Tektronix 475 Repair

[jack_elec]

Hi all,

I scored a Tek 475 scope from a car boot sale a few days ago for just £25 - I thought I'd take the risk as it was in immaculate condition externally and is a great scope. It was sold as untested, but I was up for a challenge.

Got it home, plugged it in and powered it on, a few fuzzy traces flashed up on the screen a couple of times and then it went blank - no response to front panel controls and there was a very slight smell of burning from inside the scope.

Pulled the cover off expecting to see something visibly burnt, but nothing seems untoward. Inside is in pretty good condition with just a thin layer of dust on some components. I powered it up a few more times with the cover off, and no burning smell was emitted, but still nothing appeared on the display. The only things that were working were the front panel V/div switch indicators.

I got the scope into the workshop today and had a chance to properly troubleshoot it. I checked the power supply rails first with the following results:

• +50V, voltage and ripple within spec
• +110V, voltage and ripple within spec
• +5V, voltage and ripple within spec
• +15V, no output (~26mV), measured almost dead short to GND
• -15V, no output (~0.45V), no dead short to GND
• -8V, no output (~0.73V), approx. 30? to GND

There is a problem with the +15V, -8V and -15V rails which is stopping the scope from working. As far as I can tell, a lot of the circuits in the scope are powered off the +15V/-8V rails, whilst I read somewhere that the -15V rail is only used for the external probe power option. This would tally with it having a much smaller series regulation transistor than the other rails.

To try and see if there was a short on one of the other boards which was shunting the rails to ground, I disconnected them as best I could without fully disassembling the scope:

• A8 (trigger gen & z-axis logic) - managed to fully remove/disconnect this board
• A7 (timing board) - fully removed
• A6 (fan motor board) - fully disconnected
• A4 (vertical mode switch circuit) - harder to remove, so I disconnected P300, P280 and P160 which appear to be the main (power) connections from A9 (interface board)

Powering up the scope each time after disconnecting each board in turn showed that the rails were still not there, so no luck.

A quick look at the schematic (diagram 11) shows that the op-amps which are used to regulate the -8/-15V rails are powered off the +15V rail, so, with the +15V rail being down, this could explain why those rails are not there either.

I then set about removing and checking the transistors around Q1448 (series pass transistor) for the +15V rail and checking them with my DMM in diode settings. Q1466/1444 (both 2N2222A's) checked out fine. I tried measuring the main pass transistor in circuit, but couldn't get reliable readings.

I have held off removing Q1448 so far as it looks as though it could be quite involved - but would it be recommended to give it a go and measure it out of circuit?

I am also a bit unsure as to what caused the original burning smell and why the rails failed in the first place.

Any help/guidance would be much appreciated. Also, is there anything to look out for in these scopes with their age (thinking along the lines of bad tantalums/electrolytics etc. which other Tek scopes have problems with).

Thanks in advance!

Service manual can be downloaded from Tek here: http://www.tek.com/manual/475

[tggzzz]

Tants decouplers going short-circuit is a strong possibility, but not the only possibility.

Have you followed the instructions in the service manual to help determine which rail is faulty? There should be a test point which gives a strong indication which rail is "out of balance" and drawing too much current.

I'd start by looking for all tant decouplers where the rated voltage is only a little higher than the working voltage (I've seen 15V tants on a 13V rail in a Tek scope!). If faulty, replace them with a similar value and twice the rating. There are several techniques for identifying shorted caps; I won't bother to poorly reiterate them here. However, I'll note that the scope will function with most of the tant decouplers removed. This means it is practical to use trial and error to "remove" several tants simultaneously on a suspect board/rail simply by lifting one leg.

With care, it is often possible to remove the tants without disconnecting and removing the boards.

[jack_elec]

Thanks for the tip regarding the tantalum decoupling caps - turned out that was the problem! Not too sure why I discounted them in the first place and went looking elsewhere.

C1448 (10uF/25V) tant on the +15V rail in the PSU section had failed s/c. It was the first one I lifted the leg of and tested, dead short, powered the scope on and it all sprang to life again! Bit of a fuzzy trace, but I suspect that could be down to the missing cap.

Going to replace it with another in the next few days and will probably replace the others on the PSU outputs too, just for good measure. I will probably also replace the main filter caps as they're looking a bit worse for wear and look like they might be starting to ooze electrolyte.

[velik_kazakov]

I will probably also replace the main filter caps as they're looking a bit worse for wear and look like they might be starting to ooze electrolyte.

+1  I have a short filter cap on +5v PSU and some tantalum caps on 475 that repair before 1-2 months. Think to exchange all filter caps when I have a time and to keep this scope. It work very well. Will try to persuade my wife that I need it too .

[Lorenzo_1]

I have a Tek485 that's been very useful. A good proportion of its occasional failures have been tantalum caps. I got a bit tired of stripping, diagnosing and replacing them so recapped the whole thing. It's been good since apart from one cracked ceramic resistor that was my fault. I have found it worth the effort/cost.

[tguillaume]

Hi, I have repaired a 468 some years ago.  In order to get the traces sharp I replaced all tantalum capacitors.  For the failing rails I would check the filter capacitors.  I also repaired a couple of 464 and there again I had to replace some filter capacitors.   I sold the 464 but I kept the 468 (I grew attached to it after weeks of troubleshooting) its traces are so sharp and it is a great scope.

Sent from my SM-G950F using Tapatalk

[cmcdan]

Hi, Just read your problems; I have(had) a 475 with a bad HV transf. This is fatal as it is welded in a compartment under the A9 pcb and Tek did not sell a replacement. So I converted my 475 to a Lab power supply! Cut the chassis down to 7"(cover too) and installed
5 3-terminal regulators and digital volt and current meters. I figured it would be a shame to junk a nice power transf,etc.
I have photos if anyone is interested. Clarence   cemcdan@msn.com

[Lorenzo_1]

I see Sphere.bc.ca have a Tek 475 power transformer available - don't know about the welding bit of the task, but perhaps yours could still be brought back from the dead if you wanted to.

[cmcdan]

shucks, I just wrote a reply and it disappeared. I referenced the HV transformer not the power transf. I don't think I could put it back after sawing i in two.... i see ads on eBay where a blob of light is on screen. I wonder if the potential buyers of 475's know that no intensity/focus control is an indication that the Hi voltage tap(about 300V pp) has failed. the old saw : a pic is worth a 1000 words.. how doi get one here?
The guy that made this article is a real photo nut.
i am saving his photos as I just bought an old 465 with missing parts; my stock(junk box) has a 1000 parts. I once fixed Tek stuff and sold them but too much work, no play... Clarence, 65 rears of electronics and counting...

[Lorenzo_1]

My mistake indeed - I was looking at HV transformers and happened across it. Thought it was HV too.