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Old 2nd Jun 2017, 10:16 pm   #41
willmac
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Default Re: Tektronix 468 Repair

Now that the noise is under control I have been looking more carefully at the analogue behaviour of this scope. I can actually see and measure signals now, which is a big step forward from the vague fuzz that I was previously seeing.

Ch2 seems to operate as it should. I haven't tested it over a full range of signals yet, but I get the displays that I expect, the trigger works correctly etc. The only obvious issue is that there is some noise in the rotary switch selecting the volts/division. The light that illuminates this dial also flickers on and off occasionally. I suspect dirty contacts, which I should be able to fix failrly easily.

Ch1 does not behave consistently. As I rotate the Volts/division switch I see no change in the vertical height of the trace on the screen over quite a range of positions. The vertical height of the trace does not match the voltage of the signal. It seems as if there is insufficient gain. As with Ch1, there is noise from the rotary swith, and the LED does not light up consistently. The VAR control does work correctly in all switch positions and illuminates the UNCAL light as expected.

I think there might be several possible reasons for this behaviour.

1) The switch is so dirty that some positions are not making contact at all.

2) One or more of the attenuators in Ch2 front end are faulty and are higher resistance that they should be.

3) One of the amplifier stages has failed.

I guess a combination of failures is quite possible.

Any thoughts on these symptoms would be very useful.
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Old 2nd Jun 2017, 10:54 pm   #42
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Default Re: Tektronix 468 Repair

The input attenuator modules are easily fried on them and/or the AC coupling switch contacts get cruddy. If you touch the resistor that joins them to the front end FET metal can it should generate a wobbly 50hz mess on the screen. If that happens it's the attenuator modules that are fried. If it doesn't it's the amplifier. Work forwards/back from there poking

I have a full set of attenuator modules lying around in a mule scope if you need them. Yours for postage.
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Old 3rd Jun 2017, 7:43 am   #43
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Default Re: Tektronix 468 Repair

The cam operated switches are notorious but usually respond to cleaning with a narrow strip of paper soaked in IPA. Insert with switch open, close switch, pull the strip out through the closed contacts. If you get a pair of bad switch contacts (ie both 'in' and 'out' are bad) on one attenuator it throws everything out since it breaks the series chain of attenuators.

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Old 3rd Jun 2017, 11:41 am   #44
willmac
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Default Re: Tektronix 468 Repair

Quote:
Originally Posted by MrBungle View Post
I have a full set of attenuator modules lying around in a mule scope if you need them. Yours for postage.
That is an extremely generous offer. I need to do a lot more investigation and cleaning before I could take you up on it, but it is very kind of you to offer.

I will need to remove the vertical amplifier board and the attenuator assembly to get to the switch for cleaning, so this will take a little time.

I understand in principle what the attenuators do and that they are precision devices that are intended to be be temperature and frequency independent. Is there any way I can test these using just a DVM to see if there is an obvious failure? I have another set on Channel 2 which seem to be fine, so could I just compare readings?

I assume that the rotary control switches the various attenuators in and out of circuit to achieve the various calibrated volts/division on the knob? Looking at the schematic (A23) I can see that if any of the switches in the attenuator chain fails to close due to dirt, I could get symptoms similar to the ones that I observe. Is there more to this or is this basically correct?
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Old 3rd Jun 2017, 11:52 am   #45
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Default Re: Tektronix 468 Repair

It's usually a case of working out which range the attenuators are switched in on and comparing channels. Then you swap the attenuator between the two channels and if the bad behaviour is swapped then you have found the dead one. I just apply a square wave to both inputs and switch back and forth between channels. At then lowest mV range there are no attenuators switched in.

Dirty contacts can indeed cause those problems. It only takes one to go open circuit and it's a problem.

The modules just pull out (carefully!). You can check them with a DVM but I usually pop the lid off and have a look. Knackered ones have bubbled thin film resistors in them.

IPA and paper trick mentioned by Roger Evans above is recommended here too. Works every time!

Before you do this, working out if it's before or after the front end FETs is required as it may lead to a wild attenuator goose chase otherwise

Recompensating all the attenuators afterwards isn't much fun I will add.
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Old 3rd Jun 2017, 5:59 pm   #46
willmac
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Default Re: Tektronix 468 Repair

Quote:
Dirty contacts can indeed cause those problems. It only takes one to go open circuit and it's a problem.

The modules just pull out (carefully!). You can check them with a DVM but I usually pop the lid off and have a look. Knackered ones have bubbled thin film resistors in them.

IPA and paper trick mentioned by Roger Evans above is recommended here too. Works every time!

Before you do this, working out if it's before or after the front end FETs is required as it may lead to a wild attenuator goose chase otherwise

Recompensating all the attenuators afterwards isn't much fun I will add.
I have tried the touch test and both channels go wild in just the same way, so I have to assume that the problem is in the attenuators or switches for channel 1

I have had a good look at the attenuators and switches for Channel 1. I have cleaned the switch contacts on the top of the board that can be reached without further dismantling and there does seem to be some improvement. Still not right, so I am guessing there are switches on the other side of the board that I can't see at the moment. If I put light finger pressure on the X100 attenuator module, I seem to get full contact and a proper sized trace appears for more switch positions. My guess is that there are switch contacts under that part of the board which are now making properly. Is this correct - further switches underneath?

I looked inside the X100 module out of curiosity. I won't be repairing anything inside there, given the tiny size and the way they are made. I didn't see any bubbling on the resistor in that module though.

I have no idea what is involved in recompensating the attenuators. I hope that is covered in the manual; I will need to look that up.

Thanks again for the help.
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Old 3rd Jun 2017, 6:16 pm   #47
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Default Re: Tektronix 468 Repair

Found a photo of the attenuator module in a 465B for reference...

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The only contacts are the ones either side of the attenuator module and the AC/DC coupling switches at the front. I've marked them on the above. If pressure is doing it, then it's probably the thin film modules got damp as well. Another thing that causes them to die. Not the most reliable things on the planet. If you swap the one you are pushing down on with the one from the other channel (mark the good one with a marker first!) it should confirm if it's the contacts needing further cleaning or the attenuator module. Be careful when removing and reinstalling - the legs are small and fragile.

They definitely are not repairable

As for as re-compensating goes, you basically connect a 10x probe (with the compensation set to mid way) to the channel and to a function generator with a decent rise time kicking out a square wave. A 555 circuit will do the job to be honest if you don't have one. You then start at the lowest volts/div and work out what attenuators are switched in on each setting, then adjust each one for best square wave shape progressively. Takes a while and a bit of trial and error. There is probably a more scientific method described in the manual but I didn't use it.

Edit: when I think about it, it's probably feasible with modern components to knock up a replacement attenuator module on some thin FR4 with SMD components. Problem is knowing what the module does precisely as they are functional units with no technical information.

Last edited by MrBungle; 3rd Jun 2017 at 6:24 pm.
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Old 3rd Jun 2017, 6:59 pm   #48
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Default Re: Tektronix 468 Repair

The reason that I thought there were more contacts on the underside is from looking at the schematic for the vertical attenuator board in Volume 2 of the manual.

I count 20 sets of contacts, and I think there are 10 such contacts in the photo you show. Each cam seems to operate a NC and a NO contact in the schematic.
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Old 4th Jun 2017, 2:39 pm   #49
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Default Re: Tektronix 468 Repair

As well as blowing resistors, thenattanuators suffer from cracking around the pins where they are soldered into the bottom of the attenuator. This is SOMETIMES repairable if you have a very fine iron. If you can look at the logic of the switching, it's described on the preamp circuit diag, then you can usually work out which ones are failing.

I've managed to repair a couple where the cracked pin joint has been the culprit but otherwise they're scrap.

Robin
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Old 13th Jun 2017, 7:46 pm   #50
willmac
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Default Re: Tektronix 468 Repair

I haven't posted on this repair for a while because I have had a large design project which needed a lot of work (not electronics). I have now got back to it and made some progress.

The 468 attenuators do have switches on both the upper side of the board and the lower side. The upper side you can reach for cleaning with the attenuator board in the scope. I could not reach the underside switches so decided to remove the board to do a thorough job.

This is easier said than done.

First the Vertical pre-amplifier board must be removed. This has a mass of connections. I found 2 neighbouring co-axial connections that were reversed from the description in the manual. Can the manual be trusted or are there errors??

The attenuator boards are attached to the vertical logic board. That must be rermoved in order to get to the lower side of attenuator boards. The screws that attach the attenuator assemblies to the vertical logic board are underneath. Not the most maintenance friendly arrangement.

There are further switch contacts on the vertical logic board. These are activated by the lower side of the same cams that actuate the switches on the lower and upper sides of the attenuator board. I expect that these switches are scanned by the microprocessor, but I haven't followed the schematics to confirm this. These switches are a bit more robust than the attenuator switches, but use the same gold plated fingers.

So in total each channel has 26 switch contacts!!

I found that the attenuator assemblies were very dirty. I suspect that a previous owner was a heavy smoker. I have used IPA to clean everything including the contacts.

See attached photos for anyone not familiar with this model.
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Last edited by AC/HL; 13th Jun 2017 at 11:58 pm. Reason: Images uploaded
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Old 13th Jun 2017, 11:06 pm   #51
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Default Re: Tektronix 468 Repair

Ouch. Well done on getting through it though. I've got to be honest, one of the reasons I no longer own Tek scopes is that they are pretty difficult to get access to certain bits, particularly the HV multiplier! The HP 17xx and Philips PM3xxx models are a joy to work on in comparison!

(Apart from the 453, which was really nice)
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Old 14th Jun 2017, 12:16 am   #52
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Default Re: Tektronix 468 Repair

I have repaired these sorts of attenuator/switch assemblies many times in Tek 464 scopes, they are very similar. As noted the board needs to be removed to get at the contacts on the opposite side. What happens, is despite the fact that gold is non reactive, a glassy oxide appears on the surface of the gold contacts. Because with this design of switch there is primarily a touch, versus a wiping action, the contacts become insulated. Spraying them with IPA won't fix this.

The best suggestion on this thread is the method I also use, which is to cut small strips of paper (I just use A4 printer paper) wet it with IPA, slip it between the surfaces and close the contact. Drag the paper out. The mild abrasive effect cleans off the insulating layer. I have found for some contacts this has to be done 4 or 5 times, with fresh strips of paper as it softens after 1 or two goes, to get a reliable connection again. So what I'm saying here is that while the board is out treat the underside contacts at least 4 or 5 times each, as once you put the board back in, if there is still a problem, it will have to come out again which is much more time consuming.

Last edited by Argus25; 14th Jun 2017 at 12:21 am.
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Old 14th Jun 2017, 6:55 pm   #53
willmac
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Default Re: Tektronix 468 Repair

Mr Bungle -

Yes - I agree - these scopes were not designed with easy maintenance as an objective. I am in so deep in this one that I have to keep telling myself that I CAN put it together again successfully - and if it doesn't work after all this, it didn't cost me anything. And I am learning a lot along the way.

Argus25 -

I used the method that you describe. The trouble is that I will not know if I have reliable connections until I put it all back. I guess if I still have switch problems I will just have to do it all over again.
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Old 20th Jun 2017, 7:46 pm   #54
willmac
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Default Re: Tektronix 468 Repair

I re-assembled everything - and that was a major piece of work. At first things looked good - I was getting crisp LEDs etc, but then there was a smell. It looks like one or more of the Tantalum capacitors on the vertical amplifier board has failed suddenly - at least they got extremely hot, and a nearby transistor with it. Now nothing works except the POST.

I am going to park this project. I don't have the skills needed to track down all these problems and it is possible that I am doing more harm than good. It is really discouraging to work on parts that are extremely inaccessible and then find that something else has failed.
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Old 20th Jun 2017, 8:32 pm   #55
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Default Re: Tektronix 468 Repair

Don't give up the fight yet but taking a break is a good idea. I've learned as much from fighting these battles as I have from reading a book on the subject.

If it's any consolation the first bit of gear I repaired back in about 1997 was a Tek 453. When I say repaired I nearly got it working and then it ate a tunnel diode and the power transformer blew up thanks to a duff capacitor that went short. I had spent 30 hours on that unit and it just went up in smoke. I took it to the tip and left it there because I couldn't face it any longer. Yours isn't quite there yet
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