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
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.
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.
no problem.
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:
HTTP 301 This page has been moved
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:
HTTP 301 This page has been moved
Ok. So just to set the record straight I was initially measuring VT at the arrow in the diagram you posted. Now measuring from the proper place, I am getting a constant voltage of 26.5V irrespective of tuned frequency on the display. I also measured voltages at every pin on each transistor and very few are reading correct values on all pins. The power supply values are OK as are the values on the Reset transistors. The really odd ones are Q31 and Q32 which are associated with the Mute. The voltage values on the bases are supposed to be -0.5V but they seem to fluctuate and even go positive when you engage auto tuning and cycle through. The bases connect to pin 11 on the LA3401 and this is also varying. Glenn
This is worth doing to 'prove' the charge pump is OK. We can do this by applying either 0v or 5 volts to the FET Q4.
If you lift the end of resistor R17 at the end where it connects to pin 17 then we can apply a test voltage of either 0v (grounding the resistor) or 5 volts which we can get from pins 15/16 of the chip.
With R17 grounded I would expect the tuning line to be close to zero volts. With +5 volts applied to R17 the tuning volts should rise to near 30 volts.
Another test that can be done is to isolate the tuning line (VT) and connect it instead to the wiper of a pot or preset pot (say 10k or 22k) and connect the pot across the 30 volt line. That would give you a manual tuning voltage from 0 to 30 volts and should allow stations to be tuned.
If you lift the end of resistor R17 at the end where it connects to pin 17 then we can apply a test voltage of either 0v (grounding the resistor) or 5 volts which we can get from pins 15/16 of the chip.
With R17 grounded I would expect the tuning line to be close to zero volts. With +5 volts applied to R17 the tuning volts should rise to near 30 volts.
Another test that can be done is to isolate the tuning line (VT) and connect it instead to the wiper of a pot or preset pot (say 10k or 22k) and connect the pot across the 30 volt line. That would give you a manual tuning voltage from 0 to 30 volts and should allow stations to be tuned.
Hi,
It is sounding as if your controller isn't getting the LO frequency. This comes from the tuner pack into a prescaler IC (normally anyway) and then on to the controller. There is often a ceramic DC blocking capacitor to the input of the controller for the FM LO. If the capacitor goes leay or shorted, it will disable the LO frequency input. If it goes open, same thing.
* LO = Local Oscillator.
You absolutely require an oscilloscope good for 150 MHz or better at this point in time. There isn't any way around it save very creative use of a spectrum analyser. That is an even more specialized piece of equipment (and more expensive).
The LO signal into the prescaler chip will be the tuned frequency + 10.7 MHz. The input to the controller will be this number divided by 8 or whatever the prescaler chip divides by. That should be more in the range of a normal 100 MHz oscilloscope. Time to look up chips unless Karl feels like examining the schematic for answers.
-Chris
It is sounding as if your controller isn't getting the LO frequency. This comes from the tuner pack into a prescaler IC (normally anyway) and then on to the controller. There is often a ceramic DC blocking capacitor to the input of the controller for the FM LO. If the capacitor goes leay or shorted, it will disable the LO frequency input. If it goes open, same thing.
* LO = Local Oscillator.
You absolutely require an oscilloscope good for 150 MHz or better at this point in time. There isn't any way around it save very creative use of a spectrum analyser. That is an even more specialized piece of equipment (and more expensive).
The LO signal into the prescaler chip will be the tuned frequency + 10.7 MHz. The input to the controller will be this number divided by 8 or whatever the prescaler chip divides by. That should be more in the range of a normal 100 MHz oscilloscope. Time to look up chips unless Karl feels like examining the schematic for answers.
-Chris
Thank you both. I did post earlier on lifting resistor 17 at the pin 17 end but it seems to have gotten lost. With 5V connected, zero voltage on VT. Pins on Q4 reading 5.3, 26.7 and 26.7V. With resistor earthed, 26.5V on VT. Pins on Q4 reading 2.7, 26.5 and 26.5V. So opposite to what you forecast Mooly. Unless that provides any more insight, given your most recent comments, I will shelve this attempted repair. Not worth my while to invest in a CRO at this stage. As I said earlier, I have learned a lot about tuners in trying to sort this one out. I have other tuners to listen to which are still working fine touch wood. Thanks to you both and to the others who contributed to this thread. Glenn
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