HP 1707B Scope - No Horizontal Sweep

[DaddyMac]

Vertical sweep appears to work. All push buttons OUT (auto sweep mode), 10:1 TEK probe connected to CAL output, A Input on AC, 0.1 v/div. With sweep mode in EXT HORIZ or DELAYED, I get 2 dots 1cm apart (height) on the screen. The height tracks the Volts/Div. In MAIN or MIXED, no display. Both Horiz and Vertical position adjustments work. I do not have another scope (that works), only a DVM and counter. I do not have the extender boards needed check voltages, etc. So it looks like all I can do is pull the likely circuit boards for inspection and to check resistors, replace any likely capacitors and perhaps shotgun some likely failed transistors. Any suggestions?  I have the HP Operating and Service Manual on CD and some hard copies (can make more). I do have an HP 1703A scope for parts, but little or no common boards.