[Teardown] Longest fully loaded shaft - Page 1
Forum section: electronics / testgear

author: Alex
author_rank: Regular Contributor
timestamp: September 13, 2013, 08:37:50 pm
content: 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.jpgBack of unit. Yes, it is that old.The technician's soldering iron touched the cap during assemblyAfter 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.

author: kizzap
author_rank: Unknown
timestamp: September 14, 2013, 01:30:45 am
content: Did I see some text near the power input saying that the Chassis is floating, yet see definative grounding connections to the chassis?-kizzap

author: Fsck
author_rank: Super Contributor
timestamp: September 14, 2013, 02:47:15 am
content: Quote from: kizzap on September 14, 2013, 01:30:45 amthere's a switch, either chassis or floating.

author: TerraHertz
author_rank: Super Contributor
timestamp: September 14, 2013, 05:29:43 am
content: 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.

author: kizzap
author_rank: Unknown
timestamp: September 14, 2013, 08:04:24 am
content: Quote from: Fsck on September 14, 2013, 02:47:15 amPicture 5, there is a Green wire tied directly to the chassis via the Rivet.Unless that panel is isolated from the chassis....-kizzap

author: Wim13
author_rank: Frequent Contributor
timestamp: September 14, 2013, 10:25:35 am
content: Quote from: kizzap on September 14, 2013, 08:04:24 amI 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.

author: kizzap
author_rank: Unknown
timestamp: September 14, 2013, 12:34:10 pm
content: Quote from: Wim13 on September 14, 2013, 10:25:35 amAh, Well I just learnt something new...-kizzap

author: olsenn
author_rank: Frequent Contributor
timestamp: September 14, 2013, 04:25:02 pm
content: "Longest fully loaded shaft"... Beavis & Butthead would love that one!

author: Alex
author_rank: Regular Contributor
timestamp: September 15, 2013, 12:23:09 am
content: Quote from: Wim13 on September 14, 2013, 10:25:35 amThat'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:Quote from: TerraHertz on September 14, 2013, 05:29:43 am...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.