5AHP7A Vintage SSTV The Deep image

[Harry Dalek]

True, the combination of the CRT internal last anode coating and the external 'Aquadag' coating do form a capacitor. The Aquadag being at ground/0V. This can be several hundred pF on larger TV tubes and with the flyback transformer running at 15.625kHz results in a sufficiently smoothed supply, though not regulated.

They can hold a nice charge of Kv for the unwary a big glass capacitor for sure .

On a smaller CRT like this one there may be a need to add some form of additional smoothing. Again this is an area where I have little experience.

I looked back on a past try and i had no problems ,but i will look into it back then i didn't even think about a capacitor i'll see what i can do it will not hurt and i can see the benefit.

Because this appears to be half-wave rectification the diode(s) PIV rating needs to be more than 14kV if your supply is to be 7kV. If you've used microwave oven diodes in the past successfully I suggest sticking with them. I don't think having many 1kV diodes in a string is a good idea.

OK Yes microwave Diode has worked before less messy than with a string of KV diodes there might be a sparking hazard

The only thing nagging me is the speed of microwave oven diodes. These are intended to be used on 50/60Hz mains frequencies, I'm not sure how good they might be at several/dozens of kHz. I guess there's only one way to find out...

I had used it on my flyback transformers at that frequency with no problem ,its pretty rugged device

Steve A.

Afterthought...if you had four of these diodes and the output of the HV supply is truly isolated from ground a bridge rectifier would be better. Lower PIV rating required on the diodes, half the ripple, and no DC current in the windings. Even my local supplier has some 8kV diodes in stock which are quite fast, 100ns switching time. Around US$0.30 each in small quantities. Datasheet attached.

[/quote]

That would be nice ~! i only have 2 i think not many ....not easy scrapped only one per microwave oven and they would all be different makes and ratings ! i don't enjoy scrapping those much as i tend to just want the diode and the tray geared motor so a lot of rubbish for the little parts the caps would more than likely hold a few KV ,have a few of those but never had a use for them and they are pretty big.
Looked at your data sheet they would come in handy i will have to look into those for the future its an interesting idea and i see your point of the bridge rectifier for this it would be spot on for what i am doing ,i will just have to make do with what i have this time around ...i will keep that idea in my mind as i like it !

[Harry Dalek]

I looked into if there was any data on my module and it seems not rather annoying but reading the questions and answers from the sellers site i now know i was wrong about it being Ac its DC one fellow was happy with it using it on hes Crookes Tube so that makes life less messy no diode needed i double checked and it sure is ,the reason the out put leads are not colour coded it seems its used for a bug zapper and there was no need ......No consideration for those doing crt work
As far as if theres a capacitor i have no idea ! i will see how it go's only way to find out pointless putting one in if theres no need.

It fits next to the CRT and getting a nice 7000 volts .

I next have to make the dual 12 volt power supply then some testing on the deflection side of things .

[Steve Anderson]

Yes, how inconsiderate of them not to colour-code the output leads! DC out, good, that's one issue dealt with.

As far as an external/additional capacitor goes I would wait and see if one is actually needed. If it was meant for use in a bug-zapper I would suspect there is one, that's what gives you that satisfying 'crack' when you zap the buggers.

Steve A.

[Harry Dalek]

Yes, how inconsiderate of them not to colour-code the output leads! DC out, good, that's one issue dealt with.

As far as an external/additional capacitor goes I would wait and see if one is actually needed. If it was meant for use in a bug-zapper I would suspect there is one, that's what gives you that satisfying 'crack' when you zap the buggers.

Steve A.

Yes Steve a few people who did buy this thing had the same problem not enough information any way give it a go when its time to fire it up we will see .

No bugs for this one to zap !

I will start working on the dual supply tomorrow have a bit one today the birthday season has started not mine yet thank goodness .

[Steve Anderson]

A reminder. Just as before with the 3BP1 I would keep the CRT heater disconnected (and the several kV) until you're sure that the other CRT voltages are 'about correct' and have reasonable waveforms where required.

Looking at the datasheet for this CRT it should be possible to use a large percentage of the 3BP1 circuits on this, with the obvious exception of deflection. But there are quite a few published designs that appear equally suitable.

Steve A.

[Klaas Robers]

Good work Harry!
Can you measure the ohmic resistance of the two deflection coils? Mine are 5 ohm (H) and 10 ohm (V). However the two H coils were originally routed in parallel. I opened the deflection unit to change this and put them in series. So the H was originally 1,25 ohm. This is done to make the needed current smaller. The voltage is no problem any way.

For the deflection: make saw tooth voltages, one for H (about 16 Hz) and one for V (about 8 sec.) and make two identical deflection amplifiers. My amplifiers run on +6V and -6V. That is sufficient. The currents needed are about 300mA peak. That is positive and negative. See that the amplifiers get a current output. So the currents should be saw tooth shaped, not the voltages. I did this by routing the 'cold' end of the coil to ground via a resistor of 5 ohm (two resistors of 10 ohm in parallel). Then the voltage over this resistor is compared with the saw tooth voltage. Kind of power-opamp circuit.

My HV supply is a fly back converter with a 88 mH "Pupin" coil. It gives +100 V for the video amp, +250 V for the focus and +5kV via voltage multiplication. I should see if I can send you the circuit diagrams.

[Harry Dalek]

A reminder. Just as before with the 3BP1 I would keep the CRT heater disconnected (and the several kV) until you're sure that the other CRT voltages are 'about correct' and have reasonable waveforms where required.

Looking at the datasheet for this CRT it should be possible to use a large percentage of the 3BP1 circuits on this, with the obvious exception of deflection. But there are quite a few published designs that appear equally suitable.

Steve A.

Yep i want to know i have deflection on the yoke coils ,i don't want to burn my P7 screen .
I will leave the 7Kv off and heater till the deflection looks good enough to do a test run thats all still a ways off .
I know it could be done a lot simpler but i liked the look of DCX/CLG CQ magazine DEC 1975 issue .
Made the circuits some time ago did some testing ..i have no problems changing things here and there if i have a problem.
Only really around the modulation side i will have to fiddle with as this sstv circuit uses a different CRT to mine and cathode modulation and I am using Ralph's example on the CRT Power supply and grid modulation ,the opto coupler like the 3BP1 would be perhaps a better option .
And again its a little ways off till i get to that ,just taking it a step at a time ,climb that ladder till each bit of it works .

[Steve Anderson]

Hmm, I don't appear to have a copy of that December 1975 CQ article. Though it could be tucked away in some obscure folder...I should really do a tidy-up!

Well I think you'll do just fine on this after the recent 3BP1 exercise. The only real difference is the speeds involved (much slower), the magnetic deflection and the higher voltages required for the CRT. So I'll be watching this with interest for sure!

Steve A.

I've already got designs on one of the bug-zappers here!

[Harry Dalek]

Good work Harry!
Can you measure the ohmic resistance of the two deflection coils? Mine are 5 ohm (H) and 10 ohm (V). However the two H coils were originally routed in parallel. I opened the deflection unit to change this and put them in series. So the H was originally 1,25 ohm. This is done to make the needed current smaller. The voltage is no problem any way.

For the deflection: make saw tooth voltages, one for H (about 16 Hz) and one for V (about 8 sec.) and make two identical deflection amplifiers. My amplifiers run on +6V and -6V. That is sufficient. The currents needed are about 300mA peak. That is positive and negative. See that the amplifiers get a current output. So the currents should be saw tooth shaped, not the voltages. I did this by routing the 'cold' end of the coil to ground via a resistor of 5 ohm (two resistors of 10 ohm in parallel). Then the voltage over this resistor is compared with the saw tooth voltage. Kind of power-opamp circuit.

My HV supply is a fly back converter with a 88 mH "Pupin" coil. It gives +100 V for the video amp, +250 V for the focus and +5kV via voltage multiplication. I should see if I can send you the circuit diagrams.

HI Klaas

I will measure those for sure tomorrow and let you know what ever they are ,it has 5 wires so this is interesting and meter test should explain whats what ~! i should of done this when i was fixing the casing around the coil connections.

So i am looking for a reading higher the better,the defection push pull circuit is runs on dual 12 volts yet to be tested in anger .

I will see how the flyback circuit works seems an easy fix ,i would be interested in seeing your circuit if you track it down it sounds very elegant ...i don't know if they are the same but i still have the original 88mH toroidal coils from my first sstv .....all thats left and a scrapped circuit board ...think i did what most people did when it looked like every thing was going colour and computer .
But Yes any circuits from your SSTV would be of interest to me and i am sure others !

[Harry Dalek]

Hmm, I don't appear to have a copy of that December 1975 CQ article. Though it could be tucked away in some obscure folder...I should really do a tidy-up!

Well I think you'll do just fine on this after the recent 3BP1 exercise. The only real difference is the speeds involved (much slower), the magnetic deflection and the higher voltages required for the CRT. So I'll be watching this with interest for sure!

Steve A.

I've already got designs on one of the bug-zappers here!

I will post you the pdf i know what its like after time things go missing just when you want them !

I hope so I feel since i did it before and pre internet days i should be able to do it again ,its so much easier now ,as i recall the thing worked first go unheard of for me but i did do the make and test as you go so i knew it should in theory do something ..what i saw BTW was an OWL test card short wave recording on tape ...No one to tell it worked back then .

Here on this go the deflection circuit looks ok but ,i swapped the dual 555 ic for 2 single 555's i have not got around to double checking due to the work on the 3bp1 but i think there was a mistake in the pin numbering on one as i was comparing it to a todays 556 and swapping the connections for 2 555's may be that ic was different back then or i just need to relook at it .

Any good on the bug zapper ,i don't think i ever saw one !

[Harry Dalek]

Good work Harry!
Can you measure the ohmic resistance of the two deflection coils? Mine are 5 ohm (H) and 10 ohm (V). However the two H coils

Just did the measuring on the yoke its connections are

Yellow green wires 38.3 ohm

Red orange 28.6 ohm
red blue 14.3
or
orange blue 14.3

That sounds easier to drive these so yellow green is vertical and others Horizontal .

[Harry Dalek]

Finished the low power dual 12 volt power supply and a positive 5 volts....now have to feed the wiring from the back of the case high voltage socket and wire the AC supplies to the high and low regulation circuits then thats out of the way .

In a worst case scenario as far as problems with main SSTV circuits i have made it can be used for any thing else i do as in modifications in the up coming circuits ....fingers crossed not .

Once the AC power supply i used for (the 3Bp1) is hooked up to this i can do some tests with the sawtooth circuits ,but thats a job for tomorrow now.

[Steve Anderson]

Harry, I've started reading through that article and in addition to the error in the -12V PSU there's something very wrong around the sync filter, U6a...or at least it looks like 6a, it's not a very good copy. It's the 741 with the two pots (500 Ohm + 50 Ohm) between its inputs. It's near the edge of the page adjacent to the date, "December, 1975", (Page 19).

At first I thought they had forgotten to show the '+' input of the op-amp grounded. So I simulated it, it becomes a filter when the '+' input and the right-hand end of the 50 Ohm pot are grounded - well, a filter of sorts, but nowhere near the 1200Hz it should be, but around 80Hz. I'm also very suspicious of the two caps marked .037 (37nF) - a non-standard value.

As a result I'm going to go through the whole thing to find what else may be wrong. One wonders if an errata was published in subsequent issues of CQ. Harry I suggest downing tools for a while on this.

Steve A.

Later...OK sorted out that filter. The caps should actually be .027 (27nF) and there are two errors around this op-amp. I'll sketch up the changes and add them here shortly...done...it's not going to be easy to find a 50 Ohm front-panel mounting pot these days I would have thought...

I've now also checked out the 6-pole low-pass filter which appears to be correct (centre of same page). I'll keep reading/looking.

Hmm, this is up to you Harry - but these days a large proportion of the logic could be simplified by changing to the HC series of chips and changing some of them to more recent versions which weren't available in 1975. Your call...it would also save you a lot of +5V power.

[Harry Dalek]

Harry, I've started reading through that article and in addition to the error in the -12V PSU there's something very wrong around the sync filter, U6a...or at least it looks like 6a, it's not a very good copy. It's the 741 with the two pots (500 Ohm + 50 Ohm) between its inputs. It's near the edge of the page adjacent to the date, "December, 1975", (Page 19).

Sorry the the late reply Steve bit hectic Friday afternoons...First up this circuit is on the first ic board 1 of 3 i made and it was the circuit ic u6a sync filter that was giving me trouble from the start ,if you have a look a page back number 2 i mention i played around a bit with the circuit as i recall the pot connections to the ic seemed wrong an did changed it ,also had change the 56k resistor to the filter to a lower valve much lower .

I also used a 741 for the limiter instead of the 709 circuit as posted on page 2

At first I thought they had forgotten to show the '+' input of the op-amp grounded. So I simulated it, it becomes a filter when the '+' input and the right-hand end of the 50 Ohm pot are grounded - well, a filter of sorts, but nowhere near the 1200Hz it should be, but around 80Hz. I'm also very suspicious of the two caps marked .037 (37nF) - a non-standard value.

I think i used 2 caps close as i could get this value ,i managed to get it to work i will jot down what i did when i have a look at it i hope tomorrow ....like always you think when a circuit gets to a magazine stage its been checked .Electronics Australia you had to show a schematic explain the circuit and send in the working device and they would check the circuit to the wiring ,I wish i had some readers comments to CQ that tried to make this SSTV would be interesting ,Still the original must of worked perhaps just forgetful notes drawings well it was the 1970's

As a result I'm going to go through the whole thing to find what else may be wrong. One wonders if an errata was published in subsequent issues of CQ. Harry I suggest downing tools for a while on this.

Steve A.

Yes i got ahead of my self in answering Yes there must of been a fair few Not so happy American Hams who gave it a go to find a few mistakes ,as i said i think one of the sawtooth oscillator 555s have a wiring mistake that i noticed ....

Later...OK sorted out that filter. The caps should actually be .027 (27nF) and there are two errors around this op-amp. I'll sketch up the changes and add them here shortly...done...it's not going to be easy to find a 50 Ohm front-panel mounting pot these days I would have thought...

OH thats great you worked it out for me ! i will check it against what i did some time back and change it to yours .i could parallel a resistor over a pot i have handy to what get that perhaps.

I've now also checked out the 6-pole low-pass filter which appears to be correct (centre of same page). I'll keep reading/looking.

That i think pretty much worked i recall testing it

Hmm, this is up to you Harry - but these days a large proportion of the logic could be simplified by changing to the HC series of chips and changing some of them to more recent versions which weren't available in 1975. Your call...it would also save you a lot of +5V power.

I used what i had handy Steve i have to have a look if any really are the old 5 volt versions ,i made a 5 volt supply for it without thinking did i use any old one versions or not ! you just reminded me ! I will look tomorrow ...This is the trouble restarting where i left off a few months ago ...bad memory

[Steve Anderson]

...I am very aware of the bad memory problem...I probably forgot you had built up some of the circuits for this.

If what you've got already is working as it should I would leave it, no point in re-working it all. If you spot errors or something refuses to cooperate let us know...

Steve A.