Date   
Re: TDS460A Will not turn on?

Mlynch001
 

On Sat, Feb 22, 2020 at 12:56 AM, <toby@...> wrote:



Fantastic! As a fellow TDS 460A owner, I am quite impressed and pleased
you got it going again. Once again the list collects an important repair
story for posterity...

--Toby
Toby,

Thank you! I really like the TDS460A as it has some very useful features. I bought this one on E-Pay No Reserve Auction for less than $50 USD, including freight. It was listed as "not-working" and "parts only". To my surprise, it fired right up as soon as I unboxed it and Passed all the self tests. I used it for about a year, until the Power supply suddenly died. So I have almost nothing invested in a very capable scope and have received a very important education into SMPS design and repair. I now have two 460A's and both working,with almost no monetary investment.


--
Michael Lynch
Dardanelle, AR

Re: TDS460A Will not turn on?

Mlynch001
 

In some Tek power supplies, I have observed that a degrading opto-coupler demands more and more current from a TL431A, eventually causing the TL431A to fail.<
John,

Thanks for that information, I have precious little experience with these very complex TEKTRONIX SMPS's. That is confirmation of my suspicion as well. Since there was so much discussion by others that pointed to the opto-couplers, I was compelled to replace them. Those little things are cheap, so out with the old and in with the new. Same with the 3844A, One simply cannot risk using the old part, since those are available and cheap. I am sure that you are on the right track, those failures very likely were related to stress and age. By a process of elimination, I had gone through most of the other discreet components and struck them from the list of possible sources of the problem. I even pulled the filter caps from the Output side of the supply and they checked perfect and were without any leaks. I did find one diode related to the 3844A that checked "Questionable" with the DMM, but when placed in the curve tracer (out of circuit), it checked good when compared to a known good part. After exhausting all the "usual suspects", I felt that it MUST have come down to a failure of one of the very few IC's in the circuit, at least this was my reasoning. As far as component cost, It was less than $5 USD to repair the supply, unless one counts the hours that I spent making the repair. Seeing these power supplies listed on E-Pay for $100-$300 inspired me to at least try to make a repair. Most importantly, I learned some valuable lessons, that was my "payment" for those many hours of work. All that being said, I would not have had the nerve to tackle this project before becoming a member of this group. I appreciate the wonderful advice and guidance that I have received.

--
Michael Lynch
Dardanelle, AR

Re: Weird 466 horizontal glitch

Edward Prest
 

Good idea.
With beam finder pressed, it does not show the problem.This is an important clue!
in xy mode it does glitch too, as every other setting.

I have checked Z output and its clean. (it leaks the z sum even though its an input)

more reading the manual with emphasis on the differences with beam on and off next i guess

Re: Repairing my Tektronix 454 oscilloscope need some advice

Victor
 

Albert,Thank you. I was looking for some information regarding the HV transformer terminals and during the search I read a post from a member that repair the HV circuit after found that C1460 has leakage current. Before made the measurement that you recommend I decide to disconnect C1460 and see what happens. I was very surprise when I energize the 454 and see flashing the terminals open at least 1cm (1/2 inch) distance. I believe if this happens is because the HV increase maybe to 12kV...Then I disconnect the power and measure the C1460, between ground and the terminal open with my Fluke, and get around 1Mohm...So my question is do you think C1460 is dead ? How can be sure ?
Thanks,Victor

-----Original Message-----
From: Albert Otten <aodiversen@...>
To: TekScopes <TekScopes@groups.io>
Sent: Thu, Feb 20, 2020 11:05 pm
Subject: Re: [TekScopes] Repairing my Tektronix 454 oscilloscope need some advice

Victor,
I would first of all test the waveforms at terminals 7, 9 and 10 and check that the amplitudes have more or less the same  ratios to each other (and perhaps to the 1 V pp) as in my case. Your 52 V is really to low, so check it against the (negative) peak voltage at terminal 9. The secondary waveforms may show a slightly flat negative peak because of diode conduction, but only slightly. At terminal 9 the negative "flat" peak was about 5 V smaller than the "round" positive peak.
Can you measure the primary current? Perhaps your generator can show the current drawn?
For test HV diodes see the message by Chuck Harris or see message  #163125).

Albert

On Thu, Feb 20, 2020 at 09:42 PM, Victor wrote:

Albert,
This afternoon, I was reviewing the measurement done yesterday and found a
problem (intermittent connection) with the cable connecting the wave generator
to the Q1430 collector. After repair it I test again my 454 and now with a
1Vpp on the collector of the Q1430, I get 52V at the HV test point.So, with
this result I believe my HV transformer is Ok. I have a doubt about the HV
diodes... what do you think ? By the way how do you test HV diode,I understand
you cannot do it with a regular tester.Tomorrow before made the test that you
recommend today. I would like to review the HV CRT circuit to see if any
resistor or capacitor are out of specs (transistors have been test and look
good).
Thank you,Victor

Re: TDS460A Will not turn on?

John Gord
 

Michael,
In some Tek power supplies, I have observed that a degrading opto-coupler demands more and more current from a TL431A, eventually causing the TL431A to fail.
--John Gord

On Fri, Feb 21, 2020 at 09:26 PM, Mlynch001 wrote:


After 6 months of on and off searching, I may have found the issue with my
SMPS for my TDS460A. No schematics are available, so it comes down to trying
to understand what goes on in one of these supplies and, more importantly,
getting some great guidance and advice from some of the many sharp people in
this group. I replaced UC3844A and both opto-couplers, this did not repair
the problem. So I started thinking, perhaps one of the individual shunt
regulators on the regulation board for the 4 supplies was bad? I removed all
four of these TL431A’s from the regulator board, checked them against known
good parts, found 2 that tested as resistors and replaced them all. I was not
sure exactly how to use the curve tracer to test these, but placing them in
the instrument and testing known good on one side and suspect parts on the
other side, back to back, I was able to see that the parts tested radically
differently. So the ones that did not agree with the known good parts could
safely be judged as “defective”. After installing the 4 new shunt
regulators, I put the supply back together, reinstalled it, powered it up and
the scope comes back to life. So my thought was that these two bad regulators
caused the supply to shut down or prevented it from starting in the first
place. Not sure why they failed, but at least I have my scope back in
operation.
--
Michael Lynch
Dardanelle, AR

Re: TDS460A Will not turn on?

toby@...
 

On 2020-02-22 12:26 AM, Mlynch001 wrote:
After 6 months of on and off searching, I may have found the issue with my SMPS for my TDS460A. No schematics are available, so it comes down to trying to understand what goes on in one of these supplies and, more importantly, getting some great guidance and advice from some of the many sharp people in this group. I replaced UC3844A and both opto-couplers, this did not repair the problem. So I started thinking, perhaps one of the individual shunt regulators on the regulation board for the 4 supplies was bad? I removed all four of these TL431A’s from the regulator board, checked them against known good parts, found 2 that tested as resistors and replaced them all. I was not sure exactly how to use the curve tracer to test these, but placing them in the instrument and testing known good on one side and suspect parts on the other side, back to back, I was able to see that the parts tested radically differently. So the ones that did not agree with the known good parts could safely be judged as “defective”. After installing the 4 new shunt regulators, I put the supply back together, reinstalled it, powered it up and the scope comes back to life. So my thought was that these two bad regulators caused the supply to shut down or prevented it from starting in the first place. Not sure why they failed, but at least I have my scope back in operation.
Fantastic! As a fellow TDS 460A owner, I am quite impressed and pleased
you got it going again. Once again the list collects an important repair
story for posterity...

--Toby


Re: TDS460A Will not turn on?

Mlynch001
 

After 6 months of on and off searching, I may have found the issue with my SMPS for my TDS460A. No schematics are available, so it comes down to trying to understand what goes on in one of these supplies and, more importantly, getting some great guidance and advice from some of the many sharp people in this group. I replaced UC3844A and both opto-couplers, this did not repair the problem. So I started thinking, perhaps one of the individual shunt regulators on the regulation board for the 4 supplies was bad? I removed all four of these TL431A’s from the regulator board, checked them against known good parts, found 2 that tested as resistors and replaced them all. I was not sure exactly how to use the curve tracer to test these, but placing them in the instrument and testing known good on one side and suspect parts on the other side, back to back, I was able to see that the parts tested radically differently. So the ones that did not agree with the known good parts could safely be judged as “defective”. After installing the 4 new shunt regulators, I put the supply back together, reinstalled it, powered it up and the scope comes back to life. So my thought was that these two bad regulators caused the supply to shut down or prevented it from starting in the first place. Not sure why they failed, but at least I have my scope back in operation.
--
Michael Lynch
Dardanelle, AR

Re: Scope auction

Robert Simpson
 

It might be bought for a reasonable price except for shipping.
Bob

Re: Weird 466 horizontal glitch

Mlynch001
 

Edward,

I am going to ask what may seem a stupid question. Have you cleaned the "Beam Finder" switch? I have a 465 that was doing something similar. It had a dirty beam finder switch. The switch SEEMED to be releasing, but in reality, it must have contained some sort of conductive crud inside, this caused some of the contacts to remain on (or partially on) when they should have been off. Clean with recommended cleaner, I use DE-OXIT after cleaning and vigorously exercise the switches. These switches can do some crazy things. Go after the simple stuff first, you might be pleasantly surprised.

--
Michael Lynch
Dardanelle, AR

Re: Repairing my Tektronix 454 oscilloscope need some advice

Jim Ford
 

Roger that, Roger!  At least somebody is on the ball these days.Thanks for the correction. JimSent from my Verizon, Samsung Galaxy smartphone

-------- Original message --------From: "Roger Evans via Groups.Io" <very_fuzzy_logic=yahoo.com@groups.io> Date: 2/21/20 7:28 AM (GMT-08:00) To: TekScopes@groups.io Subject: Re: [TekScopes] Repairing my Tektronix 454 oscilloscope need some advice True, but the period of the 50kHz oscillation is 20 microseconds not 20 milliseconds so the recovery time is indeed 200 x Trr.Roger

Weird 466 horizontal glitch

Edward Prest
 

My new to me 466 2nd version has a strange issue.
I see a background image related to the display. Hard to describe so there is a picture in photos under Edward Prest.
It may be a blanking issue or a horizontal issue. I see two lines at the peak and valley of all signals across the CRT. There is no DMM. I have to turn up the intensity some to see them. The bright parts do interfere with the display. Sine or square - same issue - only at the peak and valley. In the images the positions are shifted far to the side to show the artifact clearly. Shifting it left or right has a tiny effect but more or less glitch remains in place.

I have the service manual but its a big read and it will take me a while before i dig in. I have measured the power supplies, all low voltage rails are are just barely above the book spec except for 140v which is 127V - a few volts under min spec.

(p.s. I am using 60hz line noise for the sine wave in the image -that's all I have for the moment)

Re: Repairing my Tektronix 454 oscilloscope need some advice

Roger Evans
 

True, but the period of the 50kHz oscillation is 20 microseconds not 20 milliseconds so the recovery time is indeed 200 x Trr.

Roger

Re: Repairing my Tektronix 454 oscilloscope need some advice

Roger Evans
 

Indeed, but the period of a 50kHz is oscillation is 20us not 20ms so Ernesto's times 200 was actually correct.

Roger

Re: Scope auction

Bob Headrick
 

I see they also have a Tek 7854 in the previous picture, lot 19 in the catalog here: https://bid.tmcauction.com/auctions/catalog/id/23165/

I wish I was a bit closer to Sacramento.

- Bob Headrick

-----Original Message-----
From: TekScopes@groups.io <TekScopes@groups.io> On Behalf Of Robert Simpson via Groups.Io
Sent: Sunday, February 16, 2020 7:40 PM
To: TekScopes@groups.io
Subject: [TekScopes] Scope auction

About the middle of the page:
https://www.estatesales.net/CA/Rocklin/95677/2475516

No connection, but is in a neighboring town.
That 7904A looks like it has 7S12 in it with modules Bob

Re: Scope auction

Bob Headrick
 

-----Original Message-----
From: TekScopes@groups.io <TekScopes@groups.io> On Behalf Of Robert Simpson via Groups.Io
Sent: Sunday, February 16, 2020 7:40 PM
To: TekScopes@groups.io
Subject: [TekScopes] Scope auction

About the middle of the page:
https://www.estatesales.net/CA/Rocklin/95677/2475516

No connection, but is in a neighboring town.
That 7904A looks like it has 7S12 in it with modules Bob

Re: Repairing my Tektronix 454 oscilloscope need some advice

Chuck Harris
 

Wonderful! I just wanted to be sure that you and others were
aware of the issue.

Also, many of these HV diodes are designed to be used in old
color TV's, and CRT type monitors, so they need to be faster
than they would for the old style of transformer driven microwave
oven.

New style microwave ovens use inverters, and as such could be
pretty high frequency... I have never taken one apart.

-Chuck Harris

Ernesto wrote:

Hi Chuck,

You are very right about the importance of the diode recovery time. The specification on Amazon for this cheapy diode is Trr = 100 nS maximum. Other "fast" HV diodes specify 60 or 70 nS, not a big difference.
At 50 kHz the period of the rectified sine wave is 20 mS, 200 times longer than the Trr. During these 100nS the peak of the sine wave will barely comes down, so the operation should be safe. Indeed when I installed these diodes the efficiency of the supply increased, and it behaved correctly.

- Ernesto



Re: Repairing my Tektronix 454 oscilloscope need some advice

 

Thank you Jim!

I need to be more careful with my postings and avoid dumb mistakes! I try not to think that it is the years...

The period of a 50 kHz wave is 20 uS, not 20mS. So this period is still 200 times longer than Trr. And in Trr = 100nS, the wave has not significantly receded from its peak, so the capacitor will not be significantly discharged during this time.

Regards,
Ernesto

Re: Repairing my Tektronix 454 oscilloscope need some advice

Jim Ford
 

Hi, Ernesto.

Actually, 20 ms is 200,000 times longer than 100 ns. Just sayin'.

Jim Ford

------ Original Message ------
From: "Ernesto" <ebordon@...>
To: TekScopes@groups.io
Sent: 2/20/2020 6:41:48 PM
Subject: Re: [TekScopes] Repairing my Tektronix 454 oscilloscope need some advice

Hi Chuck,

You are very right about the importance of the diode recovery time. The specification on Amazon for this cheapy diode is Trr = 100 nS maximum. Other "fast" HV diodes specify 60 or 70 nS, not a big difference.
At 50 kHz the period of the rectified sine wave is 20 mS, 200 times longer than the Trr. During these 100nS the peak of the sine wave will barely comes down, so the operation should be safe. Indeed when I installed these diodes the efficiency of the supply increased, and it behaved correctly.

- Ernesto


Re: Repairing my Tektronix 454 oscilloscope need some advice

 

Hi Chuck,

You are very right about the importance of the diode recovery time. The specification on Amazon for this cheapy diode is Trr = 100 nS maximum. Other "fast" HV diodes specify 60 or 70 nS, not a big difference.
At 50 kHz the period of the rectified sine wave is 20 mS, 200 times longer than the Trr. During these 100nS the peak of the sine wave will barely comes down, so the operation should be safe. Indeed when I installed these diodes the efficiency of the supply increased, and it behaved correctly.

- Ernesto

Re: DS1742W-120 replacement adapter?

victor.silva
 

BTW, I downloaded KiCAD 5.1.5-3 and was able to completely browse the design files from the EEVBlog converter.
Very nice for the price (free).

--Victor