Hack an analog video in into an VHS recorder - Page 1
Forum section: electronics / projects

author: cBaer
author_rank: Contributor
timestamp: November 10, 2019, 06:13:11 pm
content: Hi there,I have this Quisy 220 VHS Video Cassette Recorder (PAL) lying around.It has RF in and out and a SCART output but no audio/video input.So I got it added.I had no schematics but there was a plug in board in the VCR with a TDA 9800:(http://pdf.datasheetcatalog.com/datasheet/philips/TDA9800T.pdf)The plug in board mostly followed the datasheet.The TDA 9800 is in charge for demodulating the IF signal to video and audio.I don't want to use the RF-demodulator since analog TV is out of service.If you look at the datasheet page 3 the section of interest are pin 9, 14 and 7.So what i did (see the attached schematic):- I disconnected pin 9 (audio out) and added a RCA jack (white) input instead. That was the audio part.- I removed the "audio trap" (a three pin filter and an inductor which removes the FM audio carrier from the video signal), this disconnected pin 13 and pin 14- Added a yellow RCA jack + 75 Ohm terminator/resistor to ground- Fed the terminated signal from the RCA through a 100uF 10V elco into pin 14 (video buffer input), the video signal is now buffered out on pin 7.This _almost_ works good. On very bright images it craps out.I tested it with a Super Nintendo + Tiny Toon Adventures: Buster Busts Loose (this game is not the best but my favorite to debug analog video problems).So when the Konami Logo appears (white background) the VCR switches to blue screen (no not the BSOD) and in the recording the horizontal and vertical syncronisation are unstable. It recovers when the image gets "darker".As far as I understand this has to do with the 100uF capacitor. Bright images have a high average voltage and the AC coupling causes the sync signal to go down in voltage and hitting the noise clipping of the TDA 9800.So now I am looking for a fix. I have to DC-couple it and I need to offset the the video signal. I could do it by swapping the terminator and capasitor with a resistor network but this leads to an 75 Ohm input port into an elevated voltage. I'm not sure that this will not work with all AV equipment. Any comments?

author: m3vuv
author_rank: Super Contributor
timestamp: November 11, 2019, 02:35:56 am
content: if the scart is fully populated it should have viv and audio inputs,cant you use those?

author: PKTKS
author_rank: Super Contributor
timestamp: November 11, 2019, 02:05:45 pm
content: Quote from: m3vuv on November 11, 2019, 02:35:56 amYes probably you can.Back in the old days of fully integrated decoders theTDA line was specially suited to to do.Being there at least with the TDA3561 (PAL only) whichcan be paralleled with the NTSC companion and BOTHaccepting SCART inputs.My rough draft for such hack follows..the auxiliary parts were directly available on the SERVICEMANUALS for each device... so chances are good ifyou have them to be all set..usually the boards were even for sale as kits on retailPaul

author: cBaer
author_rank: Contributor
timestamp: November 12, 2019, 10:46:32 pm
content: Quote from: m3vuv on November 11, 2019, 02:35:56 amGood idea, SCART has separate pins for in- and output. On this VCR device you can only choose RF-Channels, no EXT or something like that. If i remember correctly VCR that have EXT in do it either by dedicated RCA jacks or a second SCART Connector. This devive has an unpopulated second SCART footprint, so maybe that was an option on a more expensive model of the series. But no luck for meQuote from: PKTKS on November 11, 2019, 02:05:45 pmIm not sure if I understand you correctly but I'm not after the SCART RGB or S-Video. And I don't have the schematic, or service manual, or user manual, or even a remote.Now a update: I did the resistor network thing. See attachment. I removed the 100uF capacitor and replaced it with 470 Ohm and added ~909 Ohm between the supply voltage (5V as i believe) and Pin 14. This way its DC-Coupled, Pin 14 has the offset and the AC coupling gets attenuated by 1/3.In the attachment V2 and R2 are the video source, R1 is 75 Ohm termination. The net where R3, R4, R5 and R6 are connected is the input of the TDA 9800 Pin 14 (Video buffer in).And it works... ...Almost...Picture quality is fine etc but this time if the VCR is in passthrough it craps out (blue screen) if the picture is darkbut recovers.The recording seems fine thou. This is the Problem with the elevated termination voltage I was afraid of. In the atachment (upper part) the 0.132V at R1 would be the sync voltage if the video source sinks a bit of the resistor network current. But the SNES I used to test it can not sink current. So the sync voltage rises to 0.257V (lower part in the attachment). On the SNES the sync is normal at 0.1V and black at 0.4V (measured). So sync is getting halved.One possible solution would be a current sink into a negative voltage. But without service manual I don't know where to get a negative supply, if there is even one.For now I leave it as it is, because the devices it prepared it for should be able to sink current. If not I can get a propper VCR anyway.So thank you for your help and have a nice day.