Retro SSTV anyone?

[Harry Dalek]

...In any way don't make the EHV capacitor too large.

True, but it's a balancing act between this issue and ripple. Harry's solution of using a relay to cut-off the supply isn't that bad, in this case only 700 or so volts needs to be disconnected. In the case of a much larger CRT you're looking at many kV's and relays to handle that are expensive.

Currently the time-constant of the -700V supply is about 0.5 seconds, the +300V deflection supply about 2 seconds and the -ve grid supply over 20 seconds, by which time the -700V supply is too low for the CRT to produce any light output and the cathode is also now too cool. So for now I hope I've got it covered.

There probably won't be many updates here in this thread for a week or two, I'm currently in Hong Kong then off to Myanmar (what was Burma) at the weekend.

Steve A.

I did have one worry about the relay idea and it was sparking across the terminals...i leave they idea with you and Klaas wiser minds than me .

The Crt is really one big high voltage glass capacitor cutting the supply to the crt it will still be charged ,i was thinking as with some high voltage power supplies a resistor is placed across the supply so when its turned off it earths its voltage away slowly or in this case at the speed needed to stop spot burn from happening .
I wonder if a neon lamp with resistor would do the same job.
I like your new logo Steve reminds me of 1970s for some reason.

[Steve Anderson]

The Crt is really one big high voltage glass capacitor...

Well even a fairly large CRT it's only going to be a few hundred pF so a permanent bleed resistor after the relay of even a few Meg. will collapse the supply in under a millisecond. But a relay is a bit of an overkill in this case, just biasing the CRT off should do it. The drive to OPTO1 is simply shut off. 3A and 3B are shorted, the AND gate of the opto goes Hi (open collector) and via the 2N3906 shuts off the 2N5401. Maximum -ve bias on the CRT.

I like your new logo Steve reminds me of 1970s for some reason.

Yea man! (Or was that the 60s?)

Steve A.

[Harry Dalek]

Very good Steve

Thats a different use of an opto switch and nice to see this part of the monitor schematic.

The logo could be 60s looks a bit like this old show. i remember the 60s thats when we had antennas for our tvs

[Steve Anderson]

Thats a different use of an opto switch...

I did use an almost identical grid-mod arrangement for the 36/48/72-line CRT NBTV monitor in the past using a 3" DG7-32. I say the past...I've just opened one of the drawings and the date is April 2008!

In this case I'll be dropping the PWM frequency somewhat as the modulation is of a lesser bandwidth. Which has allowed me to raise circuit impedances resulting in reduced current consumption and simpler power supplies. However, this is still a CRT. At least this one doesn't have a PDA/accelerator which would require an additional +ve supply of perhaps 1500V, e.g. a 3FP7. But a 3FP7 would probably be much brighter and with better focus. In that respect I doubt that the 3" DP7-5 will actually do the SSTV 120/128 line structure justice.

But as I have progressed on this I've always had a 5" 5ADP7 at the back of my mind. That will require a PDA voltage of +1500V and the cathode needs to sit at -1500V, a total of 3kV across the tube. But beyond that all the rest can be identical. No need to re-build or re-design...except the chassis!

Steve A.

[Harry Dalek]

Oh Steve your thinking of using a bigger better crt yes thats a bit of a conundrum sounds like theres a size difference ?
Thats a good system if it works reuse the same circuit idea ,i also liked Klaas advice to some one some time back its easier to reuse some thing designed to do the same job well than try and copy it badly .
Theres some good advice on the old forum .

[Steve Anderson]

The mechanical difference between the 3" DP7-5 and the 5" 5ADP7 is quite large. If I were at home I would take a photo of them together. For an increase of 67% in screen diameter the 5ADP7 is 2.7 times longer at 425mm - plus socket. The DP7-5 (and the DG/DB/DR variants) were made short tubes at 160mm for compact equipment. Because the screen is quite close to the deflection plates it needs a lot of deflection drive. The 5" tube should require significantly less drive than the 3" one.

Whatever, it still would need a new chassis.

Steve A.

[Harry Dalek]

The mechanical difference between the 3" DP7-5 and the 5" 5ADP7 is quite large. If I were at home I would take a photo of them together. For an increase of 67% in screen diameter the 5ADP7 is 2.7 times longer at 425mm - plus socket. The DP7-5 (and the DG/DB/DR variants) were made short tubes at 160mm for compact equipment. Because the screen is quite close to the deflection plates it needs a lot of deflection drive. The 5" tube should require significantly less drive than the 3" one.

Whatever, it still would need a new chassis.

Steve A.

Is the main problem with the smaller one your using the possible focus size to a point, it is electro static focusing ? my old one was magnetic and i was never happy with that form of focusing as the coil got warm i lost a bit of focus after a while more than likely needed more windings .

Are the voltages about the same ? OH i see the deflection is a problem or Perhaps you would not want to do this but send the voltages to a plug and have the large tube in a empty case just having the voltages cabled to its plug....i wonder if both crts could work side by side ,i suppose its a bit like how a FSS system used this sort of idea to have a camera monitor to use the same scanning electronics.

[Steve Anderson]

All the CRTs I have are both electrostatic deflection and focusing. The DP7-5 datasheet says that the spot size is 0.7mm maximum so on a 3" tube face the limit of resolution is going to be 60-70 lines. The 5" CRT has a spot size of 0.5mm at the voltages I intend to run it at so the resolution will just be good enough for 120/128 lines.

The larger deflection drive required for the smaller tube isn't a problem per se, it simply needs it due to its mechanical layout. The +280/300V HT for the amplifiers is ample...well there's a comfortable margin.

The voltages for the two CRTs are quite different, but in the area of deflection, modulation, focus and astigmatism they are quite similar, indeed most CRTs of this nature are. The main difference is in the acceleration voltage(s) required depending on if the tube has a PDA or not.

As I mentioned before, as I've gone along with this I've had the 5" tube in the back of my mind. But my intention was/is that after the 3" display is completed was to move on to a up-converter for SSTV-625 lines. So I may do the 5" version or I may not, still undecided.

Steve A.

[Harry Dalek]

All the CRTs I have are both electrostatic deflection and focusing.
Steve A.

Yes Steve i got a Scope a while back, early 90s vintage this one has a rotation coil what ever that is as its fixed in place perhaps a once only adjustment !

Bit off topic here but i will be naughty

i picked this up a while back for 5 bucks when i got it home all the panel lights up but not that crt so its been sitting to be looked at for ages i was thinking oh the CRT is gone but i noticed the other day from a quick look in low light the heater is lighting up .

So i am fault finding now ,i removed the crt thinking the case where the High voltage section was under it but it was the under reverse side , i will do a homer simpson DOH !!!!!!!!!!!!!!!

Had seen it looked to be opened before me to fix ? but looks clean undamaged ,i saw in the high voltage section it has two neons which i would expect to light up if it had the correct anode voltage .

Theres nothing no tube high voltages only circuit main voltages ,i found this schematic for this scope the part of the pdf with the crt i started to test for correct voltages ,the top left hand corner theres a plug co404 the +14 is not there its -13.5 same as the correct -13.5 on the same plug other connection ,i then checked where these voltages came from and there is a +14 at the power supply ,i am thinking some ones made a soldering mistake as this oscillator for the high frequency transformer is not going to work without that +14 volts so i have the 120 and the -13.5 for some reason also theres no fuse in the circuit as shown ,thinking of disconnecting the wrong -13.5 and running a wire from the power supplys 14 volts to the where the problem is .

Just wanted to know those neons would light up with it working ? ,i like Visional rewards best of all would be a green trace on the CRT.

Funny this thing has a long light bottom of the CRT face that you can adjust i have not seen that before and not sure why its there ,its dual trace and i think it has z connections at the back if blanking plugs are the same thing ?

[Steve Anderson]

The trace rotate coil allows you to offset the slight tilt of the trace caused by the earth's magnetic field. All CRTs suffer from this, but it's only those with an internal graticule etched into the tube face that usually have this control...otherwise you simply rotate the whole tube...or, as in this case, it has a rectangular face. It is usually a pre-set pot which only needs setting on occasions when the instrument is moved large distances, say from the equator to Australia.

Document what you do, remember this did once work and unless there's obvious signs that someone has been 'fiddling' with the innards I would look for other reasons.

The neons might not glow under normal operation, they're often used for flashover protection, a sort of poor-man's zener. The fact there is a 2kV capacitor right next to them implies this.

Steve A.

Having downloaded the pdf, it does look like those neons are used for flashover protection.

A good find at only five bucks!

[Harry Dalek]

The trace rotate coil allows you to offset the slight tilt of the trace caused by the earth's magnetic field. All CRTs suffer from this, but it's only those with an internal graticule etched into the tube face that usually have this control...otherwise you simply rotate the whole tube...or, as in this case, it has a rectangular face. It is usually a pre-set pot which only needs setting on occasions when the instrument is moved large distances, say from the equator to Australia.

Arrrrr thats interesting i will have a peak in my other scope to see if its got one ,i was thinking at first it had some thing to do with focusing till i saw the schematic.

Document what you do, remember this did once work and unless there's obvious signs that someone has been 'fiddling' with the innards I would look for other reasons.

OK i have to double check but there are signs of an invader ! ,some ones been in there around the CRT as the bolts that held it in bolted to the case were bent and one was missing why they were bent is puzzling to me .least they didn't damage the crt .
I am not sure where this scope came from i got it at a reuse shop so some one gave up onit perhaps they were just looking for a fuse which this thing does not have ! theres also a extra little 240 volt transformer which looks to me to have been added and thats also only half bolted on have to look into what the hell its been added for, all the power supply voltages are correct from the main transformer so again puzzling .
At the moment all i am doing is taking readings and looking ,for me the High voltage section as an obvious reason the crt has no trace after i saw the main power supply was fine and the crt heat was lighting .

The neons might not glow under normal operation, they're often used for flashover protection, a sort of poor-man's zener. The fact there is a 2kV capacitor right next to them implies this.

OK no flickering gratification for me if i get it going ,i did double check here its all o voltage after that high frequency transformer .

Steve A.

Having downloaded the pdf, it does look like those neons are used for flashover protection.

A good find at only five bucks!

[/quote]

I nearly gave up on it my self till i saw the heater light up .
i was lucky that day they have no set prices for the junk they sell just have to pick the right day some one who does not care or doesn't pocket extra charge !
let you know how it gos .

[Steve Anderson]

OK, so all +/-8V is there, +5V present and about +14/-13.5V are present, +120V and +220V are OK? If that's the case and they are present around the inverter circuit, power off and make sure the windings of the inverter transformer aren't open-circuit or shorted where they shouldn't be.

As for the mystery extra transformer - who knows? I would trace out what it does, maybe one of the windings on the original power transformer has gone open circuit.

It looks in too good a condition to scrap even if it might need some serious internal modifications.

As you do this work, run it without the CRT - put that somewhere safe. Generally a scope will work and you can test all voltages and waveforms without the CRT. You did say you had another scope - often you need a scope to fix a scope!!

Steve A.

[Harry Dalek]

OK, so all +/-8V is there, +5V present and about +14/-13.5V are present, +120V and +220V are OK? If that's the case and they are present around the inverter circuit, power off and make sure the windings of the inverter transformer aren't open-circuit or shorted where they shouldn't be.

I haven't checked the 8v or +220 so i can't say on those yet ,but i have checked the high frequency transformer windings and they seem fine i looked into that a few days ago ,as to shorted windings in that transformer mmm this i have not looked into .

As for the mystery extra transformer - who knows? I would trace out what it does, maybe one of the windings on the original power transformer has gone open circuit.

Well heres 2 pictures you see the little transformer and the thick white double cable go to that little board ,i have not checked the voltage to the heater of the CRT really perhaps this is for that but just guessing need to check that , the little board it gos to looks out of place to me looks like an add on .

It looks in too good a condition to scrap even if it might need some serious internal modifications.

Nice mid sized scope at the back it has a plug for external Blanking and channel 1 output that i am not sure about what thats for or useful for .

As you do this work, run it without the CRT - put that somewhere safe. Generally a scope will work and you can test all voltages and waveforms without the CRT. You did say you had another scope - often you need a scope to fix a scope!!Steve A.

Its in that draw safe with my spare HDS

These are my scopes remember the 50 year old one you helped me with a a year or 2 back i fired it up the other day still works ,the other 2 are a 80s trio and a very old valve scope and my newer 90s dick smith scope ....thats the one worth using to check this one .

I will get stuck into it later this week but i can check this and that after work next few days .

[Steve Anderson]

Check all those voltages and make sure they are there, from the +/-8V, +5V, +14V/-13.5V and the +120V, +220V. If they are present you can be sure it's either the inverter has failed or there's some sort of short/failure on its output.

As I mentioned, trace back and find out what the heck that additional transformer is doing. That will probably give you a big clue as to other potential problems. Do not give up at this stage, it is worth the effort.

It looks like the instrument has suffered a blow or been dropped at some time in the past due to the bending/buckling of PCB supports, and the mention of your own observations of bent CRT retention screws. However, CRTs are quite rugged and there's every likelihood that this one has survived intact.

This is worth pursuing.

I don't mind this thread diverging for a while as the technology is all related. If the CRT has a rectangular face without an internal graticule - absolutely ideal for NBTV even if not for SSTV - that depends on the phosphor, most likely P1-like.

It's also quite a low-voltage CRT for its size and the rectangular face...count your blessings...

Steve A.

[Harry Dalek]

Check all those voltages and make sure they are there, from the +/-8V, +5V, +14V/-13.5V and the +120V, +220V. If they are present you can be sure it's either the inverter has failed or there's some sort of short/failure on its output.

It doesn't show up in the schematic and i just noticed it but there was a blown or open fuse on the circuit board at around the + of the 47uf cap that deals with the +211 voltage now i took the below readings after i replaced it so really need to check again with and with out as readings seem different to what i checked the other day.

Ok i had a little time this after after a mower repair this is more enjoyable to work on it ,finding all the voltages on the main power supply board is a bit tricky all the part codes are on the other side i don't want to pull every thing to bits ,with my glasses on and a light viewing at strange angles i can make out where most of it is , can see where all the Voltage regulators diodes are least its easy to test the solder side of the board .
Now what i found so far on the High voltage side
+120 is showing up as +117 volts
+220 is +211
I checked again the voltages at the inverter the plug i mentioned i am now getting
+12.8
-13.4
+117

I was hoping the fuse was the problem plugged the crt back in and still diddly squat checked for any thing out of the inverter and nothing mmm

As I mentioned, trace back and find out what the heck that additional transformer is doing. That will probably give you a big clue as to other potential problems. Do not give up at this stage, it is worth the effort.

Its a mystery yes i must find out, i do know its putting out 9.6 volts ac to that little circuit board .

It looks like the instrument has suffered a blow or been dropped at some time in the past due to the bending/buckling of PCB supports, and the mention of your own observations of bent CRT retention screws. However, CRTs are quite rugged and there's every likelihood that this one has survived intact.

OH yes its got a big ding in the back around the plugs ,i am lucky that CRT is still lighting up had rubber holding it in place with the brackets so that might of saved it .

This is worth pursuing.

With out the CRT i would of given up but i want to get it going knowing its ok .

I don't mind this thread diverging for a while as the technology is all related. If the CRT has a rectangular face without an internal graticule - absolutely ideal for NBTV even if not for SSTV - that depends on the phosphor, most likely P1-like.

IT does have that internal graticule a pain but i can live with it

It's also quite a low-voltage CRT for its size and the rectangular face...count your blessings...

Steve A.

Thanks Steve ,i didn't want to take over your subject with this little side project ,if it gets going it will be used in projects for sure ,i'll swap over on the next post to its own ,i have a feeling this is going to be a tricky one .