Unaohm EP 685A GB TV pattern generator.

[Vintage Engr]

I acquired this unit recently, as part of a lot of mixed test equipment. It seemed like a good candidate for restoration. I seem to have a soft spot for test equipment.

It had not been stored under ideal conditions, & failed a safety test due to corrosion & breakdown of the mains input wiring to the power supply transformer.

Yesterday, after gently drying out the transformer, it was apparent that the insulation on the wiring to the windings on the transformer had decomposed, gone very hard & in so doing had released a very acidic substance, This had corroded the copper on the stranded lead-out wiring to the primary. After removing the transformer & carefully cutting the old wiring back, I managed to extend the leads & heat-shrink the joints, as close to the transformer as possible.
I replaced the transformer back into the chassis, & reconnected it.
It now read o.k., or seemed to, so I powered it up.
I was missing one supply, a negative rail, which I was going to look into later today.

However, on powering up the unit, there was a growling from the transformer, together with the burning varnish smell.. & now there is a partial shorted turn/s somewhere.

If I had any service information, I would know what the various supply rails are & could deduce what might be suitable as a replacement.
That's not a current option, so I'm hoping that there is someone on the forum who may have a working unit, so that the outputs from the mains transformer/ PSU could be measured.

There are five connections to the secondary of the mains transformer. One is fairly obvious & is probably around 12v. As there is an electronic tuner in the generator, there's also likely to be a 33v or thereabouts supply. There is also 74 series logic inside, so a 5v supply must come from somewhere, although not immediately obvious as to where it is derived from.

So, I'm hoping someone has a working example of this unit that they would be willing to check the supply voltages on!

Thanks,

David.

[MotorBikeLes]

The 33v line is more likely to come off a 200v line with a dropper(27K to 47K) and a TAA or ZTK 33v stabiliser.
Likewise for a 5v supply you will need about 8v before a 7805 or similar.
Les.

[Vintage Engr]

There's no 200v line, it not a TV... but thanks. I have now traced the wiring from the transformer, it is simply derived from a 28v winding, a 1N4007, a couple or R's & C's and the ubiquitous TAA550.

All I need to do now is source a suitable small mains transformer with a centre tapped winding of around 9-0-9v and a 28 - 30v- ish separate winding.

David.

[Ed_Dinning]

Hi David, if you have plenty of space it may be better to fit 2 separate transformers, as an alternative you could get yours rewound

Ed

[Vintage Engr]

Thanks Ed,
I wondered if you'd pop up on this discussion. It seems that this was a common fault on these transformers, They look well made, but are physically on the small side. I had considered two transformers, so will look at what I have here.

I have so many transformers here, & unfortunately never catalogued them, so it's a case of going through them to see what I have. There's not much space available inside the pattern generator, the original is a C-core type. I hoping I might have a small toroidal that will fit.

David.

[Vintage Engr]

Having now acquired a cct diagram, it was easier to sort out the voltage requirements for the transformer, & also to do a quick test with a bench PSU to see if the repair was worth progressing. For testing, I ignored the -33v line, as I was only using the composite video o/p.

Having connected a +/- 5v supply to the power rails, I was rewarded with some, if not all of the test patterns. A quick clean of the function switches, and all the patterns were there, albeit in monochrome. A check with a 'scope revealed no burst, so I checked the 4.433 oscillator.

It wasn't, - oscillating that is. The DC voltages around the oscillator were sensible, so I replaced the crystal, & up came the colour. After allowing the osc. to warm up a bit, I managed to get it fairly stable (it's not ovened!) at 4.433619 where it surprisingly stayed. The circles where a bit odd, but after making some preliminary alignment, the whole unit is working well. Because the circle generator in particular is very fussy, power-rail-wise, I shall leave the final setup, until I have replaced the mains transformer, and the unit is working on it's own discrete regulators. Likewise the encoder, which I will check & set up on the vectorscope once the basic repair is completed.

All good so far, I may decide to use a single 9-0-9v mains transformer, together with a DC-DC converter to obtain the -33v for the tuning system.
This would save on space as there's not a lot of room for a large dual output mains transformer.

Now to search for the right transformer, I have quite a collection, it just finding the right one!

David.

[No.1 Labrat]

I have worked on one of these recently.
You might find that all switches are dirty and the toggle switches open. By-pass the R.F. attenuator.

[Vintage Engr]

Thanks.
Yes, I already found that the switches can be troublesome. Once I have the new mains transformer, I can re-connect the -33v line & will know whether the RF modulator is working, as well as the attenuator.

Very hairy the way the mains switch is positioned & connected to the LED indicator! A trap for unwary fingers. I will put a boot on the rear of the switch & power the indicator from the +5v line...

David.

[Vintage Engr]

Latest update. Having found a very suitable mains transformer, I have managed to fit this into the original transformers' mounting frame. That has provided the +/- 5v supplies. For the 33v tuning supply, I sourced a small PCB - mount 15-0-15v transformer. This was small enough to mount on a board, bolted to the main transformer, so fits neatly.
On switch-on, as expected, the composite video output worked, as this had been previously tested on a separate bench PSU.
However, there was no RF output from the modulator. Also the small transformer was starting to get rather warm... I therefore switched off immediately.
The reason for this fault was immediately traced to the reservoir capacitor on the -33v line having gone totally dead short circuit. Replacing this resolved the problem.
I now have to unearth a suitable TV to check the RF output with. ( I can see one, but can't get to it!). I do have an Off-Air Tuner somewhere, but cannot remember where it is. Definitely time for a clear-out.

David.

[Vintage Engr]

Have found a suitable TV, (In this case a Radio Shack 12v portable CTV) I was instantly rewarded with a good quality display from the RF output of the pattern generator, but no intercarrier sound.

I checked the output of the intercarrier sound section, no R.F., but the internal 1KHz audio generator was working o.k. At this point I referred back to the recently obtained copy of the circuit diagram to find that unfortunately, the copy was missing the part that switched the intercarrier sound on & off. I found that there was no DC power to the 6MHz oscillator, although operating the 'int. mod' switch did toggle the audio oscillator on/off. Tracing the DC for the I/C osc led to the MOD switch. One pole of this was permanently open.

No amount of switch cleaner had any effect, so I replaced the DPCO switch, & now had intercarrier sound. As kindly suggested by No1 Labrat, I did at this time check the RF attenuator switches, having had the I/C sound switch fail. Fortunately all the attenuator switches were perfect, with the attenuation steps as expected.

So, finally, & after some re-alignment, this unit is back to its full operating specifications. Whilst obviously not up to broadcast standards, (there is no SC/H relationship, & no 17.7MHz master osc) it does provide a useful set of test patterns, including a circle, for use when restoring domestic TV receivers.
As previously mentioned, this unit was purchased as as a job lot of old test equipment, & not particularly well stored, so I am pleased with the result.

For reference, should any other member have the same problem, which is a common fault, with the failing mains transformer, the one I used as a replacement was a Vigortronix VTX-126-012-4075. (Plus the small 15-0-15v PCB one).

Was it worth it? Financially no, but then I can't help wanting to rescue old test gear that would otherwise end up as skip fodder!
Now on to it's mate, a Unaohm field strength meter, which appears to use the same or similar mains transformer...

David.