Ekco T161 Timebase problems

[thermionic]

Hi everyone.

I picked up this set over a year ago now, but only now really got around to looking at it. Initially, when powered up the line stage remained inactive, but after replacing the 20P1 grid coupling capacitor (0.01uF) and the 20L1 anode resistor (470k) which had risen to nearly 700k, the line stage is trying to work.

The oscillations are weak and erratic. The frequency is way above 10khz. I have tried replacing the 20P1 and 20L1 and have checked the line hold pot and series resistor and these are ok. My set uses the later timebase circuit and I am working from a poor fuzzy copy. Even so, my circuit still appears slightly different. How many variations did they produce?

My CRT is running from a boost/isolation transformer so before I get in too deep, I want to get first light if possible. Any pointers will be most helpful.

Thanks. SimonT.

[HamishBoxer]

Check all caps and resistors in line osc stage including the Line Hold pot. Make double sure you put in correct value caps and do things one at a time, not en masse.

[Freya]

C63 2uF had the same effect on mine. The oscillator is triggered by phased pulses from V12.
Mine too is running tube transformer but only at the standard heater voltage.
Hope you got C75 round the right way

[Heatercathodeshort]

What a clean chassis! LOPT will be OK as it is the arched type Perspex case that does not suffer the problems of the later ones. Don't forget to disconnect the EHT smoothing Visconol. Very reliable line timebase. The circuit is in the workshop but I'm sure the T161 uses half the U801 as a boost diode. Worth a look. Regards, John.

[thermionic]

A bit of time tonight and some progress. Checked C62, that was OK.

Freya kindly emailed me a slightly clearer version of the T161 timebase modification sheet so I could now carry out some voltage checks. Cathode of the 20P1 was hovering around -20 volts when it should be -145V. HT is only a few volts off what it should be, so the U801 seems to be behaving himself at the moment!

The anode voltage at V12 was low at 80 volts, when it should be 200 volts. The main culprit here was the grid coupling capacitor, which once replaced rectified this problem. The line stage then started up. Why? If my understanding of the circuit is correct, then sync pulses from V12 only syncronise the line oscillator, surely this would not stop it from running at nearly the correct frequency. The result was first light, shown below. The photo shows the raster brighter than it really is, the CRT is putting on a brave face, but I only have just over 5KV measured on the Scalamp. John, The Visconol has been disconnected, and you are correct regarding the U801, two anodes for the HT rectifier, the other 2 anodes strapped for the boost rectifier.

My euphoria was short lived, however. After switching off to try a substitute U25 EHT rectifier, on re powering up, the line stage would not oscillate again! Pulling out V13 (20L1) and re fitting whilst the set was still on, started the line oscillator again. I have subbed the valve, cleaned and carried out continuity tests on the valve holder which proves fine, but the line stage will only kick in by removing and re fitting this valve several times! My circuit varies marginally from the addendum sheet, R63 in my set is 470K, and I have a 300pF capacitor connected from the LOPTY side of C69 to HT. I think the design engineers at Ekco, were having a bit of fun with this series of chassis.

SimonT.

[Freya]

Do you have another 20L1, it sounds like it could be low.

[Heatercathodeshort]

These usually oscillate when everything around them is leaky or S/C!
If the frequency is high the coupling between the 20L1 and the 20P1 must be very weak.
C69 12pf and C70 40pf are suspect. The cathode decoupler C75 50uf will also need to be checked. R68 in series with the line hold control may have risen in value together with R70 the 1M grid leak, 20P1. The anode load on my Ekco circuit is 1M [R63] so I guess a few mods were carried out probably to centralize the line hold control. Great receivers! John.
PS Remember some of the smaller caps look like resistors..

[thermionic]

Finally, after raiding boxes of old TV valves and locating as many 20L1 'pulls' as possible (6), I have a reliable line oscillator! Only 2 of the valves were happy to work, but at least progress!

EHT is still below the stated 10kV, at just over 6kV. With the Aurora connected, and switched between channels 1 - 5, there is no sign of any signal getting through, just a blank raster. I will now turn my attention to the RF deck to see if I can weed out any criminals.

The poor old CRT's heater is running at 3 volts, due to the boost transformers primary wired on the 200 -220 volt tapping. The mains tapping on the sets chassis being set at 230 -250, so I guess they were trying to get as much out of the failing CRT as possible.



SimonT.

[Freya]

Keep a lookout for those nasty EGEN capacitors, 0.001 and 0.003 from memory and they were nearly all short on mine.

Ooops, I see John mentioned them already.

[thermionic]

I managed to spend some time on this set today, and so set about finding out why I had a blank raster & no sound. The lack of video was soon traced to an open circuit contrast control. With this temporarily bridged across, I was rewarded with Test card 'C' from the Aurora, albeit with poor focus, brightness, linearity & height!

I was just about to snap a photo when the line oscillator quit - again! This time, no amount of cajoling will get it runnng correctly. It is oscillating, but at a very low frequency, see the attached photos! This was measured at the grid of the line output valve. The voltage reading on the Avo here is -36 volts.

I have re tested all the components around the line stage, including subbing the line output & oscillator valves. The only way I can get the oscillator running at near the correct frequency is by fitting a 47pF capacitor across the 12pF feedback cap. This brings a weak line whistle, but is not strong enough to generate any EHT. Therefore I can only assume that my LOPTy is poorly, a shorted turn maybe? What do the EKCO experts think?



SimonT.

[DangerMan]

Looking at those waveforms it actually looks like the line oscillator is running, but not continuously.

Between the bursts of operation, there is a period when it stops, with roughly 18millisecond period.

These sets have a negative boost rail which feeds the line output valve cathode and its grid leak resistor and also cathode of the frame output valve (not shown in your diagram).
This boost rail is decoupled with a 50uF electrolytic, C75 in your circuit. If this capacitor goes open circuit, you can imagine it could cause the line oscillator and line output to start and stop in step with frame output valve's current consumption.

C75 has been mentioned several times above, have you swapped it?... it's much more likely than the transformer. Also, note it's positive to chassis.

If you have, the main smoothing capacitor could be suspect.

[Freya]

I would imagine the lopt would be getting quite warm if it had a shorted turn on the primary, the problem must lie elsewhere.

As its a fairly simple task, you could try ringing the lopt to eliminate it.

[thermionic]

Had some time today.

C75, was tested again, and also bridged with a known good component and this made no difference. bridging C69 with a 47pF capacitor brings the oscillator almost back to the correct frequency but the whistle is weak and the boost rail very low. The main resevoir / smoothing capacitor I have already replaced due to an open circuit smoother section.

I decided to disconnect the LOPTy from the set and carry out a ring test. the result is shown below. I also dug out my old 'JABCO' LOPTy tester from the early eighties, and this also was not happy. I know these methods are not 100 per cent truthful, but I fear my transformer is poorly. I am confident I have exhausted all other options. The circuit is pretty straightforward and contains few components. If it wasn't for the fact that the transformer seems to have an excellent reliability record, I would have pointed the finger here long ago!

What are your opinions on the ring test?



Thanks. SimonT.

[Freya]

Not good; for anyone else interested

Image courtesy of Chris on VRAT of good ring on lopt
second is shorted turns and bad ring.

It is a shame but there could be a few about as they are so reliable; fingers crossed.

[PJL]

The ring test looks strange but not conclusive. Is that with a 1KHz signal?

From your earlier scope images of the grid, it starts oscillating but cuts out. Dangerman has already made a suggestion that is worth checking. I don't have a schematic and it may be that a faulty LOPT will cause this behaviour but its worth a try.

[Leon Crampin]

Set up your test for the unknown transformer with a few turns of connecting wire around the core. Short the ends of the connecting wire and observe the change (if any) in the ring amplitude and duration.

Leon.

[Heatercathodeshort]

Hello Simon,
The part number for the transformer is usually printed in blue on the top. I have in my Ekco stocks a SA4069/1. You are more than welcome to borrow it and give it a try just to confirm one way or another. They are incredibly reliable as all Ekco transformers are and these early 'arched bridge' type are formed from Perspex not that strange plastic material used from model T221 that breaks down causing burning of the case. [The windings usually survive in good order and a new case can be easily fabricated.]
I can post it given a couple of days as the local office is a few miles away.
Regards, John.

[thermionic]

Hello all.


I have re rung the transformer as Leon has suggested with a couple of turns around the core & shorted, the results are below - a slight change. In fact, this looks more promising! The calibration frequency is 2 KHz on my 'scope.

The odd 'gated' waveform I measured at the grid of the 20P1 shown in my previous post was before I tried bridging C69 (12pF) with a 47pF cap. doing this, the low frequency gating disappeared.

John, can I take you up on your very kind offer, please? At least if I can eliminate the LOPTy I know where I stand. I cannot see any identification marking on the transformer at all. If you are intending to visit Royal Wootton Bassett in a couple of weeks time, then perhaps we could meet then, if not, I will advise of my address via. PM. (PS. Nice Bike, by the way!)

According to the book 'Ekco Sounds', the T161 'triple link chassis' had circuitry designed by a Mr Maynard, ' a rather severe looking man with a moustache and heavy glasses and a clenched pipe between his teeth' , along with a Mr. Norman Atkinson, John Bussell & George Baerselman. I wonder what these gentlemen would say now, seeing me struggling to get the line oscillator running properly? Its fun to imagine......


SimonT.