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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:

If they are part of the curve tracer, they should appear in
the mechanical parts list with a description like, insulating
washer...

-Chuck Harris

Michael W. Lynch via groups.io wrote:

Yeah, the fans in my HP 8566A spectrum analyzer and 8350B sweeper are pretty darn loud, too.? Don't have a 7904A to compare, just a 7904.? FWIW.?Jim Ford?Sent from my Verizon, Samsung Galaxy smartphone

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:

Michael,

Yes, used on all, my B36xxxx instruments have them as well.

Bob,

Excellent! Thank you.

Were these used on the much later model 576? I also have a 576 that dates from the mid 1980’s and I do not recall seeing these in that unit.
--
Michael Lynch
Dardanelle, AR

Hi Eric,

I think in the case of the +150% - 100% rated capacitor,
they were using the "-" as a hyphen, not a minus sign.

I think they meant to convey that the capacitor could range
in value from 150% of the marked value to 100% of the marked
value... otherwise it makes no sense to me.

In most cases, the range is as you would suppose.

-Chuck Harris


Eric wrote:

Hi Reg-

What y'all need to solve the fan noise problem on the 7904 is a Type 555 mainframe! Just turn it on for a little while and when you turn it back off you won't even realize the 7904 is on! <laughing>

The 555 is my favorite scope, but it does have two very serious fans!

73

Tom AE5I

On Fri, May 29, 2020 at 06:31 PM, Dennis Tillman W7pF wrote:

My apologies if I missed something important because my daily SPAM reports
stopped coming over a week ago.

No need to apologize, Dennis. On the contrary, it all stems from the work you do for us.

Raymond

Last night I noticed it had been over a week since I got the last daily list
of possible SPAM emails my ISP was holding up. I go through every one of
these daily SPAM reports I get and unblock the legitimate emails and mark
the sender as trusted. It can sometimes be a tedious job if there was a lot
of potential SPAM in that day's daily catch.
Quite a few TekScopes messages were waiting for my OK in the SPAM filter for
me to release them but I had no idea they were there since my daily notices
stopped coming over a week ago.
My ISP just reset my SPAM Filter settings and sent me a list of what has
been waiting for over a week for me to check. It was a very long list. I
just finished going through it. There were about 50 legitimate emails that I
release as legitimate.
My apologies if I missed something important because my daily SPAM reports
stopped coming over a week ago.

Dennis Tillman W7pF

For the life of me I cant remember the peace of gear that cap was it but it was a filter and VERY old and I cant forget the cap cause I was shocked at the tolerance spread as well. But even in the 576 some of the filter tolerances is +100% -10% and some of the ceramics is +80% -20%

On Fri, May 29, 2020 at 05:13 PM, Roger Evans wrote:

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.

The 7A29 was a much better (and newer) amplifier than the 7A19 altogether. Much has been written about that and can be found on TekWiki and other sources.

Raymond

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

If anyone in Southern Arizona is interested, I have a 7904A
with two 7A26, 7B92A and 7B70 Plug-Ins.

Contact me off list to discuss.

Motivated Seller.

73, Dick, W1KSZ
________________________________
From: [email protected] <[email protected]> on behalf of Roger Evans via groups.io <very_fuzzy_logic@...>
Sent: Friday, May 29, 2020 8:13 AM
To: [email protected] <[email protected]>
Subject: Re: [TekScopes] Advice about buying a 7904

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

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: [email protected] [mailto:[email protected]] On Behalf Of Jamie
Ostrowski
Sent: Wednesday, May 27, 2020 9:09 AM
To: [email protected]
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@...> 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: [email protected] [mailto:[email protected]] On Behalf Of
Roy Thistle
Sent: Tuesday, May 26, 2020 10:52 PM
To: [email protected]
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: /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