MC1377 Measurements on Mini Apple IIe Prototype video board

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

Update: Turns out, R9 in the schematic above wasn’t fully soldered. After doing that, the computer now boots… at least, until it locks up.

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!

00c-side-by-side uncropped

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

MC1377 circuit on VGC based on IIgs design

Full VGC Protoboard KiCad files are available here.

 

Screenshots from VGC Protoboard rev1

 

 

01 – RGB from DAC

 

02 - Compare IIgs signal at 10-29pm

02 – Compare IIgs signal at 10-29pm

 

03 - Fig 8abc - RGB Input - One Signal

03 – Fig 8abc – RGB Input – One Signal

 

04 - Fig 8d - Composite Out LOADED

04 – Fig 8d – Composite Out LOADED

 

05 - Fig 8d - Composite Out UNLOADED

05 – Fig 8d – Composite Out UNLOADED

 

06 - Fig 8e - Sync Pulse Input

06 – Fig 8e – Sync Pulse Input

 

07 - Fig 8f - Chroma Output DC

07 – Fig 8f – Chroma Output DC

 

08 - Fig 8f - Chroma Output AC

08 – Fig 8f – Chroma Output AC

 

09 - Fig 8g - Chroma Input

09 – Fig 8g – Chroma Input

 

10 - Fig 8f - Luma Output Zoomed Out

10 – Fig 8f – Luma Output Zoomed Out

 

11 - Fig 8f - Luma Output

11 – Fig 8f – Luma Output

 

12 - Fig 8i - Luma In Zoomed Out

12 – Fig 8i – Luma In Zoomed Out

 

13 - Fig 8i - Luma In

13 – Fig 8i – Luma In

 

14 - Fig 8i - Luma In AC Coupled

14 – Fig 8i – Luma In AC Coupled

15- Luma In AC Coupled, showing activity

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