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Please forgive me my ignorance, but for most of my life these things were pure
unobtainium for budgetary reasons. And as I was not an EE and not allowed to
touch the ones at work, there was no incentive to torture myself by looking at
catalogs of things I could not afford. Getting a 465 was a huge event for me.
And I don't mean all the work it took to repair it. I am acutely aware of how
fiddly adjusting the vertical amplifiers is. But when you get it right, wow!
Though in all honesty, I like my Dumont 1062 better as it does not have a fan.
It's only 60 MHz, but otherwise matches the 465 in features.

Can I use my 7104 plugins in a 7904 mainframe, albeit with reduced BW and
sweep speeds? That would greatly ease my concerns about using the 7104 on a
regular basis. And make getting a 7904 mainframe pretty much a no-brainer.
Just a question of getting a good deal.

Is there a matrix which shows which plugins will work in which mainframes? I'm
really only interested in the vertical and horizontal plugins. For the other
stuff I have HP gear. I'm not aware of anything Tek made that can compete with
an 8566B or 8510C.

Thanks,
Reg

I find all of the fans get quieter as I get older.

I wonder why that is?

-Chuck Harris

Bob Koller via groups.io wrote:

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.

Hi Raymond!

Wow! Thanks for the thorough explanation. I had been wondering about the
so-called capacitor "reforming."

Bruce

On Fri, May 29, 2020 at 10:58 AM, Chuck Harris wrote:

Hi Raymond,

Old fashioned electrolytic capacitors have a fairly reactive,
(to aluminum) highly conductive, water based electrolyte.

The capacitor's leakage current creates an electrolytic cell
with the aluminum plates, and removes the oxide layer from the
cathode plate, and builds an oxide layer on the anode plate...

The oxide layer on the anode plate is the dielectric (insulator)
for the capacitor.

This reaction is the "reforming" process that lives on in
electrolytic capacitor lore... Even today. It is also a process
known colloquially as anodizing aluminum.

The aluminum oxide dielectric layer has competing issues:

As the oxide grows thicker, it became a better insulator, and works
to stop the leakage current necessary to grow the oxide layer thicker.

As the oxide layer ceases growing, the electrolyte dissolves the
oxide layer, allowing the leakage currents to increase... growing the
oxide layer thicker.

In normal operation, a balance is reached between leakage current
and growing the oxide layer.... A "working voltage" rating results.

The only way to increase the thickness of the oxide layer once
it reaches equilibrium, is to increase the voltage across the
electrolytic cell, making the cell's current increase, and in turn,
the oxide grow thicker... creating a new higher voltage equilibrium,
and a new "working voltage".

As long as the heat created by the leakage current doesn't raise
the temperature of the electrolyte to a point where the it boils,
the capacitor is fairly happy.

This means that higher than working voltage surges will start to
heat the electrolyte, but as long as the surge goes away before the
electrolyte boils, the capacitor will live to see another day.

One other factor needs mentioning:

The thinner the oxide dielectric layer, the higher the capacitance,
and the lower the working voltage.

The thicker the oxide dielectric layer, the lower the capacitance,
and the higher the working voltage.

The manufacturer had to balance all of these conditions when they
wrote the specifications for their old style electrolytic capacitors.

If you used these old electrolytic capacitors at a lower than working
voltage, their capacitance would increase, and their ability to
operate safely at their specified working voltage would diminish.

Unless you reformed the capacitor... safely limiting the current
until the oxide layer thickened...

Modern electrolytic capacitors use a nonreactive (to aluminum)
electrolyte and as a result, reforming is no longer necessary.

Modern electrolytic capacitors have their oxide layer created, and
their voltage rating determined, before the capacitor is even
assembled.

-Chuck Harris

Raymond Domp Frank wrote:

On Fri, May 29, 2020 at 04:07 PM, Eric wrote:

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.

I don't think anyone would have accepted caps with +150/-100% tolerance,

not

even electrolytics...

Any information on the inability of modern caps to withstand overvoltage

more than vintage caps used to? Maybe WV was just very conservatively

spec'ed?

I'd rather have known the actual max.

For (wet) Al electrolytics, choosing a spec-voltage very much higher than

it's ever going to experience, has a deformation effect, so is not
recommended.

Raymond

Hi Raymond,

Old fashioned electrolytic capacitors have a fairly reactive,
(to aluminum) highly conductive, water based electrolyte.

The capacitor's leakage current creates an electrolytic cell
with the aluminum plates, and removes the oxide layer from the
cathode plate, and builds an oxide layer on the anode plate...

The oxide layer on the anode plate is the dielectric (insulator)
for the capacitor.

This reaction is the "reforming" process that lives on in
electrolytic capacitor lore... Even today. It is also a process
known colloquially as anodizing aluminum.

The aluminum oxide dielectric layer has competing issues:

As the oxide grows thicker, it became a better insulator, and works
to stop the leakage current necessary to grow the oxide layer thicker.

As the oxide layer ceases growing, the electrolyte dissolves the
oxide layer, allowing the leakage currents to increase... growing the
oxide layer thicker.

In normal operation, a balance is reached between leakage current
and growing the oxide layer.... A "working voltage" rating results.

The only way to increase the thickness of the oxide layer once
it reaches equilibrium, is to increase the voltage across the
electrolytic cell, making the cell's current increase, and in turn,
the oxide grow thicker... creating a new higher voltage equilibrium,
and a new "working voltage".

As long as the heat created by the leakage current doesn't raise
the temperature of the electrolyte to a point where the it boils,
the capacitor is fairly happy.

This means that higher than working voltage surges will start to
heat the electrolyte, but as long as the surge goes away before the
electrolyte boils, the capacitor will live to see another day.

One other factor needs mentioning:

The thinner the oxide dielectric layer, the higher the capacitance,
and the lower the working voltage.

The thicker the oxide dielectric layer, the lower the capacitance,
and the higher the working voltage.

The manufacturer had to balance all of these conditions when they
wrote the specifications for their old style electrolytic capacitors.

If you used these old electrolytic capacitors at a lower than working
voltage, their capacitance would increase, and their ability to
operate safely at their specified working voltage would diminish.

Unless you reformed the capacitor... safely limiting the current
until the oxide layer thickened...

Modern electrolytic capacitors use a nonreactive (to aluminum)
electrolyte and as a result, reforming is no longer necessary.

Modern electrolytic capacitors have their oxide layer created, and
their voltage rating determined, before the capacitor is even
assembled.

-Chuck Harris

Raymond Domp Frank wrote:

On Fri, May 29, 2020 at 04:07 PM, Eric wrote:

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.

I don't think anyone would have accepted caps with +150/-100% tolerance, not

even electrolytics...

Any information on the inability of modern caps to withstand overvoltage

more than vintage caps used to? Maybe WV was just very conservatively spec'ed?

I'd rather have known the actual max.

For (wet) Al electrolytics, choosing a spec-voltage very much higher than

it's ever going to experience, has a deformation effect, so is not
recommended.

Raymond

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:

On May 29, 2020, at 3:23 PM, Dennis Tillman W7pF <dennis@...> wrote:

?In a recent post the 7A29 was recommended as a much better plugin that the 7A19 which apparently has a poor reputation among members of TekScopes.
I don't disagree with that assessment. But if you know a few of the details behind its remarkable development and its strategic importance to Tek you might appreciate it more.

Tek was always known for how much attention they paid to the "Human Factors" of their products. Howard Vollum's goals for the 7000 series forced the "Human Factors" engineers to get much more creative. The result was a totally new look in scopes. In 1969 Tek introduced their brand new 7000 series lab scopes at the 1969 WESCON industry trade show in San Francisco. Tek's new top of the line 150MHz 7704 was 3 times faster than their current 547 lab scope. Tek was justifiably proud of the 7000 series. Here are a few examples of the new features Tek was so proud of:
* The new triggering control (one of Howard's requirements) combined the level and slope into one knob can turn through 360 degrees. This made it possible to trigger anywhere on a waveform while the knob showed the corresponding level and slope of the trigger point.
* The Tek 7704 had 4 plugin slots for much greater flexibility. It could display up to 4 signals simultaneously at two different sweep speeds.
* The new scopes had lighted push buttons to make them easier to use.
At the same show HP introduced their new line of lab scopes. But it quickly became apparent that the new HP scopes were superior in many ways. Here are two features that proved to HP that they had the superior products:
* The new HP scopes had much better specifications and they cost much less than the 7000 scopes.
* When you applied an AC signal that went from low frequency up to the limit of the scopes triggering capability the HP scope triggered perfectly across the entire frequency range without ever missing a trigger point. By comparison the 7000 trigger setting had to be constantly adjusted to get it to trigger. HPs triggering capability was so good it left Tek embarrassed by their trigger circuits.

HP wasted no time touting the superior performance and lower cost of their new lab scopes with an industry wide advertisement. Within Tek this became known as the HP "Foxtail" ad. It didn't mention Tek but it mocked the new features of the 7K scopes as irrelevant "bells and whistles". One of those bells and whistles was Barrie Gilbert's revolutionary on-screen readout.
I uploaded this ad to TekScopes at
/g/TekScopes/files/Infamous%20HP%20%22Foxtail%22%20advertisement

Internally the WESCON show was described as a big success to the Tek employees. But Howard Vollum and the rest of management knew they were in trouble. The 7K introduction was a flop. What Tek did next would cement their reputation as the industry leader in oscilloscopes for decades to come.
The ultimate goal of the 7000 series was never meant to be the 150 MHz 7704 that was introduced at WESCON. The 7000 series was designed from the beginning to have the capability to go much faster. But faster scopes could not be done with discrete parts. Tek would have to develop their own 1GHz+ analog IC process to make faster scopes. That would be extremely difficult. Meanwhile the 7000 design team was working on the next mainframe scope and plugins to leapfrog HP. The 7704 was 3X faster than the previous 547 50MHz lab scope. The next scope was going to be 10X faster if they could do it.

Thor Hallen, an excellent vertical amplifier designer was given the task of creating a vertical amplifier with 500MHz performance in the 7904. That meant his design had to have a standalone bandwidth in excess of 700MHz. The amplifier was required to use the new ICs Tek was making. Thor had less than 2 1/2 years to do this. When the date was decided for the announcement Tek still couldn't make the ICs Thor was supposed to use to achieve the unheard of specs the 7A19 had to meet. The ICs were nowhere to be seen. At the last minute Thor had to substitute discrete transistors for the ICs and still make it work at 500MHz in the 7904 it would be announced with. I have that 7A19 plugin in my collection that Thor made, with the transistors Thor used, for the announcement. As if that wasn't enough, all by himself, Thor designed the 7A19 Option 4 plugin which has variable delay so two extremely fast signals can be displayed at the same point in time on the CRT. The 7A19/7A19 Opt 4/7B90/7934 combination was the fastest lab scope ever made and it was ahead of its time in 1972.

In spite of the inferior performance of the 7000 products Tek announced at WESCON 3 years earlier, engineers liked what they saw and wanted features like on-screen readout. Ironically the bells and whistles HP mocked in the Fox Tail" ad were a strong selling point for the new 7000 series. By the time Tek introduced the 7904, HP was no longer crowing about their superiority in scopes. The development of the 7A19 and 7A19 Opt 4 by Thor Hallen is an astounding achievement for 1972. Its 500MHz performance in the also astounding 7904 lab scope was so much faster than anything else at the time that Tek reclaimed the crown as the undisputed leader in oscilloscopes. They kept the crown for over 20 years after that.

Dennis Tillman W7pF



--
Dennis Tillman W7pF
TekScopes Moderator