GRUNDIG SUPER COLOR W6232 IT CHASSIS 29301-373.21 INTERNAL VIEW.

The GRUNDIG GSC100/GSC200 chassis consists of a large, vertical main panel with a number of modules that take care of various circuit functions. The tubes are of the 90° in -line gun type while the line output stage is of the thyristor type. Unlike earlier Grundig colour chassis that employed a transductor for width/e.h.t. regulation, in this chassis a thyristor driven by a rather unusual circuit (more on this later) is used for the purpose. There are no less than six thyristors dotted around the chassis - e.h.t. regulator (Ty503), line scan and line flyback (Ty508 and Ty501), line generator start-up (Ty607), excess current trip (Ty615) and overvoltage trip (Ty2517).

 Power Supply:

Fig. 1 shows the basic power supply arrangement (in addition, 200V and 18.6V supplies, fused by Si629 and Si627 respectively, are derived from the line output stage).

 As soon as people see a thyristor they throw up their hands 
in horror!

In normal operation, a supply (+B13.5V) derived from the combi coil is used to power the line generator and e.h.t. control modules. So a start-up supply is required. This is provided by Ty607 whose anode is fed via the fusible resistor R607. The output is limited by the 10V zener diode Di607. Once the line timebase has come into operation, Di511 rectifies pulses developed across a secondary winding on the combi coil, feeding the Darlington series regulator transistor Tr635 via fuse Si511 and the fusible resistor R632. Assuming that there's no fault condition, the regulator circuit produces a 15V supply and Tr608 switches on, shorting the gate of Ty607 to chassis to disable the start- up system.
One of the most common conditions is no results due to R607 having sprung open. In this event, check the following: Ty607 short-circuit; Tr608 open -circuit or low gain; R608 high in value; R633 high or open -circuit; no 311V h.t. supply; no output from the line generator module; no drive to Ty503 from the e.h.t. control module; Si511 or Di511 open -circuit; Ty503 open -circuit; Di636 low voltage. This covers 90 per cent of faults causing R607 to ping.
The cause of R621 in the h.t. supply being open -circuit is usually excess current trip operation due to a line output stage fault !

Excess Current Trip:

 The excess current trip module gives relatively few problems. If Ty615 has gone open -circuit there'll be no h.t. supply of course; if it's gone short-circuit there'll be no protection until R621 pings. The operation of this circuit is as follows. If a fault condition causing an increase of 100 per cent in the h.t. current occurs, the voltage developed across R621 will increase from approximately 9V to 13V plus. As a result zener diode Di619 will conduct, turning on Tr618 to short Ty615's gate -cathode junction so that it switches off. The time -constant of C618/R618 is approxi- mately 120msec, so that the trip "oscillates" until R621 pings. To check the operation of this module, connect a 10kfl resistor from the junction of R619 and D619 to chassis: the module should now oscillate at the trip frequency. EHT Control Module The e.h.t. control module (see Fig. 2) may come as a surprise. TTL in a line timebase! Very useful actually. Here's how it works. IC2511 is a monostable multivibrator which is triggered at pin 5 by pulses from the line generator module. The multivibrator's on time is set by the time -constant network R2514/C2513. It's output at pin 1 is capacitively coupled to Tr2506 which provides a transformer coupled drive to the regulating thyristor Ty503. Pulses from the line output transformer enter the module at pin 9 and are rectified by Di2521/C2522. The resultant supply controls transistor Tr2516 which in turn controls the supply to the monostable's time -constant network, thus providing e.h.t./width regulation. The same line output transformer derived pulses enter the module at pin 8. Under excess voltage conditions Di2517 and in turn Ty2517 conduct, shutting the whole operation down. The usual fault conditions are as follows. C2507 changes value, reducing the drive to Tr2506 which gets hot and dies due to the slower turn-off time. Ty2517 goes short-circuit, with the result that the monostable doesn't trigger. Zener diode Di2502 goes low which upsets things because TTL devices like a supply of 5V or thereabouts. If you can't adjust the set e.h.t. control R2523, change the 9.1V zener diode Di2516. Then set the control midway, reinsert the module, monitor tag b on the line output transformer with an AVO 8 or 9 and adjust R2523 for 49V d.c. This will give correct e.h.t. and width. If you wind the control too far Di2516 will snuff it, so be careful. Line Output Stage Thyristor line output stages are not the easiest circuits for fault finding.
The problem is that it either works or it doesn't, no half ways. A very useful tool is the transistor/ thyristor tester , since this enables you to check the power devices in situ before substitution. If you don't have a tester, the following checks and observations are worth making. R621 and maybe R607 in the power supply will usually have pinged. When resoldered, the trip module will "plop" repeatedly,  proving that excess current is flowing. If disconnecting the anode of the flyback thyristor Ty501 stops the tripping, it's probably short-circuit. If the set continues to trip, replace the scan thyristor Ty508 as it may be open -circuit. Also check the efficiency diode Line out put stage) Di508. Try disconnecting the tripler. Check the continuity of the scan coils, and the scan -correction capacitor C526 (2.3µF).
It's also worth inspecting the solder around R502 (18012, 11W) in the scan thyristor's gate drive circuit - it gets a bit hot and tends to get dry -jointed. I always replaced  devices in this area with exact Grundig  replacements and not other types, though alternatives may be o.k.
 The line output transformer and combi coil don't readily fail (Never changed one !), though I suppose some engineers will have found duff ones.
A useful tool, essential when running up a GSC100 line output stage, is a 2A variac. It saves on fuses and nerves.

Line Generator:

 The only problems I've had with the line generator module concern the TDA2591 chip. If the module fails to oscillate at start up the chip may be faulty - some are a bit funny about the voltage when cold. The Field Timebase The field time base module employs a TDA1170 i.c. There've been some odd faults in this area. C441 (0.22µF) leaky causes poor field sync. Tr467 leaky causes funny field flyback blanking - sometimes almost anywhere dur- ing the field period. The field scan coupling capacitor C473 is on the main panel: when it's leaky or short-circuit the result is field collapse with the line shifting upwards to near the top of the screen.



Audio Module:

There's little to report about the audio module apart from the TBA800 i.c. occasionally dying. IF Module The smoothing capacitor C2321 (10µF) on the i.f. module is a tantalum type and can go short-circuit, R607 eventually pinging. Don't forget that there are separate chroma and luminance outputs, with the chroma signal inverted. If you suspect that the SAWF is out of specification, if have  access to a sweep generator especially designed for use with this module should be used.





The Tuner:
 The tuner used  is of the v.h.f./u.h.f. type, part no. 29500.
For many people, delving into tuners is taboo. If certain ground rules are observed however many common faults can be cleared. Tuning drift or failure to tune is caused by one of the varicap diodes going leaky. These diodes normally have a very high impedance and any leakage at all will cause drift. The best method of tracing this fault is to connect a sensitive d.c. meter, switched to 50µA, in series with the varicap control line, disconnecting each of the diodes in turn until the current returns to zero. Replace these diodes with the exact type - no substitutes.
The r.f. amplifier transistor Tr118 tends to go sick after a thunderstorm. To confirm this, inject a signal via a loop into the output tuning area - some sort of signal should then be evident.

To check that the mixer circuit is operating, use the diode sniffer shown in Fig. 3. Insert the probe near the tuning elements and check for r.f. from ch. 21 to ch. 68. If any component has to be replaced, observe exactly how the original was fitted before removing it. Fit the replacement in the same way, otherwise severe mistuning may occur.



RGB Module:

The RGB module can present difficulties due to the feedback paths.
 If a number of panels need repairing it's worth finding a good one and fitting a 24 -pin i.c. socket so that the TDA2800 i.c. can be proved before making further investigations. Most faults occur in the RGB output stages however.
Here are one or two odd faults: no luminance, C907 (22µF) or the delay line (on the main panel) open -circuit; no luminance and low brightness, C977 (2.2µF) leaky.
There are obviously many internal faults that could occur in the i.c., causing obscure symptoms. Some less common faults I've had on the module are as follows. R1919 open -circuit, no contrast control. Zener diode D1948 open -circuit or L1920 high -resistance, uncontrolla- ble brightness.
Other faults depend on which output stage is involved.
 For the red output stage, R1904 open -circuit causes a tint of that colour on the background and loss of h.f. response; C1912 or C1914 leaky causes no red; R1911 open -circuit results in full beam current, as does T1908 going leaky or short-circuit; T1901 going short- circuit causes no colour. The relevant components in the blue and green output stages give analogoui faults.

Chroma Module:

Many chroma module faults are due to the two i.c.s (TDA2510 and TDA2521). There seem to have been difficulties with the TDA2521 as at least three versions were made, the TDA2521/3 being the latest. If there's no colour, check the colour burst level at pin 7 of IC861. It should be 0.5V peak -to -peak. Under fault conditions it may rise to 2.5V p -p and not be controllable with R827. Check C833 which could be leaky, IC861, and C823/832 which could be open -circuit.
If there's still no colour, check the reference oscillator and its tuning.
As with the RGB module, if you've many panels to look after it's worth fitting i.c. sockets to a known good one for use as an i.c. test bed. If the R -Y or B -Y signals are missing, suspect IC861 and either L854 or L857 for being open - circuit. If R828 has burnt up, check whether C831 is short-circuit. Di881 (12V zener diode) leaky causes green flashing lines while C809 causes weak flashing colours.
As mentioned in a letter (June issue) C843 must be changed to 0.0047µF if you have a colour locking problem with the Sinclair Spectrum microcomputer. this post has shed light on the problems that can be encountered with the GSC100 chassis. Most of the comments also apply to the GSC200 chassis which differs in only minor respects from the GSC100 (vision i.f. module, tuning system and the inclusion of a relay board).