Using Hedghog Mk1 as a modulator
 

I built two Hedghog Mk1's a couple of years ago and very good they are too!

They have a 405 line video output socket, but it would be handy to be able to feed 405 line baseband video into the unit and use it as a stand alone modulator. The idea being to use VHS tapes recorded in 405 directly.

Looking at the circuit of the Hedghog, analogue video is fed into pin 9 of the modulator chip, IC5 (MC44BS373).

Could external video be fed in at the same point via a blocking cap and a level control pot, or is it not that simple?

I'm not sure what other inputs the mod chip needs which might not be present without a 625 line input to the unit.

Many thanks
Nick

Re: Using Hedghog Mk1 as a modulator
 

I’ve not thought about that before, I know you can do it with the Aurora converter, as I used mine to try out an American set on 525 lines.

Regards
Lloyd

Re: Using Hedghog Mk1 as a modulator
 

Hi Nick
The Hedghog II can be used as a modulator only. The original Hedghog had not this feature.
However what you were asking could be done.

I would suggest injecting the video signal at the junction of C37 and R21. A normal 1Vpp video signal can be injected at this point without any level control.
But! if doing this the signal coming from IC2 must be isolated. Probably the easiest way to do this would be to lift R66.

If you wanted to do this and have it a permanent switchable feature.
You could mount somewhere convenient a SPDT toggle switch and a phono socket for the modulator only video.
Wire the common of the switch to the junction of C37 and R21.
One side of the toggle switch going to the new phono socket.
R66 could be changed to a through hole component and mounted between IC2 pin 7 and the other end of the toggle switch.
The switch would switch between the converter output and the new phono socket.

Frank

Re: Using Hedghog Mk1 as a modulator
 

That sounds quite simple to do, I may well fit it to one of mine!

Regards
Lloyd

Re: Using Hedghog Mk1 as a modulator
 

Hi Lloyd
It should be straight forward. The hardest part will be finding somewhere to mount the switch and socket.


Frank

Re: Using Hedghog Mk1 as a modulator
 

How about a 3.5mm mono earphone jack socket? The built in switch can disconnect the internal video when the jack is inserted?

Re: Using Hedghog Mk1 as a modulator
 

If you used the larger housing, there is space for a switch and an RCA socket:

Attachment 263458

Jac

Re: Using Hedghog Mk1 as a modulator
 

Many thanks for all the replies and suggestions!

I used the larger case to cater for the possibility of future modifications! It should be straightforward to fit a switch and extra phono socket. I quite like the idea of a 3.5mm switchable jack, but I've sort of standardised on phono for video connections, so I'll stick with that.

I think it will be a useful modification and will make the unit even more useful!

Many thanks
Nick

Re: Using Hedghog Mk1 as a modulator
 

It is always good to have some extra room.


Nick, let us know how it goes as I haven't tried it myself.

If anyone is thinking of using a jack socket. Just bear in mind that a toggle switch contacts are break-before-make but the jack sockets that I have seen are make-before-break.
IC2 will probably put up with this but the equipment that is been connected to the modulator may not.

Frank

Re: Using Hedghog Mk1 as a modulator
 

That's exactly what I did. The board lends itself to doing this quite easily. I'm pleased to say that it works very well so will become a permanent mod with a switch.

The chain of equipment was as follows:

Video out from DVD player into Aurora converter. 405 out from Aurora into Line In on a 3V22 VCR. Line out from VCR into new video input on Hedghog. RF output on CH5 into Marconiphone VT161 405 only TV.

I used the VT161 as it uses direct line sync and therefore is very tolerant of jittery line sync that comes from VCR signals. I really don't like flywheel sync as it causes problems with using sets with VCR's which I like to do.

Once everything was set up and the TV was displaying a good picture, I inserted what I thought was a cassette with a 405 line recording on it and pressed play.

I was somewhat surprised to see a slightly narrow, but otherwise perfect 625 line picture appear from the 625 line cassette I'd inserted by mistake!!

I didn't even need to adjust the line hold - the line timebase just cranked itself up to 625 speed without any fuss!! I was rather amazed!

The VT161 has always been a brilliant TV and gives superb pictures - the Mullard tube has bucket loads of emission!

The other thing that I proved was that the 3V22 works perfectly recording and playing back 405 line video. There are no issues as with later machines such as the 3V29.

Many thanks to Frank for the information and for designing the Hedghog in the first place - it's a superb piece of kit!

Cheers
Nick

Re: Using Hedghog Mk1 as a modulator
 

Hi Nick
Thanks for letting us know how you got on and confirming that it works OK.
A useful mod to the converter.

It was something that I had thought about including in the original Hedghog as well as some other things that I would have liked to have tried.
But the development of it had already gone on so long that I felt I had to draw the line somewhere or I may never have got it completed.
The modulator only function was then included in the Hedghog II and PSC converters.

Frank