Author Topic: [Teardown] Longest fully loaded shaft  (Read 4442 times)

0 Members and 1 Guest are viewing this topic.

Offline Alex

  • Regular Contributor
  • *
  • Posts: 170
  • Country: gb
[Teardown] Longest fully loaded shaft
« on: September 13, 2013, 08:37:50 pm »
I saved a Krohn-Hite 3342 two channel tunable filter from certain 'death by the skip'. Think of this as a decade resistor, but adjusting the cut-off of a filter. I knew it had not been used for a very long time and it was in the rain for two days. The selector switches were stuck beyond hope. Here it is:



As part of its restoration, I first wanted to revive the switches. So, I took it apart and was presented with what must be world's longest, fully loaded rotary switch shaft:



Each shaft is about 40cm in length and is loaded with no fewer than 15 switch plates. Here is another view:



We have all seen long shafts in e.g. old Hameg scopes, but never loaded all the way to the back. Size comparison with aerosol can:



The unit has a passive 4-pole RC mode covering 1mHz to 100kHz and now we know how they did it. But since I was inside, here are a few more teardown pics:

Main entry point and wiring. When electrical safety was no source of concern.



Power supply section after cleaning. 5W per channel.



Naturally for equipment of the time, it had to be battery powered too. Here are the battery holders. Ni-Cd or Lead-acid:



The circuit boards at the bottom can rotate outwards and be locked in place for repair. Very nice. The large value film caps could not fit on the switches like the smaller caps.



Close up of the board locking mechanism:



More film caps hidden away and mounted on insulating standoffs.



Circuit board detail for the active filter mode. It appears to be segmented, possibly each segment forming a pole of the filter:



Board detail. All discrete, of course. Note the unusual way of indicating the polarity for the bipolar transistors in to-92 using copper traces. It is strange as the boards have silkscreen.
 


Solder side.



High-res version for your desktop:
http://s23.postimg.org/w9qsljp8r/DSC04250.jpg


Back of unit. Yes, it is that old.



The technician's soldering iron touched the cap during assembly  ::)



After all the cleaning and tweaking, here is the unit working.  :-+ 1kHz quare wave fed in (top), high pass mode at a cutoff that I cant remember (bottom).



Alex.
 

Offline kizzap

  • Supporter
  • ****
  • Posts: 477
  • Country: au
Re: [Teardown] Longest fully loaded shaft
« Reply #1 on: September 14, 2013, 01:30:45 am »
Did I see some text near the power input saying that the Chassis is floating, yet see definative grounding connections to the chassis?  :scared:

-kizzap
<MatCat> The thing with aircraft is murphy loves to hang out with them
<Baljem> hey, you're the one who apparently pronounces FPGA 'fuhpugger'
 

Offline Fsck

  • Super Contributor
  • ***
  • Posts: 1157
  • Country: ca
  • sleep deprived
Re: [Teardown] Longest fully loaded shaft
« Reply #2 on: September 14, 2013, 02:47:15 am »
Did I see some text near the power input saying that the Chassis is floating, yet see definative grounding connections to the chassis?  :scared:

-kizzap

there's a switch, either chassis or floating.
« Last Edit: September 14, 2013, 05:34:23 am by Fsck »
"This is a one line proof...if we start sufficiently far to the left."
 

Offline TerraHertz

  • Super Contributor
  • ***
  • Posts: 3707
  • Country: au
  • Why shouldn't we question everything?
    • It's not really a Blog
Re: [Teardown] Longest fully loaded shaft
« Reply #3 on: September 14, 2013, 05:29:43 am »
I don't know which is harder to believe, that someone threw that in a skip, or the um... 'other' pics on your image hosting site.

I do wish the skips around here had such stunning... er um, I mean remarkable finds.
Collecting old scopes, logic analyzers, and unfinished projects. http://everist.org
 

Offline kizzap

  • Supporter
  • ****
  • Posts: 477
  • Country: au
Re: [Teardown] Longest fully loaded shaft
« Reply #4 on: September 14, 2013, 08:04:24 am »
there's a switch, either chassis or floating.

Picture 5, there is a Green wire tied directly to the chassis via the Rivet.

Unless that panel is isolated from the chassis....

-kizzap
<MatCat> The thing with aircraft is murphy loves to hang out with them
<Baljem> hey, you're the one who apparently pronounces FPGA 'fuhpugger'
 

Offline Wim13

  • Regular Contributor
  • *
  • Posts: 225
  • Country: nl
Re: [Teardown] Longest fully loaded shaft
« Reply #5 on: September 14, 2013, 10:25:35 am »
there's a switch, either chassis or floating.

Picture 5, there is a Green wire tied directly to the chassis via the Rivet.

Unless that panel is isolated from the chassis....

-kizzap

I have a tester from Leader for audio, that does the same, the mains earth is connected to the chassis,
and the boards are floating. There is also a switch to ground the circuits if you want.

But for safety the chassis has to be grounded by the mains.
 

Offline kizzap

  • Supporter
  • ****
  • Posts: 477
  • Country: au
Re: [Teardown] Longest fully loaded shaft
« Reply #6 on: September 14, 2013, 12:34:10 pm »
I have a tester from Leader for audio, that does the same, the mains earth is connected to the chassis,
and the boards are floating. There is also a switch to ground the circuits if you want.

But for safety the chassis has to be grounded by the mains.

Ah, Well I just learnt something new...

-kizzap
<MatCat> The thing with aircraft is murphy loves to hang out with them
<Baljem> hey, you're the one who apparently pronounces FPGA 'fuhpugger'
 

Offline olsenn

  • Frequent Contributor
  • **
  • Posts: 993
Re: [Teardown] Longest fully loaded shaft
« Reply #7 on: September 14, 2013, 04:25:02 pm »
"Longest fully loaded shaft"... Beavis & Butthead would love that one!
 

Offline Alex

  • Regular Contributor
  • *
  • Posts: 170
  • Country: gb
Re: [Teardown] Longest fully loaded shaft
« Reply #8 on: September 15, 2013, 12:23:09 am »
I have a tester from Leader for audio, that does the same, the mains earth is connected to the chassis,
and the boards are floating. There is also a switch to ground the circuits if you want.

But for safety the chassis has to be grounded by the mains.

That's right, the chassis is permanently earth grounded and the input/output connectors can be tied to chassis or not using that switch. This allows control of ground loops and connection of signals with a high common-mode level. Here is the back:




I don't know which is harder to believe, that someone threw that in a skip, or the um... 'other' pics on your image hosting site.
I do wish the skips around here had such stunning... er um, I mean remarkable finds.

...along with, amongst other things, a Yokogawa 0.05% voltage/current standard model 2554, also restored. :-// Do note the ceramic insulators for the banana connectors to minimise current leakage.



Alex.
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf