I'm repairing an Ekco TMB272 which in 1956 was claimed to be the only TV set that can work on AC mains and if required will operate from a 12 volt battery.
Switching on and after the longish warm time was greeted with a rather distorted raster. First job replace those TCC waxies in the frame timebase, all are leaky.
Little improvement after replacing the waxies. Next picture shows the unusual line output transformer case, it's an Ekco part for sure but certainly isn't the original.
Supply receiver with the test card from the Aurora converter. The results are a negative picture, AGC fault?
Till Eulenspiegel.
Hi Till, I think there is an AGC preset hidden in the depths of the chassis towards the front. It may need adjusting if it has been altered in the past. But quite likely loads of decoupling capacitors in the IF stages will need replacing.
To understand the black art of electronics is to understand witchcraft. Andrew.
Hi Andrew, the pre-set control on the IF amplifier unit has no effect on the performance of the AGC system. The cause of the fault will most likely be leaky decoupling capacitors on the AGC lines. Not forgetting of course that 0.1mfd capacitor in the tuner unit.
The original Mazda CRM93 CRT was replaced with a tube which was regunned by a firm here in the North-East of England. The Dunelm tube has a straight gun assembly not requiring an ion trap magnet, it differs from the original CRM93 in having a 6.3volt heater. A 21ohm resistor is inserted in series with the heater supply to the tube.
When repaired this set will display a bright and well contrasted picture.
Till Eulenspiegel.
Good progress today. The vision AGC system is fixed, changed leaky capacitors including the often overlooked 0.1mfd component in the tuner unit. The excessive brightness was caused by faulty decoupling capacitors in the vision IF amplifier, but patterning persists. The patterning fault turns out to be instability in the sound IF amplifier, again most likely the decoupling capacitors. Sync separator function improved by replacing the coupling capacitor between the video amplifier and sync valve grid. The frame linearity could be better but I'm sure I can get that sorted.
Till Eulenspiegel.
Patterning caused by instability in the sound IF amplifier
Hi Frank, today, I'll turn my attention to the frame timebase with the intent to improve the linearity.
The 0.25mfd coupling capacitor is suspect, see the circuit diagram.
Useful info: The connections to the mains lead plug.
The two 6F1 sound IF amplifier screen grid decoupling capacitors were replaced last night and that action solved the patterning fault.
Till Eulenspiegel.
These are very good little sets, I consider my one to be my best 405 set, I used it to test my 2 Hedghog’s when I built them too. Once all the caps in the frame stages are replaced it should be possible to get the linearity spot on.
The only thing that lets mine down is quiet muffled sound, not sure what’s causing it, one day I’ll get to investigate it.
Regards
Lloyd
Good progress! However will the fact that both the CRT and lopt are not original affect the performance in any way?
Hi Derren, it's possible that the line output transformer is the original part and perhaps only the Perspex housing was replaced after the original had disintegrated as most did in 1950s Ekco TV sets.
The replacement CRT is excellent but there is no indication when it was fitted. It is assumed that tube was rebuilt at the works near Crook in County Durham.
Till Eulenspiegel.
Frame timebase sorted out.
This set had an additional frame linearity control which was in the form of one of those strip like variable resistors often seen as the focus control in TVs with electrostatic focusing. I've replaced it with a fixed resistor, A.O.T. - adjusted on test, 82Kohms was the chosen value for optimum linearity. Replaced the 0.25mfd coupling capacitor and also another of 0.1mfds.
Till Eulenspiegel.
Smashing! Looks like this set has had quite a hard life or put in good service whichever way you look at it, and usually a good sign if they've been used in relatively recent years. Then again these were built for rough service I guess.
Last time I had one of these on the bench must be 50 years ago, all I remember it was an excellent performer. No idea what the fault was now.
I replaced a lot of disintegrated line output transformers in the Ekco sets, if the windings were ok I think the cases were available. At least the transformers were easy to replace in the house on most of the Ekco sets.
Frank
Blimey, that picture is so sharp you better mind you don't cut yourself Till ?
Excellent results !
To understand the black art of electronics is to understand witchcraft. Andrew.
Hi Frank, the replacement CRT must have been fitted late in the life of the set.
A company search yielded these results: Dunelm Television Tubes Ltd. Company number:00868645. Incorporation date: 11/01/1966. Company ceased trading sometime in the 1980s?
So it follows the set was quite old when the CRT was fitted. The original selenium bridge rectifier was replaced with four silicon diodes fitted on a piece of group board.
It's possible the set was still in regular use well into the seventies.
Till Eulenspiegel.
Great result Till, that tube must have got little use.
Not necessarily, Frank. Back in the 60s, we noticed that the tubes in sets that were switched on when the kids came in from school and stayed on until the parents went to be bed often seemed to go on for ever, whilst the more selective viewers, who might watch the evening news, switch off, then watch say, a drama later on in the evening then switch off again were more likely to need a new tube after relatively few years in service.
Although I couldn't continue to monitor the domestic situation after I left the trade in late 1969, I was in close contact with around 2,000 receivers based on the ITT VC100 and VC20x series chassis for 5 years or so. These ran all day five days a week (and, I suspect, some were left on 24 hours a day!) but none needed a new tube ...
When all else fails, read the instructions
