HPAK 8569A Microwave Spectrum Analyzer: display fix, A6 assembly

No display – this can be bad news, because spare CRTs are hard to come by. Let’s see what we can do here.

Overview of the assembly – red: XYZ driver boards; green: high voltage (and other supplies) for CRT; blue: digital circuits-video signal generator.
8569a display unit

The digital part and XYZ driver – seem to be fine, can get a nice video signal when probing around.

Well, the HV power supply – no signs of life. You can easy check, just switch off the unit – there are two neon bulbs that discharge the CRT – but in the current case, all remains dark.

Removed the HV power supply assembly (A6) – fuse is blown! A quick look at the schematic:

8569a partial schematic a6 assy

Only three options, either the 26 V filter, the transformer, or the Q1 driver transistor. Driver transistors are a common fault – high voltage transients can damage them. But, Q1 is testing fine.

The transformer, looking good.

The filter – well, there we go, a classic fault, a defective cap.

8569a hv power supply

This is a 50 µF, 50 V, Sprague (now: Vishay), 30D series. Date code: 1980, 26th week! “Made in USA”

8565 30d cap

This exact series – still available today, after nearly 35 years! These don’t fail so easily, but the A6 assembly, there will be some stress on these parts. The failed part is capacitor C2 – the part that is doing most of the filtering. Maybe it would be a good idea to put a 1 µF foil capacitor in parallel with the electrolytic caps, to absorb the spikes…

For the time being, just replaced the defective cap, and ordered some 105°C rated long-life caps. 30D series caps aren’t cheap. So I settled for a Kemet brand type. Will be good enough.

After some soldering, powered the thing on:
8569a working display

We have a working display!! And, it is actually a very good CRT – no signs of wear! Most likely, sitting in some lab at Iowa State University, as one of the labels on the case suggests, and then, stored away in a nasty shed or garage for years.

Now, repair can move forward – without a display, at would be only half as useful.

The culprit, and the dead fuse:
8569a hv pwr supply culprits

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