Tektronix 485: How to get to vertical attenuator?

snipped-for-privacy@stanfordalumni.org wrote in news: snipped-for-privacy@y43g2000cwc.googlegroups.com:

You pull out the entire atten. module,not just the top PCB.

After removing the atten knob,there's a nut on a bushing that must be removed. Then there's a screw on the rear side to be removed,and a hex- nut/spacer in between the two atten assemblies,on that "T" bracket. Then you have to pull the Peltola cables out from their jacks,then the harmonica connector.Then the atten module tilts out.

The serivce manual has the procedure in it.Maybe your 'copy' omitted it.

Thanks! I'll try that tonight. The service manual I got from the downloadable manuals at

Maybe it's missing something...

Best,

- Cesar

"carneyke" wrote in news: snipped-for-privacy@t31g2000cwb.googlegroups.com:

Well,I gotta disagree with that. The power supplies (LV and HV)were very hard to troubleshoot and repair,and that's the most failure prone part of the 485. They buried the HV in the *middle of the scope* under everything else,and there were those comb connectors tying the circuit boards together... And those attenuators were a PITA.they were always getting flaky.

IMO,the 2465 series was the best of TEK. Another fav of mine was the lowly 422(non-battery version);15Mhz,but a sealed cabinet and few problems,just repaint the case,replace the worn rubber feet,and cal.The insides of the 422's were always nice and clean. It was a pleasure to take a grubby,scraped-up 422 and make it look new again.customers were always amazed when they got them back.

Thanks, I got the module off. Now to a new question:

It seems the 5k pot is stuck somehow. I removed the big shaft and the shaft/pot connector and it turns out it is the pot that is not turning. Well, it turns but it is *very* hard to do it. First, what would be the reason for such a behavior? I never had a potentiometer doing that.

Also, is there anything special about this potentiometer, or is it just an old 5k carbon film linear potentiometer? I know that the volts/div switch is all full of special things about it and is irreplaceable. I just want to be sure that's not the case with that pot (the schematic seems to point to a normal potentiometer).

Best,

- Cesar

Looks like the pot needs to be replaced or at least to be re-lubed. Since t will be difficult to get a irect replacement, try this: get teh pot out, drill a very small hole in the pot housing and apply 1 -3 drops of contact cleaner/lube "Kontakt60" into the housing. Turn the the pots shaft for a couple of times. Repaeat in necessary. Let dry at least for half an hour .

I used this procedere a dozen times with success, the 485s I repaired still work smoothly.

The pwr supply is really difficult, as Jim already mentioned. Always a challenge to repair. Great job for a rainy weekend :)

The 2465 is a class of its own. Superb scope, always a good invest to do. The 422 is nice too, but the 454A is what I would collect: A little bit difficult to repair, but rock-solid made.

Well, just a collectors view...

hth, Andreas

No, looks quite complete to me, no missing pages.

I use it too.

hth, Andreas

Jim, I wasn't refering to working on them. The 485 has a very bright / fine focused CRT. The 1 Megohm / 50 Ohm attenuator with 350 MHZ bandwidth really made the scope and it's small size was very nice. The low voltage / high voltage section is a dog but we didn't have many fail. I did a lot of calibration / performance checks on them. The attenuators were a little tricky to calibrate considering it had the 50 Ohm / 1 Megohm built into one. I have a 2465B (400 MHz) on my bench and use it a few times per week. I do like that more than a 485. Before getting the 2465B (12 years ago) my favorite was a 475A with the DM package. I really dislike digital scopes as most of my work is troubleshooting, but the cursors make critical measurements easier. Take Care, Kevin

Jim Yanik wrote:

Thanks! I can't find Kontakt60, but I can probably find something else.

The only remaining thing is the 1M/50R bulb. I wonder if it could be replaced by a LED (with the appropriate resistor). The schematic shows this bulb connected to +5V on one side and to an IC (through a 10R resistor) on the other. I'd guess the IC pin goes from +5V to GND when it wants the bulb to light up, so I could put an LED plus a 270R resistor in place of the bulb. Has anybody done this? If not I will probably remove the A channel and check what's supposed to happen.

Best,

- Cesar

snipped-for-privacy@stanfordalumni.org wrote in news: snipped-for-privacy@i40g2000cwc.googlegroups.com:

That is what I would do.The incandescent bulb draws IIRC,at least 50ma,so the IC will easily switch a LED at 5-10ma. You even have enough V to run a bright-white LED,but I would use a 3mm LED instead of the usual 5mm. I installed LEDs in other TEK test gear on my bench because I tired of dealing with hard to access lamps.(and also like to mod things!) 8-)

Done. I found that my local Fry's carries Deoxit Faderlube (the spray version). I didn't even need to remove the pots (thankfully, it would be a pain for the timing one). Amazing stuff that is. Both pots now turn very smoothly.

Best,

- Cesar