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Old 21st Aug 2019, 6:44 pm   #1
Jon_G4MDC
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Default HP 8558/8559 Switch Issues

This is not a new topic to the forum. There are previous posts about how to repair these switches.

Today I have been looking at my own 8559 Spectrum Analyser which used to work OK, albeit with a few issues with the Ref Level / Attenuator switches.

Recently it has refused to sweep which is not helpful with an SA.
I have removed the plug in from the Display Unit - HP182T.

In the bottom was a tell tale 4 contact piece of spring fingering and when I looked harder there was another sandwiched in a chassis overlap. Inside the plug in itself I found two more so I have 4 identical so far - but where did they come from?

I have a pdf of the manual - it is not brilliant - but so far as I can see.

Ref Level Switch should have 2x 4 contact pieces and they are still there - I assume this is OK then.

Freq Span/Div Switch (Front) and Resolution BW Switch (Rear) have no contacts at all that I can see. There are pimples where the contacts originally fitted by something akin to a plastic rivet. We will come back to this.

Time / Division Switch should have 2x 4 contact pieces - these are hidden from view for now so unknown until unit is dismantled further.

Trigger Switch appears to use 2x 4 contact pieces - these both seem to be missing and pimples are visible as above.

My question here is how are the switches which are shown in the circuit schematics as needing 6, 8 and 10 contacts (Freq Span) realised? Should I be looking for other finger combs with more contacts? I don't seem to have enough to go around as it is!

The Freq Span/Div switch does not look wide enough to accommodate 10 contacts. The space available on that rotor (with the hairpin spring) looks like it takes 4+4 like the others. Was some trick done to mingle the pads and get more switch work out of fewer physical contact fingers?
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Old 21st Aug 2019, 7:22 pm   #2
Jon_G4MDC
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Default Re: HP 8558/8559 Switch Issues

Here is the schematic for the switch, which apparently needs 10 contact fingers.

It seems pretty sure, by looking at the PCB itself, that there is a thicker middle ring which is probably a +15V circle.

That leaves 3 rings outside and another 3 inside for contacts which must produce the correct output code if using 2x 4 finger contact combs on the rotor.

Could a cunning PCB layout produce the right output codes? There is no PCB layout diagram given unfortunately.
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Old 22nd Aug 2019, 2:14 pm   #3
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Default Re: HP 8558/8559 Switch Issues

I don't have any of the 180 series SA plug-ins, but the they all seem to shed those contacts from the switch rotors as you already know.

The 8557A has them too, I don't know if they are exactly the same as the 8559 but there are some pictures of the front switch rotors showing the contacts in this repair thread;
https://www.vintage-radio.net/forum/...ad.php?t=80757

I also found a picture showing the PCB track layout behind one of the rotary switches in a 8558B plug-in, it also looks to have another PCB behind the first one with more rotary switches.
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It's from this webpage with quite a detailed repair write-up & lots of pictures;
https://lazyelectrons.wordpress.com/...180d-hp-8558b/

Hopefully some of this will help with your repair.

David
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Old 22nd Aug 2019, 2:42 pm   #4
Jon_G4MDC
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Default Re: HP 8558/8559 Switch Issues

Thanks David, that appears to be track work for one of the other switches but the site does provide proof positive that the Freq Span/Div switch has 2x 4 contact fingers.
When I get it apart I will take my own photo of the PCB.

I don't like the homemade spring contacts - I think his PCB might have a short life. I hope to find a source of the right fingerstock (optimist!).

Anyway now I can count up how many contacts I am missing. At least it seems all are identical.

Jon
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