Old Tek 577 Curve Tracer return to life story

[mawyatt]

We've been looking into ways to create some transistor and FET characteristic curves which led to the post about using a AWG and DSO for such, and after reviewing many DIY setups starting developing our own version. Along the way a post about a ~$800 Chinese Curve Tracer showed up and jord4231 did a nice teardown & review, the build quality and attention to detail wasn't impressive, so we abandoned this item and began looking for an old Tektronix Curve Tracer while continuing the DIY route.

The Tek 576 is too large & heavy and we decided to focus upon the 577. The refurbed ones were expensive and the modest costs ones were in really bad shape from the eBay pictures and seller responses. We found a Tek 577 D2 locally that was offered as for parts, it didn't work but the CRT beam showed up as a spot. We made an offer based upon on-site inspection and picked up the 577 last week. The salvage company came from New Mexico and suspect this 577 came from Sandia Labs.

The unit was dirty, had multiple broken knobs, nothing seemed to work, and the unit had been modified. This brought back old memories as a kid when we repaired items to support school, with a scope and DMM you could just about fix anything!!

After a good cleaning we staring in on the debug, first off was the PS voltages and they looked good with little ripple and within tolerance. Then we spent some time studying the schematics, the design is very unique in some ways and likely the only solution way back, and the build seems to be 1984~5. We removed the modifications to get the unit back to square one for starters and began signal tracing. This proved to be difficult but we did uncover a couple bad op-amps which were replaced with what we had available (MC1458 replaced with LM358 and MC1456 replaced with OP-07), later we found a MC1458 in place of a MC1456. With these op amps replaced things began to become somewhat traceable except the Base Step generator wasn't behaving. After quite some time it was discovered that the Step output wasn't getting to the 177 fixture and wiring to the various connectors and then the PCB trace meanders all over the PCB underneath the complex custom switch fixtures. Fear set in that the switch fixtures were bad, these are likely unobtainable!! After more study of the schematic, layout, we decided to inject a signal to trace thru this maze, with same result. It turns out after many hours of head scratching, a few beers and choice words the P429 connector was installed backwards!!

Shame on Tek for not using a keyed connector as the P429 is a shielded cable connector has the shield is connected to the thin trace that meanders all over and the signal lead connects to a very large PCB copper plane that looks just like a ground plane that covers part the custom switch bottom!! What this is doing is providing a common mode distributed signal to the switch bottom to minimize the parasitic switch capacitive effects by means of minimizing the differential voltage across the parasitic capacitances. The cable shield actually goes to the output of a unity gain buffer amp which has a discrete dual JFETs input sensing either ground (Base Voltage Mode) or the Step Base signal (Base Current Mode) to minimize bias currents, so the cable shield is bootstrapped to the signal level to eliminate leakage currents, both galvanic and displacement (the 577 has a collector current range of 2na/div to 2 amp/div)!! The amp output also feeds a positive feedback signal to the driver amp (op amp and npn and pnp darlington buffers) to raise the driver output impedance for current source use. Very clever design indeed, but somewhat difficult to trace around!!

With the P429 connector cable properly labeled & installed and we have a working Tek 577

A trip to hardware store to find some replacement knobs (still need to get another for Collector %) and now on to the lengthy calibration procedure!!

Anyway, this was a fun (sometimes not so fun) endeavor to recover this old Curve Tracer, after a quick preliminary cal we now have some nice transistor curves in both forward and reverse operation

More to come later if folks are interested like custom fixtures for 177 since these are very scarce and expensive!!

Best,

[KE5FX]

Wow, I see what you mean about P429 (took a quick look under the cover on my own 577) -- they don't even have a triangle marking on the PCB for that one.     That would be a good interview problem for a technician.  Turn that connector around and see how long they take to find it.

I wonder if the opamps were really bad, or if the sockets were flaky until you wiggled them around in the process of replacing the chips?  Tek didn't do anybody any favors with those cheap DIP sockets.

[mawyatt]

Yep, that certainly thew a curve in our troubleshooting, I mean you actually have to see the PCB & P429 to believe it

Agree, the IC sockets are not very good and likely the op-amps may be OK. The 1458 in the 1456 socket is questionable tho, but we'll toss all in the trash rather than risk them finding there way back into something later.

Find the actual design quite fascinating and Tek created a Howland Current Source for the Base drive when the + feedback amp U380B input comprised of Dual FET Q450A & B is not grounded, as well as the "Floating PS" for Q384 and Q386 with the driver amp U380A. Clever designs indeed, but no excuse for Tek with the implementation of the P429 connector!! We just painted one side of the connector Red and the PCB large land with a Red + Sign near the connector, to spare ourselves or someone else in the future from "rediscovering" this "feature"

Did you do the full calibration yourself? We lack a few required items and may make do with what we have on hand at the moment.

Best,

[mawyatt]

Did a quick limited cal with what we have available, and so far so good!!

Still have some low current/voltage waveform and intermittent issues to chase down with the Step Generator Switches, which are the ones that are behind the main PCB as Murphy would have it

Made up a couple clips for use until we can get a proper setup fixture designed and built up.

You can tell with these old instruments from Tek and HP that they were designed by engineers that actually used them, the feature set and mode of operation convey such, and weren't designed to a cost target!! They even show how to defeat the safety interlock on the 177 manual by cutting a jumper and soldering in another jumper, although they do highly recommend against such!! Can't imagine that happening today for all the obvious reasons.

[KE5FX]

The bit about the interlock is weird.  They originally had a button you could hold down to override the interlock, but they took it away at some point.  Then, later (?), they added that page describing how to defeat the interlock, while saying "Don't do this, but..."    Would love to have been a fly on the wall in those meetings. 

My 577 was owned by an ex-Tek engineer (WA7TZY, SK sadly) and seems to be in good fettle, calibration-wise.  It still had calibration stickers when I bought it.  I was thinking that it might be nice to add PC support like the setup offered here for $4K... although I just noticed that he's selling the USB interface itself for what sounds like a fair price.  Wonder if anyone around here has any experience with that?

[mawyatt]

The 177 we have has a red override button, seen in lower left corner of 1st image above. However, it has been disabled by someone, likely same folks that added the modifications. We restored the push button override but did not enable the bypass modification per the Tek manual. 1600 volts is a lot of voltage to be floating around on the unprotected DUT test sockets, so we'll use the override for now.

There's a nice spot in the case where the Storage PCB mounts (we don't have the storage option) that would be a good place to mount a custom PCB for an external interface.

Best,

[precaud]

Nice work. Unless it was owned by a subcontractor, it is quite unusual for Sandia surplus to show up for sale in FL, and also unusual for it to be in such rough shape. They normally take very good care of their gear. (Mine came from Sandia too.)

I see your 577 has a variation of the "looping" problem, where sometimes the retrace doesn't overlap the forward sweep. Mine does as well sometimes especially at low currents. The looping control has no effect on it, and there's nothing in the cal instructions to adjust it out. Curious if you have any thoughts on it.

[mawyatt]

I mentioned Sandia because the Surplus Company came here from New Mexico and they said they had lots of equipment from Sandia and Los Alamos labs. The damage could have been in prior shipping, the broken knobs appeared to be shipping damage rather than use damage.

Yes we noticed the "Looping" issue, especially on the lower ranges, and have not figured out how to correct such. If you or anyone has any ideas we welcome any advise.

BTW the 577 mod we removed was sampling the drive signals to the H and V high voltage driver amp PCB, these two signals were brought out under the chassis with coax cables and BNCs labeled X and Y axis. Suspect this was to allow plugging into a standard DSO to get screen captures. The other mod was on the 177 where the HV override switch was defeated, we returned this to active.

We've been considering developing an ADC interface to read the X and Y axis from a RPi so we could have the curve trace data available. Our software development skills are limited tho, so this may pass.

Anyway, it's good to find others that have these 577s, they are true Masterpieces of analog design from that era, typical of Tektronix back then!!

Best,

[mawyatt]

Took a look at the signal from the X and Y 577 Driver Amp to see if one could use a DSO to "Capture" the 577 Screen. The result is inverted in the X axis, and if we could figure out how to have the SDS2000X+ plot invert the channel signals using the XY Mode this could be flipped over.

We could use a diff probe on the X Axis as shown and get the inverted result as shown, but would be nice if you didn't need to do this if the DSO allowed XY Plotting of inverted signals (hint Siglent) 

Added a couple plots of a 2N3904 NPN and 12V Zener.

Best,

[KE5FX]

The eBay seller is apparently willing to offer the USB interface by itself for $960, which strikes me as a decent deal.   A lot would depend on whether the software is any good, and whether it's a proprietary "roach motel" model that keeps you from exporting or working with the data effectively.  The listing gives no technical info, which is obviously not a great sign.

Something I'd want with a computer interface would be a way to read the control settings to allow the graph to be labeled.  Monitoring the voltages and currents seems like it would be possible at first glance, but the 577 has, what, 9 decades of control range?  Again, it seems like the graph in the eBay listing has some metadata but they tell you nothing about it. 

[mawyatt]

Took a look at that and didn't feel they actually were reading the 577 settings. Looking at the 577 schematics in detail revels it doesn't offer a simple means to read the switch settings as no processor or other "smart" device appears, and no outside comm ports either.

Our thought would be to utilize an RPi and need to keyboard input the various settings for the curve display if we decide to move ahead on the project which we are looking into now. This would be a custom PCB mounted to the available ~120mm by 60mm standoffs where the storage board resides (we don't have this option). The design would allow external DSO (with proper phasing) use with XY Mode and support an external or internal RPi utilizing a 12 or 16 bit ADC interface with proper scaling, offset and impedance levels. Internal comm to the RPi would be by wireless means, or thru the standard RPi 40 wire ribbon cable, or with HDMI and USB for internal RPi.

We'll see how this turns out, let us know if anyone is interested.

Best,

[PaulAm]

The 576 has all of that information available since they use it to drive the fiber optic display next to the screen.  The 577 just uses your eyes and the legends on the front panel.  You'll have to enter the scale information manually unless you hook up a camera ...

Amazingly, your left/right switch is intact.  They are almost always broken.

[mawyatt]

We looked into the 576 but it's too heavy and bulky for our use, actually the 577 is too big & heavy but we'll try and make room for it by moving the little utilized at the moment SAA 3021X+.

A few 577 knobs were broken, and the toggle switch cover was missing which we replaced with a thin orange rubber sock we found at Ace Hardware, along with some replacement knobs.

The more we think about it, the idea of including a digitizing capability into the 577 seems less useful since we'll need to input all the settings to create the computer screen version of the DUT Curve Trace, which kinda defeats the idea. An iPhone camera 577 screen image edited with added text as required seems like a better solution at the moment.

The hardware design is straightforward, just a couple voltage scaled differential amps sensing the V and H CRT drive signals with polarity inversion relays or SS Switches and a dual channel input ADC.  Synching up the scans might be tricky but doable as they are synched up with the Mains Line.

Anyway, some family issues just resurfaced and will prevent us from focusing on this potential project at the moment, so will make do with the iPhone camera and post editing for the DUT Curve Trace Files. We'll just look into the "looping" issue mentioned earlier by precaud. Anyone that has inputs on this issue would be helpful advise!!

Best,

[timeandfrequency]

[...]
We'll just look into the "looping" issue mentioned earlier by precaud. Anyone that has inputs on this issue would be helpful advise!!

Jack Su seems to have found a valid solution to this problem.
Be sure to read all his comments for detailed explanations.