Saved a Tek 7503, Tek 455, and BK1420 from storage - Page 1
Forum section: electronics / testgear

author: Smokey
author_rank: Super Contributor
timestamp: April 14, 2014, 02:04:15 am
content: Hey guys.I recently saved a few scopes from their long exile in storage in the back of an office. My guess is they probably haven't been powered up in like 15-20 years.1) The first one is a Tek 7503 mainframe. I'm kind of excited about this one since I've wanted a 7A22 diff amplifier plugin to play with for a while now, and I finally have the mainframe to use it.http://w140.com/tekwiki/wiki/75032) The second is a Tek 455.http://w140.com/tekwiki/wiki/4553) The third one is a really odd tiny portable BK Precision 1420. It is about the size of a modern bench multimeter (like a Keithley 2000) and has a little 2in x 2in display. This is more of a novelty.http://www.bkprecision.com/downloads/manuals/en/1420_manual.pdfSo here goes the question since some of you guys are vintage Tek experts......How do I go about powering these things up considering they haven't seen an electron in 10s of years? I remember reading somewhere (and it makes sense) that I don't want to just jam the plug in the wall and pray the magic smoke stays in. Are there caps that are pretty much guaranteed to have gone bad in that period of down time? Should I ramp up the voltage through a Variac and watch the current?Anything else I should checkout/replace before powering them up?

author: GiskardReventlov
author_rank: Frequent Contributor
timestamp: April 14, 2014, 05:07:18 am
content: Quote from: Smokey on April 14, 2014, 02:04:15 amI'm no expert but doing a visual inside couldn't hurt. And I recall reading about the variac idea on these forums once.Were they at no cost to you? Let us know what happens.

author: Smokey
author_rank: Super Contributor
timestamp: April 14, 2014, 05:17:06 am
content: They look really clean all over. Only minor plastic damage to the faceplate of the 455. I didn't see any components that were obviously damaged.Yup, free to me.

author: tautech
author_rank: Super Contributor
timestamp: April 14, 2014, 05:25:36 am
content: I would use a dim bulb tester, which is just a incandescent bulb in series with the mains.Say a 100 W bulb. It will flash bright on power up, then should dim if nothing is badly wrong.It would be wise to check all PSU's for spec and ripple after a visual inspection.Dust loading on PCB's needs to be checked and cleaned also.That's the minimum I would do, and there are a few more things, some specific to each scope that others may recommend, Tants, HV resister dividers etc.

author: PaulAm
author_rank: Frequent Contributor
timestamp: April 14, 2014, 03:04:01 pm
content: I wouldn't bother with a variac; a dim bulb tester won't hurt, but it's probably overkill.The biggest problem with that vintage Tek equipment is filter caps failing open with resultant large amounts of ripple on the supply rails. This generally screws functionality, but doesn't damage anything. If something's shorted, you'll get a blown fuse (make sure the correct fuse is in place though)I just got 3 7603s the other day. I just plugged them in and turned them on. They all had issues, but only one seems to have a power supply problem. All of those had been in storage for at least 5 years.

author: tautech
author_rank: Super Contributor
timestamp: April 14, 2014, 03:15:18 pm
content: Quote from: PaulAm on April 14, 2014, 03:04:01 pmOverkill? Its a MUST for ANY unknown old gear until one can be sure it is safe and sound.Last thing you need is for the magic smoke to escape. :-)

author: PaulAm
author_rank: Frequent Contributor
timestamp: April 14, 2014, 04:26:40 pm
content: In a general sense, I wouldn't disagree with that. I'm just saying that for Tek gear of that vintage I have not found it to be necessary and I've run multiple dozens of 5000, 7000 and 4xx scopes through my bench.I would have a greater concern if the dut had a switching ps, but Tek linear PS of that era have common failure modes that do not result in the  destruction of the scope if you just turn it on, provided the correct fuses are in place.Not saying it couldn't possibly happen and if you feel safer using a DB, by all means do so. Personally, if that 7000 scope has a linear PS, I'd just plug it in and see if it works.

author: Seg
author_rank: Regular Contributor
timestamp: April 14, 2014, 09:41:11 pm
content: Quote from: tautech on April 14, 2014, 03:15:18 pmSpeak for yourself. Some of us like a challenge!

author: tautech
author_rank: Super Contributor
timestamp: April 14, 2014, 11:16:41 pm
content: Quote from: Seg on April 14, 2014, 09:41:11 pmI was really just worried about the OP and his name. Smokey.........

author: GiskardReventlov
author_rank: Frequent Contributor
timestamp: April 14, 2014, 11:52:41 pm
content: Quote from: Smokey on April 14, 2014, 05:17:06 amCan't beat that price, they look well preserved too.

author: Smokey
author_rank: Super Contributor
timestamp: April 15, 2014, 05:45:22 am
content: The 7503 is alive. It pulls about 1.6Aac and didn't let out any magic smoke.Both vertical plugins appear to work correctly, but the horizontal timebase trigger plugin looks a little out of wack.The trace is only about 1inch wide and the triggering didn't seem to work right. Now there is a chance I'm just being stupid here and everything is fine, but I ran out of knobs to turn and its still a little tiny trace. Ill post a picture tomorrow.

author: Smokey
author_rank: Super Contributor
timestamp: April 15, 2014, 05:27:56 pm
content: This is what the little trace looks like... It's just displaying the 60hz pickup.

author: David Hess
author_rank: Super Contributor
timestamp: April 15, 2014, 05:38:03 pm
content: The sweep does not appear to be triggered either and the mainframe is unblanked for whatever reason. It could be a problem in just the 7B53A or the mainframe as well.

author: PaulAm
author_rank: Frequent Contributor
timestamp: April 15, 2014, 05:42:28 pm
content: Could be the H plugin or the H amp.Pull the sweep plugin and move the center V amp to the right hand slot. You now have an X-Y configured scope. You should be able to move the beam from top to bottom with the position control on the left plugin and right to left with the control on the right plugin. If you can do that, you have a bad sweep plugin. If a plugin will move the beam full screen in one slot and not in the other, you have a problem with the associated amplifier. Be sure the V mode selector is set for the first slot.Keep the intensity turned down so the beam is just visible so you don't burn the screen.Also doesn't look like the sweep is being triggered and there may be a visible retrace. You may need to find another sweep plugin.Edit: If you can't move the beam to the edge of the screen with either channel, check the PS voltages in the mainframe.One of the cute tricks you can play with a 7000 series is to put sweep plugins in both the V and H slots to get a raster display which you can modulate with the Z input.

author: David Hess
author_rank: Super Contributor
timestamp: April 15, 2014, 05:55:10 pm
content: Quote from: PaulAm on April 15, 2014, 05:42:28 pmThe trigger signal from the horizontal plug-in could be missing for whatever reason but the internal trigger path by itself cannot unblank the display so in addition there has to be a problem with the 7B53A or the mainframe.QuoteI start by defocusing the beam when doing this if I remember. This is a good idea whenever fooling with the z-axis circuits to prevent damage to the CRT phosphor.QuoteThe trigger indicator uses a grain of wheat incandescent bulb and could be burned out but the lack of blanking tells me the sweep plug-in is bad.QuoteThe 7000 series is amazingly flexible. Besides the vertical output, my favorite capability is dual independent sweep and alternate dual sweep. I do not know of any DSOs which can do that unless you buy two of them.

author: Smokey
author_rank: Super Contributor
timestamp: April 17, 2014, 07:59:32 pm
content: UPDATE:I swapped the 2nd vertical plugin with the horizontal and I was able to sweep the trace full screen in both directions.Then when I plugged the horizontal back in, it worked correctly for a little while then went back to the same small trace thing. So it's intermittent, which is always fun.What is more likely, bad connections on the knobs on the front of the plugin or bad connections on the backplane connector?Another interesting thing is that the intensity knob on the top right corner of the mainframe is a little loose and wiggling it around makes the trace move horizontally. Not sure what's going on there.

author: GreyWoolfe
author_rank: Unknown
timestamp: April 17, 2014, 10:45:21 pm
content: I would hazard a guess at the usual suspects. Make sure the backplane connectors are clean and do the same thing for all the rotary controls for the plugin. I would also open it up and look to see if there are any flakey components.

author: Smokey
author_rank: Super Contributor
timestamp: April 25, 2014, 05:55:30 pm
content: Bad News.I got a replacement horizontal plugin, and it does the same thing with the small trace like from the picture upthread. Looks like I'm going to have to start digging in deeper to the mainframe.Add one more thing to the project list I guess........

author: David Hess
author_rank: Super Contributor
timestamp: April 25, 2014, 06:07:35 pm
content: What kind of replacement horizontal plug-in did you try?I am thinking that maybe the load presented to the power supply by the full set of plug-ins including the 7B53A is too high because the input filter capacitors are marginal. That is easy enough to test by checking the power supply voltages with a full set of plug-ins installed.

author: Smokey
author_rank: Super Contributor
timestamp: April 25, 2014, 06:54:06 pm
content: The replacement plugin was a 7B50.Great idea about the full loading! That makes sense why the two verticals would work since the middle slot was empty. I'll try that when I get home.

author: David Hess
author_rank: Super Contributor
timestamp: April 25, 2014, 07:14:53 pm
content: Quote from: Smokey on April 25, 2014, 06:54:06 pmJust install the 7B53A into the middle slot then for a full load test. Measuring AC ripple on the power supply outputs should be sufficient but I often check the power supply inputs as well after estimating what their ripple should be just to see if the input capacitors have become marginal without measuring them directly which is more difficult.If the power supply is marginal, then it is not uncommon for some plug-in combinations to present too much load leading to problems. Some plug-ins are worse than others in this regard and so much so that it is possible to exceeded the rated power supply output and/or the mainframe cooling capacity with some configurations.With that said, any combination of 7A18s and 7A22s and a 7B53A or 7B50 should work fine in a 7503 despite it being one of the early 7000 mainframes.How did you end up with a 7B50? Its 100 MHz replacement, the 7B50A, is much more common as it was produced as a less expensive alternative to the 100 MHz 7B53A for a long time. The companion to the 7B50, the 7B51, was only produced for a short time and then both were replaced by the 200 MHz 7B70 and 7B71. One of my 400 MHz mainframes came with a 100 MHz 7B50A which I paired with a 400 MHz 7B85.

author: Smokey
author_rank: Super Contributor
timestamp: April 25, 2014, 07:31:31 pm
content: Full disclosure: I don't know anything really about tek mainframes.I just found a horizontal plugin on ebay that was local and drove over and picked it up for 25 bucks.

author: David Hess
author_rank: Super Contributor
timestamp: April 25, 2014, 07:45:45 pm
content: Well, assuming the 7B50 works, there is no reason to prefer the 7B50A to it. The 7B50 is just older.Besides mechanical differences, the 7B50 uses tunnel diodes for triggering probably making it higher performance, has a somewhat odd combined level/slope control which was shared by the 7B51, 7B70, and 7B71, and includes XY mode standard which is optional on the 7B50A and the 7B50A only supports XY mode from an internal source anyway. This means you can use the 7CT1N curve tracer with the 7B50 (at least theoretically) but not with the 7B50A.I kind of wish there had been a 7B51A to go with the 7B50A but understand why there was not.

author: Smokey
author_rank: Super Contributor
timestamp: April 26, 2014, 08:00:26 pm
content: Removing the middle vertical plugin made the horizontal kind of work better for a while. The trace covered the whole screen like it's supposed to, but the triggering didn't seem to want to work right. Then after a while it went back to having the same small trace thing like when all three plugins were installed.Which power supply voltages were you talking about checking ripple on? I checked some of the regulated voltage test points (+15, -15, etc) that were easy to get to and they all looked good. That mainframe is seriously intimidating to take apart with all the tied wire harnesses running all over the place. It looks like one of those things that I'll never get back together correctly if I start to take it apart. And it looks like you have to dig pretty deep to get to the AC power supply boards.

author: David Hess
author_rank: Super Contributor
timestamp: April 26, 2014, 08:43:58 pm
content: Quote from: Smokey on April 26, 2014, 08:00:26 pmI do not have complete 7503 documentation but I do have documentation for the 7403 which should be practically identical. The 76x3 series is very similar as well.There are a set of test points on the right side toward the rear as you face the front for the +/-50 volt, +/-15 volt, +5 volt, and +130 volt supply outputs shown on page 5-14 of the linked PDF. From your description, at least some of the DC outputs are sagging but I would expect that to be accompanied by significant ripple. The -50 volt supply is the reference for all of the others so if it is low, none of the outputs will be correct.I am including cleaned up schematics that I made for the 7503 in PDF form as attachments (*) and here is a link to the 7403 documentation as well.http://bama.edebris.com/manuals/tek/7403n/If you know where full documentation for the 7503 is, I would be interested.(*) The misbegotten forum software puked on the attachments and destroyed my post in the process so they are not included.

author: Smokey
author_rank: Super Contributor
timestamp: April 27, 2014, 02:53:16 am
content: Cool. I'll take a look.

author: Smokey
author_rank: Super Contributor
timestamp: April 28, 2014, 04:26:56 am
content: sooooooooooo....I looked through a box of data books that I rescued from the same office that the scopes came from, and check out what was in there.Two full manuals for the 7503, and manuals for the vertical and horizontal plugins it came with. I'm not sure what the difference between the two 7503 manuals is.I checked the main supply voltages and they all appeared to have ripple within the spec in the manual (5-10mV). The ripple didn't really change with two or three plugins installed.One place I did find some issues was on the "z-axis and high voltage oscillator circuit board". There is a "-1.5V" test point that measured -6.5V. There was about 125hz 0.5Vp-p ripple on that -6.5V. I included a crappy cell phone picture of that section of schematic and the layout. The resistors and caps all seem to be working around that circuit. Everything looked clean and solid (At least per the schematic) up to Q756.Some of the other voltages I noted as pretty off are:TP760=-7.13V (0.6Vpp ripple at 125hz)TP761=-10.75V (with 2.5Vpp ripple at 77khz)I don't have time to dig in deeper tonight. I'll poke around some more later.Looks like I might have some scanning to do if a full manual isn't already anywhere on the interwebs.

author: MickM
author_rank: Regular Contributor
timestamp: April 28, 2014, 06:57:53 am
content: Hi;I have a B&K 1420, if you want to sell yours I would be interested.I live near Seattle, WA.Mick M.

author: David Hess
author_rank: Super Contributor
timestamp: April 28, 2014, 07:00:18 am
content: Quote from: Smokey on April 28, 2014, 04:26:56 amGood deal.The 7403 is close but based on the photo, Tektronix added a lot of test points.QuoteDid the DC levels change?QuoteThe 125 Hz ripple is because the supply for the high voltage regulator is unregulated.QuoteI am sure Artek Media has it but I have not found a free service manual scan for the 7503.The basic design is the same down to the component values but from the looks of it, the high voltage oscillator connection to the power supply is different. On the 7403, that -1.5 volt test point is the ground return to the power supply which should be close to zero volts. Here it must return to a different point.

author: Smokey
author_rank: Super Contributor
timestamp: April 28, 2014, 07:04:23 pm
content: When I was talking about the difference, I was talking about the two different manuals I have for the 7503. They both appear to be instruction manuals. I didn't see a revision number, but I probably wasn't looking hard enough.The DC level didn't change really. I don't remember seeing a tolerance range for those voltages, but once again I probably wasn't looking hard enough.http://artekmanuals.com/manuals/tektronix-manuals/I was shocked at how reasonable the prices for those manuals are. The service manual for the 7503 is only $7.50 USD. I guess I'm used to scrolling through the $75 dollar manuals on ebay.I think I'm going to hold on to the BK Precision 1420 just to enhance my vintage test gear nerd factor. I know it might have a better home somewhere where it could actually get used, but I can just imagine showing up to a field job and pulling that thing out of the bag. Hilarity would ensure... especially if it was enough to get the job done.

author: David Hess
author_rank: Super Contributor
timestamp: April 28, 2014, 08:56:11 pm
content: Quote from: Smokey on April 28, 2014, 07:04:23 pmThere may be a difference in the back where the change notices would go but I think Tektronix would just include the most recent manual. More likely there was a second 7503 or an extra manual set was purchased.QuoteBummer. That would explain the problem and be relatively straightforward to diagnose and repair.Note that when the high CRT voltages drop, the deflection sensitivity goes up and the brightness goes down. Maybe there is just a loose connection between boards.QuoteThey have excellent prices. I figured they have the 7503 manual in electronic form but I only buy the ones for equipment I have. I am unlikely to acquire a 7503 to go with my 7603 that I have had for ages.QuoteI figured the Tektronix 455 could fulfill that role.