Another Tek 465 Problem

[Nymrod121]

Just discovered a note in my 'bits-of-paper-and-old-Post-Its-clearance' folder:

Tektronix front panel control grub-screws:
=============================

No 4-40 x 0.250 machine screw (qty x8)
No 6-32 x 0.375 machine screw (qty x 9)

(however ... these notes may have been from an earlier Tek 7xxx series mainframe restoration/plug-in overhaul rather than 465 etc.)

Guy

[HowMuch!]

Set screw problem solved. Many thanks for Mr Bungle suggesting RS Components, I didn't know they had dropped a minimum order charge. It seems the Allen key set from CPC either has the 1/16 size incorrectly supplied as a 1.5mm, or that is how they all come and they hope it will be good enough. A 1/16 from RS fitted perfectly and the knob is now off.

[MrBungle]

They're pretty handy. I ordered a £0.43 voltage reference IC yesterday and it arrived via ParcelForce 24 today. Wonder how long before they reintroduce the minimum order charge!

[HowMuch!]

Back again with more questions. This scope is now fully working (I think) with just a couple of minor problems. The pot that adjusts the variable time base is smashed and needs replacing, but I can live without that for now. I did find a thread on this site on replacing it with a more readily obtainable part. A couple of broken knobs could do with being replaced at some time too.

One thing I would like to get fixed though is a reduced brightness/flicker on some scan speeds. What happens is the trace is nice and bright and steady on all speeds below 1ms, then from 0.5ms to 0.1 ms the trace is reduced in brightness and flickering. This repeats with 50us to 10us bright... 5us to 1us dim... 0.5us to 0.05us bright again. I have noticed that the 'bright' trace can be made to dim and flicker by adjusting the variable hold-off-time control, so I'm wondering if this part of the circuit is the culprit. Any Tek gurus out there with any ideas?

[RogerEvans]

Have a look at bank 15 of the sweep speed cam operated switch S1150. It opens and closes in sync with the 'bad' sweep ranges that you observe. It is part of the holdoff timing and it looks as if it goes high resistance or open circuit it will lengthen the holdoff time and dim the trace. The standard cleaning method is a thin strip of paper soaked in IPA, insert it with the switch open and withdraw with the switch closed. These switches often give problems but usually respond to a little TLC.

I don't have a 465 so I can't comment on accessability or the lack of it.

Roger

[HowMuch!]

Thanks for the reply, yes looking at the circuit diagram I see that now. I might have a chance to look at it tomorrow. Just out of interest, what type of capacitor are these? They are huge for the values.

[MrBungle]

They are ceramic believe it or not. Amazing that the same thing with similar characteristics is a few mm long now.

I have most of a Tek 465B here if you need any parts. Yours for postage.

[HowMuch!]

That's a very kind offer I would like to take you up on Mr Bungle. One pot/switch and a couple of knobs would be most appreciated. How do I contact you?

Bank 15 of the timebase switch was indeed bent and stuck permanently closed, as was one other switch, so the scan speed would have been out on some of the ranges. It's pretty obvious that the scope had received a knock to the front sometime in it's life. This had broken a couple of pots, and presumably damaged the timebase switch bank too. Why only two switches out of the dozens were damaged is hard to imagine. Bending them back to shape was easy enough once I worked out how to remove the operating cam-bank from the board without causing more damage. I can post a picture of how the switches are mounted directly on the board if anyone is interested.

My next project is an IFR 1200S Communications Test Set with multiple faults, is this classed as Vintage yet? (looking on Ebay it seems that everything over about 5 years old is now called 'Vintage')

[cmjones01]

Originally Posted by MrBungle

They are ceramic believe it or not. Amazing that the same thing with similar characteristics is a few mm long now.

I think the reason those capacitors are so large is because they're very high precision, high stability ones upon which the accuracy of the timebase depends. I presume that small manufacturing tolerances have less effect when the dimensions involved in the capacitor are all bigger!

Chris

[RogerEvans]

This is speculation, but I have seen discussions around different types of ceramic material and the commonest ones are quite poor for temperature stability and also the change of capacitance with applied voltage which would be bad for linearity in a precision time base. I suspect the more stable ceramics have a lower K and need to be larger for the same capacitance.

Roger

[RogerEvans]

One other point about the Tektronix cam operated switches, I had one which had obviously had an impact at the front and dislodged the circlip that holds the cam in the correct position relative to the switch levers. It is worth checking yours is ok.
Roger

[1980s_john]

Originally Posted by HowMuch!

My 15 volt unregulated is actually nearly 24 volts. Should I try to reduce this, will that help with overheating?

Hi, one thing I would recommend to other UK owners of Tek 465/465B scopes is to set the input voltage to its highest setting. This reduces the unregulated voltages to their lowest thus reducing heat dissipation in the regulators.

Also check that your fan still works, these can fail.

HTH,
John