Janky BNC - Tek 455

[burgarwulf]

Long time EEV viewer, anonymous forum browser, first time poster. Bit lost on how to figure this.

I've just gotten myself a "new" Tektronix 455 CRO, but CH1 is having a bit of a connection issue.

I've got the probes from the DS1054z and a Tek P6105, and it happens with both of them.

Connect the BNC and nothing shows on screen, but if you give it just a tiny bit of pressure the connectors seems to meet and the calibration waveform will appear.

Tried a wee bit of isopropyl on a q-tip, but didn't really go crazy. Poured over the manual PDF but didn't see much if anything about this particular issue. I reckon it's the central pin, but I'm not super sure how to approach that from here.

I did also notice the Rigol probes seem to stretch quite a bit when attaching, just to get the full connection, but this still isn't enough.

CH2, no issues, works perfectly.

Any help is greatly appreciated. I find this to be quite an intriguing scope, would like to return it to full function.

[VK5RC]

Welcome!
Have you tried cleaning the inner female contacts?
I have had a little success in similar situations using a toothpick soaked in IPA, a fine round toothpick, very gently though.
Regards Rob

[burgarwulf]

The tooth pick was actually the next route I was considering but wanted to ask first ! Also did the obligatory googling but it didn't really seem to come up with much either.

I'll give it a careful proding, see if I can't get it to behave

[bd139]

Could be the coupling capacitor as well. Very well known for being generally dicky on 4xx scopes. Sometimes not even soldered! I’d crack it open and take a look. While you’re in there you can continuity check the connector as well.

[sotos]

You have to open it. Maybe the BNC or something around there is not soldered properly.

[burgarwulf]

Opened 'er up, after some inspection noticed that the ceramic cap and 43ohm resistor had become loose from the BNC post. Soldered them back on there and had it working a treat for awhile.

Went to put the case back on and somehow managed to break the lead off the resistor, too short to solder it back on. Don't actually have any on hand (do have quite a few 22ohm, considered series but wasn't sure what the extra resistance would do in full). Ordered a few from Digikey along with some other stuff.

Should I replace those odd looking ceramic caps while I'm in here? Can't find the exact one in the manual, did find the table for reading the paint marks but still a bit unsure (it appears to be brown-black-yellow-brown).

[bd139]

The caps should be fine. It’s hard to find a better cap than them to be honest.

[burgarwulf]

Okay, sounds good, if it ain't broken don't fix it.

I did end up putting two 22ohm in series, reading 43.9ohm so I went ahead and soldered em in.

The original resistor broke so close to it's body I can't measure it, just out of curiosity.

Its back to working a treat however! I thank you all for the input, glad to have it working as it should.

[bd139]

Glad it worked out. nice job