Tuesday, 13 October 2015

Tektronix 453 High Voltage

Filed under: Uncategorized — Michael @ 14:21

A while back I acquired a Tektronix 453 oscilloscope. It functioned just fine and was in quite good condition. A little cleanup and it joined my test equipment roster.


Recently, however, it developed a problem somewhere in the high voltage supply. When I turned it on, it made a sizzling sound and the display was compressed and very bright. I removed the cover over the high voltage box and saw that on power up, one of the 5642 rectifier tubes was internally arcing. (Thinking back, there was probably arcing elsewhere but I didn’t want to leave it on long enough to poke around!)
‘Aha, bad tube’, I thought. I found a couple of NOS Sylvania tubes on eBay and a few days later, the ‘scope seemed to be running fine again. Then, a couple of weeks later, the same sizzling sound. Again, a look at the high voltage box and there was arcing deep inside. On the suggestion of a couple of people on the Yahoo TekScopes group, I opened up the high voltage box and cleaned it out with IPA and put it back together. Again, the problem seemed to be fixed. But a fews days later, it was back.

Here is the high voltage box with covers off, looking at the bottom (remember, the box is ‘upside down’ in the bottom of the ‘scope).

I unsoldered the anode lead and lifted the high voltage ‘chassis’ from the plastic box so I could see the top. (Once the anode lead is free, you can gently lift the chassis out, pivoting it toward the back of the ‘scope, hinging on the row of wires at the back. I had done this before when I cleaned the inside of the box. This image is from later in the disassembly, but helps to show what I mean.)

I switched the power on and the arcing appeared across the neon lamps and the string of 5.6 MegOhm resistors. I quickly switched it off. I suspected the grid bias section. If it fails, the voltage from the cathode supply could find its way across these components. Actually, just a guess. Whatever the problem, though, it was going to mean a pretty major disassembly to get at all of the parts for testing. So, as long as I was going inside, I figured I would just rebuild the whole high voltage box.

Here is the schematic of the high voltage power section. The color highlighting indicates the component location on the chassis.

First the search for parts. I assumed that the transformer was OK, partly because the ‘scope had worked between reoccurrences of the problem and mostly because I couldn’t find one. The two high voltage diodes, Tek 152-0192-00, I found as NOS at Vernonia Northern. (Last I checked, they didn’t have any more. But they do have a ton of other Tek parts.) The high voltage ceramic capacitors and neon lamps were purchased at Mouser. The grid bias potentiometer and a few resistor values that I didn’t have came from TME. (For a complete list of the parts I used, you can view or download this PDF Tek453-HV-PartsList.pdf.) Once I had all of the parts on-hand I took a deep breath and started.

A note about the resistors. I used what I had on hand which are Yageo MF0207 series metal film, 0.6W resistors. They have a maximum working voltage of 350 V. This is actually too low for many of resistors used in the circuit. For example, in the series of five 5.6 MegOhm resistors in the grid bias circuit, each resistor has ~400 V across it. The parts I used do work but likely have a compromised lifetime. You can use 1 Watt parts which have a working voltage of 500 V or use more resistors of smaller values for the critical resistor series (grid bias and focus circuits).

I took plenty of pictures to document parts placement and wiring. The chassis has components on three layers, a separate piece at the top (held on with a Nylon screw) where the resistor strings are mounted (blue highlight on the schematic, above), the middle layer with the grid bias supply and part of the anode supply (washed out red on the schematic) and the bottom (the layer you see when you take the cover off) has the cathode supply and the 5642 tubes, and a capacitor, for the anode (orange highlight). The layers are interconnected and the whole circuit connected to the outside with numerous color-coded wires. The pictures will help when reassembling.

I only un-soldered wires as necessary. One note, the ceramic wiring blocks require a fair amount of heat and good heat transfer to get the solder to flow. I used a tip temperature of 350 and a smallish chisel tip to get into the slot. Keep your tip well tinned and use a solder that contains some silver so as not to ruin the metal-to-ceramic plating. For getting the old solder out of the slot, desoldering braid came in handy and in a few places I was able to use a vacuum desoldering iron (heating the slot with my iron at the same time).

Here is the top level, before and after.

This is the middle level.

A closeup of the anode supply capacitor placement.

And, finally, the bottom level, rewired.

The time finally came to power up the ‘scope. I held my breath and flipped the switch. Whew!, no magic smoke, no sizzling. So far, so good. A few seconds later and the trace moved into position on the CRT. Everything looked perfect. Even the focus was good. Twiddling the intensity control worked and it was obvious that the display was capable of higher intensity than before. The focus control worked as expected. All good indications that the high voltages were there and correct. I grabbed the high voltage probe and the -1950 V test point measured -1945. The anode voltage was 7750. Practically perfect. It appears that the rebuild was a total success.

So, what was the problem? As I mentioned back at the start, I suspected the fault was in the grid bias supply section. I measured all the components once I had removed them and they all tested OK. Until I came to the CRT grid bias adjustment potentiometer. It first measured open. When I tried to turn the shaft it pulled out from the pot. But the meter now showed a reasonable resistance value. The potentiometer wiper was intermittent at best. The potentiometer is wired as a variable resistor, just the wiper and one end are connected. So, an intermittent wiper would open that circuit. I am going with the assumption that that was the problem. (Of course, if that was the only problem, I replaced a lot of stuff when only two solder connections would have been necessary. But then again, I likely wouldn’t have found this without quite a bit of disassembly. Oh well.)

I’m really glad to have my 453 back in working condition. Aside from working again, the rebuild had additional payoffs. With everything new, the trace appears faster and settles down quickly. And it doesn’t grow dimmer for the first few minutes like before. Though not simple or easy, the rebuild went well. If you decide to do this, I suggest getting familiar with the circuits and to take your time. It took me a number of sessions working a couple of hours at a time. Document as you go! Don’t assume you’ll remember which wire went where or which wiring block slot this or that part was mounted to.

Thanks for reading! Feel free to post a comment with your experiences or any questions. (Comments are moderated, so it might be a few hours before they appear.)

Also, I have a set of parts for sale. This includes the high voltage ceramic capacitors, the three NE-2V (A2B) neon lamps and the grid bias control potentiometer (I always order extras, just in case). All for 30€ including post (Europe only!) If anyone is interested, let me know.

I posted a link to this post on the Yahoo TekScopes group. A number of people have responded and I want thank you all for your interest and comments. They are much appreciated.