Tek 485 repair and restoration

[Addicted2AnalogTek]

Nice... very good info here. (and good work!) 

 I recently got a 485 and a bunch of probes for a super-steal.  There is an issue on the inverter board though.  I completely isolated it from everything else and it still blows the fuse.  I tested every transistor, diode, and cap on it and found that the 32V diac is bad and am waiting for the replacement from Greece... 

The 4.7uf 35v tantalums were bad, so I replaced them with mil-spec 4.7uf 50V M39003s.  The 2uf 150V cap on the inverter board and in the main PSU were also bad so I replaced them with 3uf 150V Sprague 30Ds.  I went around measuring the ESR of every single cap in it, and for some odd reason nearly every single 22uf 15V tantalum was bad. 

Just as a precaution - and because I like to have spare parts - I ordered replacements for every semiconductor on the inverter board.  I'd rather have them while I can get the original Tek replacement parts than be stuck hunting them down or substituting later. 

I'll definitely continue following your restoration and will start a thread for my 485 next weekend (after I get my replacement parts).

 

[c4757p]

Interesting that the DIAC failed - was the resistor in series with it okay? I can't see too many possible failure modes for that.

This thing seems to chew through tantalums - some have already been replaced. Hence why I replaced the majority with good quality aluminum instead. I left a few alone, and there were a couple that I actually replaced with new tantalums, but most became Nichicon PW series aluminum.

I wouldn't be too worried about stocking replacement parts for semiconductors on the inverter board, there's nothing too exotic. For the DIAC you probably could have just used something like this. The concern I have is the vast number of custom Tek chips in this thing. I was careful to replace anything that bridged dangerous voltages or currents to the custom SMPS controller, and with well chosen replacements, because if something in there failed short I'd be pretty screwed.

Good luck with yours

I'll have some interesting bits when I get out of the "replace every bloody cap" phase, too - I've got some plans for this thing

[Addicted2AnalogTek]

You know, I didn't bother to measure any of the resistors on the board, but now that you mention it there are several that have been replaced with metal films instead of the carbon comp. I'll go thru and check them during the week and report back any interesting findings (if any)

  I'm a stickler for using original replacement parts (or higher grade very similar ones) so I have ordered Ohmite Carbon comps to put back in - and that is also why I found the original spec diac even though it was $5.   

I've never had to test a diac though, so it's possible that I did it wrong.  When I checked it, it gave me a reading of about 12V instead of the expected 32V.  It would open at about 29V and then erratically drop down to about 12V, and measured differently when I switched the orientation.  The only other part on the inverter board that could be faulty is the 110V zener (which I do not have a PSU that can output 110V to test it), but I bought a couple just in case.

I'll be going thru this thread again and deciding if I want to replace other components in the scope before attempting to power it up again. 

[c4757p]

I'm not one for insisting on original parts, though to each his own. I figure the engineers who designed it would have had no problem with putting in modern higher-quality parts, if those existed at the time.

I've never had to test a diac though, so it's possible that I did it wrong.  When I checked it, it gave me a reading of about 12V instead of the expected 32V.  It would open at about 29V and then erratically drop down to about 12V, and measured differently when I switched the orientation.

Well, it should allow you to put up to nearly 32V (they aren't typically accurate, 29V is fine) across it, and then once you exceed the trip point, avalanche and drop down to, yeah, about 12V. That's about right. I'm not sure what you mean by 'erratically', though - if it didn't do that every time or something, it could have had a bad internal connection.

It should be symmetric, so if it measured differently when you switch the orientation, it was probably bad.

[dave_k]

Great work! Just a note about putting images into the body of your post. Use the [img] BB code instead of the [url] one

[c4757p]

Great work! Just a note about putting images into the body of your post. Use the [img] BB code instead of the [url] one

Then they take too long to load. Image-heavy threads where people have large embedded images directly are seriously annoying.

[tautech]

Great work! Just a note about putting images into the body of your post. Use the [img] BB code instead of the [url] one

Then they take too long to load. Image-heavy threads where people have large embedded images directly are seriously annoying.

It's very nice the way you're doing the images. 
Nice little preview/teaser at the bottom of page and a seriously good quality full page image with the link.
That's the way you want to present them and it's classy IMHO. 

[Addicted2AnalogTek]

I'm not one for insisting on original parts, though to each his own. I figure the engineers who designed it would have had no problem with putting in modern higher-quality parts, if those existed at the time.

Touche....   however there are many semiconductors that are specially selected from the batches to be used for specific part numbers.  That's mainly why I always try to go for original Tek numbered parts.

Well, it should allow you to put up to nearly 32V (they aren't typically accurate, 29V is fine) across it, and then once you exceed the trip point, avalanche and drop down to, yeah, about 12V. That's about right. I'm not sure what you mean by 'erratically', though - if it didn't do that every time or something, it could have had a bad internal connection.
It should be symmetric, so if it measured differently when you switch the orientation, it was probably bad.

Well now I know how a diac should operate.  It definitely was not symmetric.  I also worded my description incorrectly. It did not avalanche erratically, but the LED I had connected in the test circuit began to erratically flicker when the diac was allowed to draw enough current to push my PSU voltmeters to about 55V (out of 64V).  I had a 1K resistor in series with the LED. 

I think I will have a closer look at the line filter board and likely replace any caps on it as well as have a closer inspection of the passives on the inverter board.   The fuse blew immediately upon attempted power up, so I'll check for any shorts to ground (as I already should have....).

[SeanB]

I always check old carbon composition resistors. They invariably have drifted, mostly higher in value and if they run hot ( the shiny surface as the wax is driven out to the surface) they will be out of tolerance at a minimum or up to twice the value.

[c4757p]

Perhaps I will check them more thoroughly. They have drifted a bit as usual, but the ones I checked had not drifted enough to make me uncomfortable.

[Addicted2AnalogTek]

so I managed to figure out the problem with my 485.  The 430uf 200V 36D caps are bad.  I was measuring them at the wrong freq - their ESR at 100hz is about 5ohms.  Acc to datasheet it should be nearer to 300mOhm.      I can't seem to find any that will fit properly that are in the range of 390uf to 650uf. 

Anyone have any thoughts about this?  Maybe these will work(?):
http://www.ebay.com/itm/New-2-pcs-of-Mepco-3186-series-780uF-200V-large-can-screw-terminal-capacitors/141044068674?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D20131003132420%26meid%3D7de746185dcc4ab19e4ea15e873671a7%26pid%3D100005%26rk%3D3%26rkt%3D6%26sd%3D130973530166

I also bought a bunch of Vishay ceramic caps and new resistors for the psu type circuits.  The scope itself is mechanically and physically in near mint condition, so I'd def like to find a pair of suitable caps.  If anyone has any sources please let me know.

[tautech]

I'd forgotten about Tedds:
http://www.tedss.com/Capacitors

You might be better off to deal with them direct.

[dave_k]

It's very much worth checking the carbon comp resistors. Most will have drifted up in value due to moisture absorption over the decades.

Take care when replacing them with modern metal or carbon film types. Some Tek circuits specify only to use carbon comp resistors because they are non-inductive and/or can withstand high energy pulses. Usually this will be indicated on the schematic.

[Addicted2AnalogTek]

thanks for the heads up.    I didn't know that Tedss was more than just an ebay seller - good to have another parts supplier on the list.

I always replace carbon comp resistors with carbon comps (or ceramic comp) in the critical circuits.  I usually only go for MF if I can't get the value I need otherwise, and in that situation I almost always buy KOA Speer with the highest voltage and temp rating possible for that wattage. 

I ended up dropping about $100 on all new ceramic caps, neons, and CC resistors - I bought enough to replace every last one in the 485 if need be.  I just didn't realize that I needed to replace the caps because I was still testing them at 100Khz  ( I use a FG and scope for ESR measurements).   

Once I find suitable replacements I will drop a link to them in this thread for future references.

[tggzzz]

I've got some plans for this thing

If all else fails, a "Tektronix 485 Oscilloscope 350MHz - fully working" went for £246 +£15p&p. http://www.ebay.co.uk/itm/Tektronix-485-Oscilloscope-350MHz-fully-working-/182058849812

No, I'm not thinking of selling mine, I like it too much

[Addicted2AnalogTek]

I'll be selling a couple of fully restored and calibrated 485's in the near future.  I'm not really sure what I'm going to be asking for them.  It will likely be one of the later SN range (B198XXX) and one of the mid SN range (B142XXX)

[c4757p]

Eh - haven't run into any issues other than bloody time. It's just waiting to be finished.

[tggzzz]

I'll be selling a couple of fully restored and calibrated 485's in the near future.

Our of curiosity, what do you mean (and not mean) by "fully restored and calibrated"? Traceable to some international standard?

If you just mean "twiddled with the internal settings", then what would be the appropriate words? My interest is as someone that has done that with a 465.

[c4757p]

To be fair, the vast majority of scopes are "calibrated" simply and roughly enough that I wouldn't really worry about traceable cal, just cal by someone who knows what he's doing and can follow the instructions in the manual.

[tggzzz]

To be fair, the vast majority of scopes are "calibrated" simply and roughly enough that I wouldn't really worry about traceable cal, just cal by someone who knows what he's doing and can follow the instructions in the manual.

I agree when it comes to using an oscilloscope for my personal use. But the context is about making statements describing a machine's condition when selling it.

That's different, and is the reason I asked what was meant by "fully restored and calibrated", and asked for alternative wording.

[c4757p]

Well, if we're going to argue about the definition of "calibration" I'd rather it be done in a different thread, if anybody cares. I'll probably come back here with more updates on this particular 485...

[tggzzz]

Well, if we're going to argue about the definition of "calibration" I'd rather it be done in a different thread, if anybody cares. I'll probably come back here with more updates on this particular 485...

Who is arguing? I'm not.

I did ask for a clarification of what Addicted2AnalogTek meant, and what alternative words people might have used in a specific context in which words' meanings matter more than in this forum.

Of course, if you want an argument about what we are discussing, the next question is whether that should be a 5 minute argument or a 10 minute argument: http://www.montypython.net/scripts/argument.php

[c4757p]

I was merely suggesting that you might be kind enough as to see I've created this thread with a specific purpose in mind, and save the side conversations/arguments/whatever for another thread, preferably in General Chat where it's easily ignored.

[Addicted2AnalogTek]

I'm definitely interested in how your 485 restoration comes along using the PW's as replacements.   I've recently run into the time constraints issue myself. I have these suckers just lying here on the bench with my faulty 106, and not the slightest clue when I'll have enough time to really do anything more.

I never intended this to be a thread hijacking, but it appears it nearly turned into it..

I hope you (and I) manage to slow time enough to get these things wrapped up soon!!

just to close out the "calibration question", I intend to take one of the four scopes over to my friend in the engineering dept of the local university to verify I have calibrated it to manual specifications - if I ever have the time to even calibrate them.  I don't "twiddle" with the internal adjustments now that I've got a slight clue what I'm actually doing.  There's 'calibration' and then there's 'traceable calibration'.