Date   
Re: Isolation???

 

Hi Harvey,
Regarding Mike Harmon's need for a way to measure a signal common mode without a true differential amplifier:

The devil is in the details. Amplifier design involves a variety of compromises which the end user would normally not be aware provided they used the amplifier as it was designed to be used. One of the things to be aware of with the vertical amplifiers Tek designs is their limited off-screen range. As long as the signals being displayed are on the screen or within a few divisions off screen they are in their linear range. But when you exceed that range the amplifier will saturate. You will not see this since it will be off screen. But the effect of this saturation is to slow down the recovery time of the signal considerably.

Tek lists the "Overdrive Recovery Time" in its specifications for each vertical amplifier. For instance the Overdrive Recovery Time for the 7A26 as 0.1mSec or less. Depending on what you are measuring that can be significant. As a general rule as long as the trace is on screen you don't have to worry about overdriving slowing or distorting the signal.

How successful you will be using one of Tek's general purpose dual trace plugins (7A12, 7A18, 7A18A, 7A24, 7A26) as a "poor-man's" differential amplifier will depend on attention to these details:
1) Both probe's must have exactly the same attenuation. The probes should be as closely matched as possible and be adjusted so their attenuation is the same.
2) The gain of each channel should be adjusted to be the same.
3) The signal coming to each channel must be attenuated the same amount by setting each Volts/Div knob the same if possible.
4) Each signal (by itself) should not go off screen at any point
5) The combined Add of CH1 to CH2 Inverted should not go off screen at any point.
6) The probe ground leads should be left floating. This may result in picking up a lot of 60Hz AC.

How much of a DC offset the vertical amplifier can handle between channels is not specified. The maximum each channel can take is for a 7A26 is 250VDC but that may only be under certain conditions.

Dennis Tillman W7PF

-----Original Message-----
From: Harvey White, Sent: Thursday, May 23, 2019 7:47 PM

On 5/23/2019 10:07 PM, Mike Harmon wrote:
I'm working on an old Motorola power supply. One of the adjustment steps says to connect a scope between the positive DC output terminal and a +41V test point inside the power supply, and look at the waveform. I have a 7904A with 7A26 vertical plugins. The problem is that whichever point I connect the common probe lead to is going to be taken to chassis (earth) ground! I'm not comfortable with lifting the scope chassis above ground by disconnecting the ground wire at the AC input.
What's immediately jumping out at me is that you have a 7904A with a 7A26. You can put a scope probe on channel 1 to the positive DC output, and the scope probe from channel 2 on the 41v test point. Set channel 2 to invert, then the display mode on the plugin to "ADD".

The 41 volts should be within the common mode range of the 7A26, but a quick look at the manual will tell you for what ranges.

Very standard way of measuring a differential voltage.

A 7A22 or a 7A13 has a built in differential voltage capability, so you wouldn't necessarily need the DC voltage, but with the 7A26, you should be just fine.

Harvey



--
Dennis Tillman W7PF
TekScopes Moderator

Re: 497P frequency counter question? How to turn on?

Steve Wiseman
 

On Thu, 23 May 2019 at 11:45, <radioconnection@...> wrote:

Well, two odd things. The switches are set for a 497P. I didn't fully
understand how the counter works. It apparently is enabled in this
particular 497P, regardless of the fact that the switches are set for the
correct model.
Bear in mind that the DIP-switches may well not actually work. Despite you
moving the lever, the metal gubbins inside may not move. This can become
confusing when debugging - check voltages, rather than relying on switch
positions.

Steve-the-bitter.

Re: 7904 excessive shadow/flare

 

Hi Max,
The expansion mesh was an engineering compromise which had significant drawbacks but all the alternatives had worse drawbacks.
Tek scopes were always known for their razor sharp trace. When the 7000 series was being designed it became apparent that the high frequencies the scope had to attain were going to require a new breakthrough if the length of the CRTs were going to be reasonable. The result was the expansion mesh which, unfortunately traded the super sharp trace for higher bandwidth.

An alternative is available however. The 7104 does not use the expansion mesh. It uses a box lens so the trace is finer. It also has the Micro Channel Plate which improved the brightness of the trace so much it was now possible to see a single 300pSec pulse in normal room light at 200pSec/Div sweep speed.

You need to get yourself a 7104. There is so much more it can do than a 7904.

Dennis Tillman W7PF

-----Original Message-----
From: unclebanjoman, Sent: Friday, May 24, 2019 6:21 AM

O.K.! Thanks to everyone for the comprehensive answers, very instructive to me!

Undoubtedly the driving circuits of the 7904 and 7804 CRTs are completely different and this could partly explain the different behavior regarding the halo / flare effect.
Over time I will get used to the remarkable brilliance of my new 7904 (and his associated halo /flares) which, moreover, is really in excellent condition and after careful calibration it matches perfectly the manual's specifications.

My 7904 has serial number B269718; reading the manufacture date of a pair of TI SNxxxx ICs, it should be from 1979.

If another 7904 to be repaired happens to me, I will be curious to observe the halo / flare effect, whether it is identical or not. I will let you know if the case.

Thanks again,
Max




--
Dennis Tillman W7PF
TekScopes Moderator

Re: 465B Attenuator servicing

Adrian
 

Hi Colin,

Yes you can, the Pro Power IPA is good I get 400ml spray cans from Farnell (or Element14 or whatever they want to be called today) or CPC (who are also Farnell) sometimes have offers on 10 packs of it. Farnell is around 3.75 GBP/can at 10 off I think.

Apart from the convenience I also work on the principal that if the pressure hasn't got out then the H2O has not got in!

I buy 25L bulk cans for the board washer on ebay where a bit of water is not an issue.

Adrian

On 5/24/2019 12:41 PM, Colin Herbert via Groups.Io wrote:
OK, so the existence of spray-can IPA is new to me. Perhaps I should see if
I can get it in the UK

2715 frequency counter problem

Jeff Woolsey
 

What with all this talk about frequency counters in spectrum analyzers,
it's probably time to address the problem my 2715 is having.

I find I can no longer count the center frequency, failing with

FREQ NORM SUGGESTED (1ST LO)

and when I try to do the reference frequency normalization
[UTIL][5][5][1][1] with an external 10MHz, that fails as well.  Trying
to use last norm or default values doesn't "fail", but I still can't
count center frequencies.  This used to work.

Running a full normalization finishes (eventually), but doesn't cure the
problem either. REF FREQ NORM still FAILed.

Advice?

--
Jeff Woolsey {{woolsey,jlw}@jlw,first.last@{gmail,jlw}}.com
Nature abhors straight antennas, clean lenses, and empty storage.
"Delete! Delete! OK!" -Dr. Bronner on disk space management
Card-sorting, Joel. -Crow on solitaire

494P problems - ROM and 1st LO

Sparky99
 

Hi everyone,

Some years ago I purchased a 494P needing repair and now finally have some time this weekend to investigate the faults. On power up it returns ROM 1 test failure. Pushing a button then completes the boot up process and the instrument appears to function in most modes, however decreasing the frequency span below 5MHz produces a 1st LO tuning failure, which is then replaced by Frequency control failure after a minute or so or when the frequency span is further reduced.

Unfortunately I don't have the extender cards (or manual), and before investing in these just wondered if anyone has any thoughts as to the best places to investigate for these errors (I'm not sure if these are separate problem, or a single problem which is associated with both the ROM and LO failures.)

Thanks for any thoughts.

Julian

Re: Latest firmware for TDS210 and TDS2CM?

 

On Fri, May 24, 2019 at 04:57 PM, Jared Cabot wrote:


it had the earthing recall fix already done too, so it was post recall as well.
That was done on TDS210 below B049400 and TDS220 below B041060.
If your S/N is lower than that and has the P/S MOD sticker the main board has been replaced to the newer version.
This was not a recall but it was done on extended warranty if one or more BNC connector had become loose.

/Håkan

Re: 7904 excessive shadow/flare

unclebanjoman
 

Thank you for your explanation Dennis. Very instructive!
I take this opportunity to inform you that a few weeks ago I sent you an e-mail regarding the repair of my 7854 smps (finished with success), but I have not received a reply.
Maybe it was lost due to an antispam or blacklisted?

Thanks again,
Max

Re: 494P problems - ROM and 1st LO

John Miles
 

Some years ago I purchased a 494P needing repair and now finally have some
time this weekend to investigate the faults. On power up it returns ROM 1 test
failure. Pushing a button then completes the boot up process and the
instrument appears to function in most modes, however decreasing the
frequency span below 5MHz produces a 1st LO tuning failure, which is then
replaced by Frequency control failure after a minute or so or when the
frequency span is further reduced.

Unfortunately I don't have the extender cards (or manual), and before investing
in these just wondered if anyone has any thoughts as to the best places to
investigate for these errors (I'm not sure if these are separate problem, or a
single problem which is associated with both the ROM and LO failures.)
I'd want to straighten out the ROM test failure before even thinking about any further troubleshooting. Trouble is, the original 494 models had mask programmed ROMs, all of them soldered in place from what I've seen, and I don't think they have ever been archived.

I would suggest checking ko4bb.com, but the ROM images no longer seem to be stored alongside the manual files. All I see for the 494 is a note (ROM_images.txt) that says "Firmware version info and ROM images are in 3_ROM_Images." There is no "3_ROM_Images" directory on the site, and the "Tek roms" folder under "01_Rom_images_and_drivers" is empty.

Didier, any input on how to navigate this apparent change in the directory structure?

Meantime, it's never a bad idea to have a look at the usual suspects, supply rail voltages and capacitors. Service volume 1 contains some troubleshooting guidance on the first LO failure, so it's a logical place to continue looking once the firmware has been ruled out.

Re: the caps ...

During trouble shooting, I've found several bad 50 uF at 25 VDC radial lead (not
SMD!) on the first few boards I pulled for inspection.
All of those caps showed severe leakage and many had open leads due to
corrosion. If you own a 497P it might be worth inspecting
the first few plug in boards (LO driver, Preselector driver, etc.) to see if your
version has these caps (photo posted in library.)
Serial number of unit with bad caps is B020199 version 9.7--late production run.
To be precise, those aren't radial-lead capacitors, they are axial-lead parts (think of an 'axle' with wheels mounted on the ends of the component leads.) An ESR meter is helpful for checking them, but often bad ones can simply be spotted visually as in your case.

Radial tantalums are the epoxy-encapsulated spheroidal ones that are famous for failing shorted, especially in circuits with a lot of inrush current and/or insufficient voltage derating. Both the radial and axial parts are common trouble sources, but they are constructed very differently as Chuck says and they fail in very different ways. I've rarely seen shorted axial caps or open radial ones.

-- john, KE5FX

Re: OT: App that keeps track of "Or Best Offer" auctions on eBay to tell you what a seller will accept.

Kevin Oconnor
 

FWIW, and more OT, I sell frequently on eBay, but almost never in auction format. I will use BIN with or without offers. When eBay shifted to profit on final value with essentially free listing it made no sense to me to continue the uncertainty of auction format. I know what margins I need to make an acceptable profit. If you have no cost basis in an item an auction may be ok, otherwise you are either under or overpriced.
eBays current returns policy is also onerous. eBay is becoming like amazon in that they will let a customer return an item for any reason. The seller generally can’t fight it. If your customer pays the shipping costs, you will have to credit the final value, refund the shipping costs and pay the return shipping if you want the item back.
I have sold electronics where the buyer removed components and returned the item as defective. eBay won’t do anything.
I now sell a lot of items with free shipping. I build the costs into the pricing. I pay FVF on the total and figure that into my margins. Whether or not I offer a guarantee, the customer always has to pay the return shipping before a refund is issued. Customers think twice if there is a hard return cost.
On eBay you have zero clarity on best-offers which, in my experience, range from 100% to 25%.

Kevin

"Volts/Div" knob for Tek 2235 needed [ Tek part No : 366-2148-01]

donald collie
 

I only really need the clear plastic skirt on this knob [minimum], or two
complete knobs [maximum], as I can probably repair the other one, which has
a cracked skirt. Willing to pay for these. Probably also used on other 22xx
types.
Also : I replaced the chopper MOSFET in the psu with a NZ$1-60 generic
type. It gets very hot to my way of thinking - too hot to hold your finger
on the heatsink near it- does anyone know if an exact replacement is
necessary - perhaps a faster type with lower losses. The MOSFET`s molex
socket shows signs of being quite hot over some time [browning]
Thankyou in anticipation for your
reply.........................................................Donald ZL4GX

Replacing Power LED/light on TEK 2465BCT

oakley@...
 

I just acquired a very low-hour 2465BCT. Everything seems to work perfectly with one exception, it appears that the Power LED/light is not working. Is this part easy to replace?

I'm an EE and none of the internal workings scare me, but I'd probably ignore the indicator light right now if it requires a bunch of work to replace. Thanks!

Tom

Re: OT: App that keeps track of "Or Best Offer" auctions on eBay to tell you what a seller will accept.

Jim Potter
 

I'm with you on eBay sales. I never use the auction format, and if I every buy anything that's auctioned I snipe in the last few seconds.

However, I don't understand what you mean by your last comment, "On eBay you have zero clarity on best-offers which, in my experience, range from 100% to 25%."

I usually set the lower limit for notification of best offers at the minimum I will accept, usually 75% of my BIN price, automatically rejecting lower offers. I have seen a number of instances where the potential buyer keeps increasing his offer until he gets over the threshold. Rather than dicker I always accept that offer. If I make an offer that is explicitly rejected by the seller I will counteroffer to split the difference, but I won't deal with someone who will only go $10 discount on a $500 item, unless of course it's a must have item, in which case I would have taken his BIN price immediately.

Thanks for your comments.

Jim

Re: "Volts/Div" knob for Tek 2235 needed [ Tek part No : 366-2148-01]

tom jobe <tomjobe@...>
 

One of the members of Tekscopes makes new replacement volt/div knob skirts for the 2235. I have bought some of them from him and they are very nice.
He also carries some hard to find good used/tested parts for the 2235 such as the main transformer (T948?), the high voltage multiplier (U975?), etc.
This link shows what he has in stock for sale on eBay at the moment, he must be out of those 2235 knob skirts temporarily.
https://www.ebay.com/sch/shockcore/m.html?item=273836542787&hash=item3fc1ee0b43%3Ag%3AHUcAAOSwZQRYf5YX&rt=nc&_trksid=p2047675.l2562
He is a great guy to do business with as his 100% eBay customer satisfaction rating shows.
I have bought several sets of his new replacement 2235 volt/div knob skirts and they work perfectly.
About he only way you can make a mess of the knob skirt replacement is to  use too much glue.
A few very tiny dabs of glue/epoxy at the smallest diameter of the knob register is all you need, and there is a notch that orients the skirt to the knob just like the original had.
The member name he uses on Tekscopes escapes me at the moment, but you can contact him through any of his eBay auction pages such as this page for his 2235 main transformers.
https://www.ebay.com/itm/Tektronix-120-1601-01-SwitchmodeTransformer-for-2213A-2215A-2235-Oscilloscope/273836542796?hash=item3fc1ee0b4c:g:bAQAAOSwnrJbfX~4
tom jobe...

On 5/24/2019 9:44 PM, donald collie wrote:
I only really need the clear plastic skirt on this knob [minimum], or two
complete knobs [maximum], as I can probably repair the other one, which has
a cracked skirt. Willing to pay for these. Probably also used on other 22xx
types.
Also : I replaced the chopper MOSFET in the psu with a NZ$1-60 generic
type. It gets very hot to my way of thinking - too hot to hold your finger
on the heatsink near it- does anyone know if an exact replacement is
necessary - perhaps a faster type with lower losses. The MOSFET`s molex
socket shows signs of being quite hot over some time [browning]
Thankyou in anticipation for your
reply.........................................................Donald ZL4GX

Re: Replacing Power LED/light on TEK 2465BCT

Chuck Harris
 

There is no power indicator light.

-Chuck Harris

oakley@... wrote:

I just acquired a very low-hour 2465BCT. Everything seems to work perfectly with one exception, it appears that the Power LED/light is not working. Is this part easy to replace?

I'm an EE and none of the internal workings scare me, but I'd probably ignore the indicator light right now if it requires a bunch of work to replace. Thanks!

Tom



Re: Replacing Power LED/light on TEK 2465BCT

Dan Rae
 

On 5/24/2019 10:03 PM, oakley@... wrote:
I just acquired a very low-hour 2465BCT. Everything seems to work perfectly with one exception, it appears that the Power LED/light is not working. Is this part easy to replace?
Tom, there is no Power LED as such.?? And the Hours counter can be set to zero or any number you like as part of the calibration process, so...

Dan

Re: Replacing Power LED/light on TEK 2465BCT

Tony Fleming
 

Mine 2465DMS has no light - it is a switch - when you press it in, a green
internal insert is visible so it looks like a very dim light, but there
isn't any light there - at least on mine.
BUT I don't have your model exactly, so I don't know if changes were made
in yours.

On Sat, May 25, 2019 at 12:37 AM <oakley@...> wrote:

I just acquired a very low-hour 2465BCT. Everything seems to work
perfectly with one exception, it appears that the Power LED/light is not
working. Is this part easy to replace?

I'm an EE and none of the internal workings scare me, but I'd probably
ignore the indicator light right now if it requires a bunch of work to
replace. Thanks!

Tom



Re: OT: App that keeps track of "Or Best Offer" auctions on eBay to tell you what a seller will accept.

tekscopegroup@...
 

On Wed, May 22, 2019 at 03:28 PM, Tom Gardner wrote:
Regrettably goofbid appears to return zero results, at the moment.

Maybe ebay have stymied them, maybe they have a simple server problem. I hope
it is the latter.
Another tool I use a lot and before any purchase to check out the "real" feedback story of a seller is a Firefox addon called EbayNegs. While ebay only bases the seller's current rating on the last 12 months of activity, and that is the most you will see, this app will dig up the dirt from years before. And this one too, sometimes used to report that was being blocked by ebay, but usually works quite well. As a matter of fact come to think of it its been a good while now (months) since I last saw it being blocked, so guess they figured out a way to stop the blockage from ebay. I remember some years ago when it stopped working completely because ebay had changed the format of the sellers link on the listing page, I contacted the author of the addon to let him know. I got a nice thank you within 24 hours and it took just another day or so for it to be back up and running.

Re: Replacing Power LED/light on TEK 2465BCT

Ken Winterling
 

I have a 2465DVS which is essentially the same scope.  It, too, has the mechanical green indicator built into the power button.Ken WA2LBI LG G6 ------ Original message------From: Tony FlemingDate: Sat, May 25, 2019 10:42To: TekScopes@groups.io;Cc: Subject:Re: [TekScopes] Replacing Power LED/light on TEK 2465BCTMine 2465DMS has no light - it is a switch - when you press it in, a green
internal insert is visible so it looks like a very dim light, but there
isn't any light there - at least on mine.
BUT I don't have your model exactly, so I don't know if changes were made
in yours.

On Sat, May 25, 2019 at 12:37 AM wrote:

I just acquired a very low-hour 2465BCT. Everything seems to work
perfectly with one exception, it appears that the Power LED/light is not
working. Is this part easy to replace?

I'm an EE and none of the internal workings scare me, but I'd probably
ignore the indicator light right now if it requires a bunch of work to
replace. Thanks!

Tom



Re: Replacing Power LED/light on TEK 2465BCT

Chuck Harris
 

For what it is worth, the little green indicator
is mechanically very fidgety and delicate, and
is often broken on the scopes I get for repair

The problem is a very tiny push arm moves the
shutters that hide the green spot, it breaks,
and is too small to glue.

The only way to reliably fix it is to replace it.

-Chuck Harris

Ken Winterling wrote:

I have a 2465DVS which is essentially the same scope. It, too, has the mechanical green indicator built into the power button.Ken WA2LBI LG G6 ------ Original message------From: Tony FlemingDate: Sat, May 25, 2019 10:42To: TekScopes@groups.io;Cc: Subject:Re: [TekScopes] Replacing Power LED/light on TEK 2465BCTMine 2465DMS has no light - it is a switch - when you press it in, a green
internal insert is visible so it looks like a very dim light, but there
isn't any light there - at least on mine.
BUT I don't have your model exactly, so I don't know if changes were made
in yours.

On Sat, May 25, 2019 at 12:37 AM wrote:

I just acquired a very low-hour 2465BCT. Everything seems to work
perfectly with one exception, it appears that the Power LED/light is not
working. Is this part easy to replace?

I'm an EE and none of the internal workings scare me, but I'd probably
ignore the indicator light right now if it requires a bunch of work to
replace. Thanks!

Tom