Date   

Re: TM50X transistors

um-gs@...
 

Hi,
having had the same problem (blown power transistor), I found this forum and the topic.
Just to add - there are direct replacement (same case and pin out) transistors available on ebay: NTE182/183

Best Regards, Gordian


Re: 475 questions

Harvey White
 

This is, of course, where a scope is needed to diagnose a scope.

However, with linear supplies, Ernesto is correct, you could sneak up on it.  However, let's think rectifiers for a moment. With a shorted rectifier, in either a full wave or a bridge, one diode will be effectively put across the secondary winding.  This causes large amounts of current to be drawn.  The light bulb test would indicate that you're not drawing excessive current there. Now an open rectifier would give you a lot less voltage than you'd want, with a lot of ripple, and would cause the bulk supply readings to be off.

A linear voltage regulator is a kind of voltage divider, with a pass (series) transistor for the top element, and the actual load for the bottom element.  There's a circuit that compares the actual load voltage against a reference (both likely divided down), finds out if they match, then tries to control the pass transistor's resistance to make the load voltage come out right.

Tektronix frequently used one supply as a reference for the others, and divided down that supply for a single reference.  If that supply is off or doesn't work, then the whole set of voltages (in this case) would be wrong.

But let's look at the readings you have.

With the light bulb in series, you should get some bulk voltages.  They should be off (lightbulb!) but proportional to the desired voltages.  Then check the voltages on the series transistor.  Generally, these are hooked up as NPN emitter followers, so the emitter should be the output voltage, the base should be about 0.6 volts higher, and the collector ought to be the bulk voltage.  Look also at a small value series resistor that generally has a transistor base/emitter junction across it (one way of doing this).  When the current through that resistor is such that the voltage is 0.6 volts or better, the transistor turns on, and starts to steal base current from the pass transistor, gradually shutting it down.  This limits the current of the power supply under high load or short conditions.  If the voltage across that resistor is 0.6 volts (usually), then the supply says that it's trying to deliver too much current.

Now consider where you got the 0 ohms readings.  Assuming that the supplies are fine, then there's something out there in the scope that is a short.  These are generally tantalum capacitors that have failed over time.  They'll need to be tracked down.  One way to track them down is to unplug the other boards one at a time and see if the power supplies work properly.  Others with more experience with this scope will have suggestions on how well this will work.  Nothing says that unplugging boards with the power off and seeing if the resistance changes to the better wouldn't be a good idea.  You won't damage anything here.

Harvey

On 5/22/2020 1:20 AM, ciclista41 via groups.io wrote:
Hi Ernesto!

Thanks for the excellent advice! I have been looking at new and used variacs with that in mind. However, after I built the incandescent bulb current limiter this morning, I felt that I could probably get away with turning on the scope, which I did without any (to me) known negative consequences. I let it run for several minutes. The bulb just glowed faintly. I would have preferred that it glow even more faintly or not at all, but I did not have a larger wattage bulb than a 150. My guess is that for this scope, 200W would be the ideal bulb. After seeing no ill effects, but all voltage readings low, I shut it off and replugged it into the wall receptacle. It ran fine (no trace, but nothing seemed to pop, get hot, or release smoke) for the half hour or so that I ran it. The voltage readings I took when it was plugged into the wall rose with the bulb removed, but did not reach spec. The measurements I got are recorded in my post #167295 below. My 50V didn't get above 40, so only 80% of what it should have been.
Yes, I wish I knew how to isolate the problem(s) better. At this point, I think I'm just going to start changing out old capacitors and rectifiers.



Re: Questions I have if Attachments are Permitted

Michael W. Lynch
 

On Thu, May 21, 2020 at 11:38 PM, Dennis Tillman W7pF wrote:


Hi Michael,
By now several members have posted about the all too common myth that everyone
in the US (the most high-tech country on the planet) who wants a gigabit
connection to the internet just has to ask for it. The truth is going to shock
you.

TekScopes membership is worldwide. The other thing you completely forgot is
that the United States population is only 328 million people. What do you
suppose the internet connectivity of the other 96% of the world's population
is like?
Dennis,

As I stated up front, I was looking for education, not an argument. I did not "forget" anything, I fully realize that we (the USA) are not the only place on earth, and with only a very small portion of the 8 billion or so people on the planet. It was just surprising to me to learn that more people than I could imagine, from a group of the smartest and most tech savvy people that I know (from all over the earth), are still crippled by dial up and metered connections. Thus my original question.

As for me, I don't have the option for a 100MB connection, much less a GB or better connection. But I am also not burdened by a "metered" connection. I wrongly imagined that these type of connections had gone the way of the dinosaur. I suppose the fact that I have learned that I have a relatively decent "high speed" connection is sufficient. I see these lists all the time that show many seemingly remote countries having much better access to the internet than here in the US, so I assumed (wrongly) that I must be near the bottom of the US list and thus the rest of the world.

I suppose that dial up is alive and well for many folks? I have a new appreciation for what I am blessed to have.

Thank you for your answer.


--
Michael Lynch
Dardanelle, Arkansas


Re: 475 questions

n4buq
 

Hi Bruce,

I say that because you have at least some test equipment and are already checking some of the correct things, you _want_ this to be fixed, and there are so many folks on this list that are very knowledgeable and willing to help you for as much as you need. BTDT.

Good luck with it.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "ciclista41 via groups.io" <ciclista41=yahoo.com@groups.io>
To: TekScopes@groups.io
Sent: Friday, May 22, 2020 12:23:40 AM
Subject: Re: [TekScopes] 475 questions

Hi Barry!

I appreciate the positive words! I don't quite have that level of
confidence, but since those who have gone before me in similar efforts have
been successful, I guess it's not out of the realm of possibility for me, as
well!

Bruce




Re: Questions I have if Attachments are Permitted

 

Actually I strongly suggest pictures be uploaded to the Photos area not the files area - I suggest you create a usefully named folder to put them into!

David

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Vintage Test via groups.io
Sent: 22 May 2020 11:50
To: TekScopes@groups.io
Subject: Re: [TekScopes] Questions I have if Attachments are Permitted

I agree wholeheartedly with Linda - upload to the files area and reference in the body of the text. I live in central Scotland and am lucky enough to have a 200Mbps fibre connection to my ISP, so it doesn’t bother me. Like Paul, it. Was there for day 1 of the WWW and remember the the modem chatter of the day. I later got ISDN, the most reliable connection ever, with no contention ratio as it is essentially an end-to-end connection, but only 28kbps ( never could get the channel bonding to work) Anyway Dennis, my vote is no attachments, for all the reasons discussed here - place a link in the message to the files area, that way, people can decide whether to follow the link or not. Keep up the good work - this forum is totally invaluable!🙂

All the best,
Mel

--
you can never have enough oscilloscopes, DMMs, valve testers or soldering irons . . .


Re: The schematic for the P6131 probe I use on my 2465b. I can't figure it out! T-coil

Lawrance A. Schneider
 

Sorry to be in a learning mode relative to posting my own posts.

I have up-loaded to the photos area the schematic to the P6131 probe. No mater how I re-draw it, I can't make sense of it.

AND!! That should have been "ECE student".


Re: 475 questions

Dave Hills
 

Assuming you do not have a shorted rectifier, you can determine if the rectifier filter
capacitors are functioning with an AC voltmeter. A simple analog VOM will be adequate.
If you can access the filter caps connected across the rectifiers, you should see an
AC voltage not more than approximately 10% of the expected DC level. It could be more
or less, depending on the regulation required for a particular voltage, 10% is just a rule of thumb,
so to speak.
Check the the test points you mentioned earlier for AC as well. If you measure more than
a few mV AC on the regulated outputs, then you likely have a rectifier and/or filter problem.
High ripple on the regulator inputs will cause high ripple on the outputs which will appear
just as you have observed, low regulator DC output.
Lastly, if you do have as shorted rectifier, the associated filter capacitor is likely gone as well.

Good Luck!
Dave

On Thu, May 21, 2020 at 07:02 PM, <ciclista41@yahoo.com> wrote:


I was guessing I'd have lots of ripple with bad capacitors, but I really don't
know if they're bad or how bad they are. Currently, no meter I have measures
such high μF. Guess I'd better find one.


Re: 475 questions

John Clark
 

Bruce,
Congrats on the scope! A few years ago I came in here barely knowing what a scope was, how it worked, or even what it was supposed to do. I had acquired a non working 475 from my father who had recently passed. It sat in the corner for a year with me not knowing what to do with it but not feeling quite right about tossing it out. After one day when my computer monitor quit and I was successful in reviving it with a couple of cap replacements I decided to go after the scope. The folks in this group helped me get mine going.

What I'd suggest is reading through the service manual and studying the diagrams. I did have enough knowledge of reading diagrams and with help from the great people here I had my 475 working. So, if I can do it, you can do it.

I would strategically test the power supply circuits and troubleshoot them in the order suggested by those here and the service manual. I wouldn't just throw caps in it just randomly. After I got my scope working I did replace some tantalum caps that were known for failure but only after I had it working and just for preventative maintenance. You always risk damage to the board any time you effect a repair.

I'm not good enough at all this to be much more help but wanted to give you some confidence that even an extreme novice like me was able to fix my scope. It still works to this day...again thanks to everyone here.

Good luck!
John in Charlotte

________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of ciclista41 via groups.io <ciclista41=yahoo.com@groups.io>
Sent: Friday, May 22, 2020 1:23 AM
To: TekScopes@groups.io <TekScopes@groups.io>
Subject: Re: [TekScopes] 475 questions

Hi Barry!

I appreciate the positive words! I don't quite have that level of confidence, but since those who have gone before me in similar efforts have been successful, I guess it's not out of the realm of possibility for me, as well!

Bruce


Re: Questions I have if Attachments are Permitted

Vintage Test
 

I agree wholeheartedly with Linda - upload to the files area and reference in the body of the text. I live in central Scotland and am lucky enough to have a 200Mbps fibre connection to my ISP, so it doesn’t bother me. Like Paul, it. Was there for day 1 of the WWW and remember the the modem chatter of the day. I later got ISDN, the most reliable connection ever, with no contention ratio as it is essentially an end-to-end connection, but only 28kbps ( never could get the channel bonding to work) Anyway Dennis, my vote is no attachments, for all the reasons discussed here - place a link in the message to the files area, that way, people can decide whether to follow the link or not. Keep up the good work - this forum is totally invaluable!🙂

All the best,
Mel

--
you can never have enough oscilloscopes, DMMs, valve testers or soldering irons . . .


Re: 475 questions

ciclista41@...
 

Hi Barry!

I appreciate the positive words! I don't quite have that level of confidence, but since those who have gone before me in similar efforts have been successful, I guess it's not out of the realm of possibility for me, as well!

Bruce


Re: 475 questions

ciclista41@...
 

Hi Ernesto!

Thanks for the excellent advice! I have been looking at new and used variacs with that in mind. However, after I built the incandescent bulb current limiter this morning, I felt that I could probably get away with turning on the scope, which I did without any (to me) known negative consequences. I let it run for several minutes. The bulb just glowed faintly. I would have preferred that it glow even more faintly or not at all, but I did not have a larger wattage bulb than a 150. My guess is that for this scope, 200W would be the ideal bulb. After seeing no ill effects, but all voltage readings low, I shut it off and replugged it into the wall receptacle. It ran fine (no trace, but nothing seemed to pop, get hot, or release smoke) for the half hour or so that I ran it. The voltage readings I took when it was plugged into the wall rose with the bulb removed, but did not reach spec. The measurements I got are recorded in my post #167295 below. My 50V didn't get above 40, so only 80% of what it should have been.
Yes, I wish I knew how to isolate the problem(s) better. At this point, I think I'm just going to start changing out old capacitors and rectifiers.


Re: Questions I have if Attachments are Permitted

 

Hi Michael,
By now several members have posted about the all too common myth that everyone in the US (the most high-tech country on the planet) who wants a gigabit connection to the internet just has to ask for it. The truth is going to shock you.

TekScopes membership is worldwide. The other thing you completely forgot is that the United States population is only 328 million people. What do you suppose the internet connectivity of the other 96% of the world's population is like?

Last question for you: What do you do when the Tek portable scope you were using to troubleshoot a GPS receiver craps out? The answer is easy: You send an email to TekScopes with a description of the problem and you hope for a quick answer? Now imagine you are on a coastal freighter that left Houston 2 days before bound for a research station in the Azores to drop off supplies when that Tek portable scope died. Next consider what happens if you don't get the scope fixed in time to repair the GPS receiver that the crew on the bridge was relying on to navigate the ship.

Dennis Tillman W7pF

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Michael W. Lynch via groups.io
Sent: Thursday, May 21, 2020 7:36 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Questions I have if Attachments are Permitted

Dennis,

First let me say, I am happy with whatever the Administrator and Owners of this group decide. I will adapt as needed.

I have a serious question about this and since I seem to be quite oblivious, here goes:

QUESTION #4: If almost all TekScopes members have no interest in the
napkin
sketch you attached to your message why should they have to download it to their PC and pay data charges for something they will immediately delete once they see what the attachment is about.<<

My Question: Who pays data charges in today's world? Comments seem to indicate that many here use "metered" connections, and I simply did not realize that those were that common these days. My ISP is unlimited up and download as far as I know (Yes, I realized that download is MUCH faster than upload speeds). I have not paid data charges in decades. Keep in mind, I live in rural Arkansas, so I am not even close to the "bleeding edge" of the internet or cell coverage. I'm just happy to have relatively "High Speed" internet. This applies to my phone and home ISP. I am just asking so that I can learn. This seems to be a huge issue for many on this forum.

Please advise and educate, as you always seem to be able to do.

I appreciate all that you do and the great benefit of this forum.

Sincerely,

--
Michael Lynch
Dardanelle, Arkansas





--
Dennis Tillman W7pF
TekScopes Moderator


Re: Questions I have if Attachments are Permitted

LarryS <vision1@...>
 

If you have a cellphone, you have pretty good internet.
Android phones have hotspot capabilities that rival any public access internet.
That's what I use out on 80 acres of land 20 miles from the nearest town.
Lots of folks don't have cellphones, or at least smartphones, I get that.
But those are getting to be rare.
A friend has been several years wiring the wide open spaces of farmland along I-70 with fiber optic internet. Rural remote farmhouses and one-horse towns with Gigabit speeds.
So I don't know who's being flown over, but a great many of them have better internet than many universities.

L.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Renée
Sent: Thursday, May 21, 2020 10:44 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Questions I have if Attachments are Permitted

Thanks Chuck -
I pretty much have stayed out of this as I know I am in the minority. I am one of those unknown mentioned, only in rural CA. in fact we have so many choices - one - and their service is inconsistent at best. (ok, read horrible)
Cell service- we are lucky to have one bar ( just like the town) and that ceases when the weather get bad - rain. many times one must find the right place to stand to make a call. tracfone (using verizon no service from any other carriers) is the only option for inexpensive cell service and as Chuck says it is a nickle and dime ( ok a quarter to
start) service.
Dial-up is a whopping 42K max when it works. so for emergency phone we rely on the one expensive choice landline.
....oh I forgot I travel to another town and use their library internet (once a week-shopping) or visit a friend down the road who has a sat connection for large files and stuff with lots of graphics. Thunderbird does a great job on limiting the size of emails!

Now the above being said, I will go with whatever the group decides. the info is just too valuable. Even the OT stuff!

Renée


On 5/21/20 8:09 AM, Chuck Harris wrote:
I can help you with that!

Everyone that lives in the large flyover country parts of rural US.

Not only do they get their internet over the telephone lines at
typically 56Kbits per second modem rates, they get slow or no cell
service to boot.

And, everyone that relies on tracfone, and its ilk, for their cell
service. That includes a lot of seniors. Their phones cost them
6 or 7 bucks a month, but they get nickle and dimed for each minute.

Broadband exists in all of the big cities, and their suburbs. But it
doesn't exist in the large central regions of the big farm states.

It also doesn't exist in the mountain areas, like parts of WV, CO, TN,
AK... Nor does it exist in large parts of OK, MN, WI, IA, ND, SD,
WY... need I go on?

And, even if your isp doesn't charge you a data rate for its 56K
bit/sec service, a 5.6mbyte picture takes 1000 seconds to be
transferred down your slow pipe.

If that isn't a cost, I don't know what is!

-Chuck Harris


Michael W. Lynch via groups.io wrote:
Dennis,

First let me say, I am happy with whatever the Administrator and Owners of this group decide. I will adapt as needed.

I have a serious question about this and since I seem to be quite oblivious, here goes:

QUESTION #4: If almost all TekScopes members have no interest in
the napkin
sketch you attached to your message why should they have to download
it to their PC and pay data charges for something they will
immediately delete once they see what the attachment is about.<<

My Question: Who pays data charges in today's world? Comments seem to indicate that many here use "metered" connections, and I simply did not realize that those were that common these days. My ISP is unlimited up and download as far as I know (Yes, I realized that download is MUCH faster than upload speeds). I have not paid data charges in decades. Keep in mind, I live in rural Arkansas, so I am not even close to the "bleeding edge" of the internet or cell coverage. I'm just happy to have relatively "High Speed" internet. This applies to my phone and home ISP. I am just asking so that I can learn. This seems to be a huge issue for many on this forum.

Please advise and educate, as you always seem to be able to do.

I appreciate all that you do and the great benefit of this forum.

Sincerely,


Re: Questions I have if Attachments are Permitted

Linda Davis
 

Why not just upload the picture, sketch, whatever, to the 'Files' area, and reference the uploaded file in a message?

Mark - AD7EF


Re: 475 questions

n4buq
 

Not "if" but "when" you get it working. You're over half-way there!

Thanks,
Barry - N4BUQ

Hi Albert,
<snip>

I bought two non-working scopes at once, so although the 475 is not
necessarily my first scope, neither do I have another scope that works. I'm
tempted to buy a new digital scope, but I might wind up disappointed in one
that I could justify the expense for. Maybe not. I haven't done enough
research to take that plunge yet. And if I get this one working, I will
find it even more difficult justifying such an expense.


Re: 475 questions

ciclista41@...
 

Hi Albert,

Yes, I re-read them just after turning the scope off, before my last reply, and I re-read them just now. All three are shorted regardless of the polarity with which I measured them. So it's surprising to me that the board doesn't pop, get hot, or release magic smoke when I run it.

I was guessing I'd have lots of ripple with bad capacitors, but I really don't know if they're bad or how bad they are. Currently, no meter I have measures such high μF. Guess I'd better find one. Just a month or two ago, I was checking the start and run capacitors on my roof-mount air conditioner. We Arizona desert dwellers do not want the air conditioner to quit during our summers. That can be deadly if we don't move out temporarily until it can be fixed or fix it immediately. They're predicting 108°F for this coming Wednesday, so the season is just about here. Although all capacitors measured within spec, I still bought replacements to have on hand. The only one I could not check was the compressor start capacitor, because it is rated higher than my meter will go. Another reason to find a better meter.

I bought two non-working scopes at once, so although the 475 is not necessarily my first scope, neither do I have another scope that works. I'm tempted to buy a new digital scope, but I might wind up disappointed in one that I could justify the expense for. Maybe not. I haven't done enough research to take that plunge yet. And if I get this one working, I will find it even more difficult justifying such an expense.


Re: 475 questions

 

Hi ciclista41@

You are bringing this old scope to life, and you correctly start by evaluating its power supplies.

What is very useful to have for this step, is a variac (a variable autotransformer) that allows you to increase the AC supply voltage practically from zero
This is a good precaution when not knowing what can be wrong, and what could burn out.
Your scope has a linear regulator, meaning that after the input transformer and rectifiers, the unregulated supplies will rise proportionally with the AC you apply with the variac.

If you are not in a hurry, you can then rise the AC in steps, like 25%, 50%, 75% or finer coming close to the 110 VAC, over a period of a day or so, to give an opportunity for the electrolytic capacitors to regenerate.
While you come up to the nominal voltage 110 VAC, you can watch the voltage on the +50V, which is the "master" reference for the other supplies. You can check that at some point it stabilizes, regulates, and verify that it is 50 V
If it does not reach 50 V, then you may have a failure in it, and this will be reflected in all the other supplies. The failure can be in the supply itself, or in its load (the rest of the instrument)

I don't know how much you can disconnect the power supplies from the rest of the instrument, and if you can, this helps to limit the extent of the failure to the supplies themselves, and not to some short elsewhere.

Ernesto


Re: The schematic for the P6131 probe I use on my 2465b. I can't figure it out! T-coil

Lawrance A. Schneider
 

In my own post, 'etc student' was to have been 'EEC student'.

sorry


The schematic for the P6131 probe I use on my 2465b. I can't figure it out! T-coil

Lawrance A. Schneider
 

The schematic of which I write is in the probe's manual. It uses a T-coil to extend the bandwidth. With all the discussion about attachments, ... Further, I'm not sure how to post a photo of the schematic anyway; if someone knows how and would like to post it I can send it to whomever. I've found a few urls for the T-coil:

https://www.electronicdesign.com/technologies/analog/article/21807815/whats-all-this-tcoil-stuff-anyhow

http://wwwee.ee.bgu.ac.il/~kushnero/temp/bellert/wangane.pdf

https://ibis.org/summits/feb11/ross.pdf Mr. Ross seems to have brought it to TEK

http://ibis.org/summits/may11/ross2.pdf again, Mr. Ross seems to have brought it to TEK

http://wwwee.ee.bgu.ac.il/~kushnero/temp/bellert/wangane.pdf

https://www.seas.ucla.edu/brweb/papers/Journals/BRFall15TCoil.pdf

Back a half century ago, when I worked with Hermit Polynomials and Dirac equations, I could (maybe) handle the above references; at present, I'm having a very hard time. In any event, it is more than this second year etc student can handle.

Does anyone have a more clear explanation of how the damn T-coil works?


Re: 475 questions

Albert Otten
 

Hi Bruce,

I wonder if you would again read 0 Ohm now. Maybe some caps reformed when you powered the scope up.
The combination of a slightly low +110V and a way too low +105/+160V is strange. The +110V should be pulled down by CR1489.

This 475 seems to be your first scope. Otherwise the advice would be to check the wave forms in stead of only DC measurement. You could still do AC ripple measurements (check that your meters block DC). Normally ripple is in the mV range (except perhaps for +110V). You will probably find some very excessive values.

Albert

On Thu, May 21, 2020 at 09:38 PM, <ciclista41@yahoo.com> wrote:


Hi Albert,

Thanks for replying! I only used an analog multimeter to take the readings,
and I verified them with another analog, so am confident in the values I got.
However, the value of resistance readings has been called into question,
anyway.

I put together a voltage-limiter (150W incandescent bulb in series with the
hot lead of my wall receptacle, which greatly increased my confidence that
turning the scope on would not do further damage.
So far, so good! With the scope turned on, the filament glows dimly, so I
probably should run a 200W bulb, but I don't have one handy. Knowing that the
scope would run without smoke or pops, I took readings with the voltage
limiter and then plugged directly into the wall, with the latter readings in
parentheses. Yes, a larger bulb is in order!

Checking voltages at the same test points that I got the resistance readings
from, I got:

110 = 87 (91) 83%

50 = 37.5 (40) 80%

15 = 10 (10) 67%

5 = 3.5 (3.8) 76%

-15 = -14.2 (14.5) 97%

-8 = -5.7 (6) 80%

UNREG 50 = 40 (47) 94%

105 = 7.5 (9.5) 9%

The scale intensity seems to work fine, but I get no trace at all. I put my
probe in Ch. 1 and put the negative to chassis ground and positive to the
calibrator, but still got nothing. Pushing the beam finder button didn't help,
either. Same story with Ch. 2.
All indicator lamps on the front seem to be working fine. Fan works fine.
Nothing seems to be getting hot.

Time to start desoldering, I guess.

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