Hello and welcome back. I am in need of a small black and white video monitor. And
look what I found while browsing eBay! Isn’t she a cutie? It’s a
Panasonic TR-525 5” portable TV. Panasonic used to make a whole range of portable TVs in the 1970’s,
including this TR-5046 or the TR-555 that Adrian recently converted. My personal favorite is the
TR-005 straight out of the Jetsons.
I’ll settle for this cheap eBay one instead, with its rugged and utilitarian good looks.
Problem is, it only has VHF and UHF inputs,
so I need to modify it to accept a composite video and an audio input instead.
That should be a piece of cake and provide us with a much needed break from our difficult microwave
HP plugin repairs of late. So stick around for a relaxing bit of vintage B&W CRT goodness.
[Marc] Today, on the bench of healing and modification,
a beauty from the 1970s: the Panasonic TR-525 portable TV.
And, does it work?
Seems like it does. I have sound. Oh, I have a picture.
And I can switch it to VHF or UHF. But of course,
I have nothing, because there is no such thing as TV programs over the year these days.
So instead, I have brought
my decoder, my RTTY decoder, which supposedly has a VHF modulator to it. So, let's hook it up.
So, on the back of this beauty, we don't have video in. But there is a way to go from the
default antenna to little plugs here, for VHF and UHF, by putting it on the external position.
Grab. Okay.
Let's see if that will give us a picture.
And?...
It's not the correct plug at the back.
So, here we go: two, three - oh we got something - four...
We got something even better.
And also I suppose, because this is from Europe, this is PAL
not NTSC. So, it might not be able to sync to it. Oh, it does! Here we go!
So, we have a nice display.
So, the idea is, for stuff that doesn't have a VHF modulator,
I'd like to add a direct video and direct audio input to it, so it can be a nice monitor.
So look at this beauty! It is actually able to run on 12 volts out of 6 watts,
of anything, with a CRT! So you can run it off the batteries. It's a portable one that
you would use while camping. And also you can use it from 12 volts from your camper battery.
All right, so let's try to open that one up. I see screws over here, screws over there.
Can we get into our box of 1975 goodness?
I think it's hanging up over here. So, maybe I need to do this.
Oh yeah! The knobs at the back, okay. And yeah, that was hanging up by this panel. Hop! All right.
Here we go!
Okay, doesn't seem to be much voltage into there at all. It should be safe to clean now.
So, overall tour, it's not that complicated. Power transformer, that's the antenna input,
and you have the orange one that goes to UHF, and you have a coax that goes to the
VHF. That's the flyby transformer, for the high voltage on the anode.
So, everything is buried down there, so I think I have to remove the
PCB to see where I have to hook myself up.
The board is supposed to slide out, but I don't... It's held up somewhere.
Okay, I think I found it. There is a screw right here.
Yeah, that was the secret, okay.
I think we have it, there we go.
So, my job is now to correlate this schematics with what I am seeing over here. And see,
it's related: it's a TR-505. This one obviously doesn't have the UHF.
I see that IC. That's the one on the audio section, that was back there.
And then, while we are in the audio section, let's check if the components are numbered the same.
Here is the volume pot, so that's easy to identify. Components around here: C501, R501,
502, 503. Near the pot I can see R502 right here. This is R503. This is C502. So it seems to be
following the same component numbering. So there is a chance it's the same board.
That is the VHF tuner, then there's the IF section. The automatic gain
control are those two transistors here. But what's of interest to us, is where the
intermediate frequency gets rectified. And that's that rectifier right there.
Which means, the beginning of the video amplifier is where 1 is exactly, TP13.
We would have to disconnect it right here,
and input our video signal right here, at TP13, through a capacitor. And that should do it.
For the audio, it's even simpler. It's at C207. So I disconnect C207,
and I input my audio right here into 22, or the plus of C207.
So I found C207.
It is right here, this guy.
But I haven't found the video input. And it might be because it's underneath this, over here.
Let me look at that.
Okay, I found it. It was indeed underneath here.
That's the last stage of IF amplification, and that's where the video detection is happening.
And here you see, it's TP13. That's where I think I should inject the video after I disconnect it.
Unfortunately, it's not as simple as on the schematic.
This whole thing is connected on a whole blob. So it's not that easy to separate.
I could try just to inject video here, and it might be high enough that the AGC trips,
and it suppresses both the RF and the IF noise. And that's enough, I have a clean signal.
Or, I could short - that might be easier - I could place a short right here,
and that will just quiet that whole section. I will be done
without desoldering anything. Just add a switch, a jumper that's reversible.
Okay, so for the first try, I'm going to use the simplest method, which is to inject video
at Test Point 13. And for that I have made this little adapter, that receives the video signals
from the generator, or from whatever source. 75 ohm termination, and a 47 microfarad capacitor.
So the 75 is to match the termination with the impedance of a traditional
video cable. And the 47 microfarad, I chose it so it had low impedance
compared to 75 ohms at about 2500 kilohertz. So that gives 1 Ohm,
one point something Ohm impedance, compared to 75. So it's negligibly small.
And then we'll connect that to Test Point 13 and Test Point 92 nearby, which is ground.
So let's try this, and reconnect everything, and try not to get electrocuted, and see what happens.
I don't expect very much good quality here: I don't have a coax
to go in there. This is just a quick try to see if we're tickling it at the right point,
and with about the right amount of video signal.
I see nothing happening.
Uh-huh, I think I have found the source of my power problem.
This can be powered from batteries or from 12 volts, so there's a little switch
that comes on when you introduce the AC. And I bet you, that switch here
is not working, or is not working reliably.
Deoxit to the rescue!
There we go.
Burnishing...
Try again, with a clean contact.
Yes, it has power!
Yes. Now, if I connect this.
Aha! We do get a honest to goodness picture!
I think that did it. That's all you need.
So, what happens here, is that the automatic gain control of the IF stage just kicked in,
and it just did the trick for me.
And now, I have hooked it up to my little demodulator here.
And it's not quite as good. The regular picture comes through,
because it's all dark here. It thinks that it doesn't have any signal, and the gain goes up.
So, I think I might want to explore Plan B, where I actually short the
last transistor of the IF stage, to bring the gain to zero all the time.
So, given this successful proof of concept, here are the final modifications we are going to make.
We will introduce the video signal at TP13 through our 75 Ohms termination resistor,
and 47 microfarad coupling capacitor.
We will short out resistor R111 with a removable jumper,
in order to mute any signals coming from the antenna.
While we are at it, we will make the 5.5 MHz filter removable by
putting it on a header. This filter is used to suppress the color subcarrier.
This way we can experiment if we can gain a bit of black and white resolution by removing it.
For the sound, we will replace the conveniently located factory jumper wire J7 with a removable
jumper header. That will allow us to reversibly disconnect the sound chip.
We’ll then connect the external sound input via a 33 uF capacitor on the downstream side of J7.
And that’s it. 4 simple mods,
all reversible from the top of the board with no tools. Easy peasy lemon squeezy.
So, I made more strides into my modification. First, I put the inputs where the UHF antenna
used to be. Just removed the UHF posts, but I could put them back if I wanted to.
So, that also goes into reversible modifications down here. I have now
my resistor, my cap at the end of a coax.
Over here, you have the grounding modification. So that disables the signal by shorting the resistor.
And the sound is also reversible, it's over here.
I put the four point whatever megahertz filter on a little header, so I can remove it, if I want.
So, that might improve our bandwidth. We can do an A/B test and see if that's really necessary.
Let's see if we at least didn't break it.
Okay.
All right! Where is that one? There we go.
Every time I have to switch back from PAL to NTSC.
So that works fine. Convergence... Okay.
So, let's make an experiment on that one, see if removing the filter helps any.
I tweaked the focus and I'm able to get some resolution all the way on the fine bars here.
Let me put the filter back in, and see if it makes any difference.
Okay, you stupid camera, you're blinking on me.
Okay, so this is with the filter installed.
And I'm going to try to remove the filter without zapping myself.
Hop! And this is with the filter removed. And, I didn't look at the picture while I
was doing it. So I don't know if there is much of a difference. We'll see when I review it.
Yep, it sure improved the picture. So, if the input is black and white video, that sure helps.
If you intend to input color video, you’d have to keep this color subcarrier rejection filter in.
And do we have sound?
Uh yeah, it's my computer audio thingy...
There you go.
We do have sound!
[Theme music]
Okay, I think it's a successful conversion.
Okay, let's try to hook it up to the converter, and see if we can see that.
Okay, so we go back to PAL. There we go!
And now, the background is nice and noise free,
because we... Oh here you go, oh, we have super resolution!
Let's kill that.
Okay. So, now that we have killed the intermediate frequency section, it's all totally noise-free.
So, that's what it looks like once modified. You just have your video here,
you audio here. And I put a little explanation sticker of what to do
to restore it to the original VHF, in case I forget, or somebody else gets it.
And I have hooked it up to the vidicon camera from the Apollo setup.
Hello guys, how are you?
So, this is my camera, this is me.
Let's see if I can get this.
Works fine! Okay, don't you love TV with a CRT. Black and white TV as a charm!