS IC, the custom software to load it may be dust if the device manufacturer is now defunct.

It seems much of the classic repair effort is rapidly dying out for many types of electronics. In consumer electronics, it is often cheaper to replace an entire item than to seek repair which may or may not even be possible. This is especially true for short run, bargain, Chicom electronics. (Stereo, audio, phone, toy and flatscreen monitors and TVs)

As a personal creator of nuclear electronics, myself, the use of micro controllers is the only way to offer a product that can be even slightly competetive. The key is "rolling the dice" to figure out which micro is most likely to be about the longest and to stock up on the inexpensive controller chips I am currently using. The world, today, can turn on a dime.

In the expensive nuclear instrument market, due to limited demand, a manufacturer can go under in a heart beat and if they used custom programmed IC controllers, you might be out of luck. Something to think about.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Re: Possible replacement for my now partly failed ST-360 Cou

Post by Rex Allers » Wed Mar 01, 2017 2:38 am

Richard, I hear and agree with what you say about the trends in electronics, but the ST stuff, as referenced in this thread, is generally pretty accessible. Most of it is pretty straight forward and sensible. They like lots of op amps and the digital sections are usually pretty old school. Other than the poor schematic in the manual for this particular device, it should be quite fixable for someone with pretty good electronics skills.

Other ST devices have good schematics in the available manuals on their site. Too bad the one referenced doesn't. Since it is likely the problem is in the front-end stages, maybe comparing the circuit from an older unit like the ST-350 and looking for matching patterns in the 'sort-of' schematic of the ST-360 might guide one who is of the electronic-fixer bent.
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Re: Possible replacement for my now partly failed ST-360 Cou

Post by Richard Hull » Wed Mar 01, 2017 10:25 am

Most suseptable in proportional circuit front-ends where HV is used are FET discretes or FET opamps. Some of this stuff along with the back to back switching diodes blow if the front end high voltage blocking cap fails or is discharged rapidly. Carl Willis noted this in an ancient posting on this issue. His sage admonition......"never apply the high voltage bias or remove it by switching the HV supply on and off........always begin at zero volts and slowly bring up the HV bias to its rated value, likewise, when shuting down, bring the bias back down to zero slowly." I have done this for years and have never blown a sensitive FET front end in my Princeton Gamma Tech proportional, current sensitive, pre-amp.

Unfortunately, many manufacturers of specialized gear love to either have custom house numbered components (tektronix is famous for this) or grind off the common number so as to keep repairs "in house".

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Dennis P Brown
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Re: Possible replacement for my now partly failed ST-360 Cou

Post by Dennis P Brown » Wed Mar 01, 2017 10:15 pm

First off, again, all very useful reply's!

I too am disappointed that they have no schematic for my unit - ugh! But I will be forced to look at other (rather different units) and try to figure out some common points; of course, I will trace my current circuit and try to locate anything that appears it might be worth replacing. I am fairly sure they have a pre-amp (they do in other scintillation circuits; good call there Richard! I too think that is a good starting point) and as such, if I can identify that (if it exists; then, see if I can get one and just replace it), then if that is the failure point, done. I too, now that I found those critters in other unit's schematics consider that the least likely part to handle an over-voltage (that I did ...not smart) so that is the best candidate to do a replace and see method. My oscilloscope's trace function is dead (I am really having some luck with digital devices of late) so doing a real trace of the circuit isn't gonna happen now - DMM only.

The first line Cap in the ST appears ok but I should pull it and test it in isolation (and put a 1200 volt charge on it, too; might as well see if it really is working!) Then I guess I will go from there and decide what does what in what sequence; not too hopeful since my experience is very, very limited. While I did fix my Ludlum meter, that wasn't very hard since it had an obvious problem with its HV (none!) - then just started replacing easy to obtain components in the HV circuit enabled an easy fix once the bad component was replace (third part ...lol.)

Well, I did, at least, build a shielded HV divider (about 88 mega-ohm total to ground (50 M & 33 M) so the signal will not be drawn down to significantly) and will install that in my system simply because if I do get the ST unit working again, I will need that anyway; but more to the point, I really should get that voltage for my Russian tube down to a safer scintillation voltage level that is common to those devices (around 1200 volts.) That way, if I do manage to find another useful unit (or even end up making one), it too won't be fired.

Aside: A common pre-amp they used in other units is a MC3405P (and for their HV systems) and these are readily available. That my circuit does not have - even one.

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Re: Possible replacement for my now partly failed ST-360 Cou

Post by Dennis P Brown » Thu Mar 02, 2017 1:32 pm

Well, the ST-360 device I have has essentially nothing in common with any of the older schematics that the company posted - even the one posted for the ST-350W; the chips are diverse and unrelated to those found in other units. Tracing the scintillation circuit is extremely difficult because it does not trace from the HV/sensor input to any obvious chip ... . Guess making a pre-amp & amp system, voltage comparator, and counter is in my near future (hey, at least I do have a power supply and working tube!) I have sent an email to the company to see if they will send me schematics for the ST-360 ... .

Aside: the schematic they have displayed (that has no component ID's or technical info) also is more a cartoon schematic - there are not huge number of chips on the real circuit board - so, they are also displaying the "equivalent" devices within a chip and providing the complete system rather then the real physical components. Talk about a confusing and unhelpful schematic!

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Re: Possible replacement for my now partly failed ST-360 Cou

Post by Dennis P Brown » Fri Mar 03, 2017 12:51 pm

The company sent me a real copy of the schematics for the ST-360; and yes, a number of single function chips are laid out with all their functional internal logic components but at least they are ID according to chip. I also note that the pre-amp/amp are on the first chip (their U15) inline with the high voltage cap/signal input from the scintillation tube and this, I'd think, is the most likely part in the chain that would go down under an over voltage condition; the Comparator is after this chip. I'll pull and check the cap's but even if they did go, can't see how that amplifier chip would survive. Hopefully, the chip ID (printed on it) is still available - likely since this is a fairly new device but who knows? Also, the GM tube uses the same comparator chip (U16) and I know the GM circuit/counter works fine so that confirms that section is ok. Since my previously working scintillation tube also now doesn't work on the unit, I am assuming the ST-360 is at fault and not my Russian neutron tube ... .

Aside: interesting that the GM circuit (Cap in line) output does directly to the comparator - no pre-amp or amp chip.

Aside: where do I post schematics for a specific radiation device like this?

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