I have a 7904 and a 7934 (obviously different but I believe the PSU is very similar to the 7904A). I find the fan noise of the 7934 becomes a distraction after a while when you are trying to concentrate on something else, but I agree, if you find a nice scope of either the 7904 or 4A then you should be happy with it. If you are concerned about the pulse response for very fast risetimes I would recommend the 7A29 over the 7A19. I have two 7A19s and they are very difficult to set up for clean pulse response, you need an extender (which I don't have) and decent non metallic tools to adjust the hairpin inductors which work harden slightly after several attempts to adjust them. The 7A29 has a much cleaner step response but may be getting hard to find.

Regards,

Roger

On Wed, May 27, 2020 at 10:52 AM, LarryS wrote:

I thought I had read somewhere the actual figure was 83%. You must be right!

larry

87% of all statistics are made up on the spot.
:-)

L.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Jamie
Ostrowski
Sent: Wednesday, May 27, 2020 9:09 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Encouraging beginners: What are we accomplishing?

Does anyone have any idea, statistically, of how many scopes die from
beginners trying to repair them verses those that die to tube harvesters or
relatives who have no interest in "Bob's" old scopes who send them to the
recycler?



On Wed, May 27, 2020 at 8:42 AM LarryS <vision1@southwind.net> wrote:

In answer to your question, ANY time you bring in fresh blood, it's
better for everyone. Every time. No exceptions. A rising tide floats all

boats.

Right now, Harley Davidson is facing 'massive restructuring'. Why?
Their rider base is literally dying off.
I'm involved in several other hobbies and pursuits where young blood
is not entering. They are dying. And with it comes a similar
intellectual cannibalism.

As any discipline dies, like analog scopes, the entry price will be
lower and lower. Soon, for rummage sale prices, the once-crown-jewel
pieces become affordable. In 1983, how many 60s fastback Mustangs
were parked behind gas station garages (remember those?) and could be
had for a couple hundred? Many were chopped and otherwise brutalized
in ways I can't describe. But such is the price. Today, they're
worth more than most people's financials.

There were many thousands of units produced of scopes and cars and
organs and everything else. Some will live nice lives and some will
die horrible deaths and grownups understand this. I've been the
careless kid and the fastidious curator, but at both extremes I remembered

the Prime Directive:

this thing is MINE, not yours. If you're worried about it, you can
buy it from me and store it away. If not, tough.

If we want a growing following, then we have to let the newbs make
their mistakes and learn as we did - yes, even on top tier equipment.
If it grows enough, someday my old junk might be worth as much as I
think it is now. If not, someone will speak poorly of me as they
shovel it all out into a dumpster after my demise.

Besides, if it tracks like everything else, one day some others will
join your song and these units will skyrocket - at least temporarily.
I own a Hammond console. In 1988, it was dumpster fodder. By 2010, it was

$10K.

Today, it's maybe $4k. These things have a cycle and scopes are
certainly no exception.

Anyway, help every newb you can. If they want to try their hand, the
answer is NEVER "let someone else do it". They're gonna do it wrong.
Just as wrong as you and I used to do stuff. The sooner they learn to
do it right, the sooner everything gets better.

L.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of
Roy Thistle
Sent: Tuesday, May 26, 2020 10:52 PM
To: TekScopes@groups.io
Subject: [TekScopes] Encouraging beginners: What are we accomplishing?

Hi all TekScopers:
Reading through a long thread, recently posted, caused me to wonder..
just what are TekScopers accomplishing with threads like this... and
why are we encouraging someone who is "... new to electronics..." to
dig into a 475?... one of the most complex, and compact, analog
instruments ever designed.
I suppose.. in consideration... Michael discouraged the use of a Mr.
Carlson super Weller-kluge special, on the 475's pcb(s)... but, ya know...
somewhere the thread... the 475 owner hints he paid 20.00 for 475?,
and he's also got a nonworking? PM3218 too.
So why didn't someone just recommend/... right off the bat... to take
the
475 to someone who knows what they are doing... drop another 100.00 on
it... and then he'd have one of the best scopes ever made.
Or alternatively... and better... just start in on the PM3218...itself
a very fine instrument, with a double insulated power supply... and
way overkill, for a beginner.
Look, I'm not unsympathetic... it's just that...too often.. after
parting with some scarce cash... or finding some Tek picked apart in a
basement somewhere, where its been languishing for a generation...I've
witnessed the havoc wreaked by someone trying to "fix" them.
If you want to call me a dumb ass... for thinking this way... well fine...
just PM me to do it. I can't promise I'll reply to that... but, I'll
read your message.
Best regards and wishes.
Roy

They are placed between the rectangular HV ceramic caps in the CRT HV module.

The fan on my 7904A is very quiet, a non issue on mine at least. I suggest that the most important thing will be finding the best instrument available around your location. Packing properly is difficult and shipping is expensive.

Thanks for all the responses. I also hate fan noise. That and more readily available parts make the 7904 preferable. However, I'll also look into the 7854.

As I have a Tek 11801 and HP 16702B and 16500A LAs and 8560A and 8566B SAs I'd probably use those instead anyway. I'm one of those "Tek for scopes and HP for everything else" guys.

Have Fun!
Reg

All:

Now that I have got my 576 working reliably, I was going to start calibration and I found a pair of what appear to be silicone washers laying in the bottom of the unit. I have no idea where these are supposed to go.

The color is a translucent and slightly off white.
The parts are very soft and pliable.
The appearance is like that of silicone or a similar rubber like material.
The dimensions are 22mm OD x 8mm ID x 2.25mm H (or thick).
There is no appearance of any adhesive residue on these parts.
These were laying at the bottom of the unit and had a layer of dust on top.

Photo is here: https://groups.io/g/TekScopes/album?id=247763

This unit is a very Low Serial number unit B0100169 and this is the one that I used as my "Test Mule" for the HV transformer project, it is in pristine condition inside. The unit seems to work just fine (other than needing a calibration) and I have not done anything else to disturb the internal parts of the unit, other than the HV Supplu cover and board. I do not believe that I dislodged these, since they appear to have been laying in the bottom of the unit for quite some time.

Any ideas what these might be or where they need to be installed?

Thanks in advance. .
--
Michael Lynch
Dardanelle, Arkansas

Bruce,

There are 2 fundamental differences that need to be taken in to account when replacing “vintage” caps. And I use the term vintage loosely. One is tolerance. In the worst case I can remember a capacitors value was +150% -%100 tolerance. The means that for a 1 uF the measured value of anything between 2.5 uF to .1 uF would be considered “in spec” old radios still amaze me they ever worked. Now a days +-20% is the normal. The service manual will tell you the tolerance of the filters. From memory I am guessing it is going to be +100% to -20 % so pretty wide design tolerance on the filters.

The other important spec is the voltage rating of the cap. And here vintage caps and modern caps differ greatly. Vintage caps were very tolerant of over voltage especially given how tube gear warms up before the tube’s comes in to operation the B+ will spike some times as high as 200 to 250 volts higher then when the device is operating and can hang there for about 15-30 seconds. This is not an issue in your 475 as it is solid state. However modern caps are completely intolerant of over voltage so if you have the physical space it is always good to bump up the voltage rating of the cap it wont effect anything to replace a 63V cap with a 400V cap excepta little cost in $ maybe one or 2 and physical space it will be slightly bigger then it’s modern lower voltage counterpart. However both are usually smaller then their vintage counter part even doubling the voltage the modern can will be smaller physically.

Eric

On 5/29/2020 12:50 AM, ciclista41 via groups.io wrote:

It might help if I posted the values of the capacitors that were originally in the scope. I'll do this below in the quoted text.

I have used Kemet in projects but mainly their surface mount stuff

I tend to lean to

Nichicon

Rubycon - Actual  (frequently found counter fit parts)

Veshay

and Illinois Capasitor

On Thu, May 28, 2020 at 05:59 PM, <ciclista41@yahoo.com> wrote:

‎Hi folks,

Here's my proposed parts order on Digi-Key, except that I haven't decided
between the first or second set of tantalums. The caps below those are to
replace the large C1412, 14, 42, 52, 62, and 72 capacitors. I tried to go up
a bit on the rating of both the capacitance and the voltage, but was hampered
somewhat by a lack of choice when only purchasing one or two of each. I'd had
Nichicon recommended to me, so I stuck with them where I could. Apparently,
Nichicon sold their tantalum capacitor manufacturing to AVX. I don't know
about Kemet, but couldn't find Nichicon near those specs. Also, I haven't yet
received the circular PCB's, so I assumed from the photos of them that they
can accommodate a range of lead spacings. Just thought I'd run it by you all
before I place the order in case I'd be advised to change anything.

To replace the tantalum I broke (C1091), and the other one I removed (C1093). Originals were 2.2μF 20V.

478-1870-ND‎
‎TAP225K025SCS‎
AVX Corporation
CAP TANT 2.2UF 10% 25V RADIAL
Immediate
0.46400
$9.28

or

478-8972-1-ND‎
‎TAP225M035SRW‎
AVX Corporation
CAP TANT 2.2UF 20% 35V RADIAL
Immediate
0.63500
$12.70

To replace C1414 1000μF 75VDC

‎493-17708-ND‎
‎UBY1K102MHL‎
Nichicon
CAP ALUM 1000UF 20% 80V T/H
Immediate
3.34000
$3.34

To replace C1412 350μF 75VDC

‎493-17783-1-ND‎
‎UBY2A361MHL6TN‎
Nichicon
CAP ALUM 360UF 20% 100V T/H
Immediate
3.18000
$3.18

To replace C1442 and C1462 5500μF 30VDC

‎399-19618-ND‎
‎ALC70A562BB040‎
KEMET
SNAP-IN HIGH CV 85C 5600UF 40V
Immediate
5.10000
$10.20

To replace C1452 5000μF 25VDC

‎493-14637-ND‎
‎UBY1V512MHL‎
Nichicon
CAP ALUM 5100UF 20% 35V RADIAL
Immediate
5.44000
$5.44

To replace C1472 3000μF 35VDC

‎493-14660-1-ND‎
‎UBY1V302MHL1TN‎
Nichicon
CAP ALUM 3000UF 20% 35V RADIAL
Immediate
3.02000
$3.02

Thanks for your input!

Bruce

I had a 7904A recently for a short time.
The fan was one reason I sold it - the noise is obnoxious to me. I don't think there was anything wrong with the fan, but I'm sensitive to that white noise - it bugs me.

Pete

It might help if I posted the values of the capacitors that were originally in the scope. I'll do this below in the quoted text.

On Thu, May 28, 2020 at 05:59 PM, <ciclista41@yahoo.com> wrote:

‎Hi folks,

Here's my proposed parts order on Digi-Key, except that I haven't decided
between the first or second set of tantalums. The caps below those are to
replace the large C1412, 14, 42, 52, 62, and 72 capacitors. I tried to go up
a bit on the rating of both the capacitance and the voltage, but was hampered
somewhat by a lack of choice when only purchasing one or two of each. I'd had
Nichicon recommended to me, so I stuck with them where I could. Apparently,
Nichicon sold their tantalum capacitor manufacturing to AVX. I don't know
about Kemet, but couldn't find Nichicon near those specs. Also, I haven't yet
received the circular PCB's, so I assumed from the photos of them that they
can accommodate a range of lead spacings. Just thought I'd run it by you all
before I place the order in case I'd be advised to change anything.

To replace the tantalum I broke (C1091), and the other one I removed (C1093). Originals were 2.2μF 20V.

478-1870-ND‎
‎TAP225K025SCS‎
AVX Corporation
CAP TANT 2.2UF 10% 25V RADIAL
Immediate
0.46400
$9.28

or

478-8972-1-ND‎
‎TAP225M035SRW‎
AVX Corporation
CAP TANT 2.2UF 20% 35V RADIAL
Immediate
0.63500
$12.70

To replace C1414 1000μF 75VDC

‎493-17708-ND‎
‎UBY1K102MHL‎
Nichicon
CAP ALUM 1000UF 20% 80V T/H
Immediate
3.34000
$3.34

To replace C1412 350μF 75VDC

‎493-17783-1-ND‎
‎UBY2A361MHL6TN‎
Nichicon
CAP ALUM 360UF 20% 100V T/H
Immediate
3.18000
$3.18

To replace C1442 and C1462 5500μF 30VDC

‎399-19618-ND‎
‎ALC70A562BB040‎
KEMET
SNAP-IN HIGH CV 85C 5600UF 40V
Immediate
5.10000
$10.20

To replace C1452 5000μF 25VDC

‎493-14637-ND‎
‎UBY1V512MHL‎
Nichicon
CAP ALUM 5100UF 20% 35V RADIAL
Immediate
5.44000
$5.44

To replace C1472 3000μF 35VDC

‎493-14660-1-ND‎
‎UBY1V302MHL1TN‎
Nichicon
CAP ALUM 3000UF 20% 35V RADIAL
Immediate
3.02000
$3.02

Thanks for your input!

Bruce

I just finished creating the binary files for each of the Option ROMs v2.1 and also a 16bit binary that could be used for disassembly and posted them in the ROMs folder on my github repository for the 4041:

https://github.com/mmcgraw74/Tektronix-4041-GPIB-Controller/tree/master/4041%20ROMs

I can speak of this from the 7904 perspective, since I have two that work, and some backup parts sources,

I had 7904's available, and no 7904As.

the 7A19 are the "appropriate" plugin for a 500 mhz single channel plugin.  However, they're protected by fuses which may be (impossible) hard to find.  The 7A29 has a higher bandwidth and is apparently not protected by fuses (and by something that can be reset), so I prefer them.

My general setup is a 7A26, 7D12 (with sampling plugin), 7D15, and 7B92A.

I've swapped the vertical plugins as needed, 7A22, 7A13, 7A24, 7A29.

I've used the 7B10 and 7B15, but only rarely.

Don't have the 7B8x or 7b7X to speak of, at least, I may have some of them, but don't use them.

I've had (generally) few issues, a hybrid perhaps bad, a PS bad, but nothing that I thought was system wide.

Harvey

As a precaution, some switching supplies don't work well with a voltage below the operating voltage.

For a linear supply, as the voltage to the supply rises, the regulator generally (if it has a decent reference) will turn on full until the input voltage to the regulator reaches a threshold where the output voltage would exceed the setting, then the regulator stops being a short circuit and starts regulating. Depending on the circuit, for example a 5 volt regulator may need anything from 5.25 volts to 8 volts to start regulating.

On the other hand, a switching regulator is not so well behaved.

Remember that a switching regulator has a transistor that, instead of being a variable resistor, is a switch.  It's either on or off.  The switching regulator works by pulsing the transistor on with a limited duty cycle.  That complete (on) dead short lasts for enough time to build up the output voltage, then it turns off.  There are several geometries of switching regulator, some of which, if the switching element is turned on, will draw a lot of current.

The problem in a switching regulator (or one that has two transistors driving a transformer as an inverter), is the question:  What happens when the input voltage is too low?

Some turn on and try to build a proper output voltage.  Some may not operate until they get enough input voltage to work properly.

The problem with some of them is that they draw a LOT of current before they start to operate properly (and switch on/off).

Tektronix uses both switching and linear supplies, and the switching supplies may or may not "like" low input voltages.

I have brought up scopes using a variac, but I have monitored the input current.  I've seen it go over the proper rating and then go back to a lower voltage when the supply starts to switch.

I've seen them work into a short (and the tick mode/overcurrent mode) not work well.  You find that out by monitoring the current the scope draws as the voltage goes up.

Each supply/scope may be different.  You'll need to watch what each one does.

I have a sencore unit that has an isolation transformer and a variac.  It measures both leakage current from the supply lines to ground (not neutral) and the input voltage, output to the UUT, and current.

It's been useful.  You could make one if you wanted, with the exception of the leakage current measurements.  Not sure how they did that.

I'd seriously recommend simultaneous output voltage/output current monitoring, but on this one, I know where the variac will put out full voltage, and can still monitor the current.

Harvey

I stand corrected. Sometimes you can teach an old dog new tricks! ;)

Thanks, Raymond!

Jim

Issues and features that I recall:

7904 has lots of tantalum caps, and a couple are bound to blow.
They are easy to find and fix.

7904 has a weak power supply that can't handle many of the digital
plugins, such as the logic analyzer and spectrum analyzer plugins.

7904 always ticks 3 times when it is turned on after being off for
a while.

7904 has at least one "blue" harmonica connector that will have
ossified by now. If you touch it, it will crumble to dust.

Fortunately, the so-called Dupont connectors are compatible.

7904 has a united power supply transformer that has everything
from the low voltage windings through the EHT windings on the
same core. It is reliable, but repair isn't likely.

7904A Has few tantalum capacitors.
7904A has a separate EHT transformer... but because that was too
reliable, they drove it from an AC tap on the main transformer.

Why do I care? If the main inverter goes full tilt on, the HV
and filament voltage for the CRT doubles.

Specs that matter are the same for 7904 and 7904A.

7904A has a noisy fan. 7904 has none.

7904A uses hypacon connectors on its many, many hybrids.

All this said, I prefer the 7854 as my go-to scope.

-Chuck Harris

Reginald Beardsley via groups.io wrote:

‎Hi folks,

Here's my proposed parts order on Digi-Key, except that I haven't decided between the first or second set of tantalums. The caps below those are to replace the large C1412, 14, 42, 52, 62, and 72 capacitors. I tried to go up a bit on the rating of both the capacitance and the voltage, but was hampered somewhat by a lack of choice when only purchasing one or two of each. I'd had Nichicon recommended to me, so I stuck with them where I could. Apparently, Nichicon sold their tantalum capacitor manufacturing to AVX. I don't know about Kemet, but couldn't find Nichicon near those specs. Also, I haven't yet received the circular PCB's, so I assumed from the photos of them that they can accommodate a range of lead spacings. Just thought I'd run it by you all before I place the order in case I'd be advised to change anything.

478-1870-ND‎
‎TAP225K025SCS‎
AVX Corporation
CAP TANT 2.2UF 10% 25V RADIAL
Immediate
0.46400
$9.28

or

478-8972-1-ND‎
‎TAP225M035SRW‎
AVX Corporation
CAP TANT 2.2UF 20% 35V RADIAL
Immediate
0.63500
$12.70

‎493-17708-ND‎
‎UBY1K102MHL‎
Nichicon
CAP ALUM 1000UF 20% 80V T/H
Immediate
3.34000
$3.34

‎493-17783-1-ND‎
‎UBY2A361MHL6TN‎
Nichicon
CAP ALUM 360UF 20% 100V T/H
Immediate
3.18000
$3.18

‎399-19618-ND‎
‎ALC70A562BB040‎
KEMET
SNAP-IN HIGH CV 85C 5600UF 40V
Immediate
5.10000
$10.20

‎493-14637-ND‎
‎UBY1V512MHL‎
Nichicon
CAP ALUM 5100UF 20% 35V RADIAL
Immediate
5.44000
$5.44

‎493-14660-1-ND‎
‎UBY1V302MHL1TN‎
Nichicon
CAP ALUM 3000UF 20% 35V RADIAL
Immediate
3.02000
$3.02

Thanks for your input!

Bruce

My email should have read “no well-known problems”.

I had both a 7904 and a 7904A; I gave the 7904 away to a good home when I moved across the country, reducing my collection to five 7xxx ‘scopes.

If you get a 7854 you will also want the keyboard that goes with it. Like I wrote, rabbit hole.

DaveD

On Fri, May 29, 2020 at 12:50 AM, Reginald Beardsley wrote:

which is the best choice from a long term support basis, 7904 or 7904A?

what plugins? 2x 7A19 and 7B92 + ???

Hi Reginald,

Advantages of the 7904A over the 7904:
- Newer, so possibly less failure-prone
- Easier to service because of split construction
- Front panel layout very much like the 7104 (or is that a disadvantage?)
- Internal construction and components very much like the 7104
- More modern look (to some at least)


Advantages of the 7904 over the 7904A:
- Fewer hard-to-get components
- No fan (a huge advantage according to some)
- Backlit Vertical Mode and Horizontal Mode buttons
- More versatile calibrator, includes current loop
- Can be had for very little money

Make sure that in either case, you buy one with the Alphanumeric screen-readout, which is standard.

It's very difficult to determine the better choice on long term support aspects: Do you want a younger and therefore possibly slightly more reliable instrument with more expensive, hard-to-get components or do you want an older, slightly less reliable instrument with fewer and cheaper hard-to get components? If you have room to spare, have 3 7904's for the price of one (or 2) 7904A's.

As regards plugins: For general-purpose work, put in a 7A26 (Hi-Z, 200 MHz) and a 7A19 (Lo-Z, 50 Ohm only) or, preferably, a 7A29 (resettable overvoltage protection, continuously variable attenuation), a 7D15 counter in the A Horizontal slot and a 7B92A into the B horizontal slot. Make sure you have all the other plugins nearby (7A13, 7A22, a sampler system for > 1 GHz).

Both have a switching supply. Apart from the well-known lifetime-driven weaknesses like electrolytic (mains voltage buffer) caps and tantalum (bead) caps, I'm not aware of any structural vulnerabilities.

I concur with Dave D. re. the 7854. It's more complicated and therefore more vulnerable but it's also one great piece of equipment, especially if you use sampling plugins.

Raymond

We can get the 4041 service manual scanned from the vintagetek.org microfiche library of all Tektronix documentation - after COVID-19 is over. Right now Oregon is still in lockdown. They only charge $25 an hour for scans.

Reg,

As you may know from our prior conversations, I have both a 7104 and a 7904A. The 7904A is definitely my workhorse because, as you mention, I want to preserve and protect the mcp tube in the 7104. As Dave mentioned, the chassis is different between the 7904 and 7904A. The 7904A is going to be easier to work on from an access perspective. One thing the 7904 has that I wish the 7904A carried on is a current loop calibrator for current probes. Oh well. Another thing to keep in mind is that the 7904A uses the hypcon packaged hybrids like the 7104 in the vertical amp[1], while the 7904 does not.

As far as reliability, I would imagine both have tantalum capacitors. Since these scopes have a switch mode power supply, it's easy to tell when you have a short because you get the infamous clicking as the supply tries to turn off then shuts down. Finding the short is another matter, though fortunately for me the short my 7104 developed when I got it home from the auction house turned out to be super simple and not tant related at all.

As far as plugins go, the 7A26 is a perfect vertical option for general purpose work. There's a couple different revisions hanging about. Some have a 20 MHz bandwidth switch and some do not. I have examples of both. Honestly, the one that doesn't have it has a sharper trace all around in my experience. Use two of them and have a full on 4 trace scope. The 7A19 is a good match for the mainframe if you want a vertical with a dedicated 50 ohm input. Keep in mind that unlike the 7A29's circuit breaker arrangement, the 7A19 uses fuses for input overload protection. Still useful for specialty probes that expect 50 ohms, such as current probes. Another vertical plugin in find indispensable for low frequency work is the 7A22 differential amplifier. Even single ended with a 10X probe, you can get down into microvolts/div ranges and the adjustable bandpass filter is very useful for isolating signals in noise. I have a 7B15 and a 7B92A as timebases in mine; no complaints.

[1]: http://w140.com/tekwiki/wiki/7904

The 7904A is easier to wotk pne because it has the “kickstand” feature where one can split the display section up off of the lower section to make repairs. I’ve never compared the specs of a 7904 and 7904A. I imagine they are quite similar. I’d probably buy a 7904A if I had to choose. As far as I know there are well-known problems with either ‘scope.

If it were me, I’d also consider a 7854.

Selecting plug-ins is a pretty deep subject. 7A26s with 7B92s is a good choice for general purpose work. IIRC, 7A19s are 50 ohm input only.

Once you get a 7xxx ‘scope you will probably want to get a differential amp, pulse and counter plug-ins as well. It’s a rabbit hole.

DaveD