CTC203
*IIC "Run" Bus Servicing Strategies
This Phrase means, I squared C

Preface

This chassis incorporates 3 two wire IIC system control buses. They are; Standby, Run, and Gemstar. This article only addresses the Run Bus issue. There is no reason why the same approach cannot be used to isolate troubles in either Standby or Gemstar IIC Buses. However the Run Bus occupies far more landscape in this chassis making it harder to analyze and isolate.

It is assumed by this writer that the technician using this information possesses a reasonable & fundamental knowledge on usage of the Chipper Checker, IIC bus system controlling, and logical troubleshooting skills.

The following is a strategy to help the technician troubleshoot of the "Run" Data or Clock Bus in the new Thomson CTC203 consumer television chassis. My findings so far have found the Thomson's Chipper Checker has helped to diagnose several intermittent, and "Hard" failures in this chassis. Chipper Checker is not the cure-all, Tell All instrument in the case of this IIC problem.  It was instrumental in indicating an error code of "16". This error code indicated the Run Bus was being held to a Logic "0".  The following text and an antidote might be help tech localize and isolate the device responsible for this condition.

The next step after using the Chipper Checker, is to measure the "Run"  IIC buses on U13101(3 & 4) at the instant the power is applied. There should be communications sustained for at least 45 Milliseconds on each IIC bus. At turn on there will be continuos activity on the RUN/CLK bus. These communication signals will be 5 volts peak to peak. If you discover either one of these are stuck low proceed with the following steps.

Some precautions before starting.....

Loose the solder wick. Please!  In order for solder wick to be effective the iron temperature must be very hot between 700~800 degrees. The practice of using high soldering temperatures and the solder wick tends to retain too much heat.. These high temperatures are too high and will break down the adhesive used on the copper traces. Once the adhesive is broken down the trace lifts off the board and then your are left with mess.

The Hakko 471 is an excellent choice for desoldering the components on this board. Just keep the temperature between 600 ~ 625 degrees and avoid prolonged contact to the traces. If wick must be used, use only thin stuff, and keep it trimmed back so only fresh braid contacts the trace the needs to be desoldered.

OK..... Here's the Strategy for Isolating Error "16"

Finding the load can be tricky. On two out of five occasions these lines did not measure a low resistance to ground. Instead they measured about 13K each using a Fluke 78 meter. But an analogue meter gave different readings on the bad line, 3K on the bad compared to 10K on the good line. The other two televisions had solder bridges in the tuner assembly that caused the lines to be held to ground, and the last Television  had a defective Audio MTS decoder IC U11601. Apparently on the last chassis the Stereo decoder did not send an acknowledgment to the CPU after the first polling       of the peripherals. Note: If this chassis does not receive a ACK from each peripheral the set will not turn on and it will log an error relative to the defective peripheral.

Once it has been determined which bus is low, locate jumper wires that relate to the appropriate bus and desolder them one at a time. Always checking the condition on the land of U13101. The best way is to attack this problem is to "half" the circuit. This means measure one side, and break the line  somewhere in the middle of the circuit between the CPU and the furthest point away.  Obviously always monitor the line that's in trouble. Disconnecting the jumpers will make it easier in the localize the branch & isolation the peripheral or passive component responsible for the defect. Be careful to limit movement  the free end of these jumpers it may weaken the foil on the soldered end and the pad will tear from the board.

Begin As soon as you see the troubled line go high (5.0 ~ 5.2) volts you have found the branch. At this point you will need to follow the schematic and isolate the active components i.e. F2PIP module, T-4 chip, Stereo decoder. If all else fails then you may have a  passive components. Chip capacitors should not be ruled out as potential suspects.

Jumpers related to "Run" Data are JW13122, JW13124, JW13128, and JW12112
Jumper related to "Run" Clock are JW13120, JW13123, JW13127, **JC12103

** A "JC" is a Jumper Component. It may look like a small chip resistor with the number "0" printed on it. Some manufactures call them zero ohm resistors.
 
 

 Example of a Logic "0" Data Bus Problem

I was recently involved in troubleshooting a model F36445 CTC203 where it was determined the IIC bus was held to logic "0".  After lifting jumper wire JW12112 on the data line, the load on U13101 (4) climbed from 1.9 to 5.2 volts. This jumper coupled the data line to several passive components, in the tuner and Stereo decoder circuit, along with active components U16601( 9) MTS  decoder, and the U17401(19) tuner PLL controller. A decision had to be made to cut a trace ( see silk screen)  located adjacent to the tuner shield, in order to isolate the short. If you have the Thomson ESI schematic locate the tuner section. The short was located along the data bus that controls the tuner PLL.  Capacitor C17416 in the silk screen was the culprit. Removing the capacitor and reconnecting the cut trace allowed the data line to finally go high and the set was operating again.. Capacitor was replaced wit part# 214029 a 43 pf surface   mounted capacitor.
 
 

Dennis Viereck
wa6ati@mindspring.com