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Sony STR-7055 no FM Stereo

J

John Kimrey

New Member
I have FM mono but no FM stereo. I did some basic troubleshooting and found that I do not have 17.5 volts output to PL-905, stereo lamp from pin 16 of IC 301(reads about 3 volts) but all other voltages on all other pins are within spec . When I removed the lamp(cut wires for new bulb) I possibly touched the 17.5 volt wire to lamp to ground(ouch).This situation ( I think) points to a bad IC which I assume is almost impossible to find. Sony Part number is CX-0431. Any help in locating one would be greatly appreciated .
John K
 
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Thanks, but I need a CX-0431, 18 pin with heat sink on one end. not a CX-0461, 14 pin. I can see this is not going to be easy if not impossible. As a last resort, I could use a used one from a parted out board but that in itself is going be challenging enough to find.
 
Usually the stereo decoders have an open collector bjt on the lamp pin, so to turn the stereo lamp on, it grounds the line, with current running through the lamp, thus a voltage drop from the supply. This way the supply to the ic can be different than the lamp supply, but share the same ground return. I thought this was universal.

If it is the part and you can not find the part and you want to save the rx, you could put in a different stereo decoder/mute ckt chip/pcb.
Few options as far as implementation.
Lots of Hitachi/sanyo chips to be found. Just reference other tuners/receivers using the same chip and the device data sheets. it can be done on a protoboard, aint the prettiest and best for noise, but it will do the job. use a ground plane if you can.
Hitachi HA11223 was used in a Sansui TU-X1 holy grail and they added a few tricks.
Might even be an upgrade in performance. I am not familiar with the specs of I assume a Sony CX-0431.
 
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Yes it is a Sony part also mfg. by Micron and XILNX for a Sony STR-7055 receiver . I don't have the data sheet but the block diagram of the IC appears to point to the stereo lamp switch within the chip not providing the 17.5 volt to the 8v. bulb for fm stereo. All other voltages on all of the pins are spot on or close. The bulb gets its ground through a resistor ( R306) to drop voltage and other components (tuned coil). I hear what you are saying about improvising but beyond my skills and I guess I can live with the problem until and if I find the correct chip. The only reason I say it is the chip is that all the other voltages are good at the chip but(pin 16) I may have more than one problem. Any other thoughts please reply and thanks for your prior input, much appreciated. Oh yes, I did have 17.5 volts on pin 16 but do not at this point.
 
I got the mfg. info. off the web site vintage parts.com. They have a nice pic. of the part but no part, " out of stock "whatever that means. Perhaps just perhaps they or someone will come up with one or I will live with it until one comes along. I have queries into several places but I think you are right, luck will play a big part here. Probably a used board will show up but I hate to use used parts especially ICs. I thought one of the members could point me in the right direction to find one or had one in a parts stash that they were willing to sell. Nothing ventured nothing gained as they say..
 
John Kimrey said:
I got the mfg. info. off the web site vintage parts.com. They have a nice pic. of the part but no part, " out of stock "whatever that means. Perhaps just perhaps they or someone will come up with one or I will live with it until one comes along. I have queries into several places but I think you are right, luck will play a big part here. Probably a used board will show up but I hate to use used parts especially ICs. I thought one of the members could point me in the right direction to find one or had one in a parts stash that they were willing to sell. Nothing ventured nothing gained as they say..
Click to expand...
 
hello again! I have not yet found a decoder IC sony p/n cx-0431; however, I noticed that the sony tuner st-5055 schematic shows( from a previous thread entitled "help on sony st-5055 stereo light", included a shematic of the circuit for that tuner which shows using a cx-0431 or cx-0432 for the stereo decoder chip. My question is: are these two replacements for one another? I found a cx-0432 for purchase but have no idea if it will work in the 7055 model. Any help here is greatly appreciated.
 
If the lamp is good, you should have 17.5 on both sides of the bulb, including pin 16 of IC301, otherwise I would be checking R306 (270 ohm). If you have more than a few volts across the lamp, it should be lit. The 17.5V does not come "from" the chip side, it comes from the lamp side, as rcs16 said.

I believe I have a 7055 parts unit so I might be able to help if it's really the IC.
 
After your inputs rcs 16 and DRecovery I now(think) I have a better understanding of the circuit. I went back and took some measurements. First the 280 ohm resistor R306 is good and measures 278 ohms on my meter. There is 17 volts on each side of the resistor to ground. I did not have a 8v incadescent lamp other than a led so I tied R306 resistor lead wire to stereo lamp to pin 16 of the decoder thus bypassing the lamp..which put 17 volts on pin 16 of the chip. I tried several strong fm stereo stations(good tuning meter movement) to see if the chip stereo lamp output would turn on( ground pin 16 of the chip thus creating a voltage drop across R306). Results: no voltage change/no current draw. As I mentioned earlier I also have no signal separation (fm stereo) only mono.Does this seem like a bad chip or a problem somewhere else causing the chip to remain in an open state? I haven't tried adjusting RT 301, separation adjustment. Any thoughts on where to look next?
 
Separation isn't going to do any good until you have the chip registering stereo... Your method of testing the lamp sounds good to me.

It's possible that the IF or discriminator is out of alignment and that is keeping the chip from registering stereo. That takes a fair amount of equipment though.
 
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@John Kimrey any update on your fix? I think I have the very same situation, but on a ST-5150 tuner.

No FM stereo, lamp was replaced in the past and wires were covered with some ductape which eventually crumbled down leaving the wires exposed, very close each other, so I suppose a short happened and fried the chip.
 
spaghetti said:
@John Kimrey any update on your fix? I think I have the very same situation, but on a ST-5150 tuner.

No FM stereo, lamp was replaced in the past and wires were covered with some ductape which eventually crumbled down leaving the wires exposed, very close each other, so I suppose a short happened and fried the chip.
Click to expand...
Please sir, check C301, C302, C304, C306 and C307
 
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