TV IF alignment

[jeyurkon]

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

[jshorva65]

Originally Posted by jeyurkon

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

Interesting point! I'm pretty "rusty" at circuit design in general, and even more so with Solid State and PC Board design work specifically. My last design project used tubes and a hand-wired chassis and that's been a few years ago. Last time I designed anything with a Transistor or IC was nearly 10 years ago, and just a simple Audio device built on a pre-fabbed prototype PC board. Last time I actually etched a board from a published template was over 10 years ago. It's been even longer than that since I designed anything from scratch all the way to board layout and etching. Back before the Net existed and when CAD software and a suitable PC still cost as much as a new car. Back then, photocopying hand-drawn artwork onto special transparency stock sold under a trade name of "Tech 200" and using that like an "iron-on" transfer to get the mask onto a copper-clad board blank was the only way to economically do any kind of one-shot photo-etching. The "Tech 200" stuff also worked nicely with a printed Template from a magazine article provided any flaws in the Original were touched up before making the transfer on the "Tech 200" stock. Basically, the toner deposited on the "Tech 200" would "transfer" (like an iron-on decal to a t-shirt) to a copper-clad board when heated properly with an ordinary iron (or make a serious mess, ruining the "Tech 200" sheet and virtually wrecking the board blank if not heated evenly).

[jeyurkon]

I was about to say that I just made a board like that recently, but then I remembered that recently for me is 10 years ago.

The advantage of converting the signal the way you suggested is that you would still be able to generate the 44MHZ when you need to.

An easier way to convert to 24MHZ though would be to rewind L201A. But then you'd need new marker crystals or feed an external signal generator into the external marker jack.

Given how inexpensive the 415 generator is I picked up two of them hoping to try both methods. But, I just have too many projects. That's not bad though since I won't be bored once I retire.

John

[Geist]

Hi All;
I had thought of that about a month ago, and even suggested it to another member.. But, what I didn't know, nor have I since checked out.. Is, would the older frequencies be available in readily available crystals.. Or would they need to be specially made..
THANK YOU Marty

[Penthode]

I may have missed further discussion of what I have written below on this. But the importance when adjusting the stagger tuning in a video IF amplifier is to get the 45.75MHz video carrier at the proper 50% point on the slope. The 44MHz marker is relatively unimportant: it only provides a rough midpoint of the curve.

The traps should be properly set to attenuate the adjacent sound (47.25MHz)and video (39.75MHz) carriers and co-channel (41.25MHz) sound carrier. The traps should not be used to shape the response. That is except the adjacent video trap (39.75MHz) may have been adjusted downward a bit to improve weak reception. Distant TV stations may warrant adjusting the fine tuning to bring the video carrier higher on the response to improve signal to noise and moving the adjacent video trap would improve intercarrier sound performance. But this is the only exception.

The traps should be first set before sweep aligning the VIF strip.

When sweeping the Video IF, the video carrier is placed at 50% of course because of the vestigial sideband video modulation for analog television broadcast since the 1930's.

Most color sets used the "haystack" video IF response, so named for obvious reasons. It is important to make sure the color subcarrier (42.17MHz) is at 50% on the forward slope at the same time the video carrier (45.75MHz is at 50% on the rear slope of the curve while at the same time the co-channel sound trap (41.25MHz) is accurately aligned. The 44MHz in this case is a guide and should be located roughly in the middle.