I have a Tektronix 475!

Well I have a broken Tektronix 475.. But it is getting less broken!

Why?

What is a Tektronix 475? Well it is a classic (1975ish) analogue oscilloscope. Pretty much the workhorse of its day and used for electronics development and testing. It's a beautiful and capable bit of gear that has 200MHz of bandwidth and some other cool delay and zoom features.

First of all you may be asking why I have one of these? Well I actually trained as a computer engineer so I did do a fair amount of electronics at university. As a teenager I messed around with radios and electronics as well as computers. Unfortunately I *hated* hardware at university!

I thought it was the the maths that put me off but now that I am getting back to it I think it was more that the teaching was atrocious. I was telling this to a friend of mine (Owen who is also a computer engineer from UNSW) and he was saying there was a 3rd year electronic subject where in the final exam he was able to answer maybe one and a half questions out of five and got a distinction! Essentially they scaled everybody just to get sufficient passes! I actually took all the software options from 3rd year up because I found the analogue electronics subjects so awful. I did try one subject in 3rd year but then quickly changed back. I too remembers some shockers in 2nd year where nobody I knew passed the exam but somehow we passed the course.

Well I've been playing around with this stuff again. I have been re-teaching myself circuit theory including AC analysis and basic building block circuits with transistors, op amps and FETs. I've been reading lots, watching a few youtube channels (like this guy http://www.eevblog.com/ Dave is awesome!) and reading a few forums.

I now have a cheapo Siglent SDS1062C scope, a SDG1020 signal generator and a (not so cheap) Agilent U1242B meter. I chose to get a decent meter as I think it is worth it for safety. I have a cheapo (unfortunately single channel) 30V/3A linear bench supply also and a collection of electronic components (many are quite ancient) plus a few breadboards.

Tek 475

I bought the scope as it was (a) broken (and I wanted to fix it!) and was (b) local so I didn't have to pay postage and (c) cheap! It turns on, the lights etc all seem to work and behave as you would expect but no screen display. The knobs etc are all in great condition and the front panel is clean and free of scratches and general cruft that you would expect on a 40 year old instrument.

Amusingly it comes with an extensive service manual (inch thick paper!). It's available online also from Tektronix (http://www.tek.com/manual/475). I also found another version that was edited by the US DOD and included more extensive troubleshooting guide and included waveforms.

I located the EHT circuit in the manual and looked for what components or areas could be faulty. I opened the thing up and looked around to see if there was anything obvious that might have died. At this point I was poking around with a meter while the unit was unplugged as (frankly) a couple of kV of energy scares the shit out of me!

The thing is a bit of a mechanical work of art too. There are a load of pots attached to long fibreglass extension shafts that connect them to the knobs on the front panel (presumably to minimise cable runs). Mostly the boards interconnect either using these long runs of right-angle pin connectors or using short coax connections but there are a few spots where they have just soldered something on. Also there are a load of components that look like afterthoughts tacked onto the circuit - like the capacitors around the high voltage area etc.

I had no luck finding anything obvious. I opened up the shields on the EHT area and I noted there were two neons in the high-voltage areas so I decided I would power the thing up and see if they glow. They didn't and in fact the -2450V testpoint was at.. well zero. Hmm.

PSU

I noticed that the high voltage depends on the 15V unregulated line, the -8v supply and the 110V supply. There are well marked test points for all the voltage rails so I powered on and checked each. The -8V was at +800mV! And the -15V was at -23V. The 15V unregulated output looked Ok as did the fuse that protects the power going into the primary side of the high voltage transformer.

Looking at the circuit, the chip containing the op amps that are the error amp for the -8V and -15V uses the -8V line as VSS. Ok so -8V is causing -15V to be wrong too. There is a diode (CR1468) clamping -8V from going positive so the 800mV is probably the diode drop.

Then I noticed the bridge rectifier for the 5V supply had been replaced. It was sitting above the board and had been soldered onto the top side instead of being soldered on the solder side. Also there was a link going to the filter cap for the 5V.

Turns out the filter caps have 3 legs as their can is grounded. Someone had replaced the 5V one (it was blue instead of silver) and they had bridged the centre leg spot with a link.

My best guess was the rectifier on the -8v line had gone tits-up. The main board is a nightmare to remove. 

I bought a replacement bridge from Jaycar and attacked the regulator with a pair of side-cutters (destroying it), When I got down to its legs I carefully desoldered them from the board and soldered in the new bridge slightly off the board. I turned the unit back and what do you know! -8 and -15 now work! But still no EHT :( Not that easy... Oh well.

I did a few more tests and 50V was more like 54V. The other supplies were all similarly off. The other supplies all you the 50V line as a reference voltage. I found a pot for adjusting the 50V supply and twiddled it until I got pretty close to bang on with the 4.5 digit meter (correct tongue angle was used).

The -8V is still a bit off (-8.3V) but the others are pretty close. I still haven't figured this out.

High Voltage Oscillator

The CRT circuit has an oscillator based on a transistor that switches the current through the primary side of a transformer. There is a feedback coil attached to the base of the transistor which switches the transistor off again. This change in current on the primary then induces a voltage in the feedback coil turning the transistor on again and so the cycle continues.

So my first suspicion was the transistor (of course) Q1318. Nothing else around there looked like it was having issues. The transistor was hard to find but it turns out it lives on a bit of metal down at the back underneath the main board and behind the vertical pre-amp board. The transistor looked ok on the multimeter (looked like two healthy diode junctions) but I read you can replace it with a bog standard 2N3055 so that's what I did! It was tricky to remove as I had to unscrew the preamp board and the corner of the main board and sort of bend them slightly to easy the box out. I put it all back together but no change.

While fiddling with it I noticed the graticule illumination was a bit dim. I checked the voltages and the +15V rail was in the toilet (8V). Damn. I checked a bunch of things, even disconnected the transistor but no luck. In the end I figured out I had squashed a ribbon cable when screwing the vertical board back down. I unscrewed it, pulled the ribbon out and then the short went away. In the mean while, while troubleshooting the problem I put a scope probe on one of the transistors in the 15V regulator circuit but I accidentally touched the earth ring on the probe tip to the metal transistor can. I got a small spark and the voltage *really* went off then. Luckily most of the transistors are socketed so I replaced it with a boring old BC557 and sure enough this worked.

Anyway - back to the *actual* problem! When I looked at the Q1318 collector test point on the scope, the it looked like the oscillation was just too small. It wasn't sinusoidal, the deflection was less than 2V and the frequency was only 30kHz instead of the 50kHz the service manual specifies. The average voltage was around 21V as expected (around the same as the unregulated 15V supply) but it just didn't look right. The only thing I had to go on was a very grainy photo of the waveform in the DOD service manual.

A bit stuck at this point, I talked to a friend about this and he offered to help me work on this. Graeme worked as an electronics tech for a long time and was excellent at breaking down the circuit to troubleshoot potential problems. The high voltage circuit includes regulation and will sample the -2450V output (via an impressive 24.5M Ohm ceramic film resistor) and adjust the oscillator accordingly. If the voltage is too high or low it alters the bias on the feedback coil of the transformer to suppress or enhance the oscillator. We suspected this was going wrong and clamping the oscillation down. 

The trouble was it all looked Ok. For a good while we couldn't figure out why the voltage swing on the oscillator wasn't bigger. We even tried temporarily adding more drive to the base to increase the current flow. This made the transistor warmer but the voltage swing was about the same.

Then Graeme figured out that if the transformer was effectively a dead-short then the voltage swing would be minimal

The high voltage output is fed to an EHT multiplier to generate the 15kV at the anode of the scope. I read somewhere that if the multiplier goes bad (and they do) that it will load up the secondary and the -2450V won't come up. The way to test this is to disconnect the EHT multiplier from the high voltage transformer. The next trick was finding it! Graeme guessed it was in this can below the main board. It looked like you could get to it but you have to remove the vertical pre-amp board. This had a couple of knobs, half a dozen or more coax connectors, two or three ribbons and the delay line. Annoyingly it was also soldered to resistors inside the front-end attenuators (bit probes plug into) and there was a cap soldered between the main board and the pre-amp board.

This thing also had these funky mechanical switches built right into the board. You can see the what look like tiny gold forks toward the top of the picture below. These all move when you fiddle with the bandwidth limit button. Note also the round black chips towards the bottom middle with square footprint? These are a custom ASIC! From the 1970s! That would have cost a bomb! Also the entire board is gold plated. It is a thing of beauty to be sure

Anyway we decided we didn't have the tools to pull this out and gave up for the day.

The manual listed the knobs as having a #4-40 1/8" set screw. Turns out this #4 is an American screw size, 40 means 40 turns per inch thread and 1/8" is the length in inches. #4 apparently has a hex slot size of 0.050" which is near enough to 1.3mm. I found a set of screw driver bits at Jaycar that had one of these and was able to undo all the set screws. I disconnected all the coax and desoldered what I needed to and removed the board from the scope. I removed the can around the high voltage transformer (bottom left below) and found that true to Graeme's intuition, the EHT multiplier was in there too.

EHT Multiplier is the black box on the left of the image below. Both this and the transformer were inside the can in the photo above.

I figured out which winding was which on the high voltage transformer and none appeared to be open. I decided to disconnect the EHT multiplier from the high voltage transformer to see if this was the problem (more desoldering).

I powered it on (with the veritcal board out of the scope) and it hummed a bit but nothing happened - no neons. I turned it off attached my DMM alligator to earth with the intention to check the voltage on the -2450 test point. I powered it on and as I brought my probe close it arced! So now -2450 appears to work! I turned it off, attached a scope probe to the collector of Q1318 to see what the oscillator looked like now and here it is! A beautiful sin wave with 30V amplitude!

Interestingly I got a bit of glow on the screen too which indicates the tube is essentially Ok. More good news!

So now I know what to replace but where do I get one? Some people make their own EHT multipliers as they are pretty simple - 3 voltage doublers back to back really. The trick is getting the right diodes/capacitors and then you have to pot the thing in epoxy else it will arc over.

Otherwise you essentially have to go on a transplant list and wait for another scope to die.

There is a company called QService that supplies parts for old (and even ancient) Tek scopes. They had no EHT multipliers in stock. I found a guy on ebay in Canada that sold some 475 parts and emailed him to see if he had this part. Turned out he did so I bought it off him.

That was a while ago however and I am still waiting for the part to make it via snail mail.

It's been a load of fun so far! I'm sure this won't be the last issue!

The story continues when I get the multiplier in the post!