开云体育

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 :-)

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

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

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

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"

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

(Un)fortunately enough, the frequency control section is one area that I've seldom/never had to work on, myself.

Since you (Federico) have extender boards, I think I'd try applying a combination of R134a or similar coolant spray (not sure what 's available in the EU, though) and a heat gun or hair dryer. If you are able to identify the general area where the problem is occurring, a Q-tip soaked in R134a is a good next step.

-- john, KE5FX

no if 1st if osc...I would do in this order the following..Check...
Power supplies, good, not close but right on the money. See john miles notes.
Check there is any yig output , i.e.1st IF is on front panel.2 to 6GHz.
Check the aux synth 200MHz is running and the output going to the harmonic mixer is good.
There should be around 40MHz output from harmonic mixer with yig running and 200 MHz running.
The cpu counts this 40MHz output to conrol the course loop yig control. You seem to hve 0 count.
With manual, check the aux synth 200 MHz osc is running and you have a yig output into harmonic mixer.
The cpu counts this 40MHz (after dividing by 100) and you seem to have a 0 count.

P.s. test tne above with rbw well over 200kHz. That cuts out the inner yig loop which is 2/3 of the first if hw, making debugging easier.