On 2018-Jul-03, at 5:59 PM, Jon Elson via cctalk wrote: > On 07/03/2018 03:35 PM, Ian Finder via cctalk wrote: >> On Tue, Jul 3, 2018 at 1:21 PM, Monty McGraw via cctalk < >> cctalk@classiccmp.org> wrote: >> >>> I've been repairing my Tektronix 4052. >>> >>> I've got the digital logic working - but the text and graphics are messed >>> up. >>> >>> I posted photos of the screens in my Tektronix 4052 troubleshooting thread >>> on vcfed. >>> >>> With a scope on the final X amplifier stage - it is oscillating - so I see >>> weird horizontal strokes instead of dots for text. I know from the service >>> manual that this circuit includes a feedback loop, and with the scope I see >>> oscillation all around the loop - so I haven't found the source. >>> >>> > If it is basically working, but just has oscillation, then it is almost > certainly a capacitor issue. > There are likely capacitors in the feedback loop to reduce bandwidth, and > decoupling capacitors > on the power rails. It would be fairly easy to track down the specific > capacitors from the manual and check them.
I had the same initial thought, but Monty's msgs & scope trace show a solid ramp waveform down around 37Hz, not the higher F ringing or transient sort of sine oscillation one would expect from failed small-C bypass/suppression/etc. caps. It's more like a relaxation-oscillator type of activity. He's already replaced a couple of the larger-C caps in the circuit.
