Hit or Miss
Ltd: Technically Speaking
|This extracts are taken from the engineer's letters to "The Wireless and Electrical Trader". In the early 1950's, on average, TVs broke down four times a year, so any hints you could gather about odd failure modes were most useful. I cannot vouch for their accuracy and please remember, that many electrical goods from the vintage era are very dangerous to work on, with live chassis and no earth connections.|
|Ultra V710 TV: No raster;
K. M. Sidcup, July 1951
The Ultra V710 series having an unusual voltage doubler circuit can be somewhat confusing. The full voltage from the doubler is applied to the time-base valves and the low voltage from one section of the doubler feeds the receiver valves.
One of these models came in recently with no vision or raster, sound OK. The voltage applied to the time-base valves were found to be one-third of normal, although the line generator and output stage could be heard operating.
After finding no obvious reason for loss of applied voltage the splutter resistor in the anode circuit of V1A (maker's diagram), half section of the PZ30 supplying the time-base valves was checked and found to be 3M ohm instead of 47 ohm, replacement cured the trouble completely.
Part of the Ultra V710 Circuit referred to
|Ultra V600 TV:
Cramping on Left of Picture
G. F. K., Oxford, November 1951
This fault illustrates how important a fraction of an ohm can be in the low impedance side of a line output circuit. The left hand side of the picture could be made to fold over by advancing the width control.
Feeding the disconnected output from the line oscillator to a 'scope revealed that at least the oscillator was free from distortion, but sampling the waveform when applied to the grid of the line output valve proved the valve was going into grid current at maximum setting of the width control.
The line output stage was, therefore, suspected. Voltage and current analysis was normal, but the primary winding of the line-output-transformer showed a 2-3 ohm deviation in the low direction, and on first test the scanning coils seemed OK. It was therefore decided to replace the output-transformer on suspicion for shorted turns.
I was rather taken aback when this operation had no apparent effect. Another detailed check-over of the line output stage still failed to reveal the cause of the trouble, although on checking the resistance of the line scanning coils for the second time it was noted that one side was reading 8.25 ohms, while the other 8.5 ohms which, according to the service sheet is normal. Still rather dubious of this ¼ ohm deviation, since 2-3 ohms on the output transformer had no effect, there was only one thing left; to change the scanning coils.
This time a full raster was obtained, with no cramping.
YA72, YA73 Series TV: E.H.T Fault
J. A., Stockport, June 1952
A model VA72 receiver developed a fault which produced very ragged edges to the picture. In another case the symptom was low EHT voltage. The line output transformer became suspect, although the time-base itself seemed to be working quite well, so the plastic cover was removed from the line output circuit compartment.
It was then found that the short lead from the transformer to the EHT rectifier anode was covered by a piece of sleeving which was badly charred, and the lead inside it was broken. Fortunately sufficient lead remained to permit the connection to be made again, and in doing this we discarded the sleeving.
In the second case we knew where to look, and we went straight to the transformer and removed the charred sleeving, remaking the joint without the sleeving, and in both cases the receivers have since operated without any further trouble. The fault seemed to be caused by a static charge which develops on the sleeving.
[The makers say that they are aware of this fault, which was quite unpredictable, and they agree that the remedy is to remove the sleeving. It has been omitted in production for some time, and will only be found in a a small number of early models. - Technical Editor, The Wireless and Electrical Trader.]
Series TV: Sound/Sync Interaction
L. H., Monmouth, December 1955
On one of these receivers it was found that line hold was lost during heavy sound modulation, although synchronization remained constant. In other words, although synchronizing was unaffected, heavy sound passages would trigger off the line time-base. A further symptom was a 10kc/s whistle on the sound channel.
The cause was traced to coupling taking place between the cathode circuit of the sound output valve and the cathode circuit of the line output valve. Both of these are by-passed by 50µF electrolytics (C67 and C39 respectively in Trader Service Sheet 1156/T58), but they are housed in a single container. Presumably the common earth connection became open-circuited, leaving the two capacitors connected in series between the two circuits.
|Ultra 720 Series
TV: Faulty Frame Scan
R. H. New Barnet, Herts, February 1956
An odd fault on a VA720 receiver caused the frame scan to go out of sync for an inch or so at the top and bottom of the picture while it remained perfectly firm in the middle. The difficulty with locating the fault was aggravated by the fact that it was intermittent, occurring perhaps every few days, and then only after the set had warmed up.
A new 6K25 frame oscillator valve was tried (V15 in Trader Service Sheet 1075/T34), but the fault returned. A heater / cathode short circuit was sought and new electrolytic smoothing capacitors were tried without result.
The capacitors were tried one at a time however, and the fault was found eventually to be an intermittent earth connection form the common negative lead inside the can containing C61 and C63. These are two of the three electrolytics in the frequency-doubling rectifier circuit, and replacing them one at a time had little effect on the picture.
Series TV: Line Time-base Hints
P. M. Mirfield, Yorks
In the course of a considerable amount of work on Ultra receivers, in which I specialise, I have learnt to recognise certain fault symptoms in the line time-base circuit, and I pass them on for the benefit of other service engineers.
A fairly common fault in all TV receivers generally is inadequate width, and the cause can be elusive. The first and most obvious check is on the voltages, and I have found in these particular receivers that if the HT line voltage is down by as little as 10 V, line scan amplitude will probably be inadequate. The most probable cause is fault surge limiters in the mains rectifier feed (R91-R94 in Trader Service Sheet 1156/T58) or a faulty rectifier V18.
Another cause of the same symptom is a fault in the oscillator circuit charging capacitor. The time-base uses one-half of a 20L1V8a and the 20P4 output valve V9 as the multivibrator, in which the 200pF charging capacitor C5 goes from the triode anode to chassis. A leak, a low capacitance or an open circuit can result in reduced amplitude, although usually the circuit will continue to oscillate at the current frequency.
The value of the anode load resistance R39 of V8a varies in the three different versions (-14, -15, -17) but it remains fairly critical, and if its value changes that can result in reduced line amplitude. R39 is rated at 10 per cent, tolerance, and if its value is outside this tolerance the resistor should be replaced.
Intermittent oscillation of the line time-base can result from a faulty line output valve cathode by-pass capacitor, and this fault can also cause reduced width. This is a 50µF electrolytic C39, and it forms half of a double electrolytic C39, C67, the other half by-passing the sound output valve cathode V15. It has been know for a leak to occur between the two sections, giving a sound-on-vision symptom that can be very misleading.
Finally, a faulty line output transformer can, of course, cause a reduced line amplitude, but that is not a common fault, and the foregoing components are the most probably offenders.
Note: The component numbers quoted in the foregoing hint are those used in Service Sheet 1156/T58, but the V817 is covered separately in Service Sheet 1162/T60, where the surge limiters are R97, R98, and V9 cathode by-pass is C38. The other numbers happen to be the same. - Tech Ed.
Series TV: No Contrast Control
J. H., Southsea, Hampshire
The contrast control in one of these receivers seemed to have no effect, and on a strong signal input the picture would go negative. Such things as valve changes produced no result, so as the A.G.C. was "gated" and oscilloscope was brought into use.
The A.G.C. system is dependent for its proper functioning on a supply of pulses from the frame time-base circuit, and upon following this along their course with the oscilloscope they were found to disappear at the coupling of the cathode to the diode valve through which they are fed to the A.G.C. cirucuit (V13a in Trader Service Sheet 1254/T103). The fault was found to be due to an open circuit in the 0.002µF coupling capacitor to this cathode, which is C56.