Date   
Re: help required

Craig Sawyers <c.sawyers@...>
 

dear members..
as i am planning to buy an oscilloscope of textronic make, kindly guide me which model to
purchase as
i have options to either 7904 or 7603.
what are the advantageous and disadvantageous of one over the other..
i am in the learning process... and i have just begun to learn ... i am planning to get an active
probe
p6201 with it..
aim is to learn and if possible touble shoot circuits as a hobbist.. thanks vivek
Both are fine scopes. Differences are:

7603 - three plugin bays, and the usual set up is a 7B53 dual timebase and two 7A18 dual channel
vertical amps. It has a large screen - 1.22cm per division, but has lower bandwidth than a 7904.
The advantage is that maintenance is much more straightforward, since it has a linear power supply.
The mainframe itself has a bandwidth of at leat 115MHz, and with the 7A18 75MHz bandwidth.

7904 - four plugins, 1cm/division screen (so visually much smaller than the 7603 screen), and the
usual set up is the 7B80/7B85 timebase pair and two 7A26. That will do 200MHz. But if you combine
it with the 50-ohm 7A19 you can do 500MHz. The disadvantage is that the mainframe uses Tek's arcane
switched mode power supply - and if you have a problem with that it is very difficult to fix; there
are ones that I have totally failed in fixing after long effort, and many others on this list have
likewise failed. But that power supply is in the majority of 7000 series scopes. The particular
difficulty with the 7904 is that the HT multiplier that produces the EHT for the CRT is buried right
in the middle of the power supply, which was a real dumb decision by Tek.

I guess I'm coming down to advise you to go for a 7603. There is just a lot less to go wrong, and
if something does there are a whole bunch right here to hand hold you through fixing it.

Stay away from the P6201 though. You really need to know what you are doing to use it. Instead buy
a few passive probes. I have a P6201 and hardly ever need to use it - I grab a x1 or x10 passive
probe for 99% of anything I ever do.

Craig

Re: help required

Tom Gardner
 

On 28/02/16 10:23, d.vivekj@... [TekScopes] wrote:

dear members..
as i am planning to buy an oscilloscope of textronic make, kindly guide me which model to purchase as i have options to either 7904 or 7603.
what are the advantageous and disadvantageous of one over the other..
i am in the learning process... and i have just begun to learn ... i am planning to get an active probe p6201 with it..
aim is to learn and if possible touble shoot circuits as a hobbist.. thanks vivek
Oscilloscope are specialised tools in the same way that motor vehicles are specialised tools. You wouldn't ask "what car is better" without saying what you want to do with the car - sometimes small size is important (e.g. if parking in cities), sometimes 4WD is important (e.g. driving on snow or sand). Similarly, with scopes you will get a better answer if you say what circuits you intend to measure.

Also, as with cars, beginners will probably find it easier to learn how to "drive" by learning on a modern fully-functional car. The 7603 and 7904 and plugins are both old and may well be subtly faulty.

Re: help required

Malcolm Hunter
 

On 28 February 2016 at 10:23, d.vivekj@... [TekScopes] <
TekScopes@...> wrote:

as i am planning to buy an oscilloscope of textronic make, kindly guide me
which model to purchase as i have options to either 7904 or 7603.
what are the advantageous and disadvantageous of one over the other..
i am in the learning process... and i have just begun to learn ... i am
planning to get an active probe p6201 with it..
aim is to learn and if possible touble shoot circuits as a hobbist..
​That's what I call starting at the deep end. If you're just starting
electronics as a hobby, it might be easier if you begin with a smaller
oscilloscope like the 465 or one of the 2000 series. Do you have a
particular need for an active probe? What frequency ranges are you
anticipating working on?

Malcolm​

Re: help required

stefan_trethan
 

Hi Vivek,

Both scopes are analog, non-storage.
The 7603 is only 100MHz and 3-bay, vs. 500MHz and 4-bay for the 7904.
Do you need the high frequency?
What plugins do they come with?

What on earth do you want the FET probe for?
It is not useful for general troubleshooting.

ST


On Sun, Feb 28, 2016 at 11:23 AM, d.vivekj@... [TekScopes]
<TekScopes@...> wrote:
dear members..
as i am planning to buy an oscilloscope of textronic make, kindly guide me which model to purchase as i have options to either 7904 or 7603.
what are the advantageous and disadvantageous of one over the other..
i am in the learning process... and i have just begun to learn ... i am planning to get an active probe p6201 with it..
aim is to learn and if possible touble shoot circuits as a hobbist.. thanks vivek






------------------------------------
Posted by: d.vivekj@...
------------------------------------


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

Yahoo Groups Links


help required

vivek jayaraj
 

dear members..
as i am planning to buy an oscilloscope of textronic make, kindly guide me which model to purchase as i have options to either 7904 or 7603.
what are the advantageous and disadvantageous of one over the other..
i am in the learning process... and i have just begun to learn ... i am planning to get an active probe p6201 with it..
aim is to learn and if possible touble shoot circuits as a hobbist.. thanks vivek

Re: Request for assistance from Nigel

 

Thank you for posting your results here.

The A Trigger Level Center adjustment (R643) only has a range of
+/-1.5 volts so some signals are going to be outside of the trigger
level range if you have a Vgs of 3.53 volts. Solutions include:

1. If you have a bunch of J310s, grade them for one with the smallest
Vgs.

2. Use a JFET with a smaller Vgs like a J309.

3. Install a 2.7 volt zener diode in series with the source of Q623 or
the emitter of Q624 and bypass the zener diode with a 0.1 to 0.001
microfarad ceramic capacitor. This will reduce the effective Vgs just
the right amount.

4. Put an LED in series with the source of Q623 or the emitter of Q624
and bypass the LED with a 0.1 to 0.001 microfarad ceramic capacitor.
This will reduce the effective Vgs by enough to bring the output of
Q624 within range of the A Trigger Level Center adjustment. It will
also look cool.

On 27 Feb 2016 13:42:35 -0800, you wrote:

...

I suspected the FET was the issues (Q623).

I hooked up a signal to the External Trigger input, and tried to see the signal get through to the Gate of Q623. Nothing seen, only 0 VDC.
I removed removed the FET, and tested it with a DVM. I only was able to get the S to D junction beeping on my DIode test function on the DVM.

I took a J310 FET from my junk box, and tested it, not knowing exactly how to do this, but I discovered that G to S and G to D junction gave me a reading on the DVM. So now I knew that the old JFET did not test the same as the new one.

Swapped in the J310, and tested the scope (actually used the scope probe to test the External signal)... traced it to the JFET and now the signal was triggerable on the scope!

I removed the J310.
I cut the J310 leads to match the length of the old FET, and carefully re-inserted, pushing in so the JFET was tight in the socket.

Replaced all covers, and the scope now works.

VDC on the new J310 FET is 3.53 VDC on the Source, 10.62 on the D, and 0.0 on the Gate.

Not sure if this is a good replacement for the old JFET, but it works, so that's what I am going with!

Re: 7A22 and 7A29 on the 'bay

 

Hi Nathan,
The 7A29 is a very robust plugin. It was designed by the legendary John
Addis.

Unlike the 7A24 and 7A19 which had actual fuses on their input to prevent
damage from overloads (which is very easy to do with a 50 ohm input) John
designed a brilliant overload protection circuit into the input of the 7A29.
It trips extremely fast protecting the rest of the amplifier, then it lights
a front panel light to indicate that the front end was overloaded. This
gives you the all-important feedback you need to learn from your mistake and
not do it again. Simply switching the input selector to ground and back to
DC or to AC clears the protection circuit.

Because of this 7A29 protection circuit I would expect there to be a high
probability that the 7A29 plugins on Ebay work.

SolanoTraders (one of the sellers) has been around a long time. I haven't
bought anything from them lately but I think they have a good reputation.
Judging from where they are they are probably getting a lot of their stuff
from Los Alamos National Labs.

Dennis Tillman W7PF

-----Original Message-----
From: TekScopes@... [mailto:TekScopes@...]
Sent: Saturday, February 27, 2016 4:36 AM
To: tekscopes@...
Subject: [TekScopes] 7A22 and 7A29 on the 'bay

Several 7A22 and 7A29 right now for 50obo +20 shipping. Not affiliated and
all that... Just slightly uncommon to find them at that price.
Nathan KK4REY
------------------------------------
Posted by: Nathan Johnson <jdownj@...>
------------------------------------

Re: TDS 544A and TDS Repair in General

 

Hello Barry, All

I have just picked up a TDS544A with same self-test fails, have not opened
it yet, was just curious to know how yours ended as there was no follow-up.
(At least in this thread.)

Paul

2013-03-25 19:38 GMT+01:00 baltimora86 <acuffe@...>:

In my experience the CRT usually gets weak before the LCD shutter fails.
Sometimes they develop some slightly discolored areas, but if it's small,
it won't affect usability. I've seen pictures where the shutter had
separated from the CRT leaving a mess of gel, but that can be fixed. I
would think age and storage conditions would have the most influence. I
certainly wouldn't replace a good CRT just in case it might fail in the
future.


--- In TekScopes@..., "metals2chemist" <metals2chemist@...>
wrote:


Hi Den,
Thanks for letting me know how your display upgrade went. From what you
said, it sounds like I'd be better off keeping the tube and shutter since
they appear to be in almost new condition and would give me more
resolution that the LCD.

However, there is that longevity issue. Some of those liquid crystal
shutters have shown some serious chemical breakdown. The question is, is
it mostly from age or use? Since mine has the age, but not the use, its
plausible that replacement of the crt now would be the better option unless
use is the bigger factor in its breakdown.

Does anybody know?
Thanks,
Barry




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

Yahoo! Groups Links



Re: 468 sweep offset issue

Brian Bloom
 

The B sweep waveform at TP67 now resembles the specified waveform and voltage levels much closer than before.
>I am surprised the difference is even visible.

I should have worded it better: The difference between the manual-waveform-indicated sweep start edges of the A and B sweep waveforms didn't align (X axis) on any 468 timebase setting before adjusting R758. They are almost exactly aligned now.
I'm not sure why I tossed "and voltage levels" in that sentence. Must have been the lack of sleep.

Game plan:
Since I can't make use of the 468 until I can get the vertical HF comp back near spec, I am going thru the cal procedure on my good 465 that I never touched the adjustments on. I started finding noise causing tantalums, so I'm just going through it checking ESR on all of them and replacing the bad ones. This should be the easiest scope to get into cal again.
I'm about to pick up a 7A29 and possibly a 7B10 or 15, and tomorrow I'll be digging into the 7854. Maybe I can use it to get my 106 close to cal'd so I can actually use it - since it's spec'd at 500ps calibrated resolution.

There's another 7854 on the bay... ex government and slightly beat up. Includes an 067-0961-00 as well, but it's missing 2 ICs and the bottom right one appears to have been twisted in socket. No calculator.. Possibly includes a cal-pack?? I wonder if it's even worth it since it's ex gov... everything ex gov I've obtained might as well be shredded. It could probably be had for under $200, though.


I am going to have to sit down with the schematics and read over your comments until I start to understand how these sweep circuits work. Everything I've ever chose to get into has come to me easy - except for EE. I know that it is mainly because I can't yet visualize it on the component level since I haven't fully learned most of the various modes of operation for each type of device.

Re: Request for assistance from Nigel

Nigel Maund
 

Thanks to your advice, after a few 2 hour sessions, I finally found the source of the problem.

Before I forget, here is what I did:


Turn scope upside down on bench, Removed bottom chassis, removed cover plate covering the Trigger generator .
Tested voltages around TP 656. and read 6.8 VDC.
Q623 had -9.21 on its Source.
+12 and -12 VDC were good (well, +11.69 and -11.69 in my case)


TP 636 read -3.2 VDC. TP 636 was -0.09 VDC.


Trigger pot, R643 was 5.17 VDC


I suspected the FET was the issues (Q623).


I hooked up a signal to the External Trigger input, and tried to see the signal get through to the Gate of Q623. Nothing seen, only 0 VDC.
I removed removed the FET, and tested it with a DVM. I only was able to get the S to D junction beeping on my DIode test function on the DVM.


I took a J310 FET from my junk box, and tested it, not knowing exactly how to do this, but I discovered that G to S and G to D junction gave me a reading on the DVM. So now I knew that the old JFET did not test the same as the new one.


Swapped in the J310, and tested the scope (actually used the scope probe to test the External signal)... traced it to the JFET and now the signal was triggerable on the scope!


I removed the J310.
I cut the J310 leads to match the length of the old FET, and carefully re-inserted, pushing in so the JFET was tight in the socket.


Replaced all covers, and the scope now works.


VDC on the new J310 FET is 3.53 VDC on the Source, 10.62 on the D, and 0.0 on the Gate.


Not sure if this is a good replacement for the old JFET, but it works, so that's what I am going with!


Thanks so much for giving me the area to dig into.


Rgds,
Nigel....VA2NM

Re: 7A22 and 7A29 on the 'bay

 

7A22s are surprisingly common and depressingly more so than AM502s.

They also seem to suffer from fewer age related problems.

On Sat, 27 Feb 2016 15:36:25 +0300, you wrote:

Several 7A22 and 7A29 right now for 50obo +20 shipping. Not affiliated and
all
that... Just slightly uncommon to find them at that price.
Nathan KK4REY

Sent using CloudMagic Email
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Re: 468 sweep offset issue (SOLVED)

 

On 27 Feb 2016 06:48:46 -0800, you wrote:

Just marking the topic as "Solved". Not sure if this updates the title as seen from the topics list.

Just a quick recap:

My 468 had a bothersome A/B sweep start offset that I could not figure out due to my minimal knowledge of the circuit.
B sweep began 1 minor division after A sweep.

Matching the diodes and transistors for the A & B sweep start comparators reduced the offset by half.

Changing the resistance of R758 to EXACTLY the specified resistance of 2.37Kohm eliminated the issue entirely by raising the voltage being introduced to the B sweep ramp at the emitter of Q760. At this point is where the B sweep ramp is subtracted from said voltage. This had the effect of raising the DC voltage of the B sweep ramp that is then sent to the horizontal amplifier.
Honestly if I had followed the sweep circuit as far as transistors
Q760 and Q772, I might have suggested just trimming either R758 or
R774 to align the sweep start positions. A change of 50 microamps per
minor division would not significantly alter anything else.

But the potential mismatch in junction voltages was enough to explain
the difference in sweep start positions and I tend to think in terms
of offset voltage caused by Vbe and diode mismatches so I stopped
there. Matching diodes and transistors also has fewer potential side
effects.

It could have been worse. Before getting to R758 and R774, I
considered trimming R759 and R775. This would have worked but also
would have required recalibration of the horizontal sweeps.

The B sweep waveform at TP67 now resembles the specified waveform and voltage levels much closer than before.
I am surprised the difference is even visible.

Re: 468 sweep offset issue

 

On 26 Feb 2016 15:28:04 -0800, you wrote:

Well that didn't quite do it, but it definitely helped.
Now B sweep only "lags" A sweep by 0.5 minor divisions. It also ends the same amount before A sweep, where they were both ending at the same time. According to the measurements I took, the transistor-diode combinations were within 200uV of each other. The diode pair for A sweep is also within 200uV, as are Q760 and Q772.
The sweep end points are independent of each other and the start
points. For the B sweep, Q952 and Q956 form a differential comparator
which compares the B sweep voltage to a reference produced by voltage
divider R955, R954, and R949. The input to the comparator is the
sweep voltage minus the voltage drops from Q960, CR953, CR947, and
R957.

Sweep A has the same arrangement.

The sweep lengths are unimportant as long as they extend more than 10
divisions. The sweep lengths can be adjusted by trimming the
resistive divider at the base of Q956 and Q986 or by selecting the
diodes and transistor for different voltage drops.

At least I am learning something practical about this type of circuit and how it reacts to matched and unmatched PN junction drops and such. I forgot to check the resistors again, and will do that tomorrow after work.
I guess if what has to be done to get this aligned is to trim the A sweep start circuit, I'll do it and figure out how to make it look clean.. I'll have to buy the trimmer, though, so I might as well buy an entire set of them. For now, maybe there's one that will work from the 475 main board or trigger board that I have laying around.
Probably should check for any out of tolerance resistors and any other unmatched components from A to B sweep start/miller runup, etc.
Honestly I am as well. I have not studied the 465 sweep generator
design in this much detail before.

It will be easiest to change one of the diodes and introduce a
deliberate mismatch to move the sweeps into alignment.

How close are the sweep output voltages when both are reset? The best
way to measure this is between the collectors of Q967 and Q998.

It now see another way to trim the sweep positions. R758 and R778+774
add a current into Q760 and Q772 which the sweep output voltages
subtract from. Trim the value of one of the resistors to move the
sweep slightly. A parallel resistance of about 300 kilohms will move
the trace one minor division.

Does it matter which diode I change out? If I change diodes again, should I try mismatching the diodes in the A sweep gen in an attempt to get it to delay slightly instead of trying to get B sweep to start sooner?
It does not matter which diodes are changed. The idea would be to
match the offsets as seen from the base of Q960 and Q987B.

Forgive my ignorance in understanding this circuits functionality... I am going to read through your posts, the sweep gen theory of operation, and the related section in the Tek troubleshooting your scope book again to try and obtain a better understanding of it now that I have some more clues.

"How close are the sweep output voltages when both are reset? "
By "reset" do you mean their quiescent state? I can set it to single sweep, run one sweep, and then measure that if that's what you mean.
That is exactly it. When the sweeps are reset, the comparators return
them to their quiescent state which is at the left side of the CRT
where they wait until triggered.

Setting the horizontal trigger mode to NORMAL instead of AUTOMATIC and
disabling the trigger would work also.

Putting a 300K parallel to R758 (2.37Kohm) would give me 2351ohms. This would raise the voltage of the B sweep rampslightly, thus causing the B sweep trace to move closer to the start point. Am I correct here? If doing so will move the sweep 1 minor division, then with me needing just a hair over half a minor division, I need to lower the resistance of R758 by about 11 ohms.

So if I need 2.36 ohms, what if I use a 510Kohm in parallel with R278? This would give me 2359ohms according to the math.

Am I correct in my thinking here?
Yes but I was not thinking in terms of matching the resistances. Here
is how I arrived at that value:

The sweep voltages at the base of Q960 and Q987B change by about 1
volt per division. This creates a change of current into Q760 and
Q772 through the 4.02 kilohm resistors of 250 microamps per division.
That would be 50 microamps per minor division (5 minor divisions per
major division).

R758 and R774 create fixed currents into Q760 and Q772 of about 6
milliamps. Without this, the current into the transistors from the
sweep would vary from about 3.25 milliamps to 0.5 milliamps and over
such a large ratio, their nonlinearity would be too great adding to
the sweep nonlinearity which is critical. Adding 6 milliamps makes
the range 9.25 to 6.25 milliamps which is a much smaller ratio.

These currents can be adjusted to move the sweeps by small amounts.
The voltage across R758 and R774 is fixed at about 14.4 volts. So
adding a parallel resistance of about 290 kilohms across the resistors
will add 50 microamps which is about 1 minor division of shift of the
sweep position.

Re: Changeling?

Joseph Rooney
 

At this late of a date, it is probably not worth it. If you wish, contact me at ikerooney(at)yahoo.com and I can supply a bit more information.

Changeling?

Torch Fireman
 

Thanks to this list, "my" 475 is now performing very nicely, calibrated to the best of my ability and equipment.

This scope was given to me by a friend. About 13 years ago, he bought a pallet of surplus equipment from Ontario Hydro. Included was a Tek scope on a scopemobile with an isolation transformer also mounted to the base of the scope. He knew nothing about scopes and brought it to me to see if it worked. It was at my place for a couple of weeks or a month, during which time I fiddled with it and found one channel seemed to be OK, but as I recall, the other channel was acting strange -- two traces about 6 or 8 divisions apart, IIRC. I returned it with the advice that he effectively had a nice single channel scope which could probably be repaired but the repair was beyond my ability. He stuck it in a corner of his rented airplane hangar and apparently never really did anything with it thereafter.

Flash forward to 6 months ago: His rented hangar has been sold so he has to clear out. As part of his downsizing, he offers the old scope to me rather than toss it out. When I picked it up, I was surprised -- I did not think it was the same scope. I was sure his scope had a DM-44 on top, with the scopemobile strap passed between the cabinet and the storage pouch. This one is on the same cart I remember, with the same aftermarket transformer screwed to the base, but the strap is over the pouch. He maintains it is the same scope that always sat in the corner all those years.

It's been a while, so I figure I'm remembering things wrong. I power it up, find both channels seem to be working, and start tinkering with the help of this group. Trying to figure out which version manual to use, I discover the serial number plate is missing -- like the glue let go with age and it fell off. That corner of the faceplate is slightly sprung and the A trigger outside knob is chipped, so something probably dislodged it. No big deal, you guys help me figure out the correct range based on the revisions inside, all is good and away I go.

Last night I was going through some old photos looking for something and discover a 2003 photo with the scope in the background from when it was originally at my place. Only it's not a 475 in the photo -- it's a 468. With the DM-44. On the same cart. I can even see that there is a serial number in the little opening (can't quite positively read it though.

The switch had to be deliberate. I think someone had access to the hangar and saw an opportunity to upgrade their analog scope to a DSO (if so, the joke is on them!). Maybe they removed the serial number so the switch couldn't be traced back to them or maybe the 475 wasn't their property either.

Assuming Tektronix might still have a record of who bought which serial number, that seems to be unrecoverable. It could be listed in a police database somewhere, but again, without the serial number they would need a lot of incentive to go looking after all these years.

Does anyone know of any other way to track down the rightful owner?

Re: 468 sweep offset issue (SOLVED)

Brian Bloom
 

Just marking the topic as "Solved". Not sure if this updates the title as seen from the topics list.

Just a quick recap:

My 468 had a bothersome A/B sweep start offset that I could not figure out due to my minimal knowledge of the circuit.
B sweep began 1 minor division after A sweep.

Matching the diodes and transistors for the A & B sweep start comparators reduced the offset by half.

Changing the resistance of R758 to EXACTLY the specified resistance of 2.37Kohm eliminated the issue entirely by raising the voltage being introduced to the B sweep ramp at the emitter of Q760. At this point is where the B sweep ramp is subtracted from said voltage. This had the effect of raising the DC voltage of the B sweep ramp that is then sent to the horizontal amplifier.

The B sweep waveform at TP67 now resembles the specified waveform and voltage levels much closer than before.

Re: 7A22 and 7A29 on the 'bay

Malcolm Hunter
 

They should be listed spares / repair if they can't test them. You should
be able to get at least partial refund if there's parts missing.

Malcolm


On 27 February 2016 13:52:43 "analogaddict013@... [TekScopes]"
<TekScopes@...> wrote:

Yes, I noticed there are a few of each there.

I am working on arrangements to buy a few plugins from them since I have
only 1 timebase and 1 good amplifier.

It's just a drag that they are completely untested - not even power tested.
Hopefully none are missing any internals.

Now if some more sampling related plugins would get listed......


Re: 7A22 and 7A29 on the 'bay

Brian Bloom
 

Yes, I noticed there are a few of each there.

I am working on arrangements to buy a few plugins from them since I have only 1 timebase and 1 good amplifier.

It's just a drag that they are completely untested - not even power tested. Hopefully none are missing any internals.

Now if some more sampling related plugins would get listed......

7A22 and 7A29 on the 'bay

Nathan Johnson
 

Several 7A22 and 7A29 right now for 50obo +20 shipping. Not affiliated and
all
that... Just slightly uncommon to find them at that price.
Nathan KK4REY

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Re: 7834 horizontal jittery readout

Gordon <tekscopes@...>
 

On 27/02/2016 08:03, David @DWH [TekScopes] wrote:
I am not sure if that is a problem or not. The identify command is
used to align the decoders so that the proper characters are
displayed.

You could swap U3429 with U3418 to see if that makes a difference. I
would also reseat all of the ICs and transistors. The IC sockets are
problematical.
I'll give that a try.

Thanks

Gordon