Tek 475A with DMM44 service manuals / schematic

[calin]

Back in business .. found the short. Now I just have to get to replace the power transistor on the 5V because is busted

What I did  ? Somewhat of a 50-50% between being smart and brutal. I took the time and disconnected all modules where the 5V rail was going - still shorted. Then I took all current sensitive parts of the 5V power supply out (all semiconductors and circuits are socketed) - still shorted. At this moment i decided that I've been nice enough .. time to blow the sucker up (I mean short). I put the bench PSU in parallel .. can do 6A @ 30V this way; voltage @ 5V and connect between chassis and the 5V test point; ramp current to 3A first .. just hot wires. Then ramp voltage up a tad more ... somewhere between 7 and 8V puff .. hiss .. smoke and short gone .  So 3Amp @ 8V did the required melting pretty quick.  Measured the 5 v rail .. NIIICEEE NO SHORT !!

What it was ? In some places Tek used some bare braided/stranded wire to connect grounds. A strand of wire from one of these got spliced out and I think while I was turning the scope around on my bench somehow that touched a solder joint on the 5V rail and because of the short it got itself literally soldered on that joint. It was on the un-populated part of the main board which was facing the interior of the scope and could not see it ... As you can see it took quite an hefty amount of power to get it busted out.

However the initial short has taken out almost the entire 5V power supply, opamp, transistors are gone .. it even busted the feedback transistors on the 15V rail (that uses the other half of the opamp used on the 5V). Well I changed the opamp, then the feedback transistors 15V rail is back on, 5V one not yet because the power darlington used on the 5V rail is busted (CE somewhat shorted out) - I get  only around 2V on that rail.

At least I am back on track !!! and I learned a lot. Now I know how a series regulated power supply works .

[calin]

TIme for an update ..  and to ask for some advice from more experienced peoples.

So I got all power supplies back running smooth after the dreaded short that messed up things. I almost replaced all components on the 5 and -15 volt supplies - everything is in spec .. I have 49.999 flipping to 50.000 V on the 50 V rail with sub 2mV ripple. Yeah I replaced all old opamps with new ones, transistors .. all dead caps etc. Power supply section is ironed out now no more death after X minutes. Yet I have no CRT turning ON yet

So I debugged and digged out all I could in the HV supply and Z-Axis circuits. Remember .. I had to sit on the lower voltage side as I don't have a HV probe to stick my fingers in the -2.5Kv and 15Kv areas. 

So far  I am sure that the HV oscillator works; I tested all caps, diodes and transistors all OK; voltage measurements and waveforms match the service manual very close. The thing is "humming" at a nice 51.2 ish Khz. I am almost 100% that this area works.

The -2450V supply is also looking good - not measured but I took all HV diodes out , caps measured , other diodes measured all checks OK. Again I did not measured the -2450V supply. HV diodes are fine , tested with a power supply they are not shorted and they both open somewhere between  9-10V. Also I am sure that the wingdings on the HV transformer are not shorted because I can see waveform from the output winding (there is one that gets out @ about 140V) that is used for Z-Axis and waveform matches closely what the service manual says. Like I said I am not 100% here because I cannot measure the actual -2450V.

Z-axis, measured and confirmed all working. Again i tested every single bit of part except resistors, measured voltages and waveforms. Everything by the book there.

At this stage I am thinking that the HV multiplier circuit - the 15KV one - is gone. I can hear a typical CRT "turn on" buzz and when I turn it off I can see the two neon bulbs flashing for a split second. CRT still does not turns  on .. not even a spot or faint trace coming on the screen.

But like I said, I am not a master in this so some experienced advice wold be welcome. I wonder if there is anything more I can fiddle with on the low voltage side? any idea wold be welcome. If not .. I am going to borrow a HV probe from somewhere to measure thing.

[calin]

Most likely the final update .. i found why the CRT is not turning on ... cracked .. or is should say broken in two. I don't know what to say but I think this is going to remain as a possible organ donor for another machine .. sad

Like I said, i rebuilt the low voltage power supplies are are spot on, I even tested the HV section and all is nice and dandy -2450V measures -2448 , the 15500V is 15487V .. everything looks nice. Unfortunately a replacement CRT will be much more expensive than the whole scope .. remember I paid 25$. 

Crap .. I really wanted to restore this thing back to its glory just for the cool factor .. Time to go on the "maybe one day" pile

At least in the process I got an Fluke 80K-40 HV probe for my 87-V ... man is nice the buzz of 15KV before you touch them .. with the probe of course !!!

[johansen]

i have a 475 you can have for a few bucks.

it decided to eat all the tunnel diodes on the trigger board.. so, i would swap out the whole board if i were you.
i was too cheap to buy a new trigger board off ebay for 80$ last time one was for sale.. even a parts unit costs more than that now.

given the amount of re-calibration that is required after swapping out CRTs, i wouldn't bother..

[menkelis]

I would also recommend checking out http://www.vintagetek.org for information.

There a few of the old timers there that really know the in's and out's of repairing the
old Tek scopes.

[calin]

I-ll keep my eye on Craigslit or ebay maybe an cheap CRT or dead 475 shows up somewhere.

Like I said, I did this for the journey and for the cool factor; I won't mind the calibration part or even if the thing never gets back to being "spot on". Best part of all is that I learned a lot, The old Tek manuals are literally treasures ... I learned a lot in the process about power supply design, coupling and HV circuits which I never touched before. I was too much of an idiot and the only thing I had towards HV stuff was "fear & respect"   . Especially the principle of operation sections are really really good.  Having a "patient" to dissect is nice .. not to put in balance the nice feeling you get when at least some part of it comes back to life.

BTW, the CRT had a sticker that said "repaired in Sept 17, 1979 by MS" .... cooooolll eh ? almost as old as me.