8mm Forum: Starting the Son's strip down.

This is topic Starting the Son's strip down. in forum 9.5mm Forum at 8mm Forum.

To visit this topic, use this URL:
https://8mmforum.film-tech.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=9;t=000029

Posted by Kevin Faulkner (Member # 6) on June 24, 2007, 10:28 AM:

Ok so it's a very wet day here in the UK so I have decided to start stripping down the Son projector.
Once you get started it's quite easy with each part coming off as a subassembly. I'll take some pics later.

What I'm desperdao in finding out is just how this photocell works.
It's marked as a Cinetel GS.52 from Cinema Television Ltd. It looks like a small valve and has printed on the label 100Volts.

I have a circuit for the amp but it does not show this Photocell and what i would like to know is this:

The photocell obviously has to have a 100V supply but how is that applied along with extracting the signal in other words does anyone know how this type of cell works. I have worked with the normal semiconductor cells we have now but not with one this ancient.
All tips and help welcome as I want to put a modern amp in this machine but cant figure out how to extract the signal from this cell as well as supply it 100V (DC?).

There must be some members who have done this before so it would be good to know how others have achieved the conversion.

Kev.

super8 at mrelmo dot co dot uk

Posted by Paul Adsett (Member # 25) on June 25, 2007, 03:53 PM:

Good luck Kev. Why not report on your progress (with pics) as you go along? It should be an interesting ride.

Posted by John Whittle (Member # 22) on June 28, 2007, 09:03 AM:

Kevin,

Basically all the photo-tube pick ups work the same way. Between 90 and 100 VDC are applied across the terminals and then the light developes an ac voltage. A decouplicaing capacitor then blocks the dc and allows the ac audio signal to the first stage of the tube amp.

These voltages were quite easy to come by in tube circuits with both the low filament voltages for the tubes and the B+ voltage for the grids.

To use a modern (solid state?) amp would require fixing up a little power supply to get the 90 vdc (I wouldn't start with 100 vdc) and a suitable de-coupling cap (like .1/600vdc working) to get a signal for a low level (like mic) input.

Look at a circuit diagram for an Ampro or other 50's style projector. The Ampro amps were nice because they fed dc to the filaments of the first two stages of the pre-amps where others would just use ac through out. Also many of these amps are "hot" meaning there is no transformer and the "ground" of the amp could be hot if the line is plugged in wrong (or something is amiss in an old cap to ground inside the amp). They should always be started up on a variac and then read chassis to ground with a volt meter to prevent electrocution.

The easy solution is to replace the photo tube with a solar cell and load it with a 4.8K resister and input to a mic input on a modern amp. Then all you have to come up with is a suitable exciter lamp supply.

John

Posted by Kevin Faulkner (Member # 6) on June 28, 2007, 09:37 AM:

Thanks John. I have been studying the diagram for the amp and I just might try to rebuild the original Valve amp but with modern capacitors and resistors etc.

My first job is the rubber mountings for the motor. I cant see at the moment what to replace those with.

Kev.

Posted by John Whittle (Member # 22) on June 28, 2007, 09:45 AM:

Kevin,

Post some pictures and perhaps someone will come up with a unique solution.

Depending on the size, there are wiring grommets of various sizes and maybe cutting mounts from pieces of rubber automotive hose might be a possibility.

Rubber can be ground to size, so maybe you can manufacture something.

John

Posted by Martyn Stevens (Member # 861) on July 06, 2007, 02:03 PM:

I tend to favour using photodiodes or similar in place of photcells. I've posted quite a bit about this on my website, including details of where to buy them.

Martyn Stevens

Posted by Kevin Faulkner (Member # 6) on July 06, 2007, 04:13 PM:

Martyn, How about a link to your website as I for one would be interested.

Thanks, Kev.

UBB.classic^TM 6.3.1.2