BRC 3000/3500 Diagnostics
The following article content was gathered from many articles and documents regarding the BRC 3000/3500 series chassis. Mainly from BRC service bulletins, I’ve tried to collate it into one useful document. to help assist members with diagnosing faults on these chassis. You can also follow my rebuild of Baird 8724 which should contain lots of useful as it happened scenarios.
This article will deal with diagnosing faults on the BRC 3000/3500 series chassis, isolating the fault to a particular module then providing potential fault conditions with their respective remedy. It will also deal with chrominance fault finding, video trouble-shooting and various other aspects.
The face of field servicing started to change in the 1970’s, many service departments were adopting a tier servicing system, whereby field-engineers are employed on installation and first-line servicing only, effectively becoming panel swappers.
To help isolate the fault to a particular board on the 3000, a field service engineer was provided with a field guide. This guide enabled them to locate the faulty module, replace it. They would then upon return to base pass the faulty board to the Grade 1 engineers in the back of the workshop who would diagnose down to component level and make the repair.
The field engineers were issued with a service case, this provided all the modules for the TV. To see one of these service cases look here at BRC 3000 Field Service case
Nowadays us restorers are the only guy in the field so to speak, back to the good old days when you did everything from diagnosis and component level fixing. This guide issued to the first line guys still helps us to quickly locate the faulty module, from there with our grade 1 hat on, we fix to component level.
Isolate The Fault To A Module
Likely Faults And Their Remedy
Chopper Trouble Shooting Guide
Pursuing the normal practice of localising the area of fault by a careful appraisal of symptoms and appropriate voltage and oscilloscope readings, we can examine three clear-cut conditions of their probable causes.
Mains Cut-Out Triggering
This can be caused by a severe overload on the mains transformer ( HT diodes short-circuit etc) or firing of the ‘Crowbar’ trip W621. First disconnect the 240V supply lead to VT604 collector and switch on. If the cut-out still triggers proceed with normal fault-finding, bearing in mind the possibility of a short-circuit ‘crowbar’ SCR. If however the cut-out remains ‘in” measure VT604 for collector/emitter short-circuit and W617 & W615 for short-circuit. Having ascertained that these components are satisfactory, the fault must be due to the feedback amplifier causing the monostable mark/space ratio to open-up beyond the normal operating point.
Reconnect VT604 collector lead, unsolder one end of R621, switch on and monitor the voltage at F603. The monostable should now operate at maximum mark.space and HT on F603 will be approximately 30V. Having reached this stage it is a simple matter to check the few components in the feedback amplifier, a possible cause of the trouble being open-circuit W620. Intermittent triggering of the cut-out is likely to be caused by a dry joint on the feed to VT604 collector or out from the emitter allowing surging to occur.
No HT Output From Chopper
In the first place ensure that the other HT supplies are established, fuse are intact and 240V DC is present at VT604 collector. Where F603 is blown this indicates a substantial overload and the frame and sound stages can be quickly isolated by operating the ‘Set White’ switch. The supply lead to the line output stage can be disconnected at tag A on the LTB module, and if making resistance checks don’t forget that W507 is in series with the Line output-stage HT rail. Also bear in mind that a leaky A1 smoothing capacitor C523 will not show on a resistance test of the HT rail, but may cause F603 to blow instantly in switch-on.
When F603 is intact, use an oscilloscope to monitor line oscillator waveform at PLG 3 pin 2 power supply ( or point 33 line timebase circuit diagram), and the ‘Chopper’ circuit waveforms are points 36 to 43 If a scope is not available some indication of the presence of a switching pulse is a negative voltage ( very approximately 15V on an AVO 8 25VDC range) on W608 anode.
Assuming the switching pulse is present, delay switch VT602 can be checked by measuring its base voltage which is normally 0.7V. If this reads high ( 5V – 8V ) the transistor is open-circuit, and shorting collector to emitter should restore ‘Chopper’ HT. With open-circuit VT602 the 30V line wil read approximately 35V because W605 will be inoperative, and when VT602 base also reads 35V W606 will have gone short-circuit damaging the transistor.
The monostable action is more easily checked using an oscilloscope but it is worthwhile remembering that with the monostable in stable state (i.e. no drive to the chopper) VT603 is OFF and VT606 and VT605 are ON. It therefore follows that when the circuit is in a permanently stable state due to some fault condition VT603 collector will be at approximately HT ( 30V) and VT606 and VT605 will be very low (1V to 2V) collector voltages. When operating normally VT605 collector voltage is about 10V and VT605 5.5V
Low Chopper HT
The obvious symptoms here are lack of width and frame fold-up with apparently normal sound, and where the feedback amplifier is inoperative or being over-ridden by operation of the dynamic trip W622, a 50Hz ripple on line may be apparent. Disconnect the end of R621 fed from VT608 collector (right-hand end from rear, panel in situ) and connect a supply from and adjustable 9V DC source ( a PP9 battery with a 2K to 5K pot across is adequate), positive to R621 and negative to chassis. Turn the auxiliary supply for minimum output voltage, switch on, and monitor Chopper HT out at F603.
The monostable should now operate at minimum mark/space giving approx 30V HT, and turning up the temporary voltage feed to R621 should cause HT to rise towards 60V ( do not attempt to exceed this figure). Having determined that the mark/space can be opened up by this method, the fault must lie in the feedback amp. If HT does not increase check C615, W615 and dynamic Trip W622. A check for excess current being drawn can be made by measuring the voltage developed across R907 ( beam limiter) in the case of the line output stage; and for frame output, across R442. Also the various circuits can be isolated as outlined as above.
Monitoring the waveform at point 44 Power Supply will show whether the dynamic trip is being fired by excess current, otherwise measure the voltage developed at W614 cathode which normally reads approx 0.2V to 0.3V ( AVO 8 2.5VDC range); and 3.0V depending on the excess current being drawn. Where there are no obvious signs of excess current check W622 and associated circuitry.
Power Supply Module Stock Faults
Blows Trip Or Main Fuse (F601) On Switch-On
- Mains Transformer primary shorted turns, should read 35R
- HT or LT rectifiers short-circuited
Blows Trip, R609 15R Overheating
- Chopper transistor VT604 short-circuit. Also check W606,W609, W616
- Crowbar SCR W621 short-circuit
- 72V Zener W617 short-circuit
- Feedback amplifier VT608 faulty, causing M/S ratio of monostable to allow 58V to 65V rail to increase above strike voltage of W617
- W620 open-circuit
- Mica washer on chopper transistor broken down.
No 30V Rail
- 30V Zener W605 short-circuit sometimes damaging VT602
- 30V stabiliser open-circuit
- F602 fuse open-circuit
No 58V-65V Rail
- F602 fuse open-circuit
- 30V Zener W605 and VT602 faulty
No 58V-65V Rail : Excessive Voltage At Junction Of R607, C607
- Check voltage on collector of VT605 for open-circuit driver transformer T602
- High voltage voltage on collector VT605 ( chopper Driver) would indicate VT605 open-circuit or cut-off.Check also W610, VT606 and VT603
No 58V-65V Rail: Low Voltage At Junction R607, C607 ( R607 Overheating)
- VT602 or VT603 open-circuit
- W607 open-circuit: If forward resistance of this diode is greater than 2K, Monostable will not function
Low 58V-65V Rail
- EHT preset R629 open-circuit
- W618 low resistance
- C614 open-circuit
- Dynamic Trip control R622 open-circuit
- R610 1R open-circuit
- W622 Dynamic Trip outside tollerance.
58V-65V Rail The Same Voltage As 30V Rail
- W620, OA91, short-circuit. Sometimes blows F602
- C619 140uF low capacitance or open-circuit
- C631, 0.1uF open-circuit
Rapid Brightness And Width Variation ( Similar To Traffic Flutter)
- 30V stabiliser VT601 faulty
No HT (12V) On Tuner
- VT609 open-circuit
Line Timebase Module Stock Faults
No 58V-65V Rail
- C511 25uF short-circuit
- VT501, Vt502 faulty
- C512 .015uF short-circuit
- Broken connections on L501
No EHT, 58V-65V Rail Present
(A) No Voltage Drop Across R518 (75R Double-Ended, 200R Single-Ended)
- VT503 driver short-circuit
- T501 secondary winding open-circuit
- T502 primary winding open-circuit
(B) Excessive Voltage Drop Across R518 ( Over-Heating)
- VT503 short-circuit
- C531 (print-side of board) short-circuit
- R518 high
(C) Repeated Failure Of VT503
- T502 primary winding, shorted-turns
R526 47R Overheating ( Double-Ended Circuit)
- Base/Emitter short-circuit on VT504, VT505
- C517, C518 faulty
Low 58V-65V: No EHT: LTB Drawing Excessive Current ( R907 Beam Limiter Panel Overheating)
- C514 4.7uF short-circuit
- T503 shorted-turns
- T504 Shorted-turns
- W504 short-circuit
- VT504, VT505* short-circuit (* double-ended output only)
58V-65V Present On VT505 Collector (Double-Ended Output)
- T503 secondary, open-circuit to VT505
No 58V-65V On Line Output Pair ( Double-Ended) Output)
- W507 open-circuit
Line Output Transistor(S) Open-Circuit
- Check C517 & C518 before replacing
High EHT, Low Width
- C517 or C518 open-circuit. Usually damages output transistor(s)
- C517 short-circuit
R529 82R Overheating
- C524 2.2uF open-circuit
Beam Limiter Preset R903 Overheating
- R907 open-circuit. Usually caused by faulty line timebase
Excessive Brightness: No Colour
- C519 short-circuit or open-circuit
- C520 short-circuit
Incorrect Operation Of Brightness Control
- C901 leaky
- C902 leaky
- Printed circuit connections on 3-pin PLG 22 Beam limiter intermittent
- Incorrect adjustment of beam limiter preset and preset brightness
No Colour- Returns When Horizontal Hold Control Off-Set
- C520 7500pF open-circuit
No Raster: Sound Present
A1 Voltages Absent On CRT
- W505 open-circuit. Before replacing check C523 for short
Severe Flywheel Sync Ringing ( I.E. Verticals Very Distorted, Picture Almost Completely Broken-Up)
- C506 25uF open-circuit
- C506 25uF open-circuit
Line Off-Speed ( May Set-Up Normally When Sync Test Point Is Earthed, But Will Not Lock With Short Removed)
- C502 .01uF open-circuit
C525 & C526 Overheating
- W506 short-circuit
Striations L.H. Side Of Picture
- R521 1.2K open-circuit
Frame & Sound Module Stock Faults
Fold-Over At Top
- C429 16uF leaky
Fold-Over At Bottom
- VT424 Faulty
- C432 250uF faulty
Cramping At Bottom
- C705 400uF (Convergence Board) faulty
Poor General Linearity
Suspect All Electrolytics, Output And Driver Transistors And Replace Only With Correct Types
Intermittent Frame Collapse
- VT421 faulty
Poor Frame Sync
- VT421 faulty
Sound Fades – Hissing
- Usually audio output or driver transistors
Low Distorted Sound ( Low Voltage At Junction Of Emitter Resistors R413-R414)
- C401 5uF leaky
Buzz On Sound – Interference Band Across Picture
- C431 5000pF ( 2000pF earlier production) open-circuit
- Check earthing frame output transformer mounting bolts
Chrominance Trouble Shooting Guide
First Check For Correct Tuning
Always check first that the receiver is tuned in to a colour transmission: This can be confirmed by tuning towards sound in an anti-clockwise direction and observing the inter-carrier-to-chrominance beat pattern of 1.6MHz: then turn back just enough to remove the beat pattern and it should be correctly tuned. Preferably confirm with the Test Card, observing the 4.5MHz gratings.
If colour is not obtained after adjusting the colour control to maximum, it will be necessary to override the colour killer circuit to determine which section of the decoder is not functioning.
Overriding The Colour Killer
Connect an 82K resistor between the junction of C323-C324 and chassis: This will supply a standing bias to switch on VT307. Overriding the colour killer will result in one of the following
- (a) A good Colour Picture
- (b) Venetian Blind Effect
- (c) Loss Of Colour Lock
- (d) No Colour
(a) A Good Colour Picture: Obviously the fault lies in the killer circuit and the components associated with this, e.g W505, W322.
(b) Venetian Blind Effect: The fault in this case could be in the identification amplifier VT306 in not giving the required identification signal of 7.8kHZ and therefore not operating the PAL switch from which is also derived the bias for the chrominance amplifier VT309. Alternatively the fault could be due to the PAL switch W309-W310 not switching the reference oscillator output fed into T301 to produce the required 180 degree phase change on alternate lines. This should be checked with an oscilloscope.
If an oscilloscope is not to hand, a useful method of checking whether the identification pulse is present is by utilising the audio stages in the receiver. The 7.8kHZ identification signal is of course within the audio spectrum, and by simply connecting a 47K resistor from the junction of C323 and C324 to the volume control slider, there wil be an audible whistle through the speaker at half-line frequency if the ident signal is present.
(c) Loss of Colour Lock: This could be due to the sub-carrier oscillator being off-lock, this displays itself as colours running through the picture in bands, ( barber-pole) not unlike loss of line hold but on the coloured parts of the picture. This is due to the reference oscillator not being locked to the transmitted burst and the fault could be in the burst gate amplifier circuit VT301 and VT302; or in the phase discriminator W302 and W303; or in the polarity splitter VT308, which supplies the gating pulse to switch on the burst gate amplifiers.
Before attempting any circuit checks, observe and adjust R354 to widen the gating pulse, if this is not successful return R354 to its original position and check with an oscilloscope for the burst at the collector of VT302. If this is not present , it could be due to the gated burst amplifier VT301 not functioning, in which case the gating pulses should be checked with an oscilloscope into and out of the polarity splitter, and then check around VT301 and VT302 with your meter. After clearing the fault condition check and readjust if necessary SET ACC (R308) as described in the service manual.
(d) No Colour: If there is still no colour when overriding the colour killer, either the chrominance signal or the reference sub-carrier is missing. Both the reference oscillator and the chroma signals can be substituted with a normal signal generator, but for the latter a colour-bar generator is preferred. For the sub-carrier oscillator, inject a 1V p-p minimum amplitude CW signal of 4.43 MHz into the base of VT305. If this restores colour then adopt normal methods of checking the oscillator VT304.
For chrominance amplifiers, inject at the junction of C338 and C339. Alternatively, if the sub-carrier oscillator is working it can be used as a signal source by connecting a .01uF capacitor between the emitter of VT305 and the junction of C338 and C339. In either case you should obtain a locked green or red raster.
If you do obtain a coloured raster, then there is no chroma input and checks should be made with a signal generator, oscilloscope and meter back through the chrominance take-off stage VT110 and then up to the chrominance and inter-carrier sound detector W101. If sound is present it is, however, unlikely to be W101.
If do not obtain a colour raster, when the reference oscillator is fed into the chrominance input,or there is no colour noise when it isn’t, then the fault must be in the chrominance amplifiers VT309 and VT310, and suitable checks should be made in that area.
Other Faults Apparent Without Overriding The Colour Killer
- Reversal and/or streaking of colours at the extreme left or right-hand sides of the picture may be due to ident coil L303 having moved on its former. The repositioning of this coil may be done via the Test Card but it is essential to reduce the width sufficiently to enable the colour castellations on both sides to be seen to avoid tuning too far and transferring the misidentification to the opposite side of the picture.
- Desaturation may be due to incorrect adjustment of AFC control R141 (IF Module) or ACC control R308. Desaturation can also be caused by a straightforward fault in the chrominace amplifiers.
- Venetian blind effects can sometimes occur due to phase and unequal amplitude errors, i.e. incorrect readjustment of the master phase control L309, B-Y phase control L307, or the matrix phase and amplitude controls L306 and R374. If there is a loss of either the delayed or direct signal matrix circuit in the delay line output, a reduced chrominance output would result accompanied by Venetian blinds particularly noticeable in the fully-saturated areas of the picture
- Unless the reference oscillator is faulty, it is unlikely that both demodulators will develop a faults simultaneously and so faults associated with either of these will normally show as a loss of saturation of either red or blue. A useful indication of this fault is that green will also be desaturated. This is because green has been reconstituted by proportions if red and blue, and if only one is missing sine desaturation of green will be inevitable. A colour-bar generator and and oscilloscope are required to trace signals through the circuits and it would be prudent to confirm that the reference sub-carrier input to either demodulator has not been lost.
Chrominance Module Stock Faults
Picture Goes Blue As Colour Control Increased
- C337 .47uF leaky
Colour Intermittent With Change Of Camera Shot ETC
- Check setting of Pulse-Width Preset R354
Blues And Yellows Missing From Picture
- Loss of signal input to B-Y demodulator due to bad connections to PAL Delay Line. Check DC path between W311 cathode and W318 Cathode Normally 3.5R
Video Trouble Shooting Guide
After an initial examination of the circuit it might be thought to consist of three separate signal channels driving the CRT cathodes. If you agree with such a statement it suggests that you might have fallen into the first trap. A closer look at the circuit will show that the above is only partially true and, in fact, if you attempt repairing a faulty video module with this half-truth in mind, it is almost certain that you will spend many hours of fruitless and unnecessary trouble-shooting. If we look again at the circuit it would be more accurate to state that whilst we do have three colour output stages driving three CRT cathodes, certain parts of the circuit are common to all the amplifiers and these are :-
- (a) The Luminance Drive
- (b) (G-Y) Matrix
- (c) The +200V pulse source
It is therefore possible under certain fault conditions that a faulty amplifier can affect a good amplifier via one of the above common circuits and produce very misleading symptoms on screen. To illustrate this let us consider C215: The function of this capacitor is to integrate the 200 volt pulse which is clamped to the 160 volts on the collector of VT209. If C215 went open-circuit the effect would be that the red amplifier would be cut off but the most noticeable effects on the screen would be an excessively green raster. It would not be unreasonable for an engineer confronted with this visible symptom to wrongly suspect that the fault was in the green amplifier.
One way to avoid this mistake is to turn the CRT beams on, one at a time, be means of the A1 switches and check if any of the primary colours are missing. It should be realised that the luminance signal is only applied to the output stages and its presence does not necessarily mean that the rest of the amplifier chain is operating satisfactorily. Similarly, an excessive red raster with no green could be due to an open-circuit C227, or if C231 goes open-circuit an excessively green with no blue raster would result. These effects are due to the line fly-back clamping pulse reaching the base of the colour-difference amplifier transistor in the defective channel, and then via the (G-Y) matrix components to the emitter oif the colour-difference amplifying transistor of a faultless channel.
The 400V fly-back pulse (plug14, contact 7) also plays a very important part in the operation of the video circuit. A popular misconception is that its absence will show as non-operation of the brightness control. In fact the control will operate to some extent without the pulse. The correct operation of the video circuits depend upon the presence and correct amplitude of this pulse, and the only really safe method is to check for a 200V pulse at the junction of W204-R228.
If the pulse is completely missing: for example, due to L205-R227 being open-circuit, the effect on the screen would be a very bright raster with the brightness control having very limited range of control, and to explain this let us consider the red amplifier.
The bias for VT207 is obtained by feeding the difference voltage from the junction of R233-R237 via R234 to its base. R233 is fed from the 20V positive rail but R237 is fed from a -3.8V source obtained by integrating the 200V line flyback pulse at the junction of C220-W206. If the fly-back pulse was absent the diode W206 would not conduct and the -3.8V would be lost. This would have the effect of making VT207 base go more positive and its collector voltage would fail. The base and emitter of VT208 would follow. This would turn-on VT209 more and its collector voltage would fail producing a brighter raster. Identical condition would exist in the blue green amplifiers.
It follows from the above that the DC condition in all the amplifier chains would be seriously affected if the pulse was missing and that testing for its presence should be one of the first checks made.
So far we have considered two fault conditions which could perhaps be described as not so obvious. It is quite possible that other faulty components could create a similar chain-reaction resulting in similar misleading information being produced on the CRT. However, careful cross-checking of symptoms and testing should prevent you from searching the wrong amp for the fault.
Video Module Or Tube Circuit ?
Under normal working conditions, the voltages on the CRT cathodes average 146V-160V approx and the CRT grid is zero. It therefore follows that cathode voltages in excess of this level will cut-off the CRT. However, it could happen that a particular colour is missing but the cathode voltage is perhaps around the 160V-180V level. To establish if the fault is in the video module or in the tube circuit, a quick test would be to transpose the CRT cathode lead from the missing output with that of a good one.
The CRT base socket is also particularly useful if a very bright picture or single colour is present, thus indicating that the CRT cathode voltages are low. A convenient test is to use the Set White switch to check if the cathode voltage will attain 160V. If the cathode plug is disconnected for this test, any danger of damaging the tube with and extremely bright horizontal line from a collapsed frame due to operating the Set White switch is eliminated.
General Video Servicing Hints
When servicing the video module, remember the following: –
- Establish that all the supply voltages are correct
- If one output is missing. Use one of the other outputs for test purpose by transposing the crt cathode leads
- If the fault is an over-bright colour. Depending upon the severity of the fault, switch off the A1 or disconnect that particular cathode lead whilst trouble-shooting to avoid CRT damage
- Try to avoid the initial impulse to “twiddle” all the presets which may result in valuable evidence being lost and replaced with misleading conditions.
- Voltage checks are always informative. Many faults have been located by going through the installation adjustments, a procedure which is highly recommended.
Video Module Stock Faults
Smearing At High Or Low Brightness Settings
- L205 Clamp pulse delay open-circuit
- C221 1uF leaky
Change In Background Colour
- Replace C231, C227 and C215 : 2uF
No Luminance: Colour And Sound Present
- L201, 4.4MHz trap open-circuit. VT201 cut-off
Excessive Brightness On Any Gun
- Faulty video ouput driver transistors
IF Module Stock Faults
No Vision, Good Raster, Sound Very Low
- C179 10uF AGC decoupling capacitor short-circuit
- C130 .01uF Disc-Type leaky
Bars On Picture ( Similar To Cross-Modulation Overloading Effect)
- C179 open-circuit
Low Sound With Slight Buzz
- C158, C159 both 180pF in discriminator can open-circuit
- VT110 faulty
Note: although some fault conditions may occur which do not exactly follow the patterns laid out above, the procedures outlined will enable the area of fault, and in most cases the defective component to be quickly located.
For further a further list of faults for the BRC3000/3500 chassis be sure to check the stock faults section. This is accessed via the service | TV stock faults menu above.