TACKLING THE POWER SUPPLY
One of the first parts of any detailed fault finding or restoration on any old TV set is the power supply. Here we look at common power supply configurations and problems.
If you are met with a set that gives no results, no sound or raster, first of all in the case of a valved set lean over the back and see if the valves are lit. If they are then this proves that a the mains supply is reaching the set and that the valve heater chain is intact. In this case there is probably an HT failiure.
If there is no sign of live whatsoever, look first at the simple things: is the mains reaching the set? Check the plug fuse, continuity of the mains cable, operation of the on-off switch and any internal fuses in the set.
Check also for correct operation of the voltage adjustment setting, as will be seen below in the examples shown this is either achieved by a resistor network or by taps on the Mains Transformer primary.
Valve and CRT heaters are usually wired in series and in most (but not all!) B9A valves, the heater is between pins 4 and 5. Check for continuity of the valve heater chain. In addition there may also be a Thermistor in series with the heater chain and this can fail or become brittle with age. I once got a complaint of ‘it just went off’ - sure the set was dead the tube base had been removed by the local tinkerer!
Power supply fault finding on an old TV set is not much different from fault finding on valve radio sets, however on a rather bigger scale.
Especially in BRC sets look for fusible resistors. These can be resoldered but they are safety components and should never be ‘repaired’ with high melting point solder or replaced with standard wirewound resistors.
Here are some examples of circuits that are often seen and some pointers about what to look for!
VALVE RECTIFIER ARRANGEMENTS.
In the above cases, the rectifier valve heaters are often fed via the heater chain. On the arrangement which uses 2 surge limiters, the failiure of one will result in reduced HT. When the set has a mains transformer, the rectifier valve is sometimes ran from a separate winding on the transformer secondary.
HEATER CHAIN CHECKING.
Here we see a practical example of heater chain testing. The various taps on the ballast resistor for mains supply voltage adjustment. Note also the thermistor discussed above.
When the valve with an open circuit heater is discovered and replaced, keep a hand on the on/off switch as you run the set up. In some cases not all the valves will light, and the ones that do will light up brightly. In these instances there is a short circuit either in the last lit or first unlit valve in the chain or sometims in one of the filter capacitors between the heater chain and earth.
PRACTICAL EXAMPLES
In this Pye/Ekco circuit below, look for the heater chain and also note the resistor network for mains adjustment and the way the LT supply is derived.
DC HEATER SUPPLIES: Something to look out for!
In many sets the heaters are fed on DC via a single diode, as shown in the Pye/Ekco example above and also in sets such as the Bush TV161 series and the BRC1400. If this diode should go short circuit, the valve heaters would be brighter and the lives of the valves shortened. The performance of the set my not be impaired so much as to induce the end user of the set to report a fault.
Eventually the emission of all the valves in the set could be dramatically reduced resulting in what would have been an expensive repair. So manufacturers took steps to create a fault condition that the user wound notice in this situation. The circuit below is taken from the Bush TV161 / Murphy V2314 (Rank A640 Chassis).
As can be seen from the diagram, the failiure (short circuit or ‘leak’) in the heater rectifier diode results in frame roll. In the BRC circuit a similar ripple is applied to the field output stage and this results in a cramped raster with no frame lock.
AC ONLY HEATER SUPPLY: The ‘Watless’ or ‘Capacitive’ dropper.
While we’re on the subject of heater supplies, lets take a quick look at the heater supply used in the BRC1580 series of hybrid 12 inch monochrome portable sets from about 1971. The diagram below is of the power supply arrangement used in this set. In order to reduce heat dissipation (a wirewound resistive dropper in a portable plastic case!) capacitor C85 is used to drop the supply voltage to the valve heaters. It has a very precise value of 4.43 uF, this is based on a mains frquency of 50hZ and a supply of 240v. Basic AC theory provides an explanation as to how this works, however can I ask you to trust me on this one and if you want the maths feel free to E-Mail me!! Note also the resistor connected in parallel with C85. This is a bleed resistor to discharge the capacitor when the set is off.
HT SUPPLIES.
A good example of common HT supply practice using diodes is shown here. This is from the Bush TV161 series. This is a mult-rail HT supply which feeds different HT rails to different sections of the set. Note the difference in voltage between 625 and 405 line operation.
Here some or all sections of the set are prevented from working by resistor failiure.
LOW HT, OVERHEATING AND FUSE BLOWING.
When a replacement fuse blows straight away there chances are that there is a dead short somewhere in the circuit causing excessive current drain. In some cases however the fuse may not fail even though parts of the set are in obvious distress. The components that are actually overheating or showing signs of damage can point you quite reliably in the right direction. Also other outward symptoms associated with the set can provide some sort of clue. Unfortunalely there are several instances in which cold ohme-meter checks will not reveal the problem.
This may sound silly, but are all the right valves in the right places? Its all too easy to be caught out!
If the HT is low and the rectifier is overheating, the problem is likely to be an internal short in a valve, a short circuit in an anode, screen decoupling capacitor or a coupling capacitor (akin to THAT capacitor as all vintage radio restorers know and love - C16 in the circuit below) or if you haven’t spotted it yet, a solder blob or a shorting lead somewhere.
See if the short remains if the first valve in the heater chain is removed (tends to be the Boost Diode - PY88 or something - this is usually the smaller of the two valves next to the LOPT) or if you’re feeling lazy just try removing the CRT base.
Another thing to look out for is complete absence of line drive to the control grid of the Line Output Pentode (Usually PL something). If there is no line drive this valve will be unbiased and will pass excessive current and will be seen to be glowing a nice cherry red. Line Drive faults will be tackled in the Line Timebase and EHT section of the website. For now, just lift the top cap off the valve and see if the rest of the set comes to life. If it does, transfer your attention to the other most troublesome side of old TV sets...
With Low HT and Hum, look at the reservoir capacitors first (!) and then if they are in order, check the HT thermistor or surge limiter.
Fuse blowing can be diffcult to trace if there is only one fuse which is common to HT and heater supplies. If the plug fuse blows check the on-off switch and connections to the set. In the case of internal fuse blowing, the first thing to do is break the valve heater chain (remove the first valve in the chain) and see if the fuse blowing continues. If it does then simple ohmmeter checks should point to the short circuit. I once had a Bush TV161 that had a smoothing capacitor that had a DC resistance of less than 50 ohms! An internal short in one of the first few valves in the heater chain can also cause the fuse to blow.
The most frustrating problem is intermittent fuse blowing, especially when the set seems happy enough otherwise! This can be a bad connection, frayed lead intermittently touching chassis, internal flashover in a valve. Insert an ammeter in place of the fuse (briefly!) to see how much current is actually being drawn, is the fuse ‘on the edge’? It can sometimes be an interesting exercise to insert extra fuses into the set to isolate the problem. DO NOT SIMPLY PUT A BIGGER FUSE IN AND HOPE FOR THE BEST!!!
LT SUPPLIES
These supply a low voltage rail (about 10 to 20 volts) to the solid state sections of the set and can be derived in a number of ways. They can be derived from a winding on the LOPT, the cathode resistor of the Frame output valve, the valve heater supply or simply dropped from the HT via a resisor network. LT sections of a set generally give little trouble, but here are some examples of LT arrangements.
HYBRID COLOUR TV SET POWER SUPPLIES
The principles are generally similar in the hybrid colour sets, albeit on a bigger scale! Below is a link to the power supply arrangement in a Philips G6. The solenoids at the top left are operated by a microswitch on the tuner. The G6 is a dual standard colour set & the main chassis solenoid operates the system switch and the convergence box solenoid operates a similar switch on the convergence board.
In the event of fuse blowing, it is a lot easier to isolate which part of the set is causing a problem.
Click here to look at the G6 Power supply and see how many elements discussed above in the black and white section can be seen here.
SOLID STATE SETS AND THE BIRTH OF THE CHOPPER POWER SUPPLY!
The BRC2000 chassis was a landmark in TV set design as it was the first entirely solid state colour set to emerge in the UK. The power supppy for this solid state set consisted of two modules, a power supply unit and a regulator board. The 2000 is an AC only set and the CRT heater is fed from the mains transformer. The Power Supply module provides the various LT and HT supplies for the set, some of which require a high current supply. Many of these rails are fed through fiusible resistors. The regulator board supplies the set 55v rail and also has an electronic trip circuit to shut the 55v rail down if the line timebase draws too much current.
The Chopper power supply first appeared in the BRC3000 series sets in about 1970. The power supply in the 2000 was both hot and bulky, not to mention expensive. So, a new approach was needed and the 3000 power supply, reviled my many at the time, was the solution and the forerunner of every other switch mode power supply we see.
A 30v rail is generated which drives the line oscillator circuit as well as the monostable and chopper driver transistors.
Rectified mains (about 230v DC) was applied to a chopper transistor which was turned on and off rapidly by a 15.625 kHz waveform from the line timebase panel. The mark-space ratio of the output is such that the equivalent constant DC supply is 60v. This is then smoothed and a feedback circuit is used to keep the supply voltage constant at 60v. I have simplified this below.
Power Supply faults tend to be fairly easy to diagnose and correct, with the possible exception of the BRC3000 arrangement described above. It is however an important safety measure to always use correctly rated components and never to cut corners or change fuse ratings. Not all power supply faults are entirely down to this area of the set, however, and it is important to look at the sections of a set that the PSU is feeding power to as well to avoid repeat faults and to check that one section of a set has not failed and damaged another.
