I recently tested a combo unit ( Panasonic ES30 ) to see if these units can work with external hardware. The answer is yes but it is not quite as convenient as using a separate vcr and recorder. Here is how it can be done.

Normally the combo will send the vcr signal directly to the dvd recorder side via a connection displayed as TP. External hardware cannot be used in this transfer mode.

A JVC JX-C7 color corrector and enhancer was used as the external hardware for this test. This is a composite in, composite out device. The TV was connected to the ES30 component output. The JVC JX-C7 input "1" was connected to the composite out of the ES30, then the JVC JX-C7 output was connected to the ES30 input labeled IN1.

Press the DVD button on the remote to select the dvd recorder side and use the INPUT SELECT button to choose IN1. Next set up to do an FR record using the FUNCTIONS button, then OTHER FUNCTIONS, then FLEXIBLE RECORD. Do not use the DUBBING function. At this point the recorder side should be set up to start recording as soon as the ENTER is used.

Now use the VHS button to go back to the vcr side. I do an optional step at this point. The JVC JX-C7 was connected to the composite out from the ES30 on the C7's "1" input. Nothing is connected to input "2". I select input "2" so there is no input to or output from the C7 to IN1. The TV screen should be black. Use the PLAY button to start the vcr playback, use the DVD button to switch back to the recorder side, use the ENTER key to start the recorder recording in FR mode, use the VHS key to go back to the vcr, change to input "1" on the JVC. The black screen should be gone and the vcr playback visible on the TV.

The optional steps are switching the C7 from input "1" to "2" and back to "1". The reason for this is sometimes when switching from vcr to recorder there is some extraneous signal that appears on the TV and gets recorded at the beginning of the capture. Having the C7 on input "2" creates a black screen until input "1" is selected so the extraneous signal is replaced by a few seconds of black screen.

For those who routinely copy and edit in their computer, the optional steps can be eliminated since the extraneous material can just be trimmed.

The color corrector worked fine. A procedure similar to this might work on other combos as well.

[edit] The last step with the ES30 is to switch back to the vcr side since this enables the vcr signal to come out from the composite output.