Basic B&W TV Repair Guide

Introduction

The following information contained within this article should be used with extreme caution by those starting out in vintage TV repair. If you are learning ensure you have an experienced engineer with you to advise or seek advice on the forum with the online TV engineers and experienced amateur hobbyists. As a bare minimum you should be familiar with the high voltages to be found in old TV’s, have workshop RCD protection and an isolation transformer. The greatest tool though is caution and an instinct for self preservation. The following advice is for the majority of 405 televisions between 1950-1960, not colour, as an additional word of warning do not try to draw sparks on mains EHT pre-war sets. “You Have Been Warned”.

This is by no means meant to be a definitive guide, just a quick start reference guide. Each stage will present you with many varied challenges to be overcome, it is assumed the reader has some prior understanding of vintage electronics & the safety required before attempting an old TV. Likewise If you are at all unsure or are unaware of the inherent dangers these old sets present, I would suggest you stop reading now and seek advice from the forum.

Get the service sheet and familiarise yourself with the circuit this is the road map which if studied will help you to understand how the set is designed to function, if you cannot read a schematic worry as you really need to so start learning to understand them. Have the best tools ready, your multimeter and common sense, think safety at all times never take anything for granted check and double check every stage of your interaction with a set..

1. Cold checks

First, a quick visual check for missing bits or obviously damaged components or wiring. Check heater continuity with meter, check and operate the on off switch. Ensure chassis is wired correctly if AC/DC chassis should be at neutral again use you meter to confirm this. Finally ensure the TV is set for the correct voltage, you will be surprised how often this is not and sometimes in an indicator some unscrupulous type has tried to overrun the set to eek out the last emissions.

2. If You Are Of A Nervous Disposition , Avoid The Pop

After the checks in one the set can be plugged in but be ready to switch off, in most cases it will either warm up or nothing will happen at all. Alternatively for those who are concerned about easing the old sets in, then the next approach is as follows:-

Snip the capacitor that is across mains (check the schematic), this is the mains filter cap/ RFI suppressor it’s called many things. Now with a variac, lamp limiter or both and connected via an RCD if your consumer unit is not fitted with one, give it a little taste of mains to see what happens. If you have a variac try starting it off at around 50v, increasing by 20v every 5 mins. A damp dusty smell of mains droppers and likely a bit of smoke as the dust burns off, don’t worry this is quite normal from a set that has not been powered on for a long time. If the smoke is not eminating from the dropper try to quickly determine where it might becoming from, possible an electrolytic. DO NOT stick your face or head directly over one, if these suddenly let go they can cause great harm. Switch off and disconnect from the mains, feel the can, does it feel hot, if so then it will need replacing.

3. Eyes & Ears

Use all your senses to monitor what is happening. Are the valve heaters glowing correctly or are they too bright, you don’t want to fry all your valves if the heater decouplers have failed or overrun individuals through other cap failures. Look for obvious signs of burning components or wiring or burning smells, listen for a hum from the speaker is it the deathly van Helsing rumble of main smoothing trouble or just normal/crackles, look for bubbling caps, etc. and be ready to switch off!!

If you’re very fortunate you may start to hear ( if you still can) the 10,125Hz line whistle from the line output transformer accompanied by something on screen. It helps to have the contrast and brightness set at mid point but have a twiddle just to see if you get a glimmer. The line whistle may sound odd, sort of strangulated and not healthy…. Don’t leave it like this too long as you may put the LOPT (line output transformer) under stress. More often than not the set will appear to be dead with perhaps just the valve heaters glowing.

4. Typical valve 405 television: Check points and tests

Check Points and associated tests

These following tests should help you gain a quick insight into potential faults in the modules above.

Point A; H.T. + line

Test with multimeter to chassis, AC/DC receivers with half-wave rectifiers should have a minimum of 180V, and so should many AC-only television, but expect to find exceptions with full-wave rectifiers (giving 250V or more). A low voltage on the H.T. line can be due to low emission rectifier or to a heavy H.T. current drain owing to a partial short circuit anywhere in the TV. Lack of voltage can be due to a faulty or o/c rectifier. wire-wound resistors, a blown fuse,or to faulty surge resistors (sometimes included on the mains dropper). The main H.T. line can be assumed in order if the raster is normal- but a branch H.T. feed might still be at fault.

Point B; EHT D.C.

Test at the CRT connector by drawing an arc to the blade of an insulated screwdriver held well back on the handle. If a spark is not produced, try with the connector removed to check whether the CRT is shorting the supply, in which case the EHT will reappear. The D.C. EHT spark should be blue and crackling. Lack of a spark with the EHT cap removed indicates no A.C. EHT input to the EHT rectifier or the rectifier itself is faulty.

Point C; EHT A.C.

Test at the EHT rectifier valve anode as described in test B but this time s sizzling violet arc should be produced. Try also with the CRT cap off if no spark is produced with the first test. No A.C. EHT indicates line output failure, possibly the line output valve, booster/efficiency  diode, line oscillator, line output transformer or associated circuitry.

Point D; CRT Cathode

(a) With a normal raster in which the brightness can be controlled, scratching the cathode pin with a bare screwdriver blade ( ensure used with insulated handle) should produce a few weak spots of low brightness on the screen.

(b) With no raster (EHT in order), momentarily short the grid pin to the cathode pin, if a bright raster appears, a bias fault exists.

(c) Uncontrollable brightness ; test the grid and cathode voltages with a multimeter. Bias faults can be due to shorted electrodes in the CRT, a fault in the brightness control circuit , incorrect vision valve potentials or oscillation in the vision amplifiers.

Point E; Vision valve grid

Scratch the grid pin lightly with the tip of a screwdriver and a splash of white spots should appear on the raster at the time of contact. Failure to produce spots indicates a defective video stage ( the CRT being assumed in order). If the tube circuit is untested, then the fault lies between point E, the CRT and its supply circuits.

Point F; Vision I.F. valve grid

Grid disturbance testing here will produce brighter spots than at point E and in so doing proved the vision I.F. stages to be operating. Crackles will be heard in the sound during this test. The previous vision tests described should have been made first. This test can only show whether the stage is dead or not.

(If there is a common I.F. stage in the television , disturbance testing at the grid of the common I.F. amplifier valve produces loud sounds and flashes of spots on the screen.Obviously lack of results here but results from the previous test point F, show this stage to be at fault)

Point G;Aerial input

Correct results from the earlier points up to F but no result here show the turret to be defective. The effect of a substitute aerial should be determined because many TV complaints are found to be caused by faulty aerial or cable.

Sound Tests

Point H; Output valve grid (sound)

touching this grid will produce a low hum in the speaker. Sometimes it is quicker to omit this test and go to J first as this point is more easily identified.

Point J; Volume control live tag

This point is easy to find and touching the tag with a screwdriver will cause loud hum. The volume control knob must be turned up for this test. No results here, but results from test H indicate the first audio amplifier stage is at fault. If a multiple sound output and A.F. valve is employed, then failure of this valve renders both stages ‘dead’. To test the sound I.F. stages make a test on point F, or the grid of the sound I.F. valve , to produce crackles from the speaker.

TimeBase Tests

Point K; Frame oscillator grid

When doubt exists whether the frame oscillator is operating, in the absence of vertical scan, a quick test can be made by linking a 100pF capacitor between the frame oscillator grid and the volume controls live tag. A regular 50-cycle tick should be heard in the sound that will vary in pitch as the vertical hold control is altered. This result indicates that the oscillator is operating and that only the output amplifier needs to be investigated. Remove any signals to the aerial for this test  as the sync pulses can be confusing.

Point L; Line oscillator grid

In a similar way to test K, the grid of the line time base oscillator is capacitively linked to the volume control tag when the 10,125 k/c whistle should be heard from the loud speaker. Changing the line hold setting should cause the whistle to alter pitch. This proves the line oscillator to be operating and any fault causing lack of like scan must exist in the circuit which follows the line oscillator. Lack of whistle shows the oscillator to be defective and there is no point in checking the output circuits until the oscillator has been repaired. Lack of fly-back EHT can often be traced to the line oscillator.

5. Interpretation Of Electrode Voltages

Electrode Indication Notes
Anode
  1. No HT
  2. Low HT reading
  3. High HT reading
  1. Short to Chassis within valve or capacitor. Anode load O/C
  2. Anode load high, leaky coupler to previous stage
  3. Valve not conducting, anode load low
Screen Grid
  1. No HT
  2. Low HT reading
  3. High HT readin
  1. O/C feed resistor., S/C decoupler
  2. Feed resistor high. O/C anode load
  3. Screen feed resistor low (PL81 favourite)
Control Grid
  1. Heavily negative
  2. positive reading
  1. Drive to stage satisfactory,  stage oscillating (if timebase) stage unstable (if i.f. stage)
  2. leaky coupler “soft” valve
Cathode
  1. High reading
  2. No reading
  1. Valve conductring too heavily
  2. Bias capacitor shorting. Valve not conducting at all

6. This Following Table Should Give Some Pointers

Symtoms Observations Check around Check Particularly
No picture, no sound
  1. No valves light
  2. except for eht rectifier, all valve light normally
  3. Some valves light brightly
  1. Heater chain
  2. HT Supply
  3. Heater Chain
  1. Mains lead, mains dropper,fuses, on-off switch
  2. HT Fuse, HT rectifier, main smoothing electrolytics. If still in trouble look for burned out resistors. Try disconnecting the various HT feeds systematically
  3. Heater-cathode leak in the last lit or first unlit valve in the chain
Picture satisfactory, no sound
  1. Screwdriver touched on centre tap of volume control produced no clicks or hum in the speaker
  2. Screwdriver touched on centre tap of volume control gives audible results in speaker, but no clicks are heard when channel is changed
  1. A.F. and sound ouput stages
  2. Sound i.f. and detector stages
  1. Sound output valve, o.c. lousspeaker, oc sound output transformer primary winding
  2. Sound i.f valves, detector valve or diode, screen grid decoupling capacitor on i.f. valves
Picture satisfactory, distorted sound
  1. Distortion varies with volume
  2. Distortion does not vary with volume
  1. Sound output stage
  2. Interference-Limiter stage
  1. Sound output valve, loudspeaker, grid coupling capacitor, cathode bias bypass electrolytic
  2. Resistor from HT to upper end of limiter diode, Capacitor from lower end of diode to chassis
Sound satisfactory, no picture
  1. No raster, line whistle or EHT
  2. No raster or EHT, line whistle is audible
  3. No raster, line whistle and EHT are present
  4. Raster present, varied by brightness control
  5. Raster present, but not varied by brightness control
  1. Line oscillator and output stages
  2. Line output transformer
  3. C.R. tube
  4. Video amplifier i.f. and detector
  5. C.R. tube and brightness circuits
  1. line oscillator valve, line output valve, efficiency diode, HT fuse to timebase where fitted
  2. EHT rectifier, efficiency diode capacitor, line output transformer, scancoils, EHT smooting cap if fitted
  3. CR tube, ION trap magnet (maybe loose), boosted HT line,brightness control
  4. Video amplifier valve, vision detector, Vision i.f. valves. Check that HT is present at anode and screen grids of all vision i.f. valves
  5. CR tube for inter-electrode shorts. Open circuit video amplifier anode load or choke. Unstable vision i.f. stages due to inadequate decoupling
Screen lights but no picture or sound
  1. Rotating  channel selector produces clicks and flashes
  2. Rotating channel selector does not produce clicks or flashes
  3. Hiss in sound and “snow” on screen
  1. Tuner unit
  2. Tunder unit and i.f. strip
  3. R.F. stage of tuner, aerial
  1. Mixer valve, oscillator anode load resistor high,broken tuner contacts. Failty common i.f. stage (if fitted)
  2. Tuner valves,common i.f. valves HT feeds to tuner and i.f. strip
  3. R.F. amplifier valve, aerial coil in turret tuners, aerial downlead,plug and socket, components at rear of aerial socket
Sound satisfactory, picture negative
  1. Picture dim, controls have little effect
  2. Picture bright, buz on sound
  3. Brightness normal,no buzz onsound
  1. CR tube
  2. a.g.c circuits
  3. Limiter stages
  1. CR tube with disconnected cathode
  2. A.G.C. and sync valves. Valves controlled by a.g.c. Diodes on a.g.c. line
  3. Limiter valve or diode, limiter control open circuit

Picture distorted

 

 

  1. Bottom of screen cramped or folded
  2. Top of screen cramped or distorted
  3. Insufficient height
  4. Insufficient overall width
  5. Insufficient width at left
  6. Insufficient width at right
  7. Bright band of cramping at left
  8. Verical lines bend or curve
  9. picture tilted
  10. corner shadow, picture central
  11. Corner shadow, picture off centre
  12. Line and frame hold controls need adjustment
  1. Frame output stage
  2. Frame output stage
  3. Frame output stage
  4. HT supply
  5. Efficiency diode
  6. Line output valve
  7. Line output stage
  8. Sync and timbase stages
  9. Scanning assembly
  10. Scanning assembly
  11. Scanning assembly
  12. Sync stages
  1. Frame output valve and cathode bias capacitor and resistor
  2. Frame output valve, output transformer, grid coupling capacitor, low boost volts
  3. Frame output valve, low boost
  4. HT rectifier, line output valve, screen grid feed resistor on line output valve
  5. Efficiency diode valve and its associated capacitor
  6. Line output valve and cathode resistor if fitted
  7. Low line drive from oscillator. Efficiency diode capacitor “brushing” in eht circuits
  8. Heater-cathode leak on sync seperater or line oscillator valve. Faulty electrolytic smoothing
  9. Scancoils need truning slightly around neck of CR tube
  10. Scancoils not fully forward. Displaced ION trap magnet. Faulty CR tube gun assembly (take tube out, fit upside down)
  11. Positioning magnet maladjusted , focus or ION trap magnets displaced
  12. Sync separator valve, coupling capacitor from video amplifier
Picture will not hold still
  1. Picture rolls or judders
  2. Parts of the picture more left or right as bright objects near the edge of the screen
  1. Sync stages
  2. Sync, video and aerial stages
  1. Frame sync or interlace valve or diode, faulty screen grid decoupling capacitor on sync separator
  2. Sync seperator valve and coupler. Resistor from HT to video amplifier cathode, Vidion detector diode, aerial picking up multiple (ghost) images.
Dull picture
  1. Little control over brightness
  2. Brightness control causes picture to enlarge and go dim
  3. Brightness satisfactory, but control poor
  1. CR Tube
  2. EHT supply
  3. CRT video and R.F. stages
  1. Low emission CR tube
  2. EHT rectifier valve, faulty line output valve or transformer
  3. CR tube, video amplifier, vision detector diode, r.f. amplifier valve
Sound and picture take a long time to appear
  1. Picture comes up small
  2. Sound and picture arrive together suddenly
  1. Power unit
  2. Tuner unit
  1. Low HT rectifier
  2. Local oscillator valve and anode load
Picture slow to appear
  1. Sound arrives as normal
  1. Line output stage
  1. Slow heating efficiency diode valve

Signals

A signal source is needed, an Aurora is the one to get, it provides Test Card C and also will convert 625 to 405 so you can watch freeview/sky dvds etc. If you don’t want to get one yet then a good modulated signal from a sig gen will produce bars on the screen, but to set the TV up properly you will need the Test card the aurora produces or a test pattern generator.

Safety

Get a scope and ISO T/X if you intend to use it is a must, the final word is be careful EHT won’t kill ( although mains derived EHT WILL!!) but will sting and hurt like hell, it’s the bad jump away when you react that can cause more problems. A1 volts and HT are nasty and will also kill so same care as in radios is required.

Finally

As stated at the start of this article, you should not attempt any repairs as a complete novice, indeed if you are unaware of the dangers that lurk within old TV’s and Radios and there are many,  you should leave it to someone who does.

Leave a Reply

dialog-information.png
Important Notice: To leave a comment use the box below. You must tick "I'm not a robot" and then complete the puzzle, otherwise the automated comment system will reject your post and treat it as spam.
 

Your email address will not be published. Required fields are marked *

*