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,