Manor Supplies Cross Hatch generator MK2

[dragonser]

Hi,
while sorting out some paperwork I found by chance the instructions for the Manor Supplies Mk2 Cross hatch generator.
I have one of these here that I bought from Manor Supplies [ it was a long journey to collect it in person ! ] many years ago, but had misplaced the instructions.
I was wondering, as it seems to use some 555 timers for the sync pulse timing, if it could be modified easily to either 525 line or 405 line operation ?
Of course the UHF modulator wouldn't be any good for 405 line sets.
I know that 555 timers aren't always the most stable devices but I thought it might be a budget way of being able to try out any 405 line receivers.
regards Peter

[terrykc]

If it is all based on 555 timers, it might be possible - though the crosshatch won't be square any more!

It all depends how the field sync and horizontal lines are derived, though - you could end up with 33 1/3 fields per second (625 lines but at 405 line horizontal rate)!

A decent scan of the schematic would be useful ...

[dragonser]

Hi, I'll get the schematics scanned and uploaded, you make a good point about the crosshatch pattern. Like most modifications, what can seem as being straightforward, often isn't.
regards Peter

[Alistair D]

Won't the video polarity need to be inverted for 405?

Al

[terrykc]

Originally Posted by Alistair D

Won't the video polarity need to be inverted for 405?

Yes! But that's the easy bit ...!

[Vicboduk]

Your post prompted me to have a look at my old cross-hatch generator that I bought as a kit from Tandy back in the early 80’s. It uses just three standard 4000 series CMOS IC's and has a simple on board oscillator/modulator with harmonics extending into the VHF & UHF bands. Despite its simplicity it remains quite stable. So, if you find modifying your generator difficult have a go at constructing this and change the relevant values.
If you need a higher resolution scan in colour, (easier to see the PCB layout) please PM me.

Vic

[ppppenguin]

When I got my first colour TV in about 1974 (an ex rental Thorn 2000) I needed a crosshatch generator. ISTR I built a design from Wireless World that was just a few TTL chips on a bit of veroboard. I think it took H and V sync from the set and injected into the video path. It worked well enough to converge the set.

My 2nd generator was a pirate version of a Labgear. I was in my gap year at BBC Research Labs (1974/5) and the PCB artwork etc was circulating. Somebody had arranged for a batch of PCBs to be made unofficially by the in house facility. I think most of the bits came from drawers in the lab. I made an aluminium case which somehow got spray painted in the BBC workshops.

I haven't used it for some years but it worked OK at the time. One major design nasty was that the subcarrier was dirctly divided down by 284 to approximately line rate. This was 15611Hz instead of 15625Hz. This didn't affect the H lock of the TV but the colour dot crawl patten was stationary and blatant. The PAL modulator was a TBA520 receiver chip.

[peter_scott]

If there were enough replies, DIY cross hatch generators might justify a thread on their own. I certainly know of a couple of work colleagues who made them about the same time as me.

Like Jeffrey, I made mine so as to fix-up my first colour tv back in the early 70s. It too was an ex-rental set, GEC2040 I think. Here's my effort complete with the "service manual" stuffed down the side.

Does anyone else still have their DIY cross hatch tucked away in the loft?

Peter

[ppppenguin]

Little did I know then that designing SPGs and test pattern generators would form a substantial part of my professional life. I also designed my first SPG around the same time. Again built using bits from the lab drawers. I wince when I look at the design but we all had to start somewhere. I think I may still have it somewhere, complete with a few small caps soldered on to suppress runt pulses. It was several years before I learnt the discipline of synchronous logic design.

[AC/HL]

That case looks like an HP counter I have to repair sometime!

[peter_scott]

Yes, any similarity to the HP5300 counter is purely coincidental!

Peter

[jimmc101]

Originally Posted by peter_scott

Does anyone else still have their DIY cross hatch tucked away in the loft?

Mine based on the ZNA234 (http://www.datasheet4u.net/datasheet...ctors.pdf.html) is in the wardrobe, does that count?

Jim

[ppppenguin]

Here's my first SPG from 1974/5. All standard TTL I think. 1MHz xtal. Definitely generated mixed sync, mixed blanking, line drive and field drive. May also have done burst gate and PAL square wave. All pulses just about complied with broadcast code of practice though the timings must have been a bit out with only a 1MHz clock as I don't think there were any monostables. Outputs were TTL level. I made a separate buffer board to convert TTL to 2Vp-p -ve going as used in TV studios.

[dragonser]

Hi,
sorry the the delay.
many thanks for all the interesting replies. I've scanned in the page with the 555 timers [ one for the line frequency and the other for the field frequency ]. I will find out how far it is possible to change the frequencies with the existing preset controls, and let you know.
regards Peter B

[terrykc]

As the field frequency is unchanged, you only need to adjust one!

You will probably need to increase the value of C1 (or add another capacitor in parallel with it) to get the frequency down low enough.

It is a very simple, if crude, design, so you shouldn't have any problems.

If the crosshatch part is equally simple, a similar single component change in the vertical line generator should complete the job.

As has already been mentioned, you will have to get the modulation correct if you intend to generate RF (but, presumably, you won't be doing this with the 'optional' UHF modulator shown!)

[dragonser]

Hi,
I've done a couple of quick measurements, the field frequency can be adjusted from 43.6 hz to 52.8 hz using the trim pot.
this seems ok. However the line frequency can be adjusted from 12.6 khz to 14.6 khz using the other trim pot ?
[ which doesn't seem to get high enough for the 15 khz approx 625 line frequency ].
However after looking at the resistor colour codes it seems that the resistors are 5% and measuring the resistor in circuit, but with the 555 timers removed gave a reading of 71.4k for the resistor marked 68k . I think this explains why the line frequency range isn't quite right. At least with discrete components it is easy to make changes !
many thanks again for all replies.
regards Peter
P.S anyone know what the line frequency would be for the 525 line standard ?

[FERNSEH]

The pre-colour line frequency of the US 525 line standard was 15,750Hz. field frequency 60Hz
After the introduction of colour a slightly different frequency was called for.
I believe it is something like 15,734Hz. Field rate is 59.94Hz or thereabouts.
Adjust for 15,750 and 60Hz normal service purposes.

DFWB.

[terrykc]

Originally Posted by dragonser

... it seems that the resistors are 5% and measuring the resistor in circuit, but with the 555 timers removed gave a reading of 71.4k for the resistor marked 68k ...

68k ±5% = 64.6k - 71.4k so what you have is a 68k +5% resistor, which is perfectly acceptable.

However, based on the LM555 data sheet with R2 = 68k and R3 = 18k (difficult to read in the scan but I think it's 18k rather than 15k) the nominal active period would be ~49µS and the nominal blanking period ~10µS making a total of 59µS or 17kHz. When the frequency is correctly set to 15.625kHz, the blanking period will increase to just under 11µS which is still a little on the short side.

For 625 line operation these would nominally be 52µS, 12µS, 64µS and 15.625kHz.

For 405 line operation, the timings are all ~50% greater: 81µS, 18µS, 99µS and 10.125kHz so, when you have got the 625 line timing right, increasing the value C1 by 50% should give you the correct 405 line timing.

Purists might quibble with some of these figures but, considering the simplicity of the circuitry, adding extra decimal points would be purely academic!

Playing around with this design in Excel suggests that more accurate timing would result if R3 was increased to 22k.

This gives 51.1µS, 12.5µS, 63.6µS and 15.712kHz whilst keeping your 71.4k resistor is even better! If you can also find a slightly low 22k resistor ~21k to match it, the figures are 52.5µS, 11.9µS, 64.4µS and 15.518kHz.

What was I saying about decimal points ...?

Obviously, all of this assumes that C1 is exactly 820pF - which is highly unlikely!

All of the figures above exclude the effect of RV1. If I've understood the operation of the 555 correctly, the above figures would be correct when the voltage on pin 5 is 2/3 of the supply voltage. For a symmetrical adjustment range I would suggest adding a 2k2 resistor in the earthy end of RV1

[dragonser]

Hi,
many thanks for the additional calculations. the second resistor is originally 18k.
I used some online design tools for the 555 and this suggested that for 405 line operation a cap value of about 1n2 could be used in place of the 820pf cap. But I think your calculations are more accurate.
I'll try a 1% 68k resistor and check the frequency that is produced.
Interesting comments about the effect of rv1 and getting a symmetrical range.
the supply rail to this board is regulated so the 555 frequency doesn't drift as much as it would otherwise.
on the field side of things I may try reducing the 270k resistor to a 240k as otherwise the trim pot needs to be near one end of the adjustment to produce ~ 50hz
regards Peter

[pichacker]

Nice to see one of these being used again... I had one once and by the time I had finished it had a ZNA234 providing the pulses as I found the 555's drifted too much.

Interesting to see it up and running on 405.

Do you have a picture of the inside? If I remember it had switches mounted on the board with their shafts poking through the front panel.