Thank you for dumbing it down for someone totally out of their depth!
I don’t have access to pin 5 of IC702 because the chassis is in the way. Does this “easier” way involving TP701 also require disconnecting pin 5? I hate to ask...but if you don’t mind, could you lay out the wiring/steps once again for this “easier” way?
Thanks again for your time, expertise, and patience! It’s really appreciated.
I may be able to help. I have a NAD tuner which I have not used in years. Looks the same circuit as yours. Maybe if you need it I can measure some voltages. The PCB that holds the lamps for the LCD is well charred! Maybe replace with LED's. If its IC702 that's faulty you will need a replacement.
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Connect the two outer pins of your 10K pot between 30V and ground , and connect the central pin to TP701 (having disconnected the NAD circuit from by unsodering pin17 of IC701)
Work out which front end you have (see the setup page) , there are two different versions , with different tuning volts range, you can monitor your tuning voltage on that central pot pin (the wiper)
See the setup page again (attached)
You should be able to manually tune through the FM band with your pot.... you're emulating what IC701 does.
Have FM selected on the front panel obviosuly
Attachments
Since you cannot get to the copper tracks you can just hook up the wiper directly to TP701 because the charge pump Q701/2 is not doing anything. You can even leave the "hot" end of the pot open and just connect the wiper and one end to ground. R708 will act as the connection to +30 V. You will miss the high end of the FM band though but that is OK. We only want to establish a working LO.
1/ Check Voltages on FE407-G58 front end between pin 6 and 4 (should be 12v)
2/ Check R707 (a 47 ohm FUSIBLE resistor) and that there is +30v present.
3/ Check 5V on IC701 Synth IC pins 15/16.
4/ Check voltage on Pins 7 and 4 on ic703 (Tuner sub system chip) (should be 12v).
5/ Check voltages on pins 12 and 22 of ic704 stereo decoder (should be 12v).
6/ There are a number of FUSIBLE resistors which you can check for visible signs of burning and with your ohm meter if any of the voltages above are wrong.
I've fixed a couple of NAD tuners (4300 MONITOR SERIES) and I'm currently working on a Revox B261 tuner.
I wish you well.
David.
PS If you can't measure a voltage DIRECTLY on a pin then follow the trace to a point that IS accessible and measure it THERE instead.
2/ Check R707 (a 47 ohm FUSIBLE resistor) and that there is +30v present.
3/ Check 5V on IC701 Synth IC pins 15/16.
4/ Check voltage on Pins 7 and 4 on ic703 (Tuner sub system chip) (should be 12v).
5/ Check voltages on pins 12 and 22 of ic704 stereo decoder (should be 12v).
6/ There are a number of FUSIBLE resistors which you can check for visible signs of burning and with your ohm meter if any of the voltages above are wrong.
I've fixed a couple of NAD tuners (4300 MONITOR SERIES) and I'm currently working on a Revox B261 tuner.
I wish you well.
David.
PS If you can't measure a voltage DIRECTLY on a pin then follow the trace to a point that IS accessible and measure it THERE instead.
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Fantastic, ok to ground to chassis? My store sold out of the heavier duty pots, but they have these less rugged ones made for bread boards. Would one of them work for this test? (It’s a 3-pack of “DIP Potentiometers”, 1x 1kohm, 1x 10kOhm, 1x 1x100kOhm)
Thanks again.
An externally hosted image should be here but it was not working when we last tested it.
I was able to tune with a range from ~0-15.5V with wiper attached to TP701 and one of the other two pins to ground. I was not able to tune a station using the pot. Still just a strong hiss/white noise coming from speaker.... Does this confirm I need a new fm can, at the very least?
Thanks to all who have commented to offer help.
Thanks to all who have commented to offer help.
There is one more test you can do but I don't think it will change the conclusion that the LO of the front end is borked. Take a portable radio, tune it to 100 MHz (quiet spot), pop the lid off the can and vary the tuning voltage. Hold the antenna of the radio close to the front end. If at any position your portable goes quiet the LO does work but the signal is not getting to the PLL chip IC701. But I don't think you will get that.
Repairing or replacing the front end looks like the only option if this last test fails. Fortunately you can use almost any front end that has an LO output.
Repairing or replacing the front end looks like the only option if this last test fails. Fortunately you can use almost any front end that has an LO output.