Adding A/V input to hot chassis SS B&W TV - Videokarma.org TV - Video

radiotvnut · #1 10-31-2023, 01:04 AM

Years ago, someone gave me this fairly crusty looking GE B&W TV from 1983, which is actually a re-badged GoldStar. After all this time, I decided to play around with it and discovered that its performance was very poor (very snowy picture). By using a Sencore analyzer, I determined that the IF circuit was working, the antenna isolation network was OK, and that the fault was with the tuner. Remembering from back years ago, the tuners in these cheap Korean B&W sets were crap, and I decided I didn't want to bother with it, especially since there is no analog broadcasting left.

So, I decided to add A/V input, since DTV boxes and most DVD players have composite A/V output jacks. This set uses a video processor IC and video detection is accomplished inside the IC. So, it was just a simple matter of lifting pin 3 of the IC and running my video from the DTV box into where pin 3 originally soldered to, and I had a beautiful picture. Then, I lifted the lead going to the high side of the volume control, connected the audio output of the DTV box to the high side of the volume control, and I now had clear audio.

Then, I rigged up the A/V inputs to be where the screws were for the UHF antenna, terminated the video input jack with a 75 ohm resistor, and connected a 100uf capacitor between the center pin of the video input jack and the point on the board where pin 3 of the video IC was formerly soldered.

For the audio, I simply coupled the center pin of the audio input jack to the volume control via a 22uf capacitor.

Of course, during all of these testing procedures, I had the TV connected to an external isolation transformer. Now, I need to decide on how to isolate the input jacks. If I can find an isolation transformer that's small enough to fit in the cabinet, I may go that route. However, if that does not work out, I'll see if I can come up with some way to isolate the inputs with opto-coupler ICs.

Have any of you ever done anything like this before?

JohnCT · #2 10-31-2023, 02:40 PM

Quote:

Originally Posted by radiotvnut

Have any of you ever done anything like this before?

I haven't, but I remember working on some low end TVs 40 years ago that did use opto couplers for AV isolation. All you need is a transistor or two plus an opto for each, and a suitable filtered source to power your circuits.

Maybe someone here can remember who used that layout. If we know who, we can find a schematic and simply duplicate that.

John

jhalphen · #3 11-01-2023, 03:51 AM

Hi to all,
Hi Radiotvnut,

Please have a look at this recent ARF TV section thread.
I published the complete data, including PCB files for a Video-rate Opto-isolator.

A HCPL4562 high speed opto-isolator is used, 17MHz bandwith at -3dB.
The circuit was created by Avago and available as a limited supply module about a decade ago.
It is unavailable now, hence the re-creation.
The Opto-IC is still current (Digikey, Mouser...).
The BroadCom suggested buffer has been added for low impedance video output as the Avago eval board is a high impedance output.

https://antiqueradios.com/forums/vie...p?f=3&t=430210

Best Regards
jhalphen
Paris/France

Alex KL-1 · #4 11-01-2023, 06:41 AM

Quote:

Originally Posted by JohnCT

I haven't, but I remember working on some low end TVs 40 years ago that did use opto couplers for AV isolation. All you need is a transistor or two plus an opto for each, and a suitable filtered source to power your circuits.

Maybe someone here can remember who used that layout. If we know who, we can find a schematic and simply duplicate that.

John

+1, I remember to seen some of these years ago, and also seen some with space for it in the PCB, unpopulated.

Alex KL-1 · #5 11-01-2023, 06:48 AM

Of course, with same principles (modulated CCS for the primary opto diode) is possible to use a cheap opto for audio part.

That opto solution is more safe than rectifying half wave (with ~120V mains) and grounding the TV common directly to neutral side of mains. Who remember to check the outlet grounds, and if it fails? The chassis ground becomes hot...

zeno · #6 11-01-2023, 11:08 AM

Quote:

Originally Posted by JohnCT

I haven't, but I remember working on some low end TVs 40 years ago that did use opto couplers for AV isolation. All you need is a transistor or two plus an opto for each, and a suitable filtered source to power your circuits.

Maybe someone here can remember who used that layout. If we know who, we can find a schematic and simply duplicate that.

John

NAP used it, C5 chassis IIRC. At least in the 80's, maybe other chassii.
Zeno

kf4rca · #7 11-02-2023, 03:23 PM

I remember years ago at a station, that we had a Panasonic monitor that had a video transformer in the video input. It took the external video input BNC connector and isolated it from the hot ground chassis. I didn't think that was too unusual and there must be other manufacturers using that idea if Panasonic was using it. The video quality was good. It was a Pro-line monitor. They trashed the monitor when we moved the station and I regret I didn't pull the circuit board. I've never seen that in a monitor since.

jhalphen · #8 11-02-2023, 05:23 PM

Hi to all,
Hi KF4RCA,

This device made by Kramer is a passive transformer video isolator similar to the Panasonic Broadcast device you mention in your post.
See Spec sheet enclosed.
It passes 5.8MHz bandpass (at -3dB), but the all-important tolerable Max AC & DC isolation voltage is not in the specs.

Could you trust it with 120VAC or 240VAC differential voltage to Gnd from a hot chassis?
enquire from the manufacturer.

Best Regards
jhalphen
Paris/France

old_tv_nut · #9 11-02-2023, 06:45 PM

https://proflixsales.com/tr1.html

Says insertion loss 0.8 dB (output 83% of input).
Page also has a signal to noise spec (what? it's passive!)
No hipotential breakdown spec given.

Alex KL-1 · #10 11-03-2023, 06:10 AM

https://www.amazon.com.be/-/en/Groun.../dp/B07GVGBLBC
Type used in surveillance cameras. They call it "balun" due to mode of the transformer is built. But I dunno if it isolates high AC or DC superimposed***; it is made for isolating 2 grounded devices, only for breaking the ground loop (also, I suspect the transformer being bypassed fot HF).

***like for all those sets with bridge diode rectifier from mains.

kf4rca · #11 11-03-2023, 07:27 AM

This is what I think the Panasonic used:
https://www.lundahltransformers.com/...heets/1575.pdf
Designed for circuit board mounting.

jhalphen · #12 11-03-2023, 07:40 AM

Hi to all,

Thanks! for the Amazon & Lundahl links.
interestingly, both quote a voltage isolation factor.
The Amazon states 600VDC
The Lundahl Xformer, 2kV + 11MHz bandwith, impressive!

Using them to create a video input on a hot chassis TV?
For me, 600VDC (no AC rating) is not enough (at least in 240VAC countries)
+ ahem, Amazon tech specs...
the 2kV Lundahl looks OK.

Best Regards
jhalphen
Paris/France

kf4rca · #13 11-03-2023, 08:35 AM

Beware of those large units for elimination of hum. Our station had a few for remote work. THEY READ DC CONTINUITY INPUT TO OUTPUT. They are actually a choke. Inside is a large toroid core wound with coax in to out. (I took one apart to see how it worked.)

Here is one on ebay for only $5.00. Probably see many of these for sale dirt cheap since analog video is not used in TV broadcasting anymore.

https://www.ebay.com/itm/14528665298...Bk9SR6ywl9byYg