'Improved' Mystery Switch on the 576 and 577 Curve Tracers


 

READ AT YOUR OWN RISK

Some 576 and 577 curve tracers were shipped with what I call a MYSTERY
switch because it is not described anywhere in any service manual although
it is clearly visible in the photographs of the Standard Test Fixture on the
576 and on the 177. It doesn't take long to figure out that the purpose of
this little red push button is to bypass the safety interlock that requires
you place transistors in the much-maligned safety cage with the lid closed
when voltages greater than 25V are present on the leads.



If you have one of these Standard Test Fixtures without a MYSTERY button it
is because the Tek Liability Lawyers eventually got wind of this switch and
had it removed.



Few people realize that a curve tracer is not just for testing
semiconductors. I frequently test parts of all kinds beyond 25V and find it
counter-productive to have to hold my finger on the mystery button while I
try to take a picture of the resulting curves with my other hand. And I am
fortunate enough to own one of the safety cages. There are many of you who
do not own one.



As a 'professional' and an adult I am willing to assume certain risks. So
after struggling with this arrangement for 10+ years it finally dawned on me
that the 'ideal' solution to this problem, for ME, was to replace the push
button with a single pole toggle switch that is center off, momentary in one
direction, and latching in the other direction. I believe this goes by the
nomenclature (ON)-OFF-ON or MOM-OFF-ON. C&K makes them (part number 7107) as
do many other companies. In the US Digi-Key sells the C&K 7107 for $5.13
(part number CKN1028-ND).



Now I can use the momentary position as a direct substitute for the push
button in most cases, and when I need to be hands free I can flip the switch
to the other position as long as I remember to flip it back to the center
off position when I am done. And when I forget to flip it back the curve
tracer will usually rudely remind me of my mistake.



Dennis Tillman W7PF


Craig Sawyers <c.sawyers@...>
 

Some 576 and 577 curve tracers were shipped with what I call a MYSTERY switch because it is not
described anywhere in any service manual although it is clearly visible in the photographs of the
Standard Test Fixture on the
576 and on the 177. It doesn't take long to figure out that the purpose of this little red push
button is
to bypass the safety interlock that requires you place transistors in the much-maligned safety
cage
with the lid closed when voltages greater than 25V are present on the leads.
My 576 and 577 does not have a switch - just a red lamp. And my 576 is an early one - date codes
1969.

As a 'professional' and an adult I am willing to assume certain risks.
Yup. I simply bypassed the interlocks with a soldered-in wire to short it out on both units. After
all the 575 (and 570) didn't have any of that interlock stuff - it relied on the fact that these are
machines to be driven by skilled users familiar with working with stuff that can bite back.

I'm not suggesting this route for everyone - just that this was my choice.

Craig


Ed Breya
 

I put key switches on my CT fixtures to only authorize me to bypass the interlock - I felt it was important back when there were kids running around, just in case.

You definitely have to be careful without the safety cover. I've never had one, and figured it would just be in the way anyway, but it is scary sometimes. I normally leave the key in, but always turn it off and crank the collector supply down to the lowest range and highest resistance before shutting the unit down - it's habit now - to also prevent damaging a DUT or myself the next time up.

Ed


Ed Breya
 

Another option just occurred to me - you could make it a little more complicated but safer by adding a holding relay circuit that would be set with a momentary contact, enabling bypass. Then when the session is over and the unit is shut down, the relay would be de-energized on next power-up, unless the bypass is requested again. This is by no means foolproof, but it would prevent powering up in a dangerous state. The same three-state switch action, with one side momentary, could be used - SET-HOLD-RESET.

Ed


 

Hi Ed,
That is a very nice improvement on my idea. There is DC power on the 177
Standard Test Fixture 'front porch' that could be used for the latching
relay. I do not know about power availability on the 576 Standard Test
Fixture.

Dennis Tillman W7PF

-----Original Message-----
Sent: Wednesday, December 14, 2016 7:51 PM
Subject: [TekScopes] Re: 'Improved' Mystery Switch on the 576 and 577 Curve
Tracers

Another option just occurred to me - you could make it a little more
complicated but safer by adding a holding relay circuit that would be set
with a momentary contact, enabling bypass. Then when the session is over and
the unit is shut down, the relay would be de-energized on next power-up,
unless the bypass is requested again. This is by no means foolproof, but it
would prevent powering up in a dangerous state. The same three-state switch
action, with one side momentary, could be used - SET-HOLD-RESET.

Ed
------------------------------------
Posted by: edbreya@yahoo.com
------------------------------------


Craig Sawyers <c.sawyers@...>
 

and crank the collector supply down to the lowest range and highest resistance before shutting the
unit down - it's habit now - to also prevent damaging a DUT or myself the next time up.

Ed
Absolutely. Just like setting sensitive meters to the highest voltage range, and voltage sources to
the lowest range. Should be second nature.

Craig


Craig Sawyers <c.sawyers@...>
 

I do not know about power availability on the 576
Standard Test Fixture.

Dennis Tillman W7PF
The 576 fixture is entirely passive switching, so no DC power comes in. Unlike the 577 fixture which
has DC power and active circuitry.

Craig


Ed Breya
 

Craig wrote:
"> and crank the collector supply down to the lowest range and highest resistance before shutting the
> unit down - it's habit now - to also prevent damaging a DUT or myself the next time up.
>
> Ed

Absolutely. Just like setting sensitive meters to the highest voltage range, and voltage sources to
the lowest range. Should be second nature."

Yes indeed - don't leave the states in a bad spot for next time. In the case of curve tracers, I don't set everything to zero. To avoid CRT issues, before shutdown, I normally leave the collector supply at a few volts, and set the horizontal to get at least a few divisions-long line, at medium-low intensity, and de-focused. You don't want a bright spot at an origin on power-up, unless you're attending to it right away.

If I'm looking at RF components with low breakdown voltage, I set it to zero or up to about a volt, and double-check the polarities and series R, before any DUT is applied, and always keep the beam de-focused until I get a big enough picture, then adjust the focus and intensity for good detail.

Ed


Craig Sawyers <c.sawyers@...>
 

In the case of curve tracers, I don't set
everything to zero. To avoid CRT issues, before shutdown, I normally leave the collector supply at
a
few volts, and set the horizontal to get at least a few divisions-long line, at medium-low
intensity, and
de-focused. You don't want a bright spot at an origin on power-up, unless you're attending to it
right
away.
Absolutely. It is the way of curve tracers that you can just get a spot if you turn the collector
voltage to zero. One of my 575's has a couple of burn spots as a result of a previous owner leaving
a high intensity spot at or near the screen origin.

Craig


Ed Breya
 

I think the 576 mainframe supplies are actually available, even though the standard fixture doesn't use them. I don't have access to my paper manual here, but looking at a pdf version, it looks like they are on the main connection J/P360, pins 18-22. This is on page <diamond> 7. The fixture diagram <diamond> 6 I have is a broken up schematic, and seems to have a chunk or maybe a whole page missing - it doesn't show the high voltage warning lamp or the interlock switch circuits.

So, the pins are there on the mainframe side, and as I recall, the pins on the fixture side are present and accessible within the unit, just not connected. The exception is some blank (removed) pins adjacent the collector supply pins, on both sides, to increase the voltage standoff. Likewise, there are some blank pins on J/P361 (not used on standard fixture) for more isolation from the line power available there.

Ed


Craig Sawyers <c.sawyers@...>
 

I think the 576 mainframe supplies are actually available, even though the standard fixture
doesn't use
them. I don't have access to my paper manual here, but looking at a pdf version, it looks like
they are
on the main connection J/P360, pins 18-22. This is on page <diamond> 7.
Ed
You're right Ed!

Pin 23 GND
Pin 22 +12.5V
Pin 21 -12.5V
Pin 20 +100V
Pin 19 -75V
Pin 18 +5V

Craig


Ed Breya
 

A simple way to implement the function, using the simplest SPDT toggle switch with center-off and momentary action in one direction, could be done with a small DPDT relay with 5V coil, a resistor, and a diode.

For the 576 fixture, pick either plus or minus 12.5V - whichever seems to be the least heavily loaded - to run the circuit. The relay should be a small type, to minimize coil current. Another option may be to use a reed type with two Form A (NO) contacts - the NCs aren't needed anyway. The switch pole would go to ground, and the momentary throw would go to the coil. One NO contact set would parallel the switch contacts to provide the holding function. The other end of the coil would go to the supply via a dropping resistor selected to give the right coil voltage when on. The other switch contact would go to this same coil node, to short it to ground for reset. The diode would go across the coil for kickback suppression, in the appropriate polarity. The other relay contact set then could be used as the bypass.

So, on power up, everything is off. If the switch is pushed to set, the relay energizes and holds, and the switch can return to the stable center position. If the switch is flipped to reset, or left there, it dumps the coil, and draws the supply voltage over the dropping resistor - more than the normal coil current, but not too much. The resistor will drop 0, 7.5V, or 12.5V, depending on the state. That's why you want the lightest coil power that will get the job done.

Ed


Ulf Kylenfall
 

I payed little attention to my once dead now refurbished 576 (The HV transformer...) until I read an article on the net that this instrument could be used for vacuum tube testing.


There are several articles on that subject on the net.


So I will have to find a way to safely bypass the switch since my 576 also lacks the cover...


Ulf
SM6GXV


 

Hi Ulf,
Stand by for a big surprise that changes everything. I am putting the
finishing touches on a very different vacuum tube curve tracer design
concept that works with the 575, 576, and 577. It has many advantages over
what has been done so far. I will have all the details ready in about 2
weeks.

Dennis Tillman W7PF

-----Original Message-----
From: TekScopes@yahoogroups.com [mailto:TekScopes@yahoogroups.com]
Sent: Thursday, December 15, 2016 10:35 AM
To: TekScopes@yahoogroups.com
Subject: [TekScopes] Re: 'Improved' Mystery Switch on the 576 and 577 Curve
Tracers

I payed little attention to my once dead now refurbished 576 (The HV
transformer...) until I read an article on the net that this instrument
could be used for vacuum tube testing.

There are several articles on that subject on the net.

So I will have to find a way to safely bypass the switch since my 576 also
lacks the cover...
Ulf
SM6GXV
------------------------------------
Posted by: ulf_r_k@yahoo.com
------------------------------------


Ed Breya
 

Yes, there's a lot out there about tube testing with the 576. I embarked on a project a while back, to make a tube tester fixture for the 576, using the carcass of a type 178 IC test fixture that normally goes in a 577. It can be made to fit mechanically, but took a lot of tweaking. The worst part is the back panel that carries the big blue Cinch connectors. By the time I got done milling out enough stuff to mount the connectors, it was like Swiss cheese, and needs extra hardware to finish up. The 178 already had a big cutout for its entirely different connector scheme, and I had to add P361 to pick up line power and some readout access.

I'd recommend the best way to go is to put all the tubey stuff in a separate box as big as necessary, with cabling to hook up to the 576 - you really only need the E-B-C terminals, and maybe the sense terminals if high currents are needed. The box would need heater and screen supplies, and an amplifier for the grid drive. Make it big enough to hold all the socket types you may want, and whatever setup system (switches, plug boards, etc) is needed to program the tube type. It's also good to have a meter built in for various measurements and settings.

Ed


Kevin Oconnor
 

It is interesting to hear of the "hobby" interest in safely using the high voltage ranges on tracers who the DUT safety cover. It is commendable since the current is lethal. Professionally, in 25 years at a well known US industrial R&D laboratory now based in Italy, I encountered many Tek tracers. Seldom would there be a cover. But I could generally count on finding a wooden Q-tip jammed into the little micro switch hole! That permanently solved the ambiguity.
I always assumed there could be lethal voltages present on curve tracers till I determined otherwise. I also learned early in my education and from an old GM welding electrician, keep one hand in pocket until you deactivate the lethal voltage.


 

Hi kjo,
It is one thing if I are using a curve tracer that I own and have complete control over. If I make a mistake and leave it with high voltage on the collector and I get bitten then that is my fault and I have no one else to blame but myself. I speak from personal experience on this one.

But it is entirely different if someone else has created a dangerous situation for you and your coworkers by jamming a Q-tip in the interlock and walking away without removing it or at least placing a warning sign on the curve tracer. That is irresponsible. Someone could be seriously hurt by their carelessness. Management needs to be made aware of the danger to everyone that this creates so they can at least educate the employees to remove the Q-tip when they are done. It is the least they can do.

Dennis Tillman W7PF

-----Original Message-----
Sent: Sunday, December 18, 2016 9:43 AM
Subject: [TekScopes] Re: 'Improved' Mystery Switch on the 576 and 577 Curve Tracers

It is interesting to hear of the "hobby" interest in safely using the high voltage ranges on tracers who the DUT safety cover. It is commendable since the current is lethal. Professionally, in 25 years at a well known US industrial R&D laboratory now based in Italy, I encountered many Tek tracers. Seldom would there be a cover. But I could generally count on finding a wooden Q-tip jammed into the little micro switch hole! That permanently solved the ambiguity.
I always assumed there could be lethal voltages present on curve tracers till I determined otherwise. I also learned early in my education and from an old GM welding electrician, keep one hand in pocket until you deactivate the lethal voltage.

------------------------------------
Posted by: kjo@cryden.com
------------------------------------


Kevin Oconnor
 

Hi Dennis,
Yea, I agree with u fully. Didn't mean to imply I condoned it. Problem was you would eventually learn those sticks would frequently break off and get stuck. I never trusted the switch or warning lamp. I always rotated the collector supply to its CCW stop before anything else!

Sent from kjo iPhone