Systron-Donner Spectrum Analyzer
Type 809-1 (10MHz - 12.4GHz), page 3

Ask anyone who has experience with RF Spectrum Analyzers and they will tell you that the worst-case scenario failure is a failed First-Mixer. In Hewlett-Packard units such as the the HP 8555A (mentioned earlier) the diode is bonded to a ceramic substrate and locked away in a sealed box. Click here for a brave approach to repairing one such failed diode. Systron-Donner employed a completely different style of mixer assembly. In their case, the diode itself is a replaceable component accessible from outside the unit. Thus it is easily replaced if blown. The diode is accessible through a hole in the unit side-wall … see photograph below.

But it wasn’t the mixer-diode that was faulty. Not only was the analyzer deaf, but there appeared to be a frequency discrepancy. Thus I concluded that one of the two Yig-Oscillators might be off frequency … and if off frequency, they might also be delivering a lower than normal output. That was the notion anyway … hmmm?

The service manual contains a very comprehensive fault diagnosis series of flow-charts covering no less than six A3-size fold-out pages. However there is a problem … whoever wrote them used the 809-2 with its external mixer drive socket in mind. So when I got to the bit where it tells me to measure the level of first LO drive, I don’t have the appropriate front-panel connector. My only option was to measure the level somewhere inside the RF-deck.

This introduced another issue, in that the RF-unit needs to be inside the main assembly since I don’t have the necessary extender cable … not to mention that running the RF-unit ‘split’ would be a near impossibility anyway. However, as Baldric would (almost) say, ‘I had a cunning plan’. Removing the input step-attenuator was relatively easy. This left two good size holes in the front panel since the RF-Input connector is integral with the attenuator. It was then simply a case of attaching one end of an SMA lead to the output of the second LO Yig or in the case of the first LO Yig, the associated isolator and feeding the other end out through the hole where the N-type connector once fitted. Off course the whole assembly needs to be disassembled and reassembled each time.

Tedious as it was, it worked rather well. I measured the power levels on my HP 436A Power meter and measured the frequency of each of the two Yigs. Both were in spec. So the fault lay somewhere else. One thing for certain was that it was not band-related. If it wasn’t in the RF-deck, then it was in the IF-section; in the 3rd or 4th IFs.

The third and fourth IFs are 60MHz and 6.5MHz respectively. The non-availability of any form of extender meant that chasing a fault on one of the boards in the IF-section was going to be a nightmare, so I still held out hope that the fault was still on the RF-deck. The first and second IFs are on the RF-deck. The frequency of the first IF is dependant on what band is selected, so that ruled that one out, leaving only the second IF which is 260MHz.

1st LO Yig (the red unit)

2nd LO Yig