Hi all,

...... yes, this is not a radio, nor is it valve-based .... but it can be a valuable piece of radio test gear ...... amongst many other uses ......

I have recently acquired a Tektronix 7L13 Spectrum Analyser in a Tektronix 7603 scope frame. It is in reasonable physical and and working condition.

Whilst I have both Manuals and figured out the basics, I would like to find some-one (or a Forum etc) who I can ask questions of when I get myself stuck. So I am putting out a few 'feelers' ......

Does anyone on this Forum have one?? .... or used to use one in days gone by?? One never knows who is reading this Forum and/or their experiences ....

Cheers,
Ian

In a previous life, when Hewlett Packard was a measuring instrument company, I was pretty conversant with them and may be able to fill in some of the gaps.
Harold

Hi Harold,

I am not really surprised that skills of this type are held by members of this Forum ... and I'm sure that there are others.

Do you have knowledge of the 7L13 and/or 7603 specifically, Tektronix Analysers or Spectrum Analysers in general?? .... I have questions on each level ....

Cheers,
Ian

Let me know if you get stuck. I have a similar oscilloscope mainframe to yours.

In case you missed the various warnings in the 7L13 manual, be very careful about what you feed into the input.
Any DC voltage or an AC voltage above +13dBm (around one volt) will destroy the first mixer. The max AC level specified is with the attenuator set to 0 dBm, higher signal levels can be applied with an appropriate attenuator setting.

I'm not conversant with the Tektronix gear, as I was with HP and involved with their product. My knowledge would now be more general, theory, operation and application for instance.
Harold

That's great ... a good range of experience to draw on.

Brad .... do you wish me to continue a discussion on this Forum? .... my questions will not be radio related for awhile. Or .... do you wish to shift this Thread to Tech Talk?

Cheers,
Ian

It can continue here, no worries.

Okay, don't know about everyone else, but it has been a heck few days .... family, food. presents, food, friends, food .... maybe this Thread should be about magical diets .....

A bit of background .....

Spectrum Analyser ... 7L13, to 1.8GHz, ZERO dc input, 50 ohms impedance input
... one control knob broken off but I can still use its functions
... one segment of screen readout flickers on/off during warmup
Other controls and functions seem okay ... (remembering, of course, that I am still very much a novice with this thing and haven't explored it fully).

Oscilloscope ..... 7603 .... seems fully functional except that ....
... screen image has some minor fluctuations when warming up, close to stable after 30 mins; the instability is in the vertical plane only and could be in a vertical amp in either the Analyser or the Scope (my guess only).

I am happy to leave the problems until after I have familiarised myself with the equipment properly.

I have both Manuals and can provide the web addresses if anyone wants to read them.

So far I have been using the FM and TV stations to familiarise myself with this beast. Due to the 50 ohm impedance and zero DC input requirements, I have built a 75 to 50 ohm impedance matching circuit and included a dc block. This circuit attenuates any signals, but as TV Collector points out signal input power levels must be limited to avoid damage to the front-end of the Analyser. I have also observed the signal output from my Signal Generator via an external Variable Attenuator (up to 90dB).

Questions .....
1. My primary concern is to avoid damage to the front-end. "Max input Power level is -30dBm with Attenuator at 0dB". I seem to have heaps of 'protection' for my front-end ...... internal Attenuator provides 60dB in 10dB steps ... plus my external Attenuator .... plus impedance matching DC blocking circuit ... and several charts relating dBm to power and volts etc .... start at full attenuation and work downwards etc .... HOWEVER, when observing various signals, are there any 'indicators' or 'warning signs' on the sreen that I should be aware of before I do any damage??????? Experienced users probably have a 'feel' for signals that maybe approaching the limitations of the equipment before smoke appears.

2. The main Frequency Tuning knob .... Manual states "TUNING: The center frequency is tuned by a two-speed control with a ratio of approximately 1:2.8 in the position and 45:1 in the slow or fine position" I cannot find or activate Fine tuning???????????

3. The Manual for the Analyser shows in pictures a 10dB Gain function (push button) near the Log / Lin display buttons. Mine does not have this button and I am wondering why????????? It is not just that the button is missing .... there is nothing in this position.

Any advice inbetween socialising will be appreciated,
Cheers,
Ian

I think you will find that the tuning knob will push in / pull out for the coarse and fine tuning positions. Don't force it as it may be gunked up, have a look at the mechanics of it first.
You are well ahead with the manuals available, I was going to suggest a source such as AM or FM radio, as I don't have a clue what a digital TV signal looks like. On a MW radio signal, you should be able to get down to seeing the carrier and both side bands.
Have fun
Harold

You seem to be doing all the right things and taking the necessary precautions. In answer to your questions:

1. You won't have any warning when the 1st mixer gets destroyed. There will be no smoke or any other signs of failure except the analyser will no longer respond to signal inputs. At signal levels below the damage threshold but above the maximum usable level the signal trace may be off the top of the screen. You just need to be constantly thinking about what you are doing and always start with lots of attenuation and try to avoid making mistakes! Blowing up a multimeter due to having the wrong range selected is an annoying mistake, doing the same to a spectrum analyser is a completely new level of "oops".

2. The frequency tuning knob is meant to pull out for fast tuning and push in for slow tuning. The later versions of the 7L13 did not have this instruction printed near the frequency control so they may not have this ability. See also comments in answer 3.

3. You appear to have a later version of the 7L13, as these did not have the 10dB gain switch. The manuals for Tektronix and HP test gear are serial number specific. You need the manual thats covers the range of serial numbers for your specific piece of equipment. You may need to hunt around to see if a later version of the 7L13 plugin manual is available.

The warm up instability could be due to the mainframe or plugin as you have guessed. Should be easy enough to isolate the issue to one or the other if you have access to a timebase and vertical amp plugin for the 7603 mainframe so you can use it as an oscilloscope. I have these plugins if you need to use them.

Have been hunting around for a later version of the Manual ..... and so far have downloaded the same one about 5 times.

I have Version 070-1673-00 dated April 74 ... cannot find a Serial number reference on it.

There is a later Version available 070-1673-01 and it is dated about 1977 onwards. These are hard copy from the USA, or on CD .... cannot find a web download for this later Version as yet.

As to the Frequency Tuning knob .... well, I seem to have way too many hexagon keys, but they are all too big ... and some-one has been there before me .... and not gently.

I'm not at all certain that the later 7L13 models had the dual range frequency control. If they did, I would expect there to be a reference on the front panel like the earlier ones do. What is the frequency change per revolution of the knob?

Sounds like the later version manual may be hard to get hold of unless you pay for it. At this stage, I'd make do with the early revision manual and accept that there are differences.

Frequency Tuning knob ..... about 31 MHz per turn. This is okay at larger RBW settings (ie x MHz across the screen), but way too coarse at lower RBW ( x kHz across screen).

I am caught in that old conundrum ..... too gentle and job doesn't get done, too hard and it breaks .... will err on gentle for the moment ....

Re Manual ... agreed .... although ..... I have found a reasonably priced CD version with shipping included. I will confirm the cost, then take the chance that it is for my Analyser.

Started working through the Calibration process, then I want to see if I can identify a modulated signal as suggested by Harold.

I wasn't fully aware of the plugins available for this scope .... ... they all seem to be located in USA.

Mmmmm .... the more I use this Analyser, the more I realise that the less I understand it ...

Re. Span/Div and Resolution Bandwidth control(s) .....
These controls are coupled together (... can be uncoupled if desired)
As the Span/Div knob is rotated from say 1MHz to 2 to 5 to 10 MHz, the RBW stays constant at 300kHz.
When the Span/Div knob is rotated one step further to 20MHz, the RBW increases to 3MHz.

When the Span/Div knob is rotated from 1 to 10MHz, the signal width decreases .... as one would expect. But when it rotates to 20MHz, RBW increases and the signal width increases ......

So ... when rotating the Span/Div knob through its full range, the signal width decreases as the Span/Div increases, but increases whenever the coupling mechanism increases the RBW.

Why is the signal width dependent on the RBW??

The tuning does seem to be too coarse for low bandwidths. There may be a dual range function but until certain it would be best to play safe and not force things until you know for sure. Given it's rough treatment in the past (broken knobs) something may be bent or jammed.

Have you thoroughly read chapter 2 pages 10 to 14 of the manual? It covers the basic operation and reasoning behind the freq span vs bandwidth (resolution). Basically, the controls are coupled to ensure optimum settings for the analyser to perform as specified. Selecting an inappropriate resolution for a given frequency span results in a loss of sensitivity and thus display a lower signal amplitude than is actually the case. You are free to un-couple the controls but you are then in "manual user mode" and performance depends on the user (you) knowing what they are doing.

On these old analogue analysers, a higher degree of user knowledge is required for good results. Sweep speed, span width and resolution are all inter-related and have an effect on what you are trying to display.
It will be a case of spending time to learn what everything does and the effect it has on performance. Providing you don't exceed the maximum allowable signal input, I don't think there is anything else you can do that will hurt the analyser if you set something incorrectly.