Direct Audio/Video Adapter for Vintage TVs

              

This article explains how I built an audio/video adapter for one of my antique TVs (an Admiral 24C15). With this little box, I can easily plug in standard video and audio cables from a DVD player or similar source.

The adapter bypasses the TV's tuner, audio and IF (intermediate-frequency) circuits and gives you the best sound and picture that your TV is capable of producing. Its switch can also restore the connections to the TV's native circuits, so you can toggle between a direct A/V source and the TV's antenna input whenever you choose.

Schematic and Parts List

The A/V adapter can be built in a few hours and it uses readily available parts. These photos show the finished adapter:

  

It's hard to imagine a simpler project. The adapter consists of three phono jacks, two resistors and one switch inside a little box. Here is the schematic:

When the switch is flipped up to the "External" position as shown in the schematic, external audio and video sources are routed to the TV's audio and video amplifier tubes. When the switch is flipped down to the "Internal" position, it disconnects the external sources and restores the native connections between the TV's internal audio and video sources and their amplifiers.

The schematic describes the adapter connection points in general terms (for instance, "grid of video amplifier tube") because the precise points may differ for other televisions. Later in this article, I'll show where those points are located in my set.

Here is the parts list:

Quantity Part
1 Project box
2 10K ohm resistor, 1/4 watt
3 Phono jack, panel mount
1 DPDT switch
6 ft Shielded one-conductor cable (RG-174)

For your convenience, I included links to Radio Shack part numbers for most of these items. Note that Radio Shack may change part numbers or discontinue products at any time, so if you don't find those exact parts, search their website (or another supplier's site) for equivalents.

I used RG-174 cable because that was the most appropriate cable I could find at my local surplus store. The type is not critical; any shielded cable will do. I chose RG-174 because it's thin enough to fit through an existing opening in my TV's chassis.

Building the Audio/Video Adapter

I housed my adapter in a little project box that's easy to mount on the rear of the TV cabinet. If you don't mind drilling four holes in the back of your chassis, you could mount the jacks and switch on the rear chassis apron. I chose the box as a less invasive option.

In the next photo, I have drilled holes in the box and installed the three input jacks. My surplus store had jacks with yellow, red and white cores to indicate video and audio.

Next, I wired together the jacks' ground terminals:

I used shielded cable and ground connections to prevent hum in the audio and hash in the video signal. When experimenting prior to building the box, I had used unshielded clip leads to make temporary connections in the chassis. Those seemed to produce normal picture and sound, but I will be snaking these cables much closer to other leads and components, so shielding and grounding is more important.

The following photo shows the DPDT (double-pole, double-throw) switch and the two 10K resistors that sum together the stereo inputs into a monaural audio signal. I'll use a short length of shrink wrap to insulate one of the resistors in case that junction gets pushed around.

The switch will be installed at the right side of the box and the cables will exit the box through a slot at the left.

I'll use this shielded, one-conductor cable to connect the switch to the input/output points in the chassis:

The adapter shown here is actually the second one that I built. The first version used shielded two-conductor cables—one cable for video and the second for audio—because I happened to have a spool of that cable on hand. The video looked reasonable, but when the adapter was switched to my external source, I could faintly hear the audio from the antenna signal, along with audio from the external source, caused by "crosstalk" between the two audio cables contained inside the same shield.

After I replaced the two-conductor cables with pairs of one-conductor cables, the interference between signals disappeared.

Here, I have begun to wire the switch:

Thin shielded cable is a little finicky to work with. I used a razor blade to carefully slit the insulation and remove it. A stainless steel dental pick worked well to tease apart the strands of shielding. After the shielding strands had been unwoven, I rolled them back into a single wire which I then soldered to the ground buss.

All of the internal connections have been made:

Choosing Internal Connection Points

The next photo shows where I connected the four adapter cables under the chassis of my Admiral 24C15. The cables are fed through a chassis opening that accommodates the picture tube leads. In this view, the two audio cables are connected at the two yellow arrows on the left, at the volume control, and the video cables at the arrows on the right, at the video amplifier tube socket.

The black cables carry the internal signals from their sources to the adapter and the white cables carry the signals—internal or external—from the adapter to the output tubes.

Now you can see why shielding is useful. When the adapter is switched to Internal, the audio signal travels all the way from its origin near the volume control, out the chassis to the adapter, through the switch, and all the way back to the volume control. Depending on where I secure those cables when the project is finished, they may be near sources of interference.

So, how did I choose these particular connection points?

The audio connection is easily understood. We want this adapter to substitute an external audio signal for the TV's internal audio and retain the ability to make it louder or quieter using the volume control. The following snippet of the Sams schematic shows the point that I chose. (You can download the complete 24C15 schematic from the ETF archive. It's a big PDF file, so be patient while it downloads.)

In the previous diagram, the signal flow is from left to right. This TV has a separate audio chassis connected to the main chassis through a shielded cable, as shown in the schematic. If you follow the connection through the jacks, you see that the signal goes to pin 4 of the audio amplifier tube, V24A. Whatever audio signal we feed into pin 4 will be heard through the loudspeaker.

The red X shows where I cut the connection between capacitor C37 and the "hot" end of the volume control potentiometer R3A. One cable is wired from C37 to the audio side of the DPDT switch. The blue arrow shows where I wired the incoming signal from the A/V adapter. The second cable from the audio side of the switch connects to the indicated terminal of the volume control.

Our adapter simply switches that signal back or forth, choosing either the TV's internal audio or the external audio from the adapter jacks. When the adapter is switched to External, the internal signal is disconnected and the external signal is fed to the audio amp. When it's switched to Internal, the external signal is disconnected and the internal signal is restored.

The next snippet shows where I connected the adapter's video cables, at the grid (pin 4) of the video amplifier tube V7:

When the adapter is switched to External, it feeds the external video to the amplifier tube. When switched to Internal, it restores the connection to the TV's internal source.

Not All Designs Are Created Equal

If your TV's circuitry differs from this Admiral model, as many will, you'll need to examine the schematic to determine the appropriate connection points.

The analog video signal in old TVs has a value of 1 volt peak-to-peak, with negative sync. If the correct video connection point is not obvious from the schematic, you can use an oscilloscope to search your video chain for that signal.

Caution: This Admiral TV has a transformer-type power supply. If your television has a series-string (transformerless) power supply, then it's possible—depending on which way the TV's unpolarized power plug is inserted—for the TV chassis to be connected to the "hot" leg of the AC power line. To avoid damaging your external device, you should operate the series-string TV from an isolation transformer.

Using the Audio/Video Adapter

I installed the adapter box at a convenient spot in the rear of the TV cabinet:

Now, I can plug A/V cables from a DVD player or other source into the adapter, and connect rabbit ears to the antenna input, selecting one source or the other at the flick of a switch.

This brief video clip shows the box in operation, switching from a DVD movie to an OTA signal received via antenna from my home transmitter.

My pocket camera takes lousy video, but at least you can see that the adapter works.

So, was this project worth the effort? Let's look at some photos.

Time for a Pepsi Challenge!

At first glance, the direct video looks better, but not dramatically so. The next two photos show the same DVD frame, comparing the TV's antenna input (left photo) and the direct input from our adapter (right):

  

Zooming in makes the quality difference more apparent:

  

Displaying a test pattern erases all doubt and it allows us to quantify the improvement. Again, the left photo shows the antenna input and the right shows the direct input:

  

The vertical wedges in the center are important. They comprise two scales for measuring the image quality: the bandwidth (top wedge) and the number of lines of resolution (bottom wedge).

In this context, higher bandwidth is better. If you can distinguish separate lines in the upper wedge all the way to about 3 mc (3 MHz), your TV is in decent shape. A lower bandwidth reading means that some bandwidth is being lost when the TV processes the signal through its tuner and IF sections. Your TV might benefit from a full realignment (or, it may simply be a cheap design with a lousy IF section).

In the direct video example, the bandwidth lines are clearly distinguishable all the way to 4 MHz. This translates into a very sharp picture, so sharp that I needed to back off on the Focus control to avoid an unnatural, too-crisp picture. The bandwidth is much lower in the antenna example: the vertical lines disappear just beyond 2 MHz.

It's not that this TV's antenna input looks awful at a normal viewing distance—it's actually quite good, as shown by the following picture taken during restoration—merely that the direct video input looks better.

This Admiral model is pretty well designed, but its alignment may have drifted a bit during the last 60-odd years, due to component aging. Design limitations also play a role here. The IF strip in an inexpensive TV may pass less bandwidth than the ideal, no matter how perfectly it is restored and realigned.

I can't quantify the audio improvement, but comparing signals for only a few seconds would convince anyone that direct injection sounds much better. The audio is strong and pure, with none of the annoying buzz that many vintage TVs produce when displaying high-contrast scenes such as black lettering on a white background.

Pros and Cons of Direct A/V Input

Vintage TVs were designed to receive over-the-air signals through an antenna. When I restore a set, my goal is to make it work as designed. I also use a home transmitter to broadcast programming throughout the house, so that a TV in any room can receive the program using a rabbit ear antenna.

I like using my TVs "the old fashioned way," and I also like to restore the whole television from front to back. In a sense, direct input feels like cheating, since you're bypassing roughly one-third of the TV's circuitry, using it as a monitor rather than a receiver.

On the other hand, sometimes I just want to play a DVD and enjoy the best picture and sound. With direct input, the results can be spectacular. No more furry pictures or buzzy sound. The image is crisp and rich, the sound is crystal clear.

A switchable adapter gives you the best of both worlds, versus hardwired direct connections. I can use the antenna input whenever I like, and the adapter could be removed with minimal effort, if I decided to return the TV to original condition.

People normally watch programs, not test patterns, and every vintage TV is a little different. If you're wondering whether it's worth the bother to add an A/V adapter to your set, you can experiment with clip leads, as I did before building this adapter. (I recommend that experiment in any case, to make sure you have chosen the right connection points.)

Why Not Simply Align the TV?

As noted earlier, every TV's output quality depends on properly aligned tuner, IF, and audio circuits. In practice, you'll find that most vintage TVs don't need a full realignment unless some ignorant person has messed with the alignment. However, some sets—including this one, perhaps—might benefit from a touch-up.

Of all TV restoration techniques, alignment of the tuner and IF probably requires the highest skill level, as well as specialized equipment such as a sweep generator and marker/adder. I don't own such equipment and I don't have that skill, at least not yet.

Furthermore, every design has limits, so alignment can't produce better output than a TV's circuits can support.

I'm very familiar with audio alignment, and there's a limit to the audio quality that you'll get from any split-sound system like the Admiral's. The audio from direct input is noticeably better in this TV, even though I aligned (and realigned) the audio section during restoration.

I suspect that aligning the IF strip would improve its video quality, but perhaps not quite to the level produced by direct input.

Plug 'n Play Construction

One further refinement is suggested by a 1956 article from RCA Broadcast News, describing a video adapter for the RCA CT-100 color set. Rather than cut the video connection under the chassis, the author built an above-chassis adapter using a tube socket and base.

To use that adapter, you insert its base into the socket of the video amplifier tube, and plug the tube into the adapter socket. (That adapter also included a video preamplifier, but we can ignore the preamplifier in this discussion, since a standard-level video input is sufficient to drive the Admiral's video amp.)

To build a plug 'n play A/V adapter for my Admiral TV, you could use a similar socket-and-base pair for each of the amplifiers. The cable connections from the box would be routed to the adapter sockets rather than hardwired under the chassis. To return the TV to stock condition, you would simply unplug the adapter sockets and return the tubes to their native sockets.

Speaking of the CT-100, immediately after finishing this adapter, I updated the one I had built for that set a couple of years ago. You can read all about that in the article Video Adapter for RCA CT-100 Television.


This electronic construction project, including all descriptions, diagrams, photos, and the underlying electronic design, is published here for the noncommercial use of radio hobbyists. You may print and reproduce these project instructions for your personal use. Commercial use of this material is not authorized.

©1995-2017 Philip I. Nelson, all rights reserved