SPT Video RF Modulator
 

Hello,

I have obtained a rack mount RF modulator today. I can't seem to find any information about SPT or this modulator. I have included a few photos. If you know anything about it, please let me know!

Re: SPT Video RF Modulator
 

Hello,

can't help with specifics I'm afraid, however SPT stands for Sound Powered Telephones who I believe were taken over by Redifon.

SPT originally made sound powered telephones which are used on ships as a backup means of communications, but I believe they diversified into onboard radio and TV systems.
So this may have been part of a onboard TV distrubution system, or indeed a land based system?

Mike A.

Re: SPT Video RF Modulator
 

Yes, SPT used to do distribution gear for marine use in the 1980s. They were based in Maldon, Essex. From this it looks like they were taken over by Thales after Redifon then that division was closed:
https://find-and-update.company-info...0MEL%20LIMITED

Re: SPT Video RF Modulator
 

SPT were in Malden in Essex and have been gone for around 20 years now.

I have a number of these modulators and a manual somewhere. They are high output vestigial sideband modulators to almost broadcast spec. The output is around 105dBµv. In your picture the left board is the power supply, the centre board is the actual modulator to BBC spec which gives an output at IF 38.9MHz vision. The module on the right is an upconverter to a (fixed) output channel. The digital display on the front panel is normally hard wired to display the output channel. Broadcast 2 to 12 and 21 to 69. Yours appears to show 88 which I think is cable channel S09. Be aware they may be setup with either a BG (5.5MHz) or I (6.0MHz) sound carrier.

Best way to test it is to connect it to an aerial input on a TV via around 30dB attenuator connect a video source and try to tune it in. Most modern TVs (except Panasonic) will tune cable frequencies, older ones may not.

Any more info required (including converting from BG to I) let me know.

Re: SPT Video RF Modulator
 

Could it be a possibility that one or more of the resistors on the display have gone?

I think there is also something wrong with the power supply. I am trying to figure that one out but after a while of being powered, it would trip the RCD .

Re: SPT Video RF Modulator
 

Is it possible to provide more information on the power supply? Like the output voltages or resistor value?

Re: SPT Video RF Modulator
 

The power supply is very simple. A toroidal mains transformer completely isolated with two output windings. One winding supplies a bridge rectifier, capacitor, and 18v regulator to the modulator. The other winding supplies another bridge, capacitor and -24v regulator to the upconverter. This winding also supplies an unregulated -35v supply to the display PCB.

Faulty resistors would cause display segments to go out not stay on.
Can't see how this is causing RCD trips unless the transformer is low insulation to its fixing bolt (unlikely) or possibly the mains input socket or switch.

Re: SPT Video RF Modulator
 

Perhaps the main input filter socket has leakage to earth causing an imbalance of live and nuetral, hence the rcd tripping.

Dave

Re: SPT Video RF Modulator
 

Theres a couple resistors on my power supply. I should have specified that. I will take the power supply out and see what I can find

Re: SPT Video RF Modulator
 

Broadcast kit with Belling Lee connectors. My my.

Out of interest how does it do VSB? It does not have very many components. SAW filter?

Re: SPT Video RF Modulator
 

No SAW filter. There are a set of filter cans on the modulator board. Do not tweak them.

Re: SPT Video RF Modulator
 

I'm also interested to know - what's inside that up-converter, and how come it's fixed channel?

Would I be correct to assume it's because it contains a high pass filter which is impractical to re-tune?

Re: SPT Video RF Modulator
 

The SPT Manuel provides no information on the upconverter. Only that it should be returned if faulty. I believe it was made in Germany. I never had any problems with it.

Re: SPT Video RF Modulator
 

Interesting. Whole thing looks like a maplin kitset from the 1970s but sold as a commercial product!

Definitely would like to see inside that upconverter if any owner of one has a chance to open it!

Re: SPT Video RF Modulator
 

SPT used to modify Ferguson TX9s and similar to have isolated chassis and line-level inputs - I used to see a lot of them in my past.

Re: SPT Video RF Modulator
 

When I have a chance to get around to it, I will take some photos and post it here.

Re: SPT Video RF Modulator
 

Circuit diagrams of the modulator and power supply below.


/Users/************/Desktop/Modulator.png


/Users/************/Desktop/Power supply.jpeg

Re: SPT Video RF Modulator
 

Sorry, but all you've done is told us where the pictures are located on your own PC. You'll need to upload them as described here:-

https://www.vintage-radio.net/forum/...ad.php?t=77650

Re: SPT Video RF Modulator
 

Diagrams here I hope.

Attachment 270512Attachment 270513