Category Archives: Test Equipment Repairs

For some, it is like solving crossword puzzles: fixing defective test equipment. Preferably, mid-70 to early 90s vintage.

HP 8904A Multifunction Synthesizer

HPAK (HP Agilent Keysight) 8904A Multifunction Synthesizer

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The HPAK 8904A – you will find the detailled specs elsewhere on the web, it is a one-of-its-kind instrument, an early type of arbitrary waveform generator, capable of frequencies up to 600 kHz, but its mainstay is the audio region.
Why is it so unique, well, it has a complex (some say, difficult to use, but are there any easier ways?) user interface that let’s you program all kinds of test signals without any time lost for generating complex waveform bit by bit, uploading it to a modern arbitrary generator, and so on. The HPAK 8904A support all the common modulation features, AM, FM, Phase, and additionally, DSBSC (double sideband supressed carrier) and Pulse modulation. It can gernerate FM stereo test signals, various other types of composite signals, DTMF tones, just to name a few. In fact, HPAK has published a catalog, with some of the common waveform (will try some of them later).

Most importantly, your plain 8904A might not be able to do all these things, because all the nice and special functionality is only available with option 001, which is, believe it or not, a software option (to my best knowledge, one of the earliest occurrences of a really powerful software option feature for HPAK equipment).

The unit I’m using came with option 002, two channels, but not with option 001 – well, it’s software only, all you need is a serial and key, and some intructions – which I will be happy to provide to you, when needed.

But before we start: the unit discussed here came from an undisclosed source, and failed power on test (no noise or other signs of functionality). Once opened up, big surprise, HPAK was using a Computer Products switchmode supply, 90 Watts, Model XL51-5601. Fair enough, this is a quality supply, but somethings must have gone wrong – no power.

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Troubleshooting a switchmode supply, with my somewhat limited workshop here in the US – well, it’s worth a try. No schematic, so hoping that it failed because of one of the more obvious defect modes. And, fair enough, here are the cuprits: FES8DT Schottky diodes, the secondary rectifiers – 2 of 3 were dead.

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The FES8DT are 200 V, 8 A, 85 pF, 0.95 V drop devices. 35 ns recovery time. 125 A surge current. Seems they don’t fully hold up to the requirements.

Wavetek 907A

The Wavetek 907A – initial assessment

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A Wavetek 907A – this apparatus generates microwaves from about 7 to 12.4 GHz. All the typical modulation capabilities are provided, FM, AM, pulse. Found it on xbay, USD 45. That’s less than the value of the two precision bulkhead SMA to N connectors.

Quick look inside:

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RF unit:

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You can see the YIG oscillator on the top, followed by an isolator, leveler, isolator, two attenuators, isolator. There is a side chain, starting at the leveler, and with an additional isolator: this is a non-attenuated reference signal, which is available as an auxilliary output at the front panel – quite handy to use as as a signal for PLL stabilization, or other purposes.

Some items that need attention:

(1) Power LCD display doesn’t work properly, not showing the power reading

(2) Attenuator works, but leveler doesn’t seem to work (always at maximum, unleveled power)

(3) Only have a manual with partial, cut off schematics!

(4) Some TLC required, de-dusting – done.

(5) Two pushbuttons are missing, but switches work – more a cosmetic issue, will replace, once everything else has been fixed

(6) Some switches don’t work properly – fixed with the help of DeoxIT D5.