SNAP-ON Automotive Oscilloscope MT 665

[jcrubin]

I had prepared a long list of tests conducted as we had discussed starting with the initial screw driver test , then the neon lamp test then onto various components,   luckily, I may be able to spare you all of the data ive gathered.

In the end ive discovered that the neon lamp test DOES INDEED WORK. Holding the neon near the flyback will cause it to glow.     But only when the doubler/tripler is disconnected from the flyback.   It would seem as though from what ive observed, connecting this component inhibits the operation of the flyback. 

Having now understood this.   Can I now confirm that this device is shorted? as opposed to shorted at a given voltage (arc)
What is this device (doubler, tripler)
Where can i get a replacement

[james_s]

I think it's extremely likely that is the case. I suspect it's a doubler, it's hard to say for sure without xraying it. I'm not sure if there are any off the shelf parts that will work, but it's pretty easy to build one. Some diodes like this ought to work http://www.ebay.com/itm/4-pcs-2CLG-30KV-10mA-High-Voltage-Diode-HV-Rectifier-Tesla-Ham-/281294010312?
These capacitors ought to do http://www.ebay.com/itm/5pcs-10KV-1000PF-102-High-Voltage-Ceramic-Capacitor-/161045380741?  You can put a few in parallel if needed. The second capacitor in the doubler is formed by the CRT itself. Once you have something that works, you probably want to pot it in epoxy to prevent corona and arcing. Put it in a small plastic project box with holes drilled just big enough for the wires to go through and fill it with resin.

[jcrubin]

hmmmm, Id hate to think my foray into this would be hand-jamming a doubler-trippler together without solid specification.   Given that the numbers are clearly identifiable on this particular part, is there a forum, perhaps, of the video game variety that you might know of where someone might know that particular part and a suitable replacement.  Also,  as a short is suspected, can it be confirmed, or is it possible that it is only occurring at a high enough voltage to arc across.   should i be testing from + to grn or HV to + or HV to grn?   

[james_s]

It is a custom part made for the manufacture of the scope, the part number on it is not going to be of any help anymore since it is long out of production. The Atari vector monitors were built by Electrohome and Wells-Gardner and have not been produced in 35 years. All three companies are long gone and the original doubler was never available during the time I was heavily involved in that hobby starting in the late 90s. I built several replacements just as I described above including the one in my own Asteroids which is still working fine more than 15 years later. Nobody is producing CRTs anymore at all, so nobody is building the ancillary parts either and the AC output transformer with external doubler was a 1970s thing prior to the development of "modern" vacuum potted internally rectified flyback transformers.

A voltage doubler is really not a critical part, a very wide range of component values will work fine. You're not going to be able to buy an off the shelf part that will work here unless you manage to find a 30 year old spare that is close enough. You're going to have to build something yourself.

[jcrubin]

I guess were building a doubler, and if need be, enough parts in stock for a tripler......


Again.  If there a way to prove a short in the existing unit?

[Johnny10]

All those pretty pictures.
Looks to me like a good source of parts for a tube amplifier.
Orange drops, Sprague Atoms, 1/2 watt and higher resistors!

I was going to buy one of these off Craigslist but hey, I bought a used Snap-On Verus.

More money, less fun!

[james_s]

I don't see much there that is any use for a tube amplifier. There are some decent quality capacitors but probably not in the values you'd need. Equally good quality capacitors are still readily available brand new, as well as good medium power resistors that have not drifted around in value. It would be pointless to harvest a few dollars worth of old parts when you'd already be spending hundreds for a set of decent transformers and tubes.

[Johnny10]

That was sort of tongue in cheek.

The look of the units circuit boards reminds me of the 70's era stuff I collect.
In the vintage tube amp restores orange drops are still used in re-capping old boards.
You are right, all those caps are long in the tooth and as you said resistors have drifted.
It looks to me as though they used quality parts.

I am sure this is a serious undertaking.

[jcrubin]

Yeah so anyway......

Ive ordered the requisite components as you specified and i will put together the unit for testing,  at that point ill get it back in the unit and if we can get a good result, pot it with some sort of epoxy that I image we will discuss when that appropriate.  I imagine in the mean time, there would need to be physical separation of components during the testing phase to ensure that the was no arching in the doubler during testing.

[jcrubin]

Now ebay is just out to screw with me........

eBay auction: #http://www.ebay.com/itm/HP-AGILENT-1740A-VARO-MH924A03-CRT-HIGH-VOLTAGE-MULTIPLIER-/232009615217?hash=item3604d9c771:g:znYAAOSwyDxXg8Hg



But look what Ive dug up......   The one from the device is stamped MH919

from https://archive.org/stream/VaroSemiconductorSiliconRectifiersAndBridges1981-82#page/n161/mode/2up


















and this one from another datasheet lists it as a tripler with specs, but mine doesnt have a ficus connection, the only ones missing a focus connection are doublers, model MH911, clearly mine is labels as 919 though.  perhaps this was built to the specs of snapon.



its schematic shows no resistor but a non existing focus

[james_s]

Hey check that out, I've never seen a datasheet for one of those. That's a little different than the doublers I'm familiar with but you'll have the parts either way. You might tack solder a doubler together and try that, measure the HV at the tube if you have a HV probe, otherwise look at the picture size and focus. If the image is too large and blurry then you probably need to add another stage to make it a tripler.

[jcrubin]

I just wanted to provide a quick update.

The parts started to come in, it was in 3 deliveries, unfortunately by Saturday at noon only two of the required 3 had arrived.  Not having enough components to do the onsite construction of the tripler we decided to push it out to next weekend.  On that note, Once we have a working circuit, hopefully, will fiberglass resin be sufficient to encapsulate it in a project box?   Should it be slowset to allow as many air bubbles out as possible?

[james_s]

That ought to work, or just some ordinary hardware store epoxy. The slower the set, the less air bubbles. If you have a vacuum pump, even one of those hand operated things for automotive work you can make a crude vacuum jar for potting things but it's not really necessary. Especially if you warm the resin a bit to make it thin.

[jcrubin]

Guess what showed up?

[jcrubin]

Today was a busy day,

We used the plastic case of a VW control module and door clamps to create a non metallic shell to house the new unit on the flyback housing.  I built up a doubler, then a trippler, which gave out the HV needed to drive the CRT!!

We were about to get a picture up and running, however the horizontal sweep board was bad...   the horiz/vertical driver was working though so I was happy about this.  When the horizontal sweep is fixed and im happy with the full deflection we will pot the trippler.

I drove the horizontal sweep with some low voltage DC, not optimal but worked for a test



Build start



Full Trippler



Picture

[Paul Moir]

Awesome!

Personally I wouldn't bother potting it at all.  If worried, I might coat it in silicone but that would be about it.  There's special electronics silicone that doesn't produce acedic acid when it cures, but if you cheat make sure it's completely cured before applying the HV again.
I'd be worried using polyester resin;  the heat, stresses and eventual cracking I think make it a pretty lousy potting compound.
 

[james_s]

Looks great! 

I love it when a plan comes together. 

[jcrubin]

Greetings, as id mentioned , Id forgone any potting until id gotten the horiz trigger sorted out.   This didnt take a whole lot of work to fix the failed circuitry and eventually I was able to make proper repairs as shown below.  With that behind me the final potting was completed.  The project has come far enough that Ive set the release cycle of the videos for all of this to start appearing on youtube just after my IT-1121 project, so that would be one month from now.

 


Showing a trace! Horizontal fixed







Mastic to plug holes

[maginnovision]

I know this isn't usually a popular question, but can I ask why you'd go through the trouble? Or why the owner of the equipment would? It's fairly ancient and much better tools have come about to replace it. Just a show piece? They aren't even all that rare, despite the age, because they're tough to get rid of. I've seen many shops for sale with one or more jammed in storage or some other place. Anyway looks like a fun project just curious how it happened.

[alsetalokin4017]

Nice work! But... why in the world would you need an oscilloscope to fix an old-style VW ?

(1967 factory 12V Type II (Bus) owner for 17 years, drove it over 350,000 miles, drove it in Death Valley and over Independence Pass in the Rockies, always did my own work, something like 7 complete engine overhauls, 1640cc, single-port heads, Solex 28 carb, and mech. advance Porsche distributor and Bosch blue coil, same distributor and coil the entire time)

[JPortici]

I know this isn't usually a popular question, but can I ask why you'd go through the trouble?

didn't you see? it has BLUE PHOSPHOR 

[jcrubin]

I know this isn't usually a popular question, but can I ask why you'd go through the trouble? Or why the owner of the equipment would? It's fairly ancient and much better tools have come about to replace it. Just a show piece? They aren't even all that rare, despite the age, because they're tough to get rid of. I've seen many shops for sale with one or more jammed in storage or some other place. Anyway looks like a fun project just curious how it happened.

I usually tear folks up for post project success trolling, but since this is only your second thread contribution on the whole eevblog forum, ill dial it back a notch.... this once.

1.  When you start a post with "I know this isn't usually a popular question" it really means, "I'm gonna stir the shitpot."  We all know this.  But for you i'm going to entertain this
2.  There are tens of thousands of projects on this forum which may or may not defy conventional wisdom, you ask a lot of questions here for not being a contributor to anything of value, i checked.
3.  My justifications are my own and don't require your validation.
4.  You make bold statements and arrive at your own conclusions based on them
6.  The fact that the date codes were supplied in the thread and you pointing out that its ancient, only lends in your ability to point out not only the obvious, but the readily supplied.
7.  Your statement on this devices disposition, model, make, and most importantly value , shows you are presumptuous, and shallow
8.  Id encourage you to do a lot more reading, before writing in this forum on any thread for the foreseeable future.

But yeah, fun project, thanks for asking.

[maginnovision]

I know this isn't usually a popular question, but can I ask why you'd go through the trouble? Or why the owner of the equipment would? It's fairly ancient and much better tools have come about to replace it. Just a show piece? They aren't even all that rare, despite the age, because they're tough to get rid of. I've seen many shops for sale with one or more jammed in storage or some other place. Anyway looks like a fun project just curious how it happened.

I usually tear folks up for post project success trolling, but since this is only your second thread contribution on the whole eevblog forum, ill dial it back a notch.... this once.

1.  When you start a post with "I know this isn't usually a popular question" it really means, "I'm gonna stir the shitpot."  We all know this.  But for you i'm going to entertain this
2.  There are tens of thousands of projects on this forum which may or may not defy conventional wisdom, you ask a lot of questions here for not being a contributor to anything of value, i checked.
3.  My justifications are my own and don't require your validation.
4.  You make bold statements and arrive at your own conclusions based on them
6.  The fact that the date codes were supplied in the thread and you pointing out that its ancient, only lends in your ability to point out not only the obvious, but the readily supplied.
7.  Your statement on this devices disposition, model, make, and most importantly value , shows you are presumptuous, and shallow
8.  Id encourage you to do a lot more reading, before writing in this forum on any thread for the foreseeable future.

But yeah, fun project, thanks for asking.

I really was legitimately curious, but I guess we can forget about it.

EDIT: I do want to add, I helped a friend a couple years ago fix his own, few years older model, when he bought a shop. I worked in the automotive field until I retired, and I routinely put more money and time into projects than I could ever rationally justify.

[Johnny10]

Anyway looks like a fun project just curious how it happened.

I think maginnovision had a valid question. How did it happen that you picked this project?
Something I ask myself and others quite often.
Why spend hours on equipment that has seen its day?

My answer is always, because, I need to know how it works!

[jh15]

* jh15 throws cold water on the fighting dogs.