Author Topic: Texscan VS–40(M) 1–300 MHz Sweep Generator Repair?  (Read 307 times)

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Offline Chris56000

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Texscan VS–40(M) 1–300 MHz Sweep Generator Repair?
« on: September 08, 2019, 06:11:49 pm »
Hi!

I was given a Texscan VS–40(M) Sweep Generator by a friendly colleague from a previous workplace where I once worked – it was about to go to the tip otherwise, cosmetically it was all fine externally, but there was a lot of ominous musical rattling when I brought it in out of the car!

The unit came with two Plug–In Crystal Marker Modules fitted, one for 10kHz and one for 1MHz – the price of these modules gave me a heart attack when I looked on fleabay!

Investigation revealed the lids had been removed from the 300 & 600 MHz oscillator boxes, and also the four–stage 1–300 MHz Amplifier Box, and removing them the musical rattling I heard came from lot of tiny American–thread screws, two loose carbon–compositon resistors, and a stud–mounted RF transistor with the legs all snapped off it – I think (emphasis on the think!!) it might have been Q405, the penultimate transistor in the V.A. circuit, unfortunately the book only marks it with an asterisk on the circuit–diagram and calls it up as "Factory Selected Type!!" – oh, dear, what for?!

The BNC outlet had also been removed from the assembly, and many pencilled notes on the circuit diagrams suggested a lot of midnite–oil had been burnt over 'em!!

I did get Texscan's original bookwork with this instrument, and altho' this contains full circuit diagrams and parts lists, there's no internal layout diagrams of the oscillator & Video Amplifier Boxes, nor any adjustment instructions for these assemblies!!

My questions are:–

1) What screws will I need to buy for this? They look like M2 sort of size about 3mm long, pan–head slot type – can someone suggest the correct American Size?

2) Would there be any chance of getting replacement modules – if not, how difficult a job is it to repair and realign these?

I'll post pics as soon as I can, I suspect my 'fone camera is getting near the end of it's useful life so I'll have to borrow another 'fone!

3) What equipment would I need? I currently have a Marconi 2018 RF Sig. Gen and a Farnell TM8 RF Millivoltmeter with it's probe head and adaptors – would this be enough?

There are four small trimmer capacitors in the emitter–circuit of each transistor in the V.A. box, but no info. is given on their adjustment except to say "they're adjusted at the factory prior to despatch of each instrument!"

Opinions, comments and advice appreciated please! I don't have an S.A. unfortunately – would I have to get one to help repair this instrument? – if so would my Christmas Bonus of about £700 get me something suitable?

Wideband & R.F. stage repair isn't something I'm as experienced in, mainly because of the cost of equipping a home lab with good-quality R.F. test-gear can easily run too far into four figures to appeal to me!

Chris Williams

PS!

I can redraw the circuit-extract or scan the page far better at work if need be!
« Last Edit: September 08, 2019, 07:36:49 pm by Chris56000 »
It's an enigma that's what it is!! This thing's not fixed because it doesn't want to be fixed!!
 

Offline Chris56000

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Re: Texscan VS–40(M) 1–300 MHz Sweep Generator Repair?
« Reply #1 on: September 08, 2019, 08:02:24 pm »
Hi!

Better pic of circuit diagram!

Note - the layout of my module appears to differ from the diagram shown - the chap who gave me the generator did say something about this!

Chris Williams
It's an enigma that's what it is!! This thing's not fixed because it doesn't want to be fixed!!
 

Offline Chris56000

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Re: Texscan VS–40(M) 1–300 MHz Sweep Generator Repair?
« Reply #2 on: September 08, 2019, 09:18:39 pm »
Hi!

Further to the above, I found the circuit-diagram of the Video Amplifier Module in the Tekscan VS-50 Generator is virtually the same topology as the module I have, apart from some component-value differences, presumably to tailor it's frequency response to suit the VS40 generator, which is only 300MHz max. as opposed to 500MHz for the VS50!

I'll post the exact diagram after I've worked it out!

Chris Williams
It's an enigma that's what it is!! This thing's not fixed because it doesn't want to be fixed!!
 


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