Quote:
Originally Posted by jshorva65
This thread http://www.videokarma.org/showpost.p...26&postcount=8 has a recent post from me which includes my "Top 3" recommended units for alignment of 24 MHz IF systems.
I recently scored the (presumed absolutely Unobtainium, but just REALLY hard to find) RCA WG-295B Video Multi-Marker unit and its companion WG-304A RF Modulator. I haven't found an RCA WR-70 Marker Adder yet, but I have the WR-69 Sweep and WR-99 Marker Generator units. I'm still evaluating the WG-295 / WG-304 and the "VSM" (Video Sweep Modulation) Alignment technique covered in TAB Books' volumes (and other textbooks of the era) on RCA receivers.
The WG-295 and WG-304 are both passive units, the WG-295 establishing its Marker intervals by use of an array of Series LC Traps to create "suck-out" points at its output when a Sweep of 0-5MHz is applied to its input. The output of the WG-295 then connects to one of the two inputs of the WG-304. The desired Picture Carrier frequency is to be injected to the other input of the WG-304. The result is a sweep signal with marker suck-outs at fixed intervals whose Picture Carrier frequency may then be set to any desired value (25.75 MHz for RCA 630TS and similar, 26.40 MHz for Dumont RA-103 and similar, or the RF value of Picture Carrier frequency required for an individual Channel during Tuner Alignment. This setup is adequate, but is subject to the same limitations as any setup which utilizes pre-injection of Markers as opposed to post-injection. An alternate setup using the WR-70 Marker Adder is shown in the Procedures published by TAB et al. Still, the WG-295 / WG-304 setup has proven, although adequate, a bit on the "Rube Goldberg" side.
Has anyone tried utilizing a B&K 415 coupled to one input of a Mixer circuit having a suitably flat response and a fixed Oscillator (20.000 MHz for old-band RCA, 19.350 MHz for old-band Dumont)? Such a setup should shift the B&K 415's sweep output from its 44 MHz center frequency to a center frequency of 24 MHz. Since the B&K 415 uses post-injection for its Markers, however, it seems that a duplicate Mixer would need to be inserted into the Marker channel of the 415, perhaps utilizing a single Oscillator whose output could be fed to the added Mixer stages in both the Sweep-channel and Marker-channel via a buffer amplifier having two well-isolated outputs. |
I considered something similar. But 20MHZ seemed too close to 24MHZ since you're looking at over a 3MHZ bandwidth for the TV and it seemed like the 20MHZ would be difficult to filter out and still achieve a flat response. So I was thinking about upverting it before down converting it. Adding another step.
If I understand the 415 circuitry correctly, you wouldn't have to do much to the post injection if you do the shift from 44 to 24 accurately.
John