Topics

Waking a slumbering 475


Weathers, W
 

Greetings all,

I picked up a Tek 475 with the DM44 option installed. It came with an
HP 10433A 10:1 probe marked "10MOhm // 10pF For 1MOhm // 10-16pF
inputs" (an apparent mismatch for the 475's 20pF input). No scope
manual was included with the sale, unfortunately. I know they can be
downloaded, but I'd like a printed copy. I have been perusing the
electronic copy.

The previous owner said the scope worked when he last used it. It has
been stored in a basement for a number of years. The P.O. had
recently powered it on to verify beam presence and trace on both
channels. I don't think much more has been done to it. And I don't
think it was powered on for very long.

First, a visual condition check:

The case looks to be in good condition. All knobs look to be present,
intact, and operable. The focus knob turns a little hesitantly
compared with the others, but that might be normal. A couple of the
Vertical Mode buttons show deterioration of the silk screen (the Add
and Chop buttons). The Delay Time Position knob is not a metal
graduated vernier knob but instead is an unmarked plastic knob. A
quick search shows similar knobs on some 475s out in the wild, so I
presume this is not the result of a replacement by owner.

The positive jack on the DM44 is loose.

One of the cord-wrap feet has damage to the lower-most cord channel.

There is some scuffing to the display that isn't noticeable when
looking at it head on, but can be seen at an angle. It's almost looks
as if somebody traced a noisy trace onto paper.

The manual/probe pouch smells a bit musty.

Before I do much with this 475, I thought I would check in here so
that I don't inadvertently rudely awaken it and let the smoke out.
How can I best gently wake it up? Are there best prophylactic
practices? I searched here but haven't turned up anything. I am sure
this information exists in the vast body of posts here,but I am having
trouble surfacing it.

This is the first oscilloscope that I have personally owned. I have
used both digital and analog scopes, the vast majority by Tek, though
I have never dug into one.

I am aware that these old scopes have tantalum capacitors that like to
short out. I am also aware that, in general, old electrolytics tend
to be dodgy. Do I need to throw this thing on a variac? And given
that this scope has been a basement queen, I am wondering if something
should be done to address the possibility of long term humidity
effects.

Scopes from the 475 era have a particular personal connection for me,
as my father was a Tektronix veteran. He was a technical writer for
Tek for 20+ years, spanning the 60s and 70s and into the very early
80s. He and his team produced the manuals for oscilloscopes and other
Tek products. If there are any old Tek employees from that era
reading this, yes, my Dad was Stormy.

Ward Weathers


keith@...
 

You can get replacement feet from a guy on eBay by the name of n0dy-jeff based in southern CA. I've had a set each for my 465B and 475, as the originals go crumbly with age.

Scuffing to the display is probably to the perspex screen cover - I've used some stuff called Novus Acrylic Scratch Remover & Cleaner to get light marking/scuffing off. It takes a good deal of buffing but does seem to work.

As for the delay time knob, my 465B which I've had since 1995 has just a Tek plastic knob, whereas the 475 has the vernier one. Both look like they've been on since new but who knows.

As to whether you should run it up on a variac, I'll leave that to the jury; I tend to on old valve stuff and anything I'm uneasy with. So far I've not had exploding tantalums though.


 

Ward,

Running the 475 up slowly on a variac is actually one of the performance test procedures, as it will verify that the LOW LINE indicator is working, so if you have a variac and feel like starting the 475 up nice and slow, it will be fine.

The 475 with DM44 does not have a vernier on the delay time position control, rather it has a simple knob (like yours, and as you have seen in pictures): the calibrated delay is, I believe, indicated on the DM44 LED display.

The cord wrap feet do not hold up well over time. I second the recommendation of n0dy-jeff as a source for excellent 3D printed replacements.I have several sets of his cord wrap feet for a couple of 475s and a 475A, and could not be more pleased.

I also have a personal connection to these instruments: my father was a service engineer working on laboratry equipment and data machines, and used a Tek 475 for many years. I have that 475, now 45 years old and still working: it is a wonderful instrument. I have also done some repair and restoration work on the 475 and 475A, and have found it to be enjoyable (if not always easy). I hope you get plenty of enjoyment out of your 475.

-- Jeff Dutky


Glydeck
 

Ward,

I’m a happy customer of N0DY as well. He’s also a member of TekScopes. Here’s the link to his online store.

https://www.n0dy.com/

George KD6NEW

On Feb 8, 2021, at 2:01 PM, Jeff Dutky <jeff.dutky@gmail.com> wrote:

Ward,

Running the 475 up slowly on a variac is actually one of the performance test procedures, as it will verify that the LOW LINE indicator is working, so if you have a variac and feel like starting the 475 up nice and slow, it will be fine.

The 475 with DM44 does not have a vernier on the delay time position control, rather it has a simple knob (like yours, and as you have seen in pictures): the calibrated delay is, I believe, indicated on the DM44 LED display.

The cord wrap feet do not hold up well over time. I second the recommendation of n0dy-jeff as a source for excellent 3D printed replacements.I have several sets of his cord wrap feet for a couple of 475s and a 475A, and could not be more pleased.

I also have a personal connection to these instruments: my father was a service engineer working on laboratry equipment and data machines, and used a Tek 475 for many years. I have that 475, now 45 years old and still working: it is a wonderful instrument. I have also done some repair and restoration work on the 475 and 475A, and have found it to be enjoyable (if not always easy). I hope you get plenty of enjoyment out of your 475.

-- Jeff Dutky





Jeff Davis
 

Hi guys,

Thanks for the kind words on the N0DY scope feet. It was kind of a happy accident that I started making them. I had the same problem with feet crumbling to dust on the 465s and 475s that I was repairing, so I decided to see if I could make my own.

Just as an FYI, for those folks outside of California, you can save a couple of bucks on sales tax by going to the n0dy.com site instead of eBay (no Marketplace Facilitator sales tax has to be collected). And regardless of location, most items are slightly cheaper on the web site since there's no cut that has to be paid to eBay.

Jeff / N0DY

________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Glydeck via groups.io <glydeck=aol.com@groups.io>
Sent: Monday, February 8, 2021 3:35 PM
To: TekScopes@groups.io <TekScopes@groups.io>
Subject: Re: [TekScopes] Waking a slumbering 475

Ward,

I’m a happy customer of N0DY as well. He’s also a member of TekScopes. Here’s the link to his online store.

https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.n0dy.com%2F&;data=04%7C01%7C%7Cc7fa058671d446d94c2608d8cc8a4e33%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C637484241678757361%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&amp;sdata=WpPK27B0F5mbqkttEI%2Fp2bWb9VKaJYQo3KqSfmaJe80%3D&amp;reserved=0

George KD6NEW
On Feb 8, 2021, at 2:01 PM, Jeff Dutky <jeff.dutky@gmail.com> wrote:

Ward,

Running the 475 up slowly on a variac is actually one of the performance test procedures, as it will verify that the LOW LINE indicator is working, so if you have a variac and feel like starting the 475 up nice and slow, it will be fine.

The 475 with DM44 does not have a vernier on the delay time position control, rather it has a simple knob (like yours, and as you have seen in pictures): the calibrated delay is, I believe, indicated on the DM44 LED display.

The cord wrap feet do not hold up well over time. I second the recommendation of n0dy-jeff as a source for excellent 3D printed replacements.I have several sets of his cord wrap feet for a couple of 475s and a 475A, and could not be more pleased.

I also have a personal connection to these instruments: my father was a service engineer working on laboratry equipment and data machines, and used a Tek 475 for many years. I have that 475, now 45 years old and still working: it is a wonderful instrument. I have also done some repair and restoration work on the 475 and 475A, and have found it to be enjoyable (if not always easy). I hope you get plenty of enjoyment out of your 475.

-- Jeff Dutky





 

Jeff (Davis),

Are you open to expanding your offerings? I've been looking (unsuccessfully) for a local MakerSpace with 3D printers, so that I could print a few things (e.g. some kickstands for DMM916s, top and bottom plates for TM500 plugins, TM500 latch release parts, and maybe see what can be done to produce press-on knobs for 2200 and 2400 series scopes). Would you be up for small runs like that?

-- Jeff Dutky


Steve Hendrix
 

At 2021-02-09 10:59 AM, you wrote:
Are you open to expanding your offerings? I've been looking (unsuccessfully) for a local MakerSpace with 3D printers, so that I could print a few things (e.g. some kickstands for DMM916s, top and bottom plates for TM500 plugins, TM500 latch release parts, and maybe see what can be done to produce press-on knobs for 2200 and 2400 series scopes).
I know of a new makerspace that would fit the bill, but where are you located?

Steve Hendrix


Weathers, W
 

Thank you to all who have responded to this thread. You have already given me useful information.

My intent is to take this slowly, as time allows. I did manage to slide the cover off the DM44 and tighten that loose (panel) jack nut.


 

Steve Hendrix wrote:

I know of a new makerspace that would fit the bill, but where are you located?
I'm just outside Washington D.C.

I had thought that there were makerspaces near me, but they look like shared office space and don't say anything about 3D printing or other services.

-- Jeff Dutky


Jeff Davis
 

Hi Jeff (Dutky),

I'm generally open to creating new offerings if they're something that can reasonably be done with 3D printing. I've found that knobs, in general, have too much fine detail and tolerances too tight to be good candidates for 3D printing (at least at my skill level and with my equipment complement).

The other items you mention might be possibilities. If you continue to strike out with MakerSpaces, feel free to contact me off-list and we can discuss further. I'd probably need to see photos, and if 3D fabrication appears feasible from them, eventually have a physical sample to take measurements from.

What I've found to be the challenge in creating these parts is not so much in creating the 3D model in a CAD tool. The true challenge seems to be developing and controlling the process to replicate the CAD model in ABS or whatever the material of choice is. Each part is different. I haven't found anything better than good old trial and error to do that and have gone through literally dozens of reels of filament in developing the parts that I do offer.

Good luck to you! Also feel free to contact me off-list if there's any 3D printing advice i can provide from my admittedly limited knowledge.

Jeff (Davis) / N0DY
________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Jeff Dutky <jeff.dutky@gmail.com>
Sent: Tuesday, February 9, 2021 7:59 AM
To: TekScopes@groups.io <TekScopes@groups.io>
Subject: Re: [TekScopes] Waking a slumbering 475

Jeff (Davis),

Are you open to expanding your offerings? I've been looking (unsuccessfully) for a local MakerSpace with 3D printers, so that I could print a few things (e.g. some kickstands for DMM916s, top and bottom plates for TM500 plugins, TM500 latch release parts, and maybe see what can be done to produce press-on knobs for 2200 and 2400 series scopes). Would you be up for small runs like that?

-- Jeff Dutky


Mlynch001
 

On Tue, Feb 9, 2021 at 01:01 PM, Jeff Davis wrote:


. I haven't found anything better than good old trial and error to do that and
have gone through literally dozens of reels of filament in developing the
parts that I do offer
Jeff,

I do quite a bit of 3D printing myself (non-commercial and for personal use) and you have hit the nail on the head. Most of the process is very straight forward. That is until the printing starts. I find that I must do several (sometimes dozens) prints before I am able to work through the “unforeseen” issues, orientation, surface finish and the “sizing” issues. The things that most people do not see with 3D printing are those behind the scenes details that appear when the STL file goes into the slicer. There are literally hundreds (at least) of parameters available within the slicer, with an unbelievable number of possible combinations and almost all affect the outcome. I have tried to print knobs for 24xx series scopes, not much success with FDM, as you say, the details are too fine. Some things lend themselves quite well to FDM, but small and finely detailed items are particularly difficult. I would also be glad to discuss any specific questions that people might have if they want to contact me privately.

--
Michael Lynch
Dardanelle, AR


Torch
 

I am no expert. The variac is probably a good idea.

That said, my 475 spent years in a dusty corner of an aircraft hangar. Before giving it to me, the owner plugged it in and turned it on, so the variac ship had sailed by the time I got it. It needed a transistor array for the fan motor, a knob, a good cleaning and some calibration but the rest of the innards survived intact. It does seem to be a fairly robust unit (although now I need to replace a worn-out front panel BNC connector).


Weathers, W
 

Jeff,

Someone had suggested to me ages ago that powering a long-stored scope on a voltage ramp, as a one-time thing, would help reform the electrolytic capacitors. I am trying to ascertain whether that is a common practice. It seems there are various opinions on whether this is a good idea.

One group member privately messaged me to say (I am paraphrasing here) that running the scope on a variac, well below the expected regulated voltage, was a bad idea. He noted that the regulators were not designed to run at exceedingly low voltages.

You mentioned that running the 475 up slowly on a variac is one of the performance test procedures. Do you happen to know where this procedure is documented? I would be interested in reading the procedure.

Ward


Harvey White
 

Variacs and regulators may or may not be a good idea.  Depends a lot on what kind of regulator it is, and what it does, when.

For a linear regulator, once the input voltage (courtesy of the variac) gets high enough, the regulator starts to connect, and essentially runs as a dead short (output too low!  decrease resistance!).  Since the output voltage of the regulator is way too low, the output becomes whatever the input voltage is, minus whatever drops there are in the regulator circuitry.

Generally, IMHO, the regulator doesn't care and this doesn't bother the regulator.  What the driven circuitry manages to do with the voltages (that are perhaps not balanced), that's up to that circuitry.

For a switching regulator, the subject is a bit different, and I'm simplifying.

For a switching regulator, there are two input conditions, generally:  1) input is too low, pass transistor turns on, never turns off and 2) input is acceptable (may be too low, but the transistor is switching).

For any pass transistor in a switching regulator, in general, the thing is either off (no power dissipation), or on (minimal, but designed for, power dissipation).  In these cases, the power transistor is likely to be fine.  The amount of on time vs off time controls the output voltage.  The closer you push the regulator to its limit (low input voltage) the more time the pass transistor spends on.

Now we have to consider the design of the regulator, sorry, but there are different designs and they behave differently.

If the regulator is on only for a certain time, then must switch off, then the regulator is generally going to be able to handle the situation *because* the transistor should be designed to get rid of that power....  Note the word should.

Now suppose that the regulator turns on the switching transistor, but only when the output voltage gets above the desired value. Next we have to assume (and it's not a bad assumption) that the regulator is running at a lower duty cycle, which means that it can handle the 75% or so on time, but over that, well, maybe not.

What happens in the last situation (and especially if there's no current limiting at all!) is that the transistor turns on completely, too much current flows, and the transistor overheats.

In the normal "turn on the power and see if it smokes" kind of situation, the input voltage goes through the "NOT WORKING" to the "ON FULL" to the "REGULATING" stage fast enough that the transistors involved don't overheat.

I'm sure that there are more scenarios, but this is one.

Each scenario does assume that the parts are rated for the worst case current, have sufficient power heat sinking for worst case situations, and so on.

A bunch really depends on the assumptions that the power supply designer made when he/they designed the supply.


Harvey

On 2/10/2021 6:32 PM, Weathers, W wrote:
Jeff,

Someone had suggested to me ages ago that powering a long-stored scope on a voltage ramp, as a one-time thing, would help reform the electrolytic capacitors. I am trying to ascertain whether that is a common practice. It seems there are various opinions on whether this is a good idea.

One group member privately messaged me to say (I am paraphrasing here) that running the scope on a variac, well below the expected regulated voltage, was a bad idea. He noted that the regulators were not designed to run at exceedingly low voltages.

You mentioned that running the 475 up slowly on a variac is one of the performance test procedures. Do you happen to know where this procedure is documented? I would be interested in reading the procedure.

Ward





Mlynch001
 

On Wed, Feb 10, 2021 at 05:32 PM, Weathers, W wrote:


You mentioned that running the 475 up slowly on a variac is one of the
performance test procedures. Do you happen to know where this procedure is
documented? I would be interested in reading the procedure.
Look on Page 5-25 and 5-26 of the 475 manual. They do not tell you to start from 0 volts. You start at 115 and drop the voltage until the “low voltage” light illuminates on the front panel. Supposed to come on about 103 volts on the auto transformer. This is in the section that deals with LVPS ripple as well.

--
Michael Lynch
Dardanelle, AR


Weathers, W
 

Thank you Harvey for the explanation. You've given me a good jumping off point for more reading.


Weathers, W
 

Ah, yes. My mistake. I had seen a portion of this procedure on 5-25, but somehow missed the low line indicator bit on 5-26. As a result, I thought that bit must have been somewhere else.


Harvey White
 

Remember that it's an AFAIK, so I can always be wrong.  It does seem to make sense for me, though.

I always told my students that the real fun came when you looked at a design at the boundary conditions.  Anybody (and perhaps everybody, now, with automated design tools) can design something for the middle of the bull's eye.  On the other hand, the interesting design problems come when you start to look at the boundaries of the design solution.

Harvey

On 2/10/2021 8:00 PM, Weathers, W wrote:
Thank you Harvey for the explanation. You've given me a good jumping off point for more reading.





Eric
 

I will two my 2 cents in as well. Slow ramp for linier supplies are ok. Slow ramps for switching supplies will damage them. You will over current some of the parts. I do not remember what supplies are in the 400 series but if I remember correctly it is linier/transformer based supply with regulators. The testing procedure is a ramp up and down +/- a percentage of the line voltage.

Just some thoughts.

Eric

-----Original Message-----
From: TekScopes@groups.io <TekScopes@groups.io> On Behalf Of Harvey White
Sent: Wednesday, February 10, 2021 8:09 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Waking a slumbering 475

Remember that it's an AFAIK, so I can always be wrong. It does seem to make sense for me, though.

I always told my students that the real fun came when you looked at a design at the boundary conditions. Anybody (and perhaps everybody, now, with automated design tools) can design something for the middle of the bull's eye. On the other hand, the interesting design problems come when you start to look at the boundaries of the design solution.

Harvey



On 2/10/2021 8:00 PM, Weathers, W wrote:
Thank you Harvey for the explanation. You've given me a good jumping off point for more reading.






Jim Ford
 

Yes, been there, done that with the switching regulators with too low input voltage. Happened at work with military radios, and the solution was simple; use a diode to pull up the input of the switching regulator IC to a higher voltage (fortunately there was one available). Quite a sight to get radios back from the field with blown up regulators! A multi-sensory experience; you could smell the smoke as soon as you opened them up. Some just smoked a bit, some cracked open, and some blew pins right off the DPAK! Fortunately we never got any back with PCB damage. We just cleaned up the black goo, put on a new regulator, installed the diode, retested, and sent them back out.

Jim Ford

------ Original Message ------
From: "Harvey White" <madyn@dragonworks.info>
To: TekScopes@groups.io
Sent: 2/10/2021 4:03:17 PM
Subject: Re: [TekScopes] Waking a slumbering 475

Variacs and regulators may or may not be a good idea. Depends a lot on what kind of regulator it is, and what it does, when.

For a linear regulator, once the input voltage (courtesy of the variac) gets high enough, the regulator starts to connect, and essentially runs as a dead short (output too low! decrease resistance!). Since the output voltage of the regulator is way too low, the output becomes whatever the input voltage is, minus whatever drops there are in the regulator circuitry.

Generally, IMHO, the regulator doesn't care and this doesn't bother the regulator. What the driven circuitry manages to do with the voltages (that are perhaps not balanced), that's up to that circuitry.

For a switching regulator, the subject is a bit different, and I'm simplifying.

For a switching regulator, there are two input conditions, generally: 1) input is too low, pass transistor turns on, never turns off and 2) input is acceptable (may be too low, but the transistor is switching).

For any pass transistor in a switching regulator, in general, the thing is either off (no power dissipation), or on (minimal, but designed for, power dissipation). In these cases, the power transistor is likely to be fine. The amount of on time vs off time controls the output voltage. The closer you push the regulator to its limit (low input voltage) the more time the pass transistor spends on.

Now we have to consider the design of the regulator, sorry, but there are different designs and they behave differently.

If the regulator is on only for a certain time, then must switch off, then the regulator is generally going to be able to handle the situation *because* the transistor should be designed to get rid of that power.... Note the word should.

Now suppose that the regulator turns on the switching transistor, but only when the output voltage gets above the desired value. Next we have to assume (and it's not a bad assumption) that the regulator is running at a lower duty cycle, which means that it can handle the 75% or so on time, but over that, well, maybe not.

What happens in the last situation (and especially if there's no current limiting at all!) is that the transistor turns on completely, too much current flows, and the transistor overheats.

In the normal "turn on the power and see if it smokes" kind of situation, the input voltage goes through the "NOT WORKING" to the "ON FULL" to the "REGULATING" stage fast enough that the transistors involved don't overheat.

I'm sure that there are more scenarios, but this is one.

Each scenario does assume that the parts are rated for the worst case current, have sufficient power heat sinking for worst case situations, and so on.

A bunch really depends on the assumptions that the power supply designer made when he/they designed the supply.


Harvey


On 2/10/2021 6:32 PM, Weathers, W wrote:
Jeff,

Someone had suggested to me ages ago that powering a long-stored scope on a voltage ramp, as a one-time thing, would help reform the electrolytic capacitors. I am trying to ascertain whether that is a common practice. It seems there are various opinions on whether this is a good idea.

One group member privately messaged me to say (I am paraphrasing here) that running the scope on a variac, well below the expected regulated voltage, was a bad idea. He noted that the regulators were not designed to run at exceedingly low voltages.

You mentioned that running the 475 up slowly on a variac is one of the performance test procedures. Do you happen to know where this procedure is documented? I would be interested in reading the procedure.

Ward