Complex Circuit Of Panasonic Surround System SA-PT860 And Tips For Troubleshooting
This Panasonic DVD Player Surround System was brought to me by one of my close electronic technician friends, who was concentrating only on other products. He had to accept it as it was from one of his relatives. The peculiarity of this set is that it uses wireless speakers. The complaint and history was that it was kept aside for a few months and when switched on to play a CD, only a red LED lit and no buttons from panel or remote worked. First of all I downloaded the service manual from the net. Here is the link: https://elektrotanya.com/panasonic_sa-pt560e.pdf/download.html
As my friend had already opened the set and cleaned it, I studied the structure of the set. It had AC in board, SMPS Board, Power Supply Board, Mother Board (Main control board), Audio Board etc. all of which were interconnected with strip wires, some of which were so short that I had to really struggle to dismantle the set, without which no trouble shooting can ever be done. Here is the interesting inside of this complicated set:
The SMPS circuit had two separate power supplies. One Primary, which supplied 6V to the System; let us call it PS1. The System then switches on the main power supply (let us call it PS2), if all preliminary checks are ok and then checks outputs from the PS2 too before switching on the display. The PS2 provided +30, -30, one +18V (For fan) with its own ground and another +18 (This 18V goes to various stages for DC-DC conversion stages including buck circuits) and -12V with its own ground. It had two separate DC Checks, DC-Check-1 and DC-Check-2.
These are tapped from various places from the PS to all other boards. If there is any current or voltage variation System IC won’t switch on. If everything is found ok, the PS On is enabled through Opto-coupler PC5701. Important: The grounds of the PS1 and PS2 are isolated and get joined only if we connect the Power Supply Board. Again, the ground of the PS2 from which + & – supplies go to the Audio Board get joined only if we connect the Audio Board. So, when we conduct voltage tests, we should check it with respect to its ground. Here the schematic came very handy.
Then connected the AC-In, SMPS, Power Supply Board, Front Panel Board and the mother board, keeping these on the table. My first check was whether the 6V was generated and found it to be ok. It was reaching the System at various pins as mentioned in the Service Manual. Even the PS on (PCONT-Pin 40) voltage was getting on flashing 5V, indicating that everything was ok up to that stage and there was no issue with the System IC. Then checked and confirmed that the on/off button from the front panel was working and the voltage was dropping to 0 when pressed.
So, I concentrated on the Power Supply. (I also did a thorough retouch of the solder on the SMPS Board and Power Supply Board.) When I checked the primary switching signal of the PS2 in an oscilloscope, the signals were not proper. So, combed the primary and secondary section of it and did not find any defective components except for the inverter transistor of Opto PC5701 that is controlled for PS on by System IC. Following is an extract of the circuit. You will notice that Panasonic uses its own numbers for the components and the real number is not mentioned anywhere. We have to either read it directly from the printing on the component or search in the web for the SMD Codes.
As the number on the SMD transistor was not readable, I searched the pdf file for B1GBCFLL0037 used elsewhere in the circuit, located it on the mother board and got the reading as NC C4
The SMD code C4 indicated various components, the best possible match of which was NEC’s KA4XXX series. The datasheet showed:
This transistor, therefore, had 4.7K resistors built in. But on checking, it was open. As I had no equivalent, I used C1815 NPN transistor, ensuring that the base, emitter and collector goes to the point as per circuit diagram. I drilled holes in the three points on the board and fit the C1815 from the other side.
Then applied power and checked. But situation was same. I forgot to mention that I was constantly consulting one of my regular techie-friends, who was always helpful and guiding in the process of troubleshooting. My friend suggested replacement of the STR, which I did not want to do without ensuring that the replacement would also not conk off due to some defects in the circuit.
While checking the board, I came across an IC with circuit reference number as IC5780. But there was no such circuit in the service manual that I downloaded. Following is the IC:
From the datasheet that I downloaded, this was NJM2904:
But I had no schematic of this and was not knowing its function in this SMPS. So gave a search using the IC reference number, IC5780 and got Panasonic Service Manuals of other models, in one of which this IC circuit was available and showed clearly where this is connected. I got it from totally a different model, Panasonic SAAK770pl CD Stereo System:
I then traced the inputs/outputs for this IC in the board and ensured that there were no defective components and the voltages were ok. I also combed the entire SMPS Board and Power Supply Board for any defective components, by even removing some of the through hole mounted transistors. After ensuring that everything was ok and got ready, I removed the STR with yet another Diode mounted on the same heat zinc, as the screw was not accessible because of the SMPS Transformer.
Now, I wanted to ensure that the SMPS would switch on if replaced and there were no problems in the primary or secondary section. So, I connected three wire module, connecting only its Red wire to B+ return from the SMPS transformer and Black wire to the ground:
(Forgot to mention that I had connected a 560 Ohms resistor from the 6V to PS on pin and shorted the two grounds in the outgoing connector from SMPS, as advised by my friend to enable switch on of the PS2, for checking the board isolated from other connections).
Then, when the power was applied, I got some outputs and by varying the pot in the module, I got the desired outputs in the PS2 secondary, which indicated that everything was ok with PS2 and there was no problem in the secondary up to the Power Supply Board stage. Having done this much, I replaced the STR and fit the Heat Zinc back to the SMPS and removed the module. Reconnected the Main PCB and Front Panel. Then applied power and I was glad to see the display turning on:
Disconnected the power and boards and put these back into the cabinet. Interconnected every other board including the strip wire that goes to the wireless module for speaker. Then applied power and used a headphone to listen to the audio output. Noticed that the CD tray was functioning in/out and the motors were working very well including the eye unit. So, loaded a DVD and allowed it to run fully, keeping the set in open condition. Also tried five more DVD/CDs all of which played very well. Having ensured that the set was working trouble free for long hours, put the cover back:
Thus the curtain fell on this week long strenuous work and the set was delivered to my friend, adding an intensified satisfaction to its collection bag! I was very happy that I could learn so much about circuits and functions from this experience!
Tips for troubleshooting such complicated sets:
01) Always try to fetch the schematic of a set and if necessary, pay; It is worth and will save you many hours of troubleshooting!
02) If schematics are not available, download datasheets of ICs and other related components and study the use and functions.
03) Study the schematic thoroughly, especially the block diagrams and grasp the functions of various sections and that of the system control IC. Take print outs of the relevant sections and keep it in front of you. Study the functions of the IC to know the pins that are held low (0) or high (with some voltage present). Switch system on/off holding the multimeter probes to check the low/high points. This is very important to understand the nature of the problems.
04) Don’t give up if you do not get what you want in a Google search. Vary the inputs for search and use different combinations, such as mentioned in the article to get an IC circuit that was not available in the downloaded service manual.
05) Always ensure that every component is checked before replacing the main controlling component in the primary of SMPS and conduct a temporary alternative connected test like a module that was used in this case.
06) If a print on any component is not readable, look for a similar one elsewhere in the boards and try reading that.
07) Study and conduct tests in the System IC (both cold and hot tests) so as to ensure that it is functioning very well.
08) Always look for flash presence of voltages in various points before the System IC shuts off, to know the health of various functions. This will direct us and help us focus on the trouble area/section. The reset voltage should come when switched off and can be checked by holding the multimeter probes while switching on/off.
09) Every complicated problem that comes in front of us is an opportunity to learn and improve our knowledge and so, we should not get depressed or worked up or fed up. Remember the old adage: Sincere and strenuous efforts will never go unrewarded. Do not hesitate to take-up such jobs though the monetary return might not commensurate with the efforts put in. It is a great experience that might sharpen our expertise and pay back in long term, apart from improving our confidence!
10) Develop a habit of maintaining healthy relationships with Electronic Technicians, especially living far away, so that we can exchange doubts and ideas. Knowledge is something that grows within when we share with others!
Best of luck!
This article was prepared for you by Parasuraman Subramanian from India. He is 72 years old and has more than 30 years’ experience in handling antique equipment like Valve Radio, Amps, Reel Tape Recorders and currently studying latest tech-classes conducted by Kerala State Electronics Technicians’ Association. He has done graduation in BBA degree, private diploma in Radio Engineering and retired as MD of a USA company. Presently working as Consultant to Hospital and other institutions.
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