Tektronix 465b oscilloscope CH1 & 2 issues
 

Evening folks,

I've recently had the good fortune (or at least hope it turns out that way) of acquiring a Tectonix 465b, although in a sad and neglected state.

For the past couple of days I've cleaned it and managed to make some good progress but run into a problem with channel 1 and 2.

From a non starter, I now have a trace on channels 1, 2 and external. The Tim/Div and trigger are now working, all buttons and knobs seem fine now. I've got the calibrator working and displaying the square wave correctly and accurately displaying 300mv much to my surprise.

However, when I select CH1 and measure a battery, I get a deflection with the volts/div set at 5v, 0.2v, 0.1v, 50mv only. The other settings don't work.

I've done all the obvious, i.e cleaned the very delicate switch contacts with a method shown on YouTube by an experienced individual using paper soaked in cleaner to remove any dirt. I've checked all voltage settings per the manual for the CH1 and 2 preamp and all volts are within a whisker of the values required. The main power supply voltages are also fine. I read somewhere on the forum, that removing and cleaning the transistors helps eliminate any bad connections, so I've done that as well as the "jumper connections" from one board to another.

Ch2 shows a trace but nothing else when trying to test against a battery, but at the moment I'm concentrating on getting CH1 to work fully.

Given my lack of test equipment I'm limited to how I can test this, only having a multimeter. So any ideas what to check next,? I'm hoping it's something simple as I've made good progress so far and seem to have most of the functions working.

Here's to hoping someone has a bright idea.

Regards, Richard

Re: Tektronix 465b oscilloscope CH1 & 2 issues
 

I've lost count of how many of these I've been inside. Sounds like something is amiss still. If all you have is a DMM try these steps. This is a pretty basic continuity test.

1. Power the scope down and take the attenuator cover off and set it on AC coupling.
2. Connect one probe to the attenuator end of the input 1M resistor (R3014)
3. Connect the other probe to the attenuator end of the output 1M resistor (R3039)
4. Step through the following attenuator readings (1X reading!) and write down ones that are open circuit:

5mV -> should be short. Anything else, bottom of board contacts aren't working somewhere.
.5V -> open circuit = 100x attenuator or switches on top are duff.
50mV -> open circuit = 10x attenuator or switches on top are duff
20mV -> open circuit = 5x attenuator or switches on top are duff
10mV -> open circuit = 2x attenuator or switches on top are duff.

If you suspect an attenuator module is duff, swap it from channel 2 into channel one and see if it affects it.

If it doesn't work after that then you will need to do further debugging of the attenuator. First pull all the modules out and continuity check the attenuator switches against the switch positions in the service manual. Any problems, take it to bits entirely and clean it and check that the cam shaft isn't damaged or worn. Sometimes you have to bend the lever a bit up in the middle and down at the contact end to get it to work.

Also look out for components touching things that shouldn't be. I had one recently which was just a bent resistor shorting in the attenuator :(

Attachment 185878

If you find a dead attenuator, let me know as I've got three whole BER scopes full of good ones - any spares you need are yours for postage.

Edit: sometimes you get to here and find that it's completely knackered:

Attachment 185879

I've given up buying them after the 475 I bought last week for a fiver ;D

Re: Tektronix 465b oscilloscope CH1 & 2 issues
 

I would certainly follow Mr B's very sound advice. One other thing that might be worth mentioning is the tightness of the screws holding the attenuator cover in place. The boards are very sensitive to the slightest distortion and overtightening these screws can cause all sorts of problems with the contacts. Also, I found with my 465B that the contacts needed cleaning several times before they would function satisfactorily. I assume that you used IPA or a Tek approved switch cleaner. Anything else can be fatal apparently.

Alan

Re: Tektronix 465b oscilloscope CH1 & 2 issues
 

There is a type of surface corrosion that appears on gold contacts, that is semi transparent or transparent to the eye.It is an insulator. The problem then crops up on contacts that are not a sliding type( sliding contacts self clean the gold surface). The places I have seen this effect now are:

1) the switch contacts in Tek attenuators.

2) the gold plated target electrode contact in single tube color video cameras (picture goes green)

3) the DIP switch arrays in vintage computers, which cause havoc, I replace these in all my 70's vintage S-100 computer boards now.

4) Zero insertion force CPU sockets, typically in computers also using gold plated CPU pins (like the Athlon 64) computer won't POST and erratic behavior.

The technique for sliding out contact cleaner soaked paper does work to clean the Tek attenuator contacts, but be aware in stubborn cases it can take 10 or 15 passes with the paper to do it.

For Zif sockets, one method that helps is to partially insert the IC to about 1mm of seated, partially close the locking arm for a little friction, and push it home, the sliding effect cleans the surface.

Re: Tektronix 465b oscilloscope CH1 & 2 issues
 

Thanks Mr B, Alan and Argus25 for the comprehensive advice so far and kind offer of attenuators if needed. I must admit suspecting the attenuators, so I'll have a go at those today. As far as swapping modules from channel 2 to 1 as a test, are they plug in types ? I can't find anything in the hefty manual on removal of these?

For contact cleaning I used IPA? I'm running out of that so need to order some more if I'm going to make multiple cleaning attempts, is Servisol ok as I have that too? The comment of thin film and stubbornness of removal is noted. They look so delicate I didn't want to damage anything. The fact ch2 gives no readings could be due to lack of use, I guess ch1 gets used more and film could be particularly bad on ch2.

The scope was a freebie, so I'm not losing out, and I feel more confident than when I started. They look daunting, but the manual is very good with layouts and cross referencing to a schematic, so location of components is simple if time-consuming.

It had been stored for years in not ideal conditions and even basics like the power plug were frightening, no earth connection, that cable was folded back. Why do so many people not know how to wire a plug?

If someone could just confirm my attenuator query above please.

Here's to more success today,

Regards Richard.

Re: Tektronix 465b oscilloscope CH1 & 2 issues
 

Some people disconnect the earth wire in the plug so they can float the scope, so that inputs references aren't tied to building ground.

It isn't a case of not knowing how to wire a plug. It is a generally dangerous thing to do, and leaving the earth floating afterwards is criminal incompetence. In firms where they used scope brands with non-captive mains leads in the days of rewireable plugs, people using this trick were responsible for a steady input of unearthed mains leads into the general population.

Most equipment with a mains earth relies on it. The whole thing can go live with just a single failure and give no warning.

This is common enough, and commonly forgotten by the perpetrators that anyone buying a scope is well advised to check.

David

Re: Tektronix 465b oscilloscope CH1 & 2 issues
 

Attenuators are plug in. Just pull them out. Try and do it as straight as possible as they have 6 small pins on the bottom.

Re: Tektronix 465b oscilloscope CH1 & 2 issues
 

Disconnecting the earth wire was probably done deliberately for the purposes of troubleshooting live chassis equipment. However, it's an extremely dangerous practice and you have definitely done the right thing.

I think people have used Servisol for cleaning the contacts. Personally I'd stick to simple IPA but that may just be my cautious nature. I think you're right about poor storage, especially in damp or humid conditions.

Dismantling anything in a 465B is not to be undertaken lightly as everything is so tightly packed although the manual is helpful once you've managed to find the right section. Probably best to try the repetitive cleaning route first.

Alan

PS Plug wiring comment crossed with David's.

PPS I'll bow to Mr B's greater experience regarding board removal.

Re: Tektronix 465b oscilloscope CH1 & 2 issues
 

Floating the scope is a completely dangerous thing to do.

Tek manufactured two pieces of kit to get round that. The A6902 isolated probe which can deal with signals up to 3kV remote from ground up to 20MHz http://w140.com/tekwiki/wiki/A6902 and the A6901 Ground Isolation Monitor. That allows you to float the scope, but it the delta V exceeds 40V or the ground current exceeds a trip current (selectable) the unit connects the ground and turns off the scope http://w140.com/tekwiki/wiki/A6901

Craig

Re: Tektronix 465b oscilloscope CH1 & 2 issues
 

You have to do one modification to the A6901 to make it work. As designed it checks ground continuity by putting a current down the earth and sensing the voltage drop.

That trips the circuit breakers in the consumer unit.

The work around is to remove the themistor that feeds the current and short it inside to fool the (entirely analogue) circuit into thinking there is a low impedance earth.

Craig

Re: Tektronix 465b oscilloscope CH1 & 2 issues
 

Hi,. Very interesting comments on earthing etc. I'm fairly confident earth was never attached as the wire would not reach the terminal in the plug and all ends of the 3 wires were the same length with the soldered ends as manufacturers often did before plugs were fitted as standard. Anyway all sorted now and I'll concentrate on the basics for the moment.

Thanks Mr B for confirming the plug in nature of the attenuators. Glad so much of this kit is plug in, although it may have a downside with weak contacts. I'll continue with cleaning and hope it's nothing more than dirt/film. If no joy I'll do your tests above and try a swap of the modules.

Now where is R3014?, Now where's my manual gone......?

Regards , Richard

Re: Tektronix 465b oscilloscope CH1 & 2 issues
 

Forgot to say Richard that you did really well to acquire your scope as a freebie. Tek scopes from this era really were the Rolls Royces of analogue scopes in my view. Once fettled they are a real pleasure to use. Definitely worth all the effort. :)

Alan

Re: Tektronix 465b oscilloscope CH1 & 2 issues
 

In my first job in 1981, we were bought a brand new 465B when they were in current manufacture. We thought it was Christmas and New Year all rolled into one!

Craig

Re: Tektronix 465b oscilloscope CH1 & 2 issues
 

Evening,

Just a quick update on progress today.

Following the advice from Mr B and with the manual and multimeter I now know I have 4 dodgy contacts below the attenuator boards. Naturally, the most awkward and inaccessible place to reach especially Ch1.

Alan, to were right, some of these contacts take 50 or more passes to clean, but at least now I understand the switching I can move methodically through and test each contact with my multimeter before moving to the next.

The next step unfortunately, is to remove the attenuator boards to get at the 4 dodgy contacts. Any advice on doing this to avoid unnecessary dismantling? Any risks from the cam or switches flying apart?


Regards, Richard

Re: Tektronix 465b oscilloscope CH1 & 2 issues
 

Hi.

Forgot to mention above that ch2 has now sprung to life with readings on one volt/div setting which makes sense given the known contacts not working.

So definitely moving forward

Regards Richard.

Re: Tektronix 465b oscilloscope CH1 & 2 issues
 

I haven't actually taken a 465B to bits to that level. I assume it's the same as a 465, probably without some of the design flaws and probably easier. The things come apart pretty easily. It's just time consuming. The main thing is you need a decent 1/4" nut driver to undo the nuts holding the attenuator onto the front of the unit. Access is difficult. Wera do one that just about fits:

Attachment 185916

Basic objective is to get the entire vertical board out. Take lots of pictures on your phone/camera and should be fine.

Attachment 185917

When you get it out, you can remove the attenuator cams completely and clean them up. Then assemble in reverse. They are relatively easy to disassemble and reassemble. I don't think I followed the instructions! ;D

Attachment 185918

The 4xx scopes are a lot easier than most to take to bits to this level.

Incidentally this one was beyond repair as the actual cams are worn out. Check yours before you entirely reassemble it. I didn't and had to do it all again :(

Re: Tektronix 465b oscilloscope CH1 & 2 issues
 

Thanks Mr B,

Does the entire cam come out still attached to the attenuator board, I can't quite see from the picture? Then remove the cam from the board. Those small nuts holding the board to the front look awkward to get at, I'll see what I've got in the garage.

There will be a delay before I can carry on as I've run out of IPA (I still think of that as India Pale Ale) and just ordered some more rather than risk using anything else as advised above. Mind you I might have more luck cleaning the contacts with pale ale given the time it's taken to clean them so far! I love the paragraph in the manual that says these contacts should require little if any maintenance just the occasional wipe with IPA using a camel haired brush. Still I don't expect they thought it would last 40 years and spend a chunk of that time in a barn!

Getting there slowly.

Regards Richard

Re: Tektronix 465b oscilloscope CH1 & 2 issues
 

Yes. You will need to take all the knobs off first and basically the entire side of the scope comes off in one chunk. The nuts are rather difficult to get to!

After doing attenuators I think I’d be happier with a pale ale :) They are problematic. The design is pretty good really but as suggested, 40 years have passed.

The 475 I got the other day is dated around 1982. It appears that it broke pretty soon after manufacture and sat in some stores somewhere. It’s immaculate. Unfortunately the last owner was a gorilla and did damage to the board trying to play “find the shorted tantalum capacitor”. Grr!

Re: Tektronix 465b oscilloscope CH1 & 2 issues
 

Evening,

Mr B, I'm sure you'll enjoy repairing the "gorilla's" handiwork and relish the challenge.

I checked I had all the tools to remove the small bolts which I do, although I'll have to remove a metal canned component stuck directly on front of one of the bolts to stand a chance of getting the smallest driver on it. If I'm lucky I might get some long nosed pliers on it as long as it's not too tight to unscrew. Also the Allen keys once I'd rememberered that it would need imperial ! So once the IPA arrives I can crack on.

The switch contacts look very fragile and sensitive, some seem to touch on one half of the contact but the other side has no real contact. You can tell by the pressure on a piece of paper slipped underneath. I've had to "balance" a few of them so that both halves of the contacts touch with the same pressure. Not easy!

Did some more testing today and I suspect the 2X attenuator is faulty. One works but when replaced with the ch2 attenuator nothing. Anything I can do to test further or revive this, or once gone is that it?

Still getting somewhere slowly,

Regards Richard

Re: Tektronix 465b oscilloscope CH1 & 2 issues
 

Yes you have to desolder the input capacitor to get to it. I failed to mention that earlier.

If the attenuator module doesn’t work it’s not repairable so replacement is the only option. They are thin film hybrid circuits. If you pop the lid off one you can see how it’s constructed.