Out of context screenshots are out of context
On the Mini Apple IIe project, I am testing the composite/sync amplifier, which is an MC1377. The composite output looks great, until connecting the output cable to a receiver. It outputs about 2.6 Vpp until loaded with 75 ohms, then it drops to about 400 mVpp. We have replaced most of the passives on the output and have tried 3 different MC1377s. The measurements below are from a known good MC1377 removed from a working Apple IIgs.
Here is an MC1377 datasheet mirror link. The Figure references below match Page 8 of that datasheet. The biggest suspects are the Luma signals.
Not captured, but tested, none of the input signals change amplitude when the output (pin 9) is loaded with a 75 ohm load.
Problem: Update, SOLVED
These two screenshots are the same point, RCA Out Header in the schematic. On the left is the output when the node is left open. It is about 2.6Vpp with minimal DC offset. However, when the signal is terminated with a 75 ohm resistor (or a receiver circuit) it drops to about 300 mVpp!
Circuit
The circuit is a copy of the circuit from the Apple IIgs. In that circuit, there was a “capacitive transformer” which appears to be an EMI filter. That part was omitted in the VGC protoboard. In case it did some kind of isolation, we removed the same component from a working Apple IIgs, but it did not have an appreciable effect.
mc1377 based composite/sync circuit
Full VGC Protoboard KiCad files are available here.
Screenshots from VGC Protoboard rev1
02 - Compare IIgs signal at 10-29pm
03 - Fig 8abc - RGB Input - One Signal
04 - Fig 8d - Composite Out LOADED
05 - Fig 8d - Composite Out UNLOADED
06 - Fig 8e - Sync Pulse Input
07 - Fig 8f - Chroma Output DC
08 - Fig 8f - Chroma Output AC
09 - Fig 8g - Chroma Input
10 - Fig 8f - Luma Output Zoomed Out
11 - Fig 8f - Luma Output
12 - Fig 8i - Luma In Zoomed Out
13 - Fig 8i - Luma In
14 - Fig 8i - Luma In AC Coupled
15- Luma In AC Coupled, showing activity