Old Scopes

As I finish up some high priority tasks I think about the old oscilloscope I am fixing.  I hope to spend some time working on it today.  It is one of many interesting and fun jobs I get to do at the museum.

Since the beginning of the year I have been quite involved with old oscilloscopes at the museum and I confess it has been a guilty pleasure of mine.  A guilty pleasure because I have wanted to restore an old oscilloscope for a very long time.  Far longer than I have been working at the museum and this old desire makes me forget I have legitimate reasons for restoring old scopes here and so brief twinges of guilt come to me and I have to remind myself that what I am doing is OK.

The oscilloscope tube has been removed so I can get access to parts inside the scope  I need to change out.

Old scopes don’t seem to have much to do with vintage computers at first but if  you think about them a bit you see that spending time on old oscilloscopes in a computer museum is appropriate.  Oscilloscopes are the original visual output device for electronic computers  and computers as we know them could never have been built without oscilloscopes.  Oscilloscopes have been an essential tool for seeing what happens inside electronic circuits since the electronic age began.  Without being able to see inside of and understand how electronic circuits work, computers as we know them would never have been built and compared to where we are now the highly technically advanced world we live in would resemble a stone age had they never been invented at all.

The cathode ray tube used in oscilloscopes was invented in 1897 by Karl Ferdinand Braun.  The A. C. Cosser company first started selling oscilloscopes in 1932.  The scope I am working on now and which I show was made in the 1950’s.

Not everyone is interested in old oscilloscopes and I have never seen an ‘I  Love My Old Scope‘   bumper sticker on a car anywhere but I am not alone in my passion.  Several videos have been posted on You Tube showing people repairing old oscilloscopes.  Some of us apparently like old oscilloscopes the way most dogs like a thrown ball and I suspect seeing a few working oscilloscopes can make a visit to our museum more memorable.  My guilt is shared by the old scope video makers.  Their guilt usually makes them come up with a plausible explanation of why they are doing what they are doing.  Just doing it does not seem to be OK but it should.

I have watched a couple of the You Tube videos recently which confirmed some facts I also discovered in my own journey into the world of oscilloscope repair.  I would have benefited by watching some of these videos in the beginning of my journey and will relate what I have learned from them and in my own activity in the future.

One video in particular had a rather humorous moment.  When any piece of electronic equipment is first powered up and nothing bad happens the equipment is said to have passed the ‘smoke test’ because nothing burned up and none of the vital smoke inside of the electronic parts so necessary for correct equipment operation is let out.  The narrator of the video had a thick east coast accent further colored by the crackling voice acquired from years of cigarette smoking.  The narrator powered up his oscilloscope and announced his scope ‘has passed the smoke test‘.  As soon as the narrator said this he then had an intense coughing fit too authentic to possibly have been staged.  When he was done coughing the narrator then said ‘which is more than you can say about me‘.

The smoke test is performed after certain parts inside the oscilloscope have been changed out and replaced with new parts.  I have discovered the condition of a scope restoration candidate varies depending on how the oscilloscope has been stored (temperature and humidity) and on the quality of components used.  Scopes varied in quality and capability and this also affects how well they survive the years.

The external appearance of an old scope my be very good but if the scope has aged in an unfriendly environment many components inside will have to be replaced before a smoke test should be done.  An un-restored scope may power up and appear to work fine but I have learned such scopes will fail components very quickly and then stop working.  I have learned that the statement ‘scope powers up’ in eBay ads does not tell the whole story because even if the scope works some components are old and ready to fail making the scope unreliable.

The particular scope I show in this post will be used to display a reproduction of what is considered to be the first video game ever, ‘Tennis for Two’ and has been selected because I believe this is the same model of oscilloscope used to display the original ‘Tennis for Two’ game way back in 1958 or a model very close to it.  I will be describing what I have been doing to restore this scope and the general steps of what needs to be done to restore old oscilloscopes to working condition in a future post.




Moron CDC6500 Memory

Where are we, and how did we get here? When we last left our hero, the last part for the new memory module was about to come in.

I couldn’t just sit there and admire my handiwork, I HAD to plug it in! The bad news was it didn’t work, the good news was that nothing blew up! OK, what works and what doesn’t? The addresses all seem to work, the read and write pulses seem to work, along with the chip enable.

Output data seemed to have a problem: I had decided to use ‘541 inverting buffers, with 10K ohm pull downs so I could generate small positive pulses, kind of like what the sense amp was looking at. One problem was that when the output was turned off, the level was floating at about 2V! This is not right, according to the spec, the leakage was supposed to be 5 micro-amps, which with a 10K pull down might end up with 0.05V. I swapped out the 10K’s for 470 Ohm resistors, and things looked a bit better.

I had decided, based on playing with some PS sense amplifier modules that maybe I could get away with capacitively coupling to one side of the differential pair that ends up being both ends of the sense wire through the core mat. As I looked at what the sense amps were seeing in the machine, the amps were not liking what I was doing: the levels were going all over the place.

OK, what do I do now? Do I generate the other polarity of signal? Do I shoot myself? The sense amp wants to see something like a wire between the two pins of the inputs. Hmm, we have a small boat load of core modules that we reclaimed from a gold reclaimer. Each one of the back modules has 64 nice Genuine CDC pulse transformers, would they be usable? The secondary is pretty much a wire into which a signal can be induced with the primary.

I extricated about 15 transformers from a board, and kluged them onto my memory. Does that work” No! Well, let’s see: Some of the data bits look like they have reasonable pulses, but some don’t. I found a transformer wire that hadn’t gotten soldered on properly. Eventually I found a couple of transformers wired in one pin off. Now I had pulses on all the pins, but come were positive, and some were negative.

I compared the real core module, and the pulse on the read portion of the cycles was always positive. In my investigation of the PS modules, I had convinced myself that the polarity didn’t matter. Maybe it does? I rewired all the transformers to be the same polarity, which I thought I had done, but reality came up and slapped me in the face saying: No, no!

Now all the bits are positive, except ONE! I must have missed that one somehow. Back upstairs to fix that one. They all have the right kind of pulses, but I get pulses for zeros, and the real one does pulses for ones! Yesterday I convinced myself that I had the data upside down, and changed the data drivers from ‘541’s to ‘540’s, so I guess I was wrong. I Hate it when that happens! After swapping back the ‘541’s, the pulses were all there, and in the right places!

Now SMM came up, and here is a picture of module with PMM, the PP memory test, running:

That’s Cool! What about the OS? Here we have the bench, with the laptop which has the console, and you can still see the module in Chassis 1:

Bruce Sherry 3/30/2017 2:56:10 PM

CDC 6500 Memory

We have been running the CDC 6500 for about a year, with only half of its memory. There are 170 Core Memory Modules in the machine, and so far I have declared about 20 of them as being bad. Some it might be possible to tune into working, but some are just dead.

The plan has been to design a replacement module when the time came to get the rest of the memory working, and it seems that time is now. A couple of weeks ago, I spent some Quality Time examining the Peripheral Processsor memory in Chassis 1:

Each cycle lasts about 1 micro-second and includes both a read portion, and a write portion, because reading core is a destructive process, and the data needs to be re-written. The read and write controls are each about 400 nano-seconds, the sense amplifiers look at the read data for about 100nS, and write data is available for the whole period of the write signal, so this is pretty easy with modern components.

I chose to use two 8k by 8bit static rams, and only use half of the space. Timing was produced with some gates, and a delay line.

It took a few days to come up with enough of a schematic to start layout. Here is a picture of the first page of the schematic on the left screen, and the layout with some parts still to be moved onto the board, on the right screen:

The yellow lines indicate connections that haven’t been made yet. It took a few days, but then:

Red lines represent things and wires on the top of the board, whereas blue lines are on the bottom of the board. I didn’t display the two inner planes which hold power and ground. All the yellow lines are gone because I have created the traces to connect all the components.

I sent the board out to be built, ordered all the parts, and went on vacation while everything made it to the Museum. When I got back, I had work to do:

I have the parts, and the board ready to assemble, along with a free program on the laptop, called VisualPlace, which sorts all the parts, and shows me where they go. The red dots on the image of the board are pointing to where I should put the SRAM chips. Several hours later:

I discovered a couple of problems, the first of which is circled in red, and that is I forgot to order one part. The second is that somehow, the connector on the  right ended up 0.050″ too narrow:

This is why we build Prototypes! The good thing is that I can still plug it in and learn more about how it is supposed to work, and the other mistakes I made. Here is a picture of the prototype mounted on extenders in chassis 12:

You can see 12 LED’s, in the upper right corner, lit as the addresses go to every module all the time. This being chassis 12, the other two LED’s aren’t lit because we aren’t using the upper half of memory yet. The missing part should arrive in an hour or so, but for now here is a picture of how the new module might look next to an original module:

Bruce Sherry 3/28/2017 9:03:17 AM


The SAGA of the DEC VR-14

An option for the PDP-12 was an XY point display. This replaced the oscilloscope option that was used in the LINC and LINC-8 systems. There were two models of XY point displays available from DEC: the VR-14 and VR-12. While they appear similar to each other, they are quite different internally. The PDP-12 at LCM+L came with a VR-14, and it was stock. In fact, it was so stock that it had the most common failure: blown power transistors in the X deflection amplifier and a blown main fuse. After studying the schematics in the pdp-12 directory on bitsavers, the power transistors were replaced, pads freshly greased, fuses replaced and the VR-14 came back to life .. for a while.

It seemed to run for 3 months at a time, though it would fail consistently with a blown power transistor in the X deflection. The power transistors are 2N4399 and 2N5302 and are used in pairs. When one fails, it almost always takes out the other. In fact, the DEC documentation states that all power transistors should be replaced as their failure may cause the others to become marginal.

After running the VR-14 (our exhibit for the PDP-12 is the AD-12 interactive ‘Kaleidoscope’) for about a year with some failures we needed to find a way to make the stock VR-14 more reliable. After some searching, we came across a bit of a rare but very interesting 11 page document from 1972 called: “VR14 and VR20 troubleshooting procedures“. The introduction goes a little like this:

When servicing a VR14 or VR20 Display, it is relatively easy to replace a faulty component and obtain a display on the screen. However, this method of maintenance usually fails to eliminate the problem that caused the component failure. Until the cause is found and corrected, fuses and transistors may have to be replaced almost on a weekly basis. The purpose of this document is to provide procedures to determine the cause of a component failure.

This document then goes on to describe 6 ECOs which change a few resistor values, drops the voltage of the main supply and moves the location of the X deflection amplifier to improve cooling. Even after doing all this, you may still run into failures, but our VR-14 ran for over a year without failures.

So we wanted to improve upon this and we determined that the 2N4399 can be replaced with the higher rated 2N5684G and the 2N5302 can be replaced with the higher rated 2N5686G.

The modified G836 board.

There are also ECOs for the two A225 XY deflection flipchips documented in the Troubleshooting Procedures. If you perform these ECOs and also use our suggested power transistors, you’ll make your VR14 much more reliable and enjoyable.