Imponderable question - HP400GL repair?

[swpaskett]

I have to admit it -- I am a sucker for old tech. I have this HP RMS voltmeter sitting on the shelf... it's been there for 12 years or so. It works for a while when I manage to get the range switch seated just right, but the custom rotary switch is just worn out. It has a beautiful 5 inch meter with a mirrored scale... I drool just thinking about it.
Does anyone have a source for old-fangled rotary switches or parts? Or, barring that, I could replace all the discrete stuff in the box with about 3 opamps. I would still have to fix the attenuator section, but it would require a less exotic switch I probably have in the junk box.
Finally, don't tell me it's not worth it. I know that, but Frankenstein had his monster, and I have mine.

 

[mjrippe]

I will assume you have cleaned the switch as best you can Some rotary switches (multi-deck in particular) have screws that hold the layers together. Tightening these can make a huge difference.

If that is not the case, do you know how many poles and positions the switch uses? I have a box full of vintage rotary switches and I'm sure others here do too. There are also a few surplus companies left in the USA but you really need to know what you are looking for.

Mike

 

[swpaskett]

Thanks, Mike. Yes, I cleaned it and cleaned it. Tightening the screws is a good idea. The switch itself is one of those stacked wafer things that were common 50 years ago. It has concentric shafts, the inner shaft is an SPST switch that controls a 100 kHz low pass filter. The outer shaft is a 3 wafer 8 position switch with a shield between the B and C wafers. If not for the shield, I would just replace it with a modern switch, rebuild the attenuator, and wire the LPF permanently on. The wafers are about 2 inches in diameter and separated by about 3/4 inch spacers. Not sure if that was done because they had lots of space or were concerned about coupling. Probably some of both.
Years ago I had lots of old rotarys and would sometimes swap out worn wafers in old gear, but tossed what I had 10 or so years ago -- what would I ever use that junk for? <sigh>

 

[Disco Volante]

I have very good experience cleaning contacts in a soak of mild citric acid solution (coffee machine descaler). Soak overnight and clean with water and IPA. We use it at the bronze-foundry to clean off oxides, fingerprints, etc before patina goes on. Does not bite the precious copper. Restored my old Quad33 switches to complete functionality.

Old switches sometimes need a little tightening of the forks, but you probably already know this;-)

I strongly believe in fixing the unfixable...

 

[JohnRoberts]

perhaps you can source a modern switch with same function... depends on whether you care more that it works well or looks well...

JR

 

[radardoug]

RS Components sell wafer switches that you can build custom, wafer, shields and actuators are separate parts.
Heres the actuator.
https://nz.rs-online.com/web/c/swit...tch-shaft-assemblies/?searchTerm=wafer switch

 

[MisterCMRR]

Wow, thanks for the lead to NSF! At the link, RS had only one switch section available - a "shorting" type that shorts all positions except the one selected - which certainly wouldn't work for the HP400 (I have one of those in my "to fix" collection, having a problem in its power supply).