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Yamaha TX540 tuner not tuning
Yamaha TX540 tuner not tuning
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Old 20th July 2019, 01:25 PM   #51
anatech is offline anatech  Canada
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Yamaha TX540 tuner not tuning
Hi Glenn,
I'm afraid I might have some bad news about your 'scope. You need to test it to see if my experience holds true for you. It is a very good oscilloscope and while pretty different it isn't that hard to use. I bought one 27 years ago, rebuilt it once and still can use it. I like the scope beyond one fault it causes. I think there is a modification available that will allow it to remember your settings for the next time you spark it up. That would have been handy for me back then.

So, what I discovered is that when you are using that scope, you aren't listening, much less aligning, any FM tuners. The scope interferes with the operation of tuners, so it must be putting out a mash of frequencies around 10.7 MHz, or maybe it's messing up the FM band. I tried using a PM3365 and it had the same problems. Otherwise, a beautiful trace, sharp and clean. Perfect for doing CD players and everything other than FM tuners. I hope your experience is different.

Having said that, it will not interfere with you checking control signals or the local oscillator. Only a received signal is affected.

Do as Mooly recommended and read back over the troubleshooting points we were talking about.

-Chris
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Old 21st July 2019, 10:44 AM   #52
Glenn E is offline Glenn E
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Thanks for the comment Mooley and Chris, I should have checked with you Chris before I bought the PM3070. I'll let you know how it pans out. I'll check the Yamaha tuner over the next week or so. Glenn
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Old 21st July 2019, 11:12 AM   #53
Mooly is offline Mooly  United Kingdom
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Yamaha TX540 tuner not tuning
Remember that for checking the high frequency high impedance stuff you really need to use a divider probe (a divide by 10 type) which reduces the capacitive loading as seen at the probe tip.

Use the 'Cal' output of the scope to calibrate the adjustable trimmer cap on the probe setting it for no under or overshoot on the Cal squarewave output.
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Old 21st July 2019, 11:03 PM   #54
Glenn E is offline Glenn E
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Mooley, it is a x10 probe which I assume is the same as divide by 10 type. The shape of the square wave on the cal output looks OK. Not sure what to expect when I test the LM7001 at pins 12 and 14- presumably some type of audio signal. Glenn
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Old 22nd July 2019, 01:09 AM   #55
anatech is offline anatech  Canada
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Yamaha TX540 tuner not tuning
Hi Glenn,
Use some epoxy and glue your probes into the X10 position. It's important that you do this to prevent it slipping into X1 and frying your scope input circuits. For direct connect applications you can use a BNC to clips type probe. That way there is never any doubt as to what you are hitting your scope with and what the capacitive load on the circuit will be. You can make a browsing type probe with an old pen case and some wire with your BNC cable running into it, and the ground going to a black wire and clip. You build this yourself.

What you will see is anything from an HF rounded square wave to an HF sine wave. These will be at (FMIN) the tuned frequency + 10.7 MHz. The (AMIN) will be the tuned AM station + 455 or 456 KHz (whatever the AM IF is in that set, probably 455 KHz). These frequencies will track as you change stations.

The LM7001 is a PLL controller, so no audio. It supplies the charge pump voltage control to the local oscillators for AM and FM operation and makes them agree with what the micro-controller displays on the display, so it has inputs from the micro-controller as well.

Pre-audio would be the IF (intermediate frequency) at 10.7 MHz approx. From tehre is is amplified and limited to eliminate AM components, then detected with a frequency sensitive circuit. There are a few that could detect FM signals. After that, you get audio, which then runs into the MPX (Multiplex) circuit to generate stereo, then filtered and off to your amplifier. Hopefully with a buffer in between.

-Chris
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Old 22nd July 2019, 05:27 AM   #56
Glenn E is offline Glenn E
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Chris, the probe I have is the Fluke Command. It has two push tabs on the stem that do not seem to do anything. The scope senses the probe and indicates that it is 10x. I cannot find a manual for this probe online. Thanks for the heads up on how to make a direct probe. Glenn
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Old 22nd July 2019, 05:41 AM   #57
Glenn E is offline Glenn E
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Hi Chris, Mooley, have now checked pins 12 and 14 on the LM7000N. At pin 12, no signal on FM band apart from jagged saw tooth pattern which is probably interference. At pin 12, sine wave on AM band that tracks as auto tuning cycles in search mode. Nothing at all on pin 14 apart from the jagged saw tooth pattern in both AM and FM. I eagerly await your diagnosis. Glenn
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Old 22nd July 2019, 07:18 AM   #58
Mooly is offline Mooly  United Kingdom
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Yamaha TX540 tuner not tuning
Pin 14 is the one to concentrate on. Use the metal can of the tuner circuitry as the ground for the scope and try and trace the connection from pin 14 back to the tuner.

You need to re-measure the signal where it emerges from the tuner. Amplitude of what you see is also important in helping us make a judgement.

If it is still no good then I suspect we are looking at a front end issue in the can, however it would still be interesting to know what you see on pin 17 of the LM7000. Do you see any kind of squarewave? The amplitude would be in the 4.5 volt region peak to peak region and the duty cycle could be extreme i.e. on or off for 95% of the time. Just curious to know what is on there.

What I'm getting at with this is that before condemning the tuner front end, I'd like to see what the chip is doing. If there is no squarewave present then I'd like to see a temporary bodge with you applying a random voltage to the tuning line to see if that changes anything such as sparking the osc into life.

Probes... we always say a 'times ten probe' but they actually attenuate the signal by 10:1, not multiply it. Confusing, but in tech speak both are the same.
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Old 22nd July 2019, 11:42 AM   #59
Glenn E is offline Glenn E
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Hi Mooley, I'm not seeing anything with the scope probe on OSC pin of tuner with metal can as ground on either AM or FM. On pin 17 of LM7000 I have a kind of square wave with AM cycling but nothing on FM. Not a classsic looking square wave though but definitely a signal. The wave signal reverses polarity (if I have that right?) just temporarily every so often. Glenn
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Old 22nd July 2019, 05:54 PM   #60
Mooly is offline Mooly  United Kingdom
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Yamaha TX540 tuner not tuning
Lets just go back over a couple of things as I've had more time to re read the thread.

The missing OSC signal on FM points to a problem in the tuner can but you also say in post #1 that AM reception is not working either. In the last post you mention seeing the charge pump drive signal (pin 17) cycling on AM.

On AM as you tune through the band, the signal on pin 17 should vary in duty cycle. See image. The scope should be able to lock onto this type of signal. Is the 'cycling' you mention actually a change in duty cycle and also is the amplitude good.

(Make sure you haven't got any HF filters for triggering engaged on the scope)

If the duty cycle varies (and the amplitude should be as I mentioned before at around 4.5 volts peak above the zero volt line) then the VT line should also be varying in response to that and its DC voltage varying significantly.

If AM is really dead as well then that is suspicious with regard to the missing FM OSC signal as they are unrelated and shouldn't interact in any way.

The duty cycle we expect to see will probably (on a good tuner) be even more extreme than this and go between perhaps 5 and 95%.
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