{"id":322,"date":"2014-11-09T20:24:50","date_gmt":"2014-11-10T01:24:50","guid":{"rendered":"http:\/\/paulcarbone.com\/blog\/?p=322"},"modified":"2020-12-31T22:07:27","modified_gmt":"2021-01-01T03:07:27","slug":"tektronix-564-crt-replacement","status":"publish","type":"post","link":"https:\/\/paulcarbone.com\/blog\/tektronix-564-crt-replacement\/","title":{"rendered":"Tektronix 564 CRT Replacement"},"content":{"rendered":"

I figured it was time to break this off into it’s own post.\u00a0 After some initial troubleshooting<\/a>, I’d determined that the CRT was the source of the intensity and focus flickering I was experiencing.
\nI ordered a replacement I found on eBay, and it arrived yesterday, safely and extremely well packed:\u00a0 Here it is after unbubblewrapping:
\n\"DSC_0207\"<\/a><\/p>\n

I proceeded to carefully disconnect and remove the old CRT, leaving a gaping hole in the business end of the scope:
\n\"DSC_0224\"<\/a>
\nNote the electrode connectors that press against pads on the neck of the CRT.\u00a0 in front of that is the trace rotation coil, wrapped around a black plastic bobbin.<\/p>\n

I compared the two to make sure they were in fact identical.\u00a0 Spoiler alert:\u00a0 They’re not.\u00a0 The part# is the same, but there are other markings that differ.
\n\"DSC_0231\"<\/a><\/p>\n

OK though, same part number, we should be good, right?\u00a0 Hmm.. one of these things is not like the other: \"DSC_0236\"<\/a>
\nThere are 6 pads around the old CRT, and 6 corresponding contacts on the scope.\u00a0 The new screen is missing the 2nd (or 5th?) pad.\u00a0 Additionally, it looks like there’s a conductive trace coming out of the left of the 2nd pad on the old CRT that’s attached to the 3rd trace on the new one.\u00a0 I needed to do a little more investigation before I was comfortable just shoving in the new screen and hoping for the best.<\/p>\n

These connections are all a part of the storage circuitry, as seen in the schematic below.<\/p>\n

\"Tek564Storage\"<\/a><\/p>\n

After a bit of cold metering in the scope, I was able to determine which connections were which on the CRT, and I marked them with a grease pencil:
\n\"DSC_0240\"<\/a><\/p>\n

So pins 1 & 6 (counting counterclockwise from the bottom in the scope) are STR1 & STR2, the upper and lower storage targets, while 2 – 5 are CE1 – CE5, the collimation electrodes.\u00a0 I’ve marked the path of the conductive traces so they’re more visible,\u00a0 you can see that they each terminate at the connection to their respective grids.<\/p>\n

So the new replacement is just missing the last collimation electrode.\u00a0 That doesn’t seem like such a big deal.\u00a0 Maybe they realized it just wasn’t needed it in subsequent revisions?\u00a0\u00a0 The original has the ‘T5640-200’ in it’s markings, which matches the description in the manual from BAMA (\u00a91964), but the replacement does not.\u00a0 The replacement has what appears to be a lower serial number, but I’d be surprised if it was the older one;\u00a0 It seems more likely that they removed an electrode in later models than leaving the space and adding one later.\u00a0 Build techniques also differ.\u00a0 The traces and connections on the replacement are finer and cleaner, but I don’t know if that equates to a earlier, smaller, more hand-built batch, or a later, more refined manufacturing process.\u00a0 Any insight would be greatly appreciated.<\/p>\n

\"DSC_0246\"<\/a><\/p>\n

Feeling confident enough, I carefully installed the new CRT and brought it up slowly on the variac.\u00a0 I got a beautifully sharp trace that didn’t flinch, even with gentle taps on the tube:
\n\"DSC_0257\"<\/a><\/p>\n

As you can see, intensified mode works great.<\/p>\n

One of my other recent purchases was a Heathkit TT-1 Tube tester.\u00a0 I started out the previous version diagnosing a smoking resistor in the storage section power supply.\u00a0 Now I had a way to check the tube.\u00a0 Both sides of the 12AT7 checked out OK on mutual conductance and shorts, but one side failed the grid current test (exhibiting lots of it).\u00a0 AH HA!!<\/p>\n

I suspect the grid current was causing excessive draw on the 475v supply, which is why the resistor was overheating.\u00a0 I replaced the tube and slowly brought it up on the variac –\u00a0 low and behold the resistor remained cool.
\nWait a minute,\u00a0 a 12AT<\/strong>7?\u00a0 The schematic (and nicely silkscreened chassis) calls for a 12AU<\/strong>7.\u00a0 Huh,\u00a0 I didn’t realize that upon first inspection.\u00a0 The 12AT7 has a much higher gain then the 12AU7, and I wonder if that led to the premature failure of the tube.\u00a0 It now has the correct tube.<\/p>\n

Next on the hitlist:<\/p>\n