Re: 7000 series carrying handles - how strong are they?


Ni Nenad,
Not silly. I have been carrying 7000 series scopes around for almost 50 years and I have never given the handle a second thought because I have removed so many of them from instruments I have scrapped. Every one of them is as solid as the day it was made. This applies to the outer plastic cover, the inner spring steel, the way they are attached to the instrument, and the way the rigid aluminum chassis handles all that weight. It is beautiful integration of all those parts into a very sophisticated mechanical design.

Even with the 500 series scopes, which weighed much more, and had handles that were covered in leather, I never saw a handle break. The leather might dry out and crumble in some climates but the steel hidden inside was still more than strong enough to continue doing its job of making it possible to safely lift a scope (provided you had a couple of helpers with strong backs to provide assistance).

Dennis Tillman W7PF

-----Original Message-----
From: [] On Behalf Of Nenad Filipovic
Sent: Sunday, August 18, 2019 2:41 AM

Well I suppose this sounds silly, but I just can't get over it. This thin metal sheet inside the handle is surely tough, high tensile strength steel, but somehow subjectively it doesn't nearly impress as the rugged 465 handle, for example. In my head thin metal sheets tear and crack. Every
7000 handle end I inspected showed some minor bending around the hole where it pulls against the main screw.

So my question is, has anyone ever had that handle crack or snap? 7000s are no longer costly lab instruments handled with utmost care, today we use them in our homes, shops... They get transported probably a lot more frequently than they were intended to, possibly against some rules which were in power back then, now forgotten or never even known. Should the instrument be supported by your other hand from below?

I don't use my 7104 often, but when I do I need to carry and lift it on the table. And every time I get shivers in my feet. Some reassurance would help.

Best Regards,
Nenad F.

Dennis Tillman W7PF
TekScopes Moderator

Re: Tektronix Cables Found

Craig Sawyers <c.sawyers@...>

Overnight the new listings for these current probes all jumped to over $100 and stayed there. This
seller had forced the asking price to jump almost 4X by what he did. After a while I stopped
the prices since I was not going to pay that for something I used to pay a lot less for. Prices for
things on eBay are now off the wall. $125 would be a bargain compared to what they are asking today.

Since then I have seen the same kind of thing happen with other items. But the opposite must happen
as well from time to time when the market adjusts the price down to a level where buyers see the
value of something and the demand starts to pick up. At the higher price there was no demand at all.

Dennis Tillman W7PF
It is a shame that Deane Kidd is no longer with us - he could always find what I was looking for no
matter how obscure, and on a few occasions asked for no money and shipped it free of charge (to the
UK). I am sure that he would have charged a fair price for a CT1 or CT2 independently of what the
current eBay pricing was.


Re: Tek Cover Part Number


Hi Gary,
The service manual should list the part number in the rear section of the manual where all of the mechanical parts are listed and referenced to an exploded view of the mechanical parts of the scope and how they are interconnected.
Dennis Tillman W7PF

-----Original Message-----
From: [] On Behalf Of Gary Robert Bosworth
Sent: Saturday, August 17, 2019 9:14 PM
Subject: [TekScopes] Tek Cover Part Number

Does anyone know the official part number for the cover used on the Tektronix 492/496 Spectrum Analyser?


Dennis Tillman W7PF
TekScopes Moderator

Re: Tektronix Cables Found


I learned an interesting lesson about eBay pricing trends from the CT-1 and CT-2. Back in the first half of the new decade (approximately 2000 to 2005) the CT-1 and CT-2 sold on eBay for $25 to $35 typically. I bought a few in those years and I kept track of the prices for them in case I wanted to buy one or two more.

Economists study auctions to gain an understanding about how prices are set for goods and services. There are few constraints on prices in an auction so whenever an auction for an item is successful it matches a seller who is willing to sell an item to a buyer willing to buy it for a price they both perceive to be fair. Auctions continuously establish the fair price for an item even as people's perceptions continuously change due to surrounding conditions. Most people alive today are forced to deal with a totally different system of pricing where items have fixed prices (and price stickers to reinforce this concept) and your only choice is to take it or leave it at that price. As recently as 150 years ago in the US there were no fixed prices and everything was negotiable in the form of barter which is just a variation on an auction.

eBay brought back our ability to barter for items we wanted as was done in the 1800s. One day I saw someone list a CT-1 for a starting price of $125. I send him an email telling him he should take a look at the current prices and the completed listings to realize he overpriced his listing and he would never sell it at that price. As politely as he could he told me to piss off.

He kept relisting his current probe every week and it never sold because there were always others listed for $25-$35. Most buyers knew there was almost nothing that can go wrong with these things so why pay more for one? This went on for at least 6 months then someone bought this sellers CT-1 current probe. This probably happened because that week there were no others listed and the buyer needed it in a hurry or didn't know how to look up the price of completed listings.

Overnight the new listings for these current probes all jumped to over $100 and stayed there. This seller had forced the asking price to jump almost 4X by what he did. After a while I stopped following the prices since I was not going to pay that for something I used to pay a lot less for. Prices for these things on eBay are now off the wall. $125 would be a bargain compared to what they are asking today.

Since then I have seen the same kind of thing happen with other items. But the opposite must happen as well from time to time when the market adjusts the price down to a level where buyers see the value of something and the demand starts to pick up. At the higher price there was no demand at all.

Dennis Tillman W7PF

-----Original Message-----
From: [] On Behalf Of Greg Muir via Groups.Io
Sent: Friday, August 16, 2019 10:28 PM


The offerings from distributors are poorly worded to a great extent making it rather confusing as to what the offering is.. Looking in a 1966 Tektronix catalog (yes, these things have been around for some time) on page 145 they single out the "probe" (cable) from the CT series current transformers by stating that the "probe" serves as an interconnecting cable between the current transformer and oscilloscope. And it was a jaw drop to see the prices then as compared to now:

CT-2 current transformer: $17
P6041 current probe: $12
Both together: $31

If you look on Newark, they specifically call out the P-6041 as the cable only ( and offer the CT probes together with the P-6041 for a price in the $800 range. The current price offering as compared to the 1966 one seems to have appreciated in a manner akin to that of gold.

As to why one would give a cable the label of "probe" as well as a probe model designation is beyond me. The CT-6 is a horse of a different color and also draws a hefty price close to $1k. But it's bandwidth is good to 2 GHz.


Dennis Tillman W7PF
TekScopes Moderator

Re: TOPIC CHANGE: Pro's and Con's of the 576 and 577 Curve Tracers. WAS: 5xx 'Scopes


Hi Dave,
What does a 7704A with a P7001 have to do with curve tracers???
The reason I changed the topic was to separate the discussion about 500 series scopes from the discussion going on about transistor curve tracers with the same subject.
Dennis Tillman W7PF

-----Original Message-----
From: [] On Behalf Of Dave Seiter
Sent: Thursday, August 15, 2019 11:45 PM
Subject: Re: [TekScopes] TOPIC CHANGE: Pro's and Con's of the 576 and 577 Curve Tracers. WAS: 5xx 'Scopes

I picked up a 7704A w/ P7001 about 7 years ago specifically because it had the core version of the memory and it was local. I've always wanted something with working core (and of course, it was a Tek product). Had to miss a free department lunch to pick it up. It came with hard copies of all the manuals. It had some modifications on the rear, and I've never gotten around to trying it (like some many other projects, I've been waiting for just the right time...) -Dave
On Thursday, August 15, 2019, 12:32:29 PM PDT, Raymond Domp Frank <@Raymond> wrote:

On Thu, Aug 15, 2019 at 08:45 PM, Dennis Tillman W7PF wrote:

The DPO used a DEC PDP-8 minicomputer to capture data from the 7704
lower section plugins and display the results on the 7704 upper
display section. Tek listed the DPO in its catalogs from 1974 to 1981.
In 1981 Tek introduced the
7854 which does all of this in a normal size 4-wide mainframe using a
microcomputer instead of the PDP-8 minicomputer.
This is incorrect.
The DPO that you're referring to used a 7704A, with separate sections. The 7704 had the old, one-piece construction. the later 7904/7904A have the same distinction.

Between the top section "D7704 Display Unit" and the bottom section "A7704 Acquisition Unit", that together made up a 7704A, Tek inserted the "P7001 Processor Unit".
The P7001 section provided the data capture and storage. Acquired data could be transferred to a minicomputer (Tek setup used a DEC PDP-8), processed further and if required, sent back to the P7001, which could show the result on the D7704.
A small keyboard on the P7001 may be used to store and recall waveforms and start programs on the PDP-8.

My P7001 instrument happily stores and shows acquired data, without a PDP-8 or anything like that. *And*, it contains real magnetic core memory, none of that modern-day semiconductor stuff: Switch power off with traces on the screen, switch on again and find your traces intact!


Dennis Tillman W7PF
TekScopes Moderator

Re: TOPIC CHANGE: Pro's and Con's of the 576 and 577 Curve Tracers. WAS: 5xx 'Scopes


Hi Gary,
No. There was no onscreen readout on any Tek curve tracer except (I'm mostly guessing) on their very newest ones (post 2000).
Dennis Tillman W7PF

-----Original Message-----
From: [] On Behalf Of Gary Robert Bosworth
Sent: Thursday, August 15, 2019 9:50 PM

Did any Tektronix curve tracers have labeling on the scope screen curves?
I would never buy a curve tracer without curve labeling.


On Wed, Aug 14, 2019 at 10:41 PM Dennis Tillman W7PF <@Dennis_Tillman_W7PF>

Hi Michael.
I changed the subject since it has been about the 500 series SCOPES
and not CURVE TRACERS until now.
In the next few days there will probably be many replies to this question.
I request that everyone keep the discussion dignified.

For 99% of our members the most important consideration will be which
one you can get at an affordable price. They are both wonderful
instruments especially if you have nothing else.

Since you are in the 1% category that owns both a 576 and a 577 (and
so do
I) 1 I will comment on which one meets my needs and why.

The first thing I think of, that I believe Deane also thought was
important, was the storage capability of the 577-D1. It has a drawback
as well - it can be annoying when it is not adjusted properly. I have
followed the internal adjustment procedure carefully for adjusting the
storage voltages with little success so I live with the occasional
storage artifacts. Another reason I like the 577 is because I am also
interested in testing the additional things my 178 "front porch" can
test (OpAmps, Voltage Regulators, SCRs, etc).

I personally think the 576 is the most beautiful instrument with a CRT
that Tek ever made (In that respect I'm a romantic). Many 576 owners
love the readout which helps to avoid mistakes when measuring
parameters. This is not something I need. Like my annoying storage,
the fiber optic readouts have their own issues. Most 576s have some
dark fibers by now and there are no replacement readouts that I know
of. I don't think there is a practical way to repair individual fibers
either. The 576 has its own specialized "front porch", the 176, for
testing power transistors which is important for some users. The 576
was incredibly expensive to build and Tek did not make much, if any, profit on each they sold.

The 577 was designed to fix that problem. Tek engineers borrowed
heavily where they could to keep costs down. For example, the entire
display section was borrowed from the 5000 series of scopes. My
principal complaint about the 577 is due directly to this lineage:
there is no Graticule Illumination. That is problematic when I want to
take a photo of the curves on the screen and the graticule lines do
not show up in the photo so it is almost impossible to know what the V
& I of each curve is. Instead I discovered an alternative way to add a
graticule overlay after I take the photo of the semiconductor
characteristic curves. But this is still an annoying oversight from
Tek. One final thing in the 577's favor: it was released several years
after the 576 and in the intervening years several new types of
semiconductor devices were developed and the design team made sure the 577 could test those devices.

You are probably wondering why I find the storage of the 577-D1 to be
essential. Here are four examples:
1) Junction (bipolar) FETs are notoriously temperature sensitive in a
curious way. Below a certain bias point the temperature coefficient is
negative, above this bias point the temp coefficient is positive. So
display a FET on a non-storage 577-D2, pick some typical gate bias
voltage steps, and look at the curves you get. You can't tell where
the gate bias point is exactly at the midpoint between the positive
and negative temp coefficients. If you do the same thing with the
577-D1 and turn the storage on all you have to do is change the
temperature of the FET by applying a little heat to the FET and the
curves above the critical bias point will move up and curves below
this point will move down. The storage displays the bias point where
nothing moved and that is the critical zero Temp coefficient bias point you should design for.

2) With storage turned on put a transistor in the left socket and
another one in the right socket and toggle left to right and you can
immediately tell if they match and by how much. The curves won't
overlap unless they are matched.

3) This is a little trickier but I have used storage to find two power
transistors (one NPN and one PNP) that had Beta close enough to make a
near perfect class B output stage. I had the NPN in the left socket
and I displayed the curves for it in the upper right quadrant of the
577-D1. Zero collector voltage and current was located at the exact
center of the screen. Then I stored the top half of the screen. Next I
did a similar thing with the PNP which was in the right socket. Again
zero voltage and current were set to the center of the screen and its
curves were positioned into the lower left quadrant. Then I stored the bottom half of the screen.
There was a perfectly linear load line from the negative PNP collector
voltages and currents on the bottom left of the screen to the positive
NPN collector voltages and currents in the upper right part of the screen.

4) At extremely low collector currents Miller capacitance, socket
capacitance, transistor noise, and AC (hum) pickup distort each
collector V vs. I curve into a noisy loop. As you switch to more
sensitive ranges the loops occupy a larger portion of each step until
the steps themselves are totally obscured by the capacitance,
transistor noise, and AC pickup. The solution is to switch to manual
+DC or -DC and use the display filter to reduce the noise. With
storage you can vary the collector voltage manually and trace out the
entire set of characteristic curves just fine on the CRT no matter how
sensitive the range you select. With a 576 or a 577-D2 you would have
to use a grease pencil to mark your progress on the CRT to see the curves.

Dennis Tillman W7PF

-----Original Message-----
From: [] On Behalf Of
Sent: Wednesday, August 14, 2019 7:10 PM
Subject: Re: [TekScopes] 5xx 'Scopes

On Wed, Aug 14, 2019 at 01:24 PM,
I once asked Deane Kidd, when I was in his lab, which he
576 or 577. His answer was definitive: the 577 hands down. So that
is what I found to replace my 575 10 years ago. I have had both the
577-D2 and 577-D1 over the years since and there is no question that
the 577-D1 (storage) version has more capabilities although the
storage is tricky to adjust and is sometimes annoying.

I am now fortunate to own both a 576 and a 577 and all the optional
"front porches" so I can test just about anything. . .

Dennis Tillman W7PF

Is there as specific reason that the 577 is the preferred instrument?
I have both working instruments and am looking to sell one of the two.
I do not know enough about either to say which is better. Looking for
some sage advice.


Michael Lynch
Dardanelle, AR

Dennis Tillman W7PF
TekScopes Moderator

Gary Robert Bosworth
Tel: 310-317-2247

Dennis Tillman W7PF
TekScopes Moderator

TDS 644A Power Supply problem

peter bunge

A year ago I repaired a TDS 644A power supply.with overheated VR4 and VR5 snubbers and blown Q9 switch transistor. Q9 was replaced with an MJE8501 which was pencilled in on the schematic. The transistor that I removed was a BU508A but replacements I ordered from China tested about 500v on a curve tracer so the MJE8501 was left in.
I also found that C17 was defective and replaced it.
A year later the new VR4 and VR5 show signs of overheating and Q9 is blown again.
VR4 and VR5 should never handle any current, only clip transients. Q9 should not experience excessive voltages because the regulator and all lock out circuits were tested and are working. I had adjusted R18 (A19 power factor control) slightly to set the bulk voltage at 408v as it was a bit high.
Does anyone have any experience with suggestions to repair this power supply.

Re: Need Help Troubleshooting Tektronix PS280

John Crighton

Hello Daven9ooq,

I will chuck in my 2 cents worth to help you.

One of the previous posters suggested that you print out the schematics. I am going to adopt a Judge Judy attitude here.
Have you printed out enlarged copies of the circuit diagrams? That is a YES or a NO

Another poster suggested that you start at the transformer and work your way forward.
Have you tested the transformer? That is a YES or a NO

Have you checked the bridge rectifiers as the other poster suggested? YES or NO

The previous helper asked you to make notes on your printed out circuit diagram and component layout drawings?
Have you done as he suggested?

We know your power supply is slightly different from the diagrams. This is the beginning of the "Fun Part." for us hobbyists.
Find a big piece of paper and start drawing the main parts of the circuit as you find it.
Turn this repair job into a learning excercise. Make it fun.

Start with the transformer.
Load test it with say a 2 amp load.
Note the open circuit volatge and loaded voltage as Dave suggested. Write your results down on your big hand drawn diagram.
Load test the ouput of the rectifier stage. (cut the track or wire to the filter capacitor)
Check for a nice fullwave rectifier display on oscilloscope. No Load and full 2 amp Load. This will show up any cracked tracks or
dodgy wiring connctions.
Reconnect the filter Capacitor to the bridge rectifier. Put your 2 amp load resistor across your filter capacitor.
Check the ripple, No Load and Full Load. Record your results as Dave or the other poster suggested.

I want you to tell me that you have tested the transformer, the rectifiers, the filter capacitors and it is all good because you have
test results to prove it. That to me would be a good concrete start on this fun filled interesting repair job.

John Crighton

----- Original Message -----
From: "daven9ooq via Groups.Io" <>
To: <>
Sent: Saturday, August 17, 2019 9:48 AM
Subject: Re: [TekScopes] Need Help Troubleshooting Tektronix PS280

Hi Dave, Many thanks! For your comments, I did make some headway today ,haven't found any bad components yet but did get some smooth voltage adjustment from the Master side, No the metering is not working correctly! Could be my fault got some connectors mixed up for the metering shouldn't be too hard to figure out.
The metering does light up and appears to work but not reading correctly, I dlo have extra chips if needed.I took my measurements right from the Master output terminals ,in place of relying on the internal meters, seems like at first I has full range to 31volts but when I checked again it was only half of that, not sure whats going on , when you adjust voltage the relays are clicking like a pinball machine!
Hope I dont have a bad relay could be hard to replace, actually the relays and the transformer are the only difficult components to replace everything else is commom easy to get stuff.
Earlier I took a look at tekscopes for other threads on the PS280 there were some, I would really like to hear what users think about the relibality and Quality of it , I see no one learned their lesson on using counter sunk screws in cheap plastic!
How about using a counter bore, And pan head screws!
I'm working one handed due to a couple strokes, doing well considering but work is slow!
On the plus side there is a Theroy of design in the manual I got from Tekwiki , Many Thanks to whoever did that nice work!
Unfortunately the manual is the wrong revision for the one I have. Seems there was 3 versions or revisions mine seems to be the latest and has the heatsink inside and one main board with a zillion connectors.
The schematic is usefull in a general way just the connections are wrong fot the revisions, I looked for a different manual but wasn't able to come up with anything usefull. If anyone has one Please post it!
I'll do my best to keep you posted on my progress or lack of it.
There are a ton of these on ebay most bad which makes me think there is a design flaw ? Any comments welcome!
The PS 280 is branded with the Tek Name but wasn't built or designed by Tek , It was aftet sold out to Danaher I think and built and designed by Goodwill a Taiwaneese company Wich explains the Asian capacitors and transistors in it.
Best regards to all.

This email has been checked for viruses by AVG.

Thoughts on TDS744A

Mark Schoonover


I'm interested in purchasing my first scope and looking for opinions on the
TDS744A. I'm not interested in purchasing a Rigol/Siglent, etc - not that
they are bad scopes, I've used Teks before at different employers. It's
just that I'm not familiar with this particular model. It'll be used for
working on amateur radio related radios, experimentation and general
messing around :) I know it can be modified to a 1GHz scope and might do
that in the future.

Thanks for reading.

73! Mark KA6WKE

Author: 4NEC2 The Definitive Guide EMail List::

Re: Fan direction on TEK485


All I have ever seen draw in at the back. The air passes through the filter first before being delivered to the interior of the cabinet. Here is a bit of fan/blower trivia: A propellor fan needs maximum power when the air flow is impeded or blocked, a centrifugal requires minimum power when the air flow is blocked. The motors on propellor fans are usually cooled by the air flowing over them. A clogged filter not only overloads the motor, it deprives the overworked motor of cooling. Keep the filters clean and free flowing. Don't ignore the filter if it is missing, high voltage attracts dust from unfiltered cooling air, which can cause a flashover.

Bruce Gentry, KA2IVY

On 8/19/19 8:28 PM, Kevin Oconnor wrote:
Did the TEK factory have the fan suck air in or blow it out the back?

Fan direction on TEK485

Kevin Oconnor

Did the TEK factory have the fan suck air in or blow it out the back?

Re: TDS460A Will not turn on?


On Mon, Aug 19, 2019 at 04:19 PM, Stephen Hanselman wrote:

Other folks, Monsanto, LiteOn, etc, all seemed more stable and reliable.


These optocouplers in the supply have a Small circle made with a dashed line surrounding an "M". It is not a "Motorola M", different from their LOGO. I'll bet these are made by MONSANTO. Thanks for the information!

Michael Lynch
Dardanelle, AR

Re: TDS460A Will not turn on?



Excellent information! I am not an I.E.E.E member. I'm pretty sure that they don't take old hot rodders and motorcycle mechanics turned electronics hobbyist as members. I have followed a lot of rabbit holes, so this would be no different. I get the gist of what you are saying. The optocouplers can degrade to the point that they cease to turn the device on, but still may test as "good". Sounds like I need to replace these with new ones and mark this off the list of potential failure points. After all, it is less than $2 to replace both of them.

Much Appreciated!

Michael Lynch
Dardanelle, AR

Re: TDS460A Will not turn on?

Glenn Little

Where I used to work, we used optocouplers to Isolate TELCO ring from our circuit and detect ring.
We found that as the opto aged the ability of the photo device to see the emitter decreased.
We replaced a lot of these because of this.
It got to the point that when a board came in for repair and was shown to be faulty in the test set, the first thing replaced was the opto.
This fixed about 90% of the field returns.
It may be that we used optos close to their low limits?


On 8/19/2019 5:08 PM, Chuck Harris wrote:

I have been repairing switching supplies since the
mid 1980's, and I have yet to have to replace an optocoupler.

Not one.

That is very odd, given your statement.

Perhaps, whatever a 6130 is, it uses the optocouplers in
a most severe way?

I must be the luckiest guy on Earth...

-Chuck Harris

Stephen Hanselman wrote:
If the opto-couplers are "HP" I'd have to argue with you. My boss (late '70's) at Opto Elec Div used to do the 6130C's and 6131C's and before he started he poured a little pile of couplers on his bench, and we were the div that made them. We (our company) still see regular failures of these today.

Glenn Little ARRL Technical Specialist QCWA LM 28417
Amateur Callsign: WB4UIV wb4uiv@... AMSAT LM 2178
"It is not the class of license that the Amateur holds but the class
of the Amateur that holds the license"

Re: TDS460A Will not turn on?


On Mon, Aug 19, 2019 at 04:21 PM, Chuck Harris wrote:

A properly functioning colloquially named zener diode will
have a forward conducting knee of 0.5-0.7 volts, and a reverse
conducting knee at the published zener voltage.

Any voltage below the conduction knee will result in little or
no current flow.

I think you are describing a bad zener, which could easily stop
your supply from bootstrapping.

-Chuck Harris

Since I cannot tell what the original Zener or avalanche voltage was on this part, I am calling it a Zener. I understand that these diodes over a certain voltage (about 6V) are not truely Zeners, but are actually avalanche diodes, This is something that I just learned regarding what is popularly called a "zener" Diode. It is possible to clearly see the difference in the curve tracer. As you say, Zener is the name they have been given, i suppose in the same way we call all facial tissues "Kleenex"? Strange that they used the same schematic symbol for both.

Thank you so much!

Michael Lynch
Dardanelle, AR

Re: TDS460A Will not turn on?



I realize all of what you have very clearly stated. However, All of the diodes on this board are marked as "CRxxx". I am not a complete stranger as to how TEKTRONIX schematics have been marked in the past. There are Zener Diodes in clear, red tinted glass packages as well, they look just like the Fast switching diodes (e.g.1N4148, etc.), It may take a curve tracer to tell them apart (thankfully, I have one). While yes, they are all technically "crystal rectifiers", there is no delineation on this board as to which is a Zener, Schotky, High Speed switching, Standard Rectifier or anything else. All of this could have been avoided if they had just seen fit to release a schematic. They wanted to sell complete replacement boards, new scopes and not encourage board level repairs. This really handicaps those of us who need a schematic to follow. I am not one who can look at the loaded board and figure out how it works without a schematic, i wish that I was.

I appreciate your comments and insights!

Michael Lynch
Dardanelle, AR

Re: TDS460A Will not turn on?

Roy Thistle

On Mon, Aug 19, 2019 at 01:55 PM, Stephen Hanselman wrote:

still see regular failures of these [optocouplers] today.
Hi All:
I.E.E.E., NASA, and the U.S. Navy, et. al., have published papers citing the significant "failure" of these "critical" components in causing SMPSs not to operate. The authors are claiming that there are "aging" processes that take place in an optocoupler's packaging, and an optocouplers LED. As far as I can tell, the claims are mostly that the performance of some optocouplers degrades enough to prevent the optocoupler from accomplishing its intended purpose, in the design. (They don't seem to imply the aging process causes the optocoupler to fail completely; before the SMPS stops operating.)
If you are interested in going down that particular rabbit hole, and you are an I.E.E.E. member too, there are papers on Xplore.
Best regards and wishes.

Re: TDS460A Will not turn on?

Chuck Harris


As a life long EE, I would not have put up with that for
a minute. Either the circuit would get redesigned, or the
vendor of defective parts get banned.

But, I have always been a bit of a hard-ass about stuff like

-Chuck Harris

Stephen Hanselman wrote:


He would change them 3 or 4 at a time. Now we see two or three over 5 or 6 units. The 6130C uses 22 (or so) and the OEM HP ones were unstable. HP got better, but at the time (1979) they broke at the drop of a hat.

Other folks, Monsanto, LiteOn, etc, all seemed more stable and reliable.


Re: TDS460A Will not turn on?

Chuck Harris

My point was most switching supplies have optocouplers that
send signals in both directions. Typically, one will send
a feedback signal from the low voltage side to the high
voltage side, to regulate the output voltage, and another will
send a power good signal (or other signal) from the hot side
to the low voltage side to help with orderly shutdowns, etc..

You seem to be recognizing optocoupler orientation just fine.

The power resistor is typically almost 1/4" diameter, and 3/4
inch long. Often there are two in series, standing up on the
board (in a "U" shape), and covered with a piece of braided
insulating tubing.

The power resistor is supposed to burn out safely to protect
the supply when a bootstrap failure occurs. It is operating
way over its safe power rating for just a fraction of a second.

As to being a bother, that is what the group is for.

-Chuck Harris

Michael W. Lynch via Groups.Io wrote:

I am trying to learn and NOT be a bother to anyone. I appreciate your help, cannot say that enough.

You are correct on both 1 and 2. See my response, sent before I read this message.

BTW, I have ZERO fear of taking components out of circuit. That seems to be the best and most accurate way to get good test results.

These two couplers have the #1 pin on the "Low" side. So both face the same direction? That is if I am understanding you correctly.

I learned long ago to take pictures and carefully not the orientation of the components. I have messed up in this manner before.

I have posted pictures of the board in the "Photos" section, if you need to see the board lay out.
Your Million$ Hint mentioned Zener Diodes, so I am starting to test the Zeners on this board.
I do not see any "Large" power resistors, but it is entirely possible that I do not know what I am looking for.

Re: TDS460A Will not turn on?

Chuck Harris

A properly functioning colloquially named zener diode will
have a forward conducting knee of 0.5-0.7 volts, and a reverse
conducting knee at the published zener voltage.

Any voltage below the conduction knee will result in little or
no current flow.

I think you are describing a bad zener, which could easily stop
your supply from bootstrapping.

-Chuck Harris

Mlynch001 wrote:

I would put the odds that the Opto isolators are bad
at under 1%. They are very reliable, and pretty hard
to kill.

-Chuck Harris

You are 100% correct about that optocoupler. I pulled that component out of circuit and it tests good. Don't know why I even trust "in circuit" tests? Moving back to your "million dollar hint". . . .

I do have what the board shows as CR17 and this connects to PIN 1 of UC3844 (PCM Controller). Every diode on this board is labeled as CRxx. So the labels are no help in determining what the components actually are. That being said, CR17 looks like a classic glass package Zener Diode, so I lifted one end from circuit and then hooked it up to my TYPE 576 curve tracer to see what I had, expecting to see a typical ZENER curve. C17 tests as a Zener diode, but it acts in a VERY strange manner, unlike any Zener I have tested before. When I hook it up and increase the collector voltage control, the diode begins to conduct positive at the expected .6V and exhibits the other classic breakdown "knee" fairly quickly. For lack of a better explanation, it is acting like a "variable voltage" Zener. As I increase the collector sweep voltage, the breakdown voltage knee moves farther and farther left on the screen as the positive current increases and the positive current trace stays stationary at about .4V . A "known good" 5.1V Zener does not exhibit this strange characteristic. The known good ZENER knee appears and holds at the correct Zener voltage, regardless of the collector sweep voltage. I am fortunate enough to also have a 577 curve tracer as well, and it shows the same characteristic.

Is this some sort of diode that I have never heard of? Or could this be the troublesome component? Sorry if my explanation is not technically correct or as clear as it could be. I am dealing with something that I have not experienced before.

Any thoughts?