Tek 7L12 & 7S12

[vk6zgo]

Been having a bit more of a play with the devices I got at the Hamfest.

The 7L12 seems to be OK--it is clunky as hell to use, but they were like that 30 years ago.
Earlier, I was suspicious as to whether the 50MHz cal signal was on frequency, so dragged out the "Victor" frequency counter, which promptly showed me 75MHz, then something else.
I had forgotten that "El Cheapo" frequency counters are a tad insensitive, so I had to use the 7A18 plugin as a preamp which gave it a decent signal level to work with, when it read spot on 50MHz.

I have been looking at random RF off-air & it sees the FM stations & some other stuff all right.
Back in the day, Analog TV signals were easy to recognise, as were the initial terrestrial digital TV transmissions, but there doesn't seem to be as much on highband VHF these days.

I tried the 7S12 again & fed the output from the S52 into the TA18, which showed a strong signal but, with no timebase plugin, it couldn't be triggered.

Today, I looked at the signal with the BWD 845, which showed approximately the correct signal, but had trouble with triggering when I tried to look at it on faster timebase settings.
Old mate "Dubya" has a non-functional dual timebase, making looking at fast signals very dicey.

And, so, down the rabbit hole......

[MED6753]

You're much braver than I am, probably because you are familiar with them. Analyzer stuff is all voodoo to me and I'd never buy one of those plug-in's unless I got it for free. I was looking at the 7L12 pictures and all the calibration points most likely requiring massive amounts of highly expensive TE.

[tggzzz]

I was looking at the 7L12 pictures and all the calibration points most likely requiring massive amounts of highly expensive TE.

I don't see the problem.


Of course.

[Cubdriver]

You're much braver than I am, probably because you are familiar with them. Analyzer stuff is all voodoo to me and I'd never buy one of those plug-in's unless I got it for free. I was looking at the 7L12 pictures and all the calibration points most likely requiring massive amounts of highly expensive TE.

That's the whole idea! You find the page in the manual regarding calibration and turn the 'Required Instruments' bit into a shopping list as you now have justification because you need them. (Then get manuals for THOSE instruments and repeat this process ad infinitum...)



-Pat

[mansaxel]

That's the whole idea! You find the page in the manual regarding calibration and turn the 'Required Instruments' bit into a shopping list as you now have justification because you need them. (Then get manuals for THOSE instruments and repeat this process ad infinitum...)

This so! Very much. I had my sights set on a hp 410B because I wanted a VTVM. The service manual of my parents open reel tape recorder (Tandberg 1200X) mentioned a VTVM all the time because (now I know) the usual Avometer would load the high impedance circuitry too much. So a VTVM was high on my initial TE wish list. Little did I know that the 410B manual contained such a trap -- more specifically the hp 428b! So instantly I thought I had to have one. And that is but the rim of the rabbit hole.

I now have 4 VTVM (Two hp, one Macaroni and a Sennheiser/Grundig/H&B). And that 428b.

[vk6zgo]

A few more musings & actual doings-

The 7L12 seems to be usable pretty much as it is, even if it is a bit out of spec.

In one of my old jobs, the one we had was very useful, as after I worked around its "clunkiness" I was able to use it to troubleshoot really strange things, like an interfering signal being injected into a couple of video amps.
This signal was visible on a picture monitor, but not so on an ordinary 'scope.

The 7L12 showed a solid carrier at 800kHz when I looked at the output of the amp (75 ohm to 50 ohm mismatch be damned!)
800kHz just happened to be the specified separation between FM Broadcast transmitters, of which there were a bunch right next door.
It turned out that the coax into the amps was picking up several such carriers & the first stage was acting as a mixer.

Another time, I monitored the composite stereo program source for one of the FMs we were "Hosting" under the "good corporate neighbour" scheme.
This FM Tx also had two SCA channels, one of which would periodically get smushed by main programme signals.
This was readily visible with the 7L12, although the spec said it shouldn't be!
It turned out to be a switch on the Optimod at the studio being in the wrong position, or so the Tech there said.

Our Studio had a bunch of wired RF TV channels used for distribution of programme material & "off-air" stuff, mainly for the "Suits" & other non-tech folk.
These would over time, drift, & needed tweaking, which, without an SA was a bit "hit or miss".

At the previous job, we would have probably turned our noses up at the7L12, but as a "knockabout" SA, it more than "paid for its keep".

[vk6zgo]

Update on the 7S12 TDR thingy:-

It appears that the S52 pulse generator works, as looking at its output with the BWD 845 shows a waveform like the ones shown various places on the Internet.
Unfortunately, as the Dual Timebase is faulty on the BWD, I can't look at rise time properly, but using the fastest display I can get with the ordinary setup, the rise time for the CRO appears to be around 10ns or thereabouts, which meets the 845's spec, so it would seem that the pulse is OK.

Using the S52 into the S6 yields no display on the 7603, so it seems that the S6, or something in the 7S12, is faulty.

[tggzzz]

If the sampler is anything like the 1502, then there will be some trimpots/caps that need to be adjusted to ensure sampling does occur. That's easy in the 1502 since there's no need for extender cards.

[tggzzz]

A recent PPAuction (=>sight unseen, farm barn storage) realised £400 (add +37% uplift) for this lot:

Tektronix 7854 oscilloscope with 7A26, 7A13, 2x 7B85 plug ins, 7854 waveform calculator keyboard & 4 extra plug in modules, 7L12 spectrum analyser 0.1MHz-1800MHz, 7S12 TDR/sampler with S6 & S52 heads, 7D20 digitiser, 7A26 dual trace & manuals

Does that make you feel any better?

[vk6zgo]

Distinctly!
For the price, the 7603 is a bargain, & the other stuff is just the icing on the cake!
As a bonus, the Kenwood radio that had stopped working somehow fell off the bench & now works.
In earlier times, that would have triggered an intensive investigation to find the original problem, but these days, I'm just grateful for small mercies"!

[bd139]

I’ve wrestled a 7L12 before.

Hours of fun!

[vk6zgo]

I’ve wrestled a 7L12 before.

Hours of fun!

Even when they are fully functional, as this one happily seems to be, they are not "user friendly".
It is bit like having a performing bear!

[bd139]

I’ve wrestled a 7L12 before.

Hours of fun!

Even when they are fully functional, as this one happily seems to be, they are not "user friendly".
It is bit like having a performing bear!

That's part of the fun!

Also playing find the tant at 2AM!

post15_7sAY8gf_anzWF.jpg

[tggzzz]

Realism: you didn't say "find the faulty tant"

[bd139]

Realism: you didn't say "find the faulty tant"

Hey I did find it in the end. It was however disguised as a silver mica!

post6_vJAH1z1_gpEbN.jpg

[AVGresponding]

Realism: you didn't say "find the faulty tant"

Hey I did find it in the end. It was however disguised as a silver mica!


post6_vJAH1z1_gpEbN.jpg

The yellow bead one with the brown dot?

[EC8010]

It turned out to be a switch on the Optimod at the studio being in the wrong position, or so the Tech there said.

That'll be the power switch. Set to "On". Horrible things beloved by station engineers who believe that people tune radios by stopping at the loudest station. (Maybe they did when radios were steam powered and had real variable capacitors and flywheel on the tuning knob.)

[bd139]

Realism: you didn't say "find the faulty tant"

Hey I did find it in the end. It was however disguised as a silver mica!


post6_vJAH1z1_gpEbN.jpg

The yellow bead one with the brown dot?

Oh no that'd be too easy.

Yes that is actually indeed a tant.

post6_vJAH1z1_gpEbN.jpg

[AVGresponding]

[MED6753]

Have seen lots of brown tants. Just as bad as the yellow ones.