Tektronix 7603 with 7A26 and 7B53A plug ins

I have just got back from manual hunting.
The 7603 manual was first followed by the 7A26.
The 7B53B has proved illusive however there is a 7B53A out there.
They took ages to download on my country internet but they are well scanned and clear.
The first thing I have spotted it that the manual says don't take the screws out of the switch PCB.
I will have to show it some cleaner first and see if it works.
Yes indeed I am a test gear fan and intend to use it regularly when I have got it all fixed up.

Seriously tasty scope you've got yourself there Simon. Will look forward to you bringing it back to good health

Recently I have been working on a number of TM500 series modules that use the same pad and finger switches described by Refugee. I have never had any problems removing the screws and the switches falling apart but I live and learn.

The usual method for cleaning the contacts is to slip a piece of paper soaked in IPA, no not the beer , under the open contact then rotate the switch until the switch closes and carefully slide the paper in and out to rub the contact surfaces clean. On a few occasions that has not worked. My solution is then to drip Electrolube contact oil onto the pad and leave it a day or two. After that use a fine paintbrush dipped in IPA to clean the oil out. So far it has worked 100% and seems to be a lasting fix.

Al

Once upon a time Electrolube made some stiff paper/card strips soaked in 2AX or similar designed for cleaning non-wiping contacts (e.g. older wavechange switches like those on a Murphy A24). I kept a set in my toolkit for years but I'll bet they've long dried out 🙁

Seriously tasty scope you've got yourself there Simon. Will look forward to you bringing it back to good health

Yes it is a big boys scope and I was expecting you to say that it is far too complicated for your workshop

I have begun diagnosis today.
I have powered it up and the tube is bright and as reported by the vendor there is something wrong with the timebase.
I have downloaded the 7B53A manual as a close guide for the 7B53B and it does not have an awful lot of differences.
Faults found so far are the time base only has three speeds with a little bit of variation on the settings in between the steps that work. There is a -50V supply that does not do much apart from feed a reference to the time base switching. I will have to get the covers off the frame and check it before I go in any further.
There is a bit of noise on the Y amps so capacitors in the power supply need to be checked too.
The scale lamps are not working so if these are NLA the old ones will have to be stuffed with LEDs.
The on screen display will also need to be looked at. There are little bits missing from the trace when the switch is on so some of the logic is working but not the character generator.

The paper or card method will be worth a try but to avoid stripping down I will first check with a meter to see if it really is the switches.
I have had an open circuit via hole on a Tektronix scope in the year that I worked for a test equipment repair and cal shop. I was not the first engineer to look at that one. I had to measure the two ends of the wire and then follow it in the manual circuits checking at each board entry/exit terminal only to find that it was on the main board that the plug ins go into. The tube was about the only thing I didn't have to take out to get far enough in to fix it.
There are signs that this one has not been best stored so I shall not rule out a bad via hole at this stage.

I have had an open circuit via hole on a Tektronix scope in the year that I worked for a test equipment repair and cal shop. I was not the first engineer to look at that one.

I've also encountered that one in of all things, a very expensive Sony CD radio-cassette. It was intermittent though, and caused crackling. Bridging it diminished the crackling, only then by cutting the tracks did the crackle finally stop. Likewise, it had been through a few people.

That via hole must have been arcing to an adjacent track or hole.
I have started to study the manual in order to establish what is not there between the frame and the plug in and then I will go in and measure it. I will keep the on screen display switched off to avoid confusion while I get the timebase going again.

One cautionary note about the plastics used in the plugins on the 7000 series, only use the isopropanol to clean the switches as recommended by TEK, don't be tempted to use Servisol switch cleaner as it attacks the plastic. Guess how I found out.

It is a sort of nylon like plastic in those switches.
The stuff I used does not melt plastic or contain anything oily and it has worked.
I have been testing the switches and so far all have been OK so I am still on it.

I have been off this restore doing the garden and getting all the indoor plants outside for there summer vacation.
I have made a mistake with the part number of the timebase plugin. I will PM Chris if he does not spot this post. It is indeed a 7B53A. My fault for not cleaning the finger prints off my glasses

In the switching it is the resistor bank that looks like it has something wrong in it and I am going to have to check every contact in the switch bank. I am pretty sure at this stage that the special chip is all right.

Sunday was officially bad FET day.
The FET is Q596 and I got a reading a little over 700 ohms between source and gate.
It was shunting the resistor section of the timebase switch so that the only change was seen when a capacitor was switched.

Lifting the FET out of the socket cleared the short from pin 8 of U580 and disabled the delayed timebase.
All the main timebase steps came back.
Next session is to try a 2SK43 in it to get the delayed timebase back again.

On screen displays next.

I have put a 2SK43 in place of the faulty FET and now both timebases are working correctly.
I will now start looking at the on screen display parts in the frame so it is now fast TTL time for me.
Also that spike on my pulse generator is going to have to be looked at now I have a fast enough scope to tackle it.

I have been doing some testing and a lot of reading and began to tackle U2126 first. On pin 12 the waveform is shown as roughly -6 volts and this is wrong on mine.
It varies with the display brightness control and will not budge above -0.5 volts. Pin 11 where the signal from the brightness pot connects also will not budge above -0.5 volts unless I power up with U2126 removed. This allows it to go from +4.5 volts for minimum brightness to -13.5 volts at maximum brightness on pin 11.
With U2126 in place disconnecting the Z amp cable does not restore it.
CR2124 and CR2125 have been tested as have R2122 and R2123.
Pin 12 has narrow positive going pulses and these are present also on pin 11 but these are narrow enough that a DVM ignores them and also they only come up to 0 volts.
The pulses vary with the settings on the plugins so that part is working.
Has anyone got a U2126 I could borrow?
The part No is 155-0021-01
Readings from pins 11 and 12 for maximum and minimum brightness would be useful if anyone has got the cover off there 7000 series.

Hi Simon,

Following this thread with interest, great work so far . Scope repairs are complex so its good to get some tips from those with test equipment repair experience. I've only repaired one scope, that was substituting a Gould CRT into a TEK 2205 which was great fun and learning curve working out the rewiring and subsequent re-set up.

Do you suspect the U2126? If so and it does turn out to be faulty, what on earth are you going to, aren't theses TEK custom chips?

Hi, I did a quick google search on Tektronix U2126 and there was a scope site which states an LM3086 is a generic replacement which is easier to get. Best regards, Norman

That sounds interesting.
Have you got a link to the site?
It looks like the Tektronics sites are out of stock.
A data sheet check has just revealed that the LM3086 is a 14 pin and the part I need is 16 pin so it looks like a false lead but a look at the site will still be fun.

Hi, I am not sure how to put links in so here goes, http://www.logwell.com/tech/oscilloscop ... notes.html

Hi, I am not sure how to put links in ...

You do now ...!