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Old 3rd January 2007, 06:11 AM   #1
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Default A question about votage abnormal

Hi group,

I have a tuner which was out of work few days ago.

The LCD can't show the frequency number and there's not any output signal.

So I guess may be the control IC is out of function.

I used the voltage meter to meter the Vcc of this IC. The votage is not the regular 5.0V, just about half, 2.5V.

The IC is "UPD1704C-011" which is PLL frequency synthesizer and control ic for FM tuner.

It means maybe the power circuit is broken.

I am wondering if anybody can diagnose this probelm for me.

Which part demage can cause the valtage reduce half?

maybe it can give me some clue to find the broken components?!

Thanks for your kindness in advance.

best regards,
Frank

by the way, I brought an IC "UPD1704C-012" to replace the original one.

Can I directly replace it?
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Old 3rd January 2007, 06:23 AM   #2
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make and model of tuner?

sounds like you have a shorted ic somewhere. just recently worked on a lightning damaged kenwood receiver with a shorted processor chip. replaced the chip only to find out that the mode switching chips were also fried.

i really prefer the "old fashioned" mechanical power switch, and rotary input/mode switch rather than microprocessor based controls.
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Old 3rd January 2007, 06:39 AM   #3
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Hello unclejed,

Thanks for your reply.

It's a vintage FM tuner made by Luxkit (a sub-brand of Luxman in 70's~80's).

The model is A-503

there's a picture on the following websit.

http://www.hm5.aitai.ne.jp/~t_ak/A503.html

best regards,
Frank
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Old 8th January 2007, 10:45 PM   #4
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Check all power supply voltages-may have a bad 5volt regulator or zener diode could also have a bad bridge rectifier or power supply capacitor
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Old 9th January 2007, 05:25 AM   #5
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if the power supply turns out to be good, check with an o-scope (if you have one) the signal on the processor crystal (the metal oval thing next to pin 40 of the processor chip). the signal should be at least 2.5vP-P, and be the same frequency as what's stamped on the crystal (usually 6, 8, or 10Mhz). the processor chip drives everything else, display, tuner front end, etc...
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Old 9th January 2007, 05:46 AM   #6
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Thanks for you guys kind reply.

XEAGLEKEEPER: Thanks for your advise. I have the same idea as
you. I'm still hanging on it. But unfortunately I have only very little electronic knowledge but a lot of DIY interestes, so I'll keep trying fix it by myself. If I can find a service manual which will help me a lot. Actually I'm looking for it right now.

unclejed : I don't have a scope. can I just replace the crystall to see what will happen? I think it's easier to me.
In addition to no signal, the LCD didn't function either. That's why I suspect the processing IC was out of function.


best regards,
Frank
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Old 9th January 2007, 11:11 AM   #7
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A schematic would be a definite plus. I am not familiar with this make or model. That 2.5 vdc that you were refering to earlier may be a legitamate value. But " When In doubt or without always check power supplys first........Good luck..
Dave
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