开云体育

While it may not matter for this application, the inductance of film resistors may be different depending on type, value and trimming method both from one resistor to another as well as to carbon composition.

________________________________
From: [email protected] <[email protected]> on behalf of Richard Knoppow via groups.io <dickburk@...>
Sent: Tuesday, May 20, 2025 4:53:59 PM
To: [email protected] <[email protected]>
Subject: Re: [TekScopes] Tektronix 494P 1st LO and Frequency failure(s)

FWIW Allen-Bradley carbon composition resistors are identical with Ohmite. For some reason AB marketed through Ohmite. Same with AB and Ohmite pots.They are about the best of the carbon comp resistors but despite this they do drift. Usually upward and usually higher values drift proportionally to value. It is not reversible. While heat may accelerate the drift the resistors reportedly drift on the shelf. Carbon and metal film resistors are far more stable, essentially neither drift with age or use. Carbon comp resistors have no real advantage over film resistors, there is no good reason to use them. BTW a little OT, IRC advertised heavily but I almost never see any in old gear. I don't remember their having a bad reputation and they don't seem to have been more expensive than AB/Ohmite, so a bit of a puzzle.Sent from my Galaxy
-------- Original message --------From: "Ulf Kylenfall via <ulf_r_k@...> Date: 5/20/25 1:27 PM (GMT-08:00) To: [email protected] Subject: Re: [TekScopes] Tektronix 494P 1st LO and Frequency failure(s) I have repaired a 494 as well as a 2756. Apart from what KE5FX writes in his document there are a few other things to look out for.On several PCB's there are High Ohm carbon mass resistors made by AllenBradley. Several MegaOhms.During my attempts to repair the analyzers, I have found these resistors to age/drift much above and outside their original some cases more than 50%. And these resistors can be found in places where frequencies are set.When trouble-shooting using the built in procedures I could get the instrumenst to work only to get unlock messages the nexttime one of the instruments was turned on. Eventually, I replaced all of them with 1% metal film resistors. After havingdone that the problems disappeared.Ulf KylenfallSM6GXV

Carbon composition resistors have their place where where short duration, high energy pulses are involved.
Film resistors just open up.
Carbon composition resistors increase in value with age, probably due to the way they are made.
Carbon granules, in a binder material are encase in a material like Bakelite.
One crack where the lead exits the case and moisture enters, swelling the binder and the resistance goes up.

Glenn

--
-----------------------------------------------------------------------
Glenn Little ARRL Technical Specialist QCWA LM 28417
Amateur Callsign: WB4UIV wb4uiv@... AMSAT LM 2178
QTH: Goose Creek, SC USA (EM92xx) USSVI, FRA, NRA-LM ARRL TAPR
"It is not the class of license that the Amateur holds but the class
of the Amateur that holds the license"

FWIW Allen-Bradley carbon composition resistors are identical with Ohmite.? For some reason AB marketed through Ohmite. Same with AB and Ohmite pots.They are about the best of the carbon comp resistors but despite this they do drift.? Usually upward and usually higher values drift proportionally to value.? It is not reversible.? While heat may accelerate the drift the resistors reportedly drift on the shelf.??Carbon and metal film resistors are far more stable, essentially neither drift with age or use.? ?Carbon comp resistors have no real advantage over film resistors, there is no good reason to use them.? ?BTW a little OT,? IRC advertised heavily but I almost never see any in old gear.? I don't remember their having a bad reputation and they don't seem to have been more expensive than AB/Ohmite, so a bit of a puzzle.Sent from my Galaxy

I have repaired a 494 as well as a 2756. Apart from what KE5FX writes in his document there are a few other things to look out for.

On several PCB's there are High Ohm carbon mass resistors made by AllenBradley. Several MegaOhms.
During my attempts to repair the analyzers, I have found these resistors to age/drift much above and outside their original specification.
In some cases more than 50%. And these resistors can be found in places where frequencies are set.

When trouble-shooting using the built in procedures I could get the instrumenst to work only to get unlock messages the next
time one of the instruments was turned on. Eventually, I replaced all of them with 1% metal film resistors. After having
done that the problems disappeared.

Ulf Kylenfall
SM6GXV

On Sun, May 18, 2025 at 08:51 PM, Roy Thistle wrote:

Eagle Picher LTC-7P Battery
Non-Rechargeable,Rectangular, Lithium Thionyl Chloride,3.5 VDC,750mAh
MFR Part #: LTC-7P
RS Stock Number: 70141129

Thank you! larry

This subject has been discussed many times over the years. A short answer is that the technical aspects of the CRT are quite modest in terms of speed (and therefore operating voltages), and the only thing "special" about it is the 10 x 10 division graticule on the screen, versus 10 x 8 commonly used in scopes. It's the same basic CRT as in some other Tek models except for that. Years ago I figured out the models that have the right CRT, but I forget which - I think it is certain 5000 series scopes and 600 series XYZ monitors. I reported on this here some years ago, so the results should be possible to find with some searching. I think it was around five to ten years back - I had a possible different CRT sub selected, but didn't need it yet since I did a successful (enough) rejuvenation on a 576's original CRT.

The simplest fix then is to just put in one of the alternative types, and tweak it up to register properly with the 10 x 8 graticule. The 576 original has the 10 x 10 square plus two extra horizontal divisions marked with dotted lines, which kind of help to fill in the classic 4:3 aspect ratio. The scope equivalent is 10 x 8 larger divisions. So, you get a bigger picture, but less vertical range on screen. As long as the deflection system is tweaked up to match, it will still be right, just a little different in appearance and operation. For instance, there will be only 8, not 10 on-screen vertical offset steps. You can adapt to the differences with use - it's way better than having no 576 at all.

Ed

Hi Federico
If you search the forum messages for 'John Miles' you may find some relevant information.

Ciao Federico,
se cerchi "John Miles" nei messaggi di questo forum, magari trovi qualche informazione utile
--
Roy Thistle

Hi all,
I'm new to this group, I'm italian, apologies for language errors ;-)

I have a Tek 494P (fantastic!), but as you well know it is a “time bomb”.
I have repaired it many times, but now I am facing a new problem that I cannot solve:
the SA immediately (at startup) gives the 1ST LO error, then the frequency counter error.

As you can see in this picture

the counter ... does not read

It wasn't like that right away: it would start randomly, and sometimes it depended on the “center frequency” (meaning it worked on some frequencies, and not on others) - now the fault is always there, whatever frequency range and span is used (max span, < 10Mhz/div, < 500 kHz div).

In the past I have had many problems with the (in)famous A50 board, and looking at the diagrams there were many variations between the 494, 494A and the 27xx series. But now that doesn't seem to be the problem (if I'm not mistaken).

I have extension cables for the boards (homemade), but I would need other boards for swapping (and I don't have them).

Any suggestions on where to start looking? All the obvious analysis has been done (e.g., power supplies).

cheers
Federico

fedevirgo@...

p.s. if someone have spare boards to sell ... rise your hand :-)

On Mon, May 19, 2025 at 07:45 AM, Tom Gardner wrote:

I have
not checked the amplitude/frequency is suitable for the 2465.

I didn't want to mention any particular vendor/manufacturer.
Nothing? ... at least nothing inexpensive... beats the tunnel diode pulse generator, for low level, very fast, very sharp pulses.
Tekwiki gives the Tek 067-0681-01 as
Pulse amplitude ≈250 mV into 50 Ω
Rise time ≤150 ps
Aberrations < 4% including < 8% peak-to peak ring at (typically) 5 GHz measured with 11.5 GHz bandwidth
But AFAIK, the 067-0681-01, comes up for sale infrequently... and demands a high price, for what it is.
So... supposing the 2465B has a bandwidth of 400 MHz
Then the rise-time is characterized as 875 picoseconds.
Using the '1/3 rule' ... that gives the required rise time of the source as approx. 290 picoseconds.
So according to the Tek specs, the 067-0681-01 is better.
But according to the Bodnar specs... for their "Fast rise-time pulse generator (BNC)" ... that would probably do.

--
Roy Thistle

On Mon, May 19, 2025 at 01:51 PM, Jay Czaja wrote:

Any one have a spare

If you can get the model, you can get it 3-D printed, very cheaply... maybe even free shipping.
Unless you find someone locally, with a parts mule... getting the 3-print might be cheaper than shipping domestically.

--
Roy Thistle

From your picture I note that we have different knob styles.
I only read 366-1124-00 in the manuals 070-0905-00/01 but "Tek_RPR_366-_Knobs_and_Pushbuttons" lists both 366-1007-00/01 and 366-1124-00 for the 576. Perhaps I have that 1124 style with 4 ribs.
Albert

at 6.8mm, I think it is too low

Can confirm 10.16 mm (=0.4 inch) on mine

/H?kan

Another option is to increase the CRT filament voltage. The easy way is to set the mains voltage selector to LOW.
Since the CRT filament is powered directly from the main power supply transformer, this increases the filament voltage enough to often cause a significant increase in intensity.
This will also increase the the voltage regulator dissipation, but with the large heatsink of the rear casting, this is not really an issue.
Be aware that the CRT filament is raised 3KV above ground.

There are few options for an excessively weak or dead CRT in the 576.
Years ago I had considered attempting to install a CRT from the 1420 or 528, one without an internal graticule. These are at least somewhat similar in physical size, although smaller, and use a similar low HV.

Good Luck with your effort.

at 6.8mm, I think it is too low.


left is the expected 10.16 mm.

I measured about 6.8 mm, more precise is difficult (for me). Albert

Hi Max,

On May 19, 2025, at 14:56 , unclebanjoman via groups.io <mmazza@...> wrote:

I strongly suspect that you have a bad trimming capacitor (C163 or C187). They had influence on both + and - signal path.
If there is no change at all when turning they, one strong possibility is the one trim cap is open or loose almost all it's capacity.

That was my first thought as well, having a 7A26 last year that was cursed with half a dozen bad ceramic trimmers, and last month a 492P with a bad one in the VR module. I replaced C187 with no change, though, and the old C187 measured approximately correct on the DE-5000.

Maybe I'll take a harder look at the others, though, after looking at your story. I hadn't thought about one being partly working!

thanks,
Adam

Thanks for saving the inventory from getting tossed. I'm sure it will help keep a lot of Tek and HP gear from getting parted out or worse. The benches in my shop are 75% Tek, so I'm grateful for your efforts.

I Adam,
I strongly suspect that you have a bad trimming capacitor (C163 or C187). They had influence on both + and - signal path.
If there is no change at all when turning they, one strong possibility is the one trim cap is open or loose almost all it's capacity.

I had a similar experience with my 7A16.
You can read the full instructive story (problem solved) here:
/g/TekScopes/message/180640

Cheers,
Max

I managed to get the screen looking a little bit better in the corners. After having a closer look, it turns out the clear plastic screen behind the blue filter was filthy on the edges and after a cleaning, the CRT still isn't sharp overall but the corners are much improved. While taking the front bezel off, I also managed to snap one of the knobs under the CRT by being a cavalier and just pulling on the knob (oops). I used a little bit of heat on the rest of them and didn't have any issues. Lesson learned. Any one have a spare knob or 2? haha.

Jay

Rick,

I echo what Paul says. One thing you can do is look for a donor 576. As you know from reading through the messages, the older 576's have issues with the HV (brown) transformer. I got one of these and had a replacement HV transformer professionally made. The CRT is bright and sharp and appears to have low hours.

I have a theory that the ones with HV transformer issues will have low hours on the CRT, as the HV transformer issue is difficult to troubleshoot and usually only shows up when the transformer has warmed up a bit. I would have never guessed or thought to look at the HV transformer with out the help of this group (and some members of this group that have moved over to Tekscopes2). So my theory is, the older 576s developed an issue with the HV transformers, and service was attempted (unsuccessfully) and then put on the junk pile or relegated to a back closet somewhere.

If you keep an eye out on eBay (high priced mostly ... but not always ... you can have eBay email you everytime a new 576 listing shows up) and other places (craigslist, facebook, swap meets, etc) for ones with the brown transformer and really beat up, you can get a donor at a good price (less than the kit mentioned a few months ago), plus you will get some other parts too. I agree it is still a risk the CRT will be well used or damaged, but the two "brown transformer" ones I have gotten have really good CRTs)

I hope you find one.

Dave