Texscan AL-51A Spectrum Analyser

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Hi all,

I've got a Texscan AL-51A that's been sitting on a shelf here at work for a decade or two gathering dust.  It's an early portable battery powered spectrum analyser with a CRT display, frequency range of 4 to 1000MHz, and I'm guessing it qualifies as vintage. 

Price range was $4,450 and up - in 1979!
https://archive.org/details/TNM_Communications_land_mobile_radio_magazine_Sep_20180117_0125/page/n26/mode/2up
https://archive.org/details/TNM_Communications_land_mobile_radio_magazine_Sep_20180117_0125/page/n29/mode/2up

It still fires up and displays signals although the displayed amplitude seems to be well off.  Might delve deeper into it when motivated enough.   

The main reason for this post is to provide this copy (of a copy) of the operating manual, in case anyone is interested in the design or looking for the manual. 

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Well, I was thinking it'll be a faulty input attenuator because that's the first thing 'in the line of fire', but it seems that I have a blown First Mixer. 

The 1000MHz IF output is only -80dBm with a LO input of 1054MHz @ 10dBm and RF input of 54MHz @ -40dBm (from the front panel Cal output that I've confirmed as good). 
The LO is behaving OK and ranges from 10 to 14dBm over the 1000-2000MHz tuning range.  You can set the Dispersion control to 'CW' which is like a zero span mode, which turns off the LO sweep.

I haven't been able to identify the mixer component, but the manual says 'the first mixer is of the double balanced type, using four matched Schottky-barrier diodes configured as a ring modulator'.

Which means it's something like this;



If it is, I'd expect to see the schottkys with a multimeter diode test in either polarity between IF and RF, and IF and LO.  However this one is just showing a short between all inputs and output, but this could be by design depending on the internals.  I've lifted the resistors from the LO and RF pins so there are no external paths to ground.
 
So I'll need to identify the mixer to confirm the fault, and then find a replacement, either the same component or a like-for-like with similar drive level, conversion loss and frequency ranges.  LO input range is 1000-2000MHz, RF input is up to 1000MHz, IF output is 1000MHz, drive level is around 10dBm, and that's about all I can determine for now.  Any replacement will need to be a close match to the original.

Maybe someone can recognize the mixer component from the attached photos?

It's looks similar to this one from Pulsar, even the pinouts are the same (but not the specs);
https://www.pulsarmicrowave.com/product/mixer/X2L-10-301

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If anyone did recognize the mixer, then full credits to them, because it was mounted upside down. 

It's an Anzac MD-614.
http://datasheet.iiic.cc/datasheets-1/m_a-com_technology_solutions/MD-614.pdf
 
The weird thing is, it was mounted with the 'X' or IF lead connected to the RF input, the 'R' or RF lead to the first IF 1000MHz BPF and 'L' or LO to the First LO output (which is ok).  See photo.

Was this ever going to work? I suspect...no, never...but I could be wrong. 

First obvious problem apart from the pinout transposition is that the MD-614 specified IF port frequency range is only DC-300MHz.

So the question is - can a double balanced mixer still fulfill some purpose with RF and IF ports transposed?

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Back to basics for me, and just answered my own question I think.  With the RF input path connected to the mixer IF port, then it must be doing Low side upconversion;



https://www.digikey.com/en/articles/the-basics-of-mixers

f_IF = analyzer RF input range from 0-1000Mhz
f_LO = 1000-2000MHz
f_RF1 = 1000MHz to the IF BPF and IF amp