Date   
SD-26 disassembly, how to separate sampling gate block from pcb

Albert Otten
 

Does anyone know how to remove the complete sampler block from the printed circuit board? I ask since I'd like to replace the channel select switches. The solder lips are hidden between sampler block and pcb. I removed the 6 block attachment screws but still I can't remove the block with reasonable force. Maybe more force is needed because there are several contact pins between this block and the pcb. The nice close-up repair photos by Leo Bodnar show the internals of the sampler block but not how to remove the block (the block perhaps was still in situ during repair).
It seems that the tiny plastic push pins of the switches have been cut away deliberately. I guess this has been done to prevent any accidental touches by an operator. I can still activate the switches using a small screw driver, so I don't want to take the risk of damaging the otherwise functioning head.

Albert

Re: Switching power supplies

Chuck Harris
 

SMD electrolytic capacitors are a tricky problem in several ways:

First, they are easily damaged by the heat of the reflow oven,
and solvents that may be used in cleaning flux,... though cleaning
flux isn't done much if at all, on consumer grade equipment.

Second, it is almost impossible to find out what was originally
installed. There are no standardized markings to identify the
manufacturer, or what grade capacitor was installed.

Third, it is hard for a re-worker to find a complete spectrum of
ESR values from which to select replacement capacitors.

When you grab an assortment of SMD electrolytic capacitors off
of ebay, you are unlikely to be able to match more than just the
capacitance and voltage of the original. Never the ESR or lifetime
rating.

When you buy from Mouser, or DigiKey, you will find that cute
little 20uf, 16V cap you need to replace, (because its ESR is 3 ohms
while its cohorts are all 0.5 ohm), can only be had with ESR's of
0.7 ohm, 3 ohms, and 12 ohms! Replace the 0.5 ohm ESR caps with a
0.7 ohm cap, and you have taken the expected life of the cap and cut
it in half (or more). Put in a 3 or 12 ohm version, and you will have
improved nothing.

Tricky indeed!

-Chuck Harris

Kevin Oconnor wrote:

I have to agree with Chuck. Most test equipment with imbedded/integrated switchers are going to be very difficult to qualify parts if working. Their functionality can be extremely subtle. Blanket replacement of components will more likely cause additional reliability issues.
My 1978 Tek 485 has had spot repairs, but it’s still working fine. I’d never replace caps in it without good reason.
I have an HP 500MHz digital scope with one of this 3rd party modular switchers. (I forget the HP model but 5digits). Switcher went bad. No docs, no schematics. Really hopeless to diagnose. Best option was an eBay replacement switcher. $50 and never looked back.
Now if you are talkin about bad caps in an LCD monitor or TV, that’s where “replace em all” will serve u well! Lots of crap used in those devices.

K

Sent from kjo iPhone



Re: Switching power supplies

David Kuhn
 

Switching supplies can be a problem when they fail. I recently had a
Agilent VXI E4808A chassis that the main power supply failed, at least
its 12 volts out did. That power supply is huge with a logic board mounted
about it ( I think it's a power supply monitor board). It's a lot of
physical work just to get it apart to check caps. Anyway, I doubt
HP/Agilent made that power supply themselves. I really don't know, as to
get information out of them is like pulling hen's teeth. They no longer
sell, or support, those 4 slot VXI chassis's, so it would be nice of them
to release the schematics, or service information. I think you can only
hope to buy a used chassis somewhere. I would love to have the schematics
for them.

In other switches that fail to start, I often find what I call the
"start-up" capacitor in the primary is open or leaking or has a high ESR.
I call it the "start-up cap" as I do not fully understand switching
supplies, and often if there are schematics, there is not a theory of
operation with them, but there is often a capacitor in the primary circuit
that looks to be a short to ground and then charge-up to create the initial
switch swing to get it going and then afterwards, the power supply self
sustains.

So a lot of times with a dead switcher, I have been able to fix them by
replacing the small electrolytic in the primary side of the circuit. It is
usually a 4.7uf or 10uf. Other than that, if the rest are not physically
leaking or swelled and the supply is working I don't touch it.

One instrument that I work on has an on-board +5volt switching regulator
circuit. It is very reliable. Two times (since 1999) I have seen the
switch regulator fail where the output drive to the FET shorts to ground
allowing the supply voltage (~+12volts) go through to the output with no
over-voltage protection, not even a +5.6volt zener to short. It blows
every TTL chip on the +5volt rail, and some some regulators that follow it.
Stupid, stupid German design. You have to be very urber careful probing
that +5volt switching regulator circuit. One slip of the scope probe and
you can simulate the switching regulator IC output shorting to ground
turning on the pass FET full time. So, I've learned "If it aint broke,
don't fix it"!

Sorry it that was slightly off topic.

Dave

On Wed, Mar 27, 2019 at 8:29 AM Chuck Harris <cfharris@...> wrote:

SMD electrolytic capacitors are a tricky problem in several ways:

First, they are easily damaged by the heat of the reflow oven,
and solvents that may be used in cleaning flux,... though cleaning
flux isn't done much if at all, on consumer grade equipment.

Second, it is almost impossible to find out what was originally
installed. There are no standardized markings to identify the
manufacturer, or what grade capacitor was installed.

Third, it is hard for a re-worker to find a complete spectrum of
ESR values from which to select replacement capacitors.

When you grab an assortment of SMD electrolytic capacitors off
of ebay, you are unlikely to be able to match more than just the
capacitance and voltage of the original. Never the ESR or lifetime
rating.

When you buy from Mouser, or DigiKey, you will find that cute
little 20uf, 16V cap you need to replace, (because its ESR is 3 ohms
while its cohorts are all 0.5 ohm), can only be had with ESR's of
0.7 ohm, 3 ohms, and 12 ohms! Replace the 0.5 ohm ESR caps with a
0.7 ohm cap, and you have taken the expected life of the cap and cut
it in half (or more). Put in a 3 or 12 ohm version, and you will have
improved nothing.

Tricky indeed!

-Chuck Harris

Kevin Oconnor wrote:
I have to agree with Chuck. Most test equipment with imbedded/integrated
switchers are going to be very difficult to qualify parts if working. Their
functionality can be extremely subtle. Blanket replacement of components
will more likely cause additional reliability issues.
My 1978 Tek 485 has had spot repairs, but it’s still working fine. I’d
never replace caps in it without good reason.
I have an HP 500MHz digital scope with one of this 3rd party modular
switchers. (I forget the HP model but 5digits). Switcher went bad. No docs,
no schematics. Really hopeless to diagnose. Best option was an eBay
replacement switcher. $50 and never looked back.
Now if you are talkin about bad caps in an LCD monitor or TV, that’s
where “replace em all” will serve u well! Lots of crap used in those
devices.

K

Sent from kjo iPhone





Re: Switching power supplies

Chuck Harris
 

In switcher parlence, the "start up" capacitor is called the
bootstrap capacitor because the supply picks itself up by its
bootstraps. It is often charged by using a high wattage resistor
connected to the power line, with a diode, capacitor, and zener
to limit its DC voltage. It supplies power to the control PWM
circuitry. When the supply starts, the bootstrap will be taken
over by voltage from a winding on the switcher transformer.

Half wave rectifying creates a lot of ripple current in these
capacitors, and they very often... and I mean VERY often go open
circuit, preventing the supply from starting.

I have fixed many tens of thousands of dollars of equipment ranging
from sewing machines to TV sets by replacing the bootstrap capacitor
and sometimes the bootstrap resistor.... Even HP supplies ;-)

Competently designed switching power supplies are a delight to
behold. They seem to protect themselves from everything, including
stupid technician mistakes.

-Chuck Harris

David Kuhn wrote:

Switching supplies can be a problem when they fail. I recently had a
Agilent VXI E4808A chassis that the main power supply failed, at least
its 12 volts out did. That power supply is huge with a logic board mounted
about it ( I think it's a power supply monitor board). It's a lot of
physical work just to get it apart to check caps. Anyway, I doubt
HP/Agilent made that power supply themselves. I really don't know, as to
get information out of them is like pulling hen's teeth. They no longer
sell, or support, those 4 slot VXI chassis's, so it would be nice of them
to release the schematics, or service information. I think you can only
hope to buy a used chassis somewhere. I would love to have the schematics
for them.

In other switches that fail to start, I often find what I call the
"start-up" capacitor in the primary is open or leaking or has a high ESR.
I call it the "start-up cap" as I do not fully understand switching
supplies, and often if there are schematics, there is not a theory of
operation with them, but there is often a capacitor in the primary circuit
that looks to be a short to ground and then charge-up to create the initial
switch swing to get it going and then afterwards, the power supply self
sustains.

So a lot of times with a dead switcher, I have been able to fix them by
replacing the small electrolytic in the primary side of the circuit. It is
usually a 4.7uf or 10uf. Other than that, if the rest are not physically
leaking or swelled and the supply is working I don't touch it.

One instrument that I work on has an on-board +5volt switching regulator
circuit. It is very reliable. Two times (since 1999) I have seen the
switch regulator fail where the output drive to the FET shorts to ground
allowing the supply voltage (~+12volts) go through to the output with no
over-voltage protection, not even a +5.6volt zener to short. It blows
every TTL chip on the +5volt rail, and some some regulators that follow it.
Stupid, stupid German design. You have to be very urber careful probing
that +5volt switching regulator circuit. One slip of the scope probe and
you can simulate the switching regulator IC output shorting to ground
turning on the pass FET full time. So, I've learned "If it aint broke,
don't fix it"!

Sorry it that was slightly off topic.

Dave

Re: Switching power supplies

Tom Gardner
 

On 27/03/19 06:05, Kevin Oconnor wrote:
My 1978 Tek 485 has had spot repairs, but it’s still working fine. I’d never replace caps in it without good reason.
Personally I'd replace C911: 22uF 15V tant bead on a 13V PSU rail, diagram 17 (not the PSU diagram!), and IIRC there are a couple of others on that rail, hidden on the other diagrams 7, 9, 11, 16.

When C911 shorted, it toasted R965.

Re: 11801 calibrator rise time

Albert Otten
 

Reg,

If you display the two channels simultaneously then they share the same time base speed and position. I don't see how you tried to align the rising edges. On my CSA's I do this: display both channels CH1 and CH2, then Store CH1 en turn CH1 off, then Recall the stored trace of CH1. Now the displayed stored trace is "frozen" and remains in position regardless how you change the time base of the CH2 trace. (Note you can select one of the displayed traces by touching the trace somewhere.)

Albert

On Tue, Mar 26, 2019 at 11:10 PM, Reginald Beardsley wrote:


BTW I have a 2 port divider feeding the upper channel of my SD-22s and the
waveforms look to be exactly the same. The pieces of hardline from the
divider to the heads are different lengths and I have not been able to shift
one relative to the other to see how closely they overlay as there is a 340 ps
delay between the two. The divider increases the rise time to 42 ps. It's an
MBC Technology unit. No frequency rating specified.

Re: 11801 calibrator rise time

Reginald Beardsley
 

That was my conclusion. But the UI is so bad that I thought there might be a variable delay line in there somewhere that would adjust channel skew. I've also discovered it is buggy as all hell. You pretty much have to reinitialize the scope for a bunch of operations. I tried switching from vector to dot mode. Menu said vector was off, but the display said otherwise. Once I initialized the scope I was able to set it to dot mode.

I've had the 11801 running for 4-5 hours with the calibrator feeding an SD-22 with cursors set on the peaks of the reflection in the cable. I've noticed that the internal timebase varies by a little over 1 ps over the course of 5-10 minutes. I have a postit on the screen and the cursors are not moving. The waveform is moving. I've got the internal clock output connected to my 5386A with the OCXO option and that confirms the internal time base period is changing.

From looking at the block diagram in the service manual, it appears to me that the OCXO is not as stable as the one in my 5386A which I have compared to a GPSDO over long periods.

This has me considering whether installing one of Leo Bodnar's single output GPSDOs in place of the 200 MHz OXCO would be worthwhile. In particular in improving the jitter spec from the factory 1.1 ps.

Watching the clock period vary by 100 femtoseconds over the course of 15-30 seconds with a 1 s gate time is rather interesting.


Reg

--------------------------------------------

On Wed, 3/27/19, Albert Otten <aodiversen@...> wrote:

Subject: Re: [TekScopes] 11801 calibrator rise time
To: TekScopes@groups.io
Date: Wednesday, March 27, 2019, 12:57 PM

Reg,

If you display the two channels simultaneously
then they share the same time base speed and position. I
don't see how you tried to align the rising edges. On my
CSA's I do this: display both channels CH1 and CH2, then
Store CH1 en turn CH1 off, then Recall the stored trace of
CH1. Now the displayed stored trace is "frozen"
and remains in position regardless how you change the time
base of the CH2 trace. (Note you can select one of the
displayed traces by touching the trace somewhere.)

Albert

Need Help repairing a 7104 oscilloscope

 

Hello all

I just got a 7104 for 30 dollars. With all 4 plugins. Including the 1ghz
one.

I need help fixing it. It powers on but the trace is wide and choppy . I
created a video. Please see it and let me know. I will start with
checking the volts on the different rails.


https://youtu.be/wMGDOzYwLys


Thx in advance for your help

Re: Greeting from Stan

 

Hello, I'm Armando

I'm new here.. I just bought a 7401 for 30 bucks. , it turns on, and if I play with the buttons and give a little smack , I get a distorted, extra wide and chopped up trace.
anyways. I will be doing the dismantleling soon and visual inspection, looking forward for some help, getting this puppy up and working. :}

Armando

Re: Hello World

 

Welcome, I'm not afraid, my name is Oscillator Sherman

You have any vidoes or PIctures of the monster oscilloscope
I just bought a 1ghz tektronix 7104 for 30 dollars Partially working.
Here is my video

https://www.youtube.com/watch?v=wMGDOzYwLys

Re: New: Member Intro

 

Welcome, I have 1 7401 with this Plug ins: 7A18A , 7A29 , 7B53A , 7b15
I need help rebuilding it.
It turns on, and If I give a little smack and play with the buttons, eventually I got a distored glitched out Fat and chopped trace. :(
wil start with the visual inspection and cleaning first.. we'll how to goes from there

Any one with experience on this is Local " Los Angeles, CA"

Tek 2465B Calibrator issues

Marcial Gomez Varas
 

Hi everyone,

I bought some time ago (2017) a TEK 2465B that I didn't have too much time to play with (having 2 kids and a travelling job doesn't help!)
I have been toying with it lately, and I've discovered that the calibrator is showing some really awful square signal.

It's clearly NOT square ! It has quite a bit of slope in the rise and fall lines, and bouncing on the high level.
I have checked and both the output levels and frequency seem to be OK, but somehow the signal is distorted.

Could it be some capacitor? Looking at the service manual it shows to be in the A1 board, section 5 (That should be in the middle left side, am I right?)

Any help would be appreciated !

Cheers,
Marcial

Tek 2710 focus issue

john@...
 

Hi,
I have tried adjusting the rear focus control to get a good focus on the screen, but it just 'jumps about'

I have tried the trimmer on the main PCB underside of the unit which allows me to adjust to focus, but power down and focus is blurr!

Any help would be much appreciated.

Regards

John

Need Help fixing my newly acquired 7401

 

Hello guys

I just bought a 7104 fully loaded with plugin for 30 bucks from this guy on offer up in California. But its partially working.

I posted a video of what it does.
I'm excited to get some pointers on where to star and get this Vintage Gem working again. Voltage rails any specific capacitors. It's the issue the plug ins ?


https://youtu.be/wMGDOzYwLys

Re: Tek transformer 120-0866-02

Richard Katezansky
 

According to the recently scanned and released transformer list on VintageTek it's for the 408, 412, or 414 portable patient monitors.
Mid to late 70's vintage.

Richard

TEK 475 voltage issues.

Glen Layne
 

Hey all. I'm new to the group and trying to restore 475 scope. when I got it, 2 of the voltages were way off. The +110 was about +19 and the +105/160 was coming in at +10.7. The other voltages were fine. So, I ordered and replaced the 6 main caps. Q1497 was bad and I replaced it. Now the +110 is coming at 86.1 and the +105/160 is still at 10.7.

So, the scope appears to work. Surprisingly well considering the voltage issues I still have. I can't find what's bringing down the +110 and the +105/160 at 10.7 can't be good. Q1456 (darlington) runs pretty hot and it's on the +105 rail.

On occasion, the trace will shrink to postage stamp size. I can usually adjust the Horizontal position all the way to the right and it will come out of the postage stamp mode even though it may not be a stable trace. Sometimes, I double push the 10x Mag and fiddle with the Horiz position to get things back working.

Any ideas what to do next?

Advice sought on 7934 vs 7904A mainframe

John
 

I have an opportunity to buy a 7934 mainframe. I would have preferred a 7904A but the 7934 has come up. I am not all that impressed with storage tube scopes but ignoring the storage function how do the 2 mainframes compare? They are both 500 MHz units so the question becomes are there any real or effective differences in their performance? Are there any weak spots in either of these two mainframes? On the surface ignoring the storage capability of the 7934 it and the 7904A look fairly similar. Can I get some feedback on these two units from the group.

John Proctor
VK2DLP

Re: Tek 2710 focus issue

Chuck Harris
 

There is this long string of 10M resistors that forms the
focus string... They tend to be open.

-Chuck Harris

john@... wrote:

Hi,
I have tried adjusting the rear focus control to get a good focus on the screen, but it just 'jumps about'

I have tried the trimmer on the main PCB underside of the unit which allows me to adjust to focus, but power down and focus is blurr!

Any help would be much appreciated.

Regards

John



11801 question

Reginald Beardsley
 

I've got the unit running with a pair of SD-22s fed by a 2 port splitter from the calibrator into ports 5 & 7. I was finally able to figure out how to manually adjust the skew so the two traces overlaid. The cables are about 1/2 mm different lengths

I'm seeing the displacement between the two traces vary from 0 to about 10 ps over the course of a few minutes. Is this normal or is there a fault? I've read the user manual once, but it's not great. Certainly no match for a 465 or 485 manual.

Does anyone know why this is happening? I've got the internal clock output connected to an HP 8356A and I can see the period varying. I don't have the 8356A connected to a GPSDO at the moment, but it has the OXCO option and is very stable.

Is there any reason to think that replacing the 200 MHz master oscillator with one of Leo Bodnar's GPSDOs would eliminate the interchannel drift?

I discovered this shortly after making an offer for four SD-26 heads. My intended use is to measure actual FPGA word skew of a DSP stream, so this would be a serious problem.

Thanks,
Reg

Re: Tek 2465B Calibrator issues

Roy Thistle
 

Hi:
It's obvious, so you probably are... but, are you using a properly calibrated probe?
Regards.