November | 2014 | SimonsDialogs

After quite some heavy work with the MSR-904A, we are close to completion. The last defect, supposedly last, was an issue with the 0-9 V internal tuning voltage – could not get it to change from 0-9 V even with all adjustments suggested by the manual, and looking at the schematic – no other way to adjust. There can be drifts of some resistors over time, but all checked and very much as spot-on is they can be.

So, the issue comes from a part of the AFC circuit – there is some gain switching with a 4051 CMOS switch, and as it turns out, this was missing the -6 V supply voltage – causing a positive voltage at the output, which upset the tuning assembly, even with AFC disabled. The wire, not sure if it was broken when I received it – well, easy fix, but took me the best part of 2 hours to find it.

With the machine now ready to be put back in service and fully operational, a few hours warm-up, and all the oscillators and YIG filters fine-tuned, I slipped with the screw driver – a spark – and the 250 MHz amplifier was dead. Fortunately, no major defect, just the 2N2905A that switches on the power for the 250 MHz amp. Added a “new” one, 1984 vintage…

Finally, closed the lid, to keep my screwdrivers out, and did a quick check of sensitivity at about 1.7 GHz (because I have a really well calibrated 0.1-2.1 GHz source, a HP 8642B here already on the bench and did not want to carry around heavy microwave synthesizers…).

Here, the result, which I would call pretty much satisfactory. Doesn’t get much better unless you cool down the receiver with some liquid helium.

The machine, in all its beauty.

The 8645A – see earlier post, reached my workshop in a really bad shape, electronically dead, and severely damaged. For any reasonable effort, damaged beyond repair.

However, I do this for a hobby and there is no rush, and I just can’t take this thing apart for scrap metal – the case suffered damage, but the inner parts, no major mechanical issue.

For most of their 1980-1995 equipment, HP used a so-called “System 2” modular case, mainly, die-cast aluminum alloy. The 8645A is not much different – the bottom and top lid are different (to provide better shielding than the regular System 2), but the front frame, it is the same.

Looking around at xbay – found a complete front system 2 front assembly, System 2, from a 8782B Vector Signal Generator. The Chinese vendor, he seems to make a cut by taking apart equipment, and selling it for parts – it wasn’t quite cheap, but still, for a 8645A, to get it back into service, it seems pretty much worthwhile.

It arrived already after about one week.

Not in best shape, but usable.

This assembly also has quite a few BNC connectors, and a precision N connector, all very handy to fix the replace the snapped-off parts.
As it turns out, the N connector is of a different length, but never mind, I have some more spares around.

The front panel assembly, it is apparently one of the weak points of the 864x series.
Two layers of rather soft aluminum, a ~0.8 m plastic sheet that also acts as filters for the fluorescent displays, and a thin outer layer with the labels.

This is quite easily pushed in, if force is put on the BNC connectors – it is always best to safely secure the instrument at the corners during transport, and to attach handles.

The layers are held together by glue, and it seems, HP has used a very sticky, but not permanent type – with great care, it can be separated. Key thing is to keep the sticky surfaces clean, and they are best protected by a polyethylene foil.

To flatten things out, a carefully adjust iron is a handy tool – with the plastic sheets put in between two layers of non-stick paper.