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Yamaha TX540 tuner not tuning
Yamaha TX540 tuner not tuning
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Old 18th March 2019, 09:53 AM   #21
Glenn E is offline Glenn E
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Hi Mooly, I have 5.2V on pin 17 and not changing as frequency changes (or maybe incresasing very very slowly if I let it cycle on auto tuning) but it should be 2.4V. So, the problem may be with the PLL or somewhere further back. I'm at the limits of my troubleshooting ability tracing back past the PLL. Glenn
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Old 18th March 2019, 10:09 AM   #22
Glenn E is offline Glenn E
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Hi Mooly, ignore my last reply. I was reading wrong chip. Actually no voltage on pin 17.
Glenn
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Old 18th March 2019, 10:12 AM   #23
Mooly is offline Mooly  United Kingdom
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Yamaha TX540 tuner not tuning
A DVM will not show a true reading because the voltage is a logic level that constantly changes. Scroll down this link to 'Duty Cycle'

Pulse-width modulation - Wikipedia

If you have 5 volts on pin 17, then you should find that if you short the gate of the FET o ground (that the 10k from pin 17 goes to) that the tuning line VT changes dramatically. Don't short pin 17 directly though, do it at the end of the resistor.
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Old 18th March 2019, 10:15 AM   #24
Mooly is offline Mooly  United Kingdom
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Yamaha TX540 tuner not tuning
Lol no problem.

Is the 5 volt supply to the chip OK?
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Old 18th March 2019, 10:23 AM   #25
Glenn E is offline Glenn E
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Not sure which pin is the 5 volt supply. Looks like pins 5, 15 and 16 should read about 5 volts. I'll check them. Glenn
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Old 18th March 2019, 10:24 AM   #26
Mooly is offline Mooly  United Kingdom
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Yamaha TX540 tuner not tuning
Vdd is the supply pin, Vss is ground.
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Old 18th March 2019, 10:30 AM   #27
Glenn E is offline Glenn E
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Pins 6 (not 5), 15 and 16 all reading between 5.2 and 5.5 volts. Glenn
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Old 18th March 2019, 10:31 AM   #28
Glenn E is offline Glenn E
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That is pin 15 so looks OK. Glenn
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Old 18th March 2019, 10:40 AM   #29
Mooly is offline Mooly  United Kingdom
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Yamaha TX540 tuner not tuning
So the supplies seem OK. This is getting into scope territory I'm afraid as you are limited with what you can see with a DVM. Although the scope can't reveal whether the 'data' content is good or not, it does provide massive clues in terms of what is there and the amplitude and so on. Also if data is present then its usually (in my experience) correct.

You could try connecting a 1 k resistor from the 5 volt supply to the gate of the input FET and seeing if that swings the voltage on the tuning volts line. That is a very crude test of the transistor stage.

Data sheet for LM7000:
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Old 18th March 2019, 11:18 AM   #30
Glenn E is offline Glenn E
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Not sure which is the input FET. Can't pick it on the schematic. Glenn
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