I am trying to fix this FM tuner - Page 1
Forum section: electronics / repair

author: Hextejas
author_rank: Frequent Contributor
timestamp: September 05, 2018, 06:24:21 pm
content: This is a new thread to try and fix the FM tuner. The AM portion as well as the voltages look good.It is a NAD 402 and I have had it in storage for about 25 years. It used to work just fine.I dont know where to start.If I look at the block diagrams I am able to see voltages and an oscilloscope signal coming from the LA 1266 and LA 3401, FM out and L and R output.The thing is that I don't know what a good FM signal is supposed to look like, plus I am a noobie with the scope and I just spin dials until I see something.Anyhow, here they are.

author: Specmaster
author_rank: Super Contributor
timestamp: September 05, 2018, 06:30:12 pm
content: I think that you are looking at the wrong things here. I assume that it worked before you put it into storage so therefore the things I'd be looking at are the capacitors, there are now at the age where they start to go leaky or even short circuit. Yopu say you checked the voltages and they all seem to be OK so it capacitors all day long as the first thing to check with an LCR meter.

author: coromonadalix
author_rank: Super Contributor
timestamp: September 05, 2018, 08:16:12 pm
content: why did you double post ??

author: SilverSolder
author_rank: Super Contributor
timestamp: September 05, 2018, 08:18:10 pm
content: What's actually wrong with it? No sound? Distorted sound?

author: Hextejas
author_rank: Frequent Contributor
timestamp: September 05, 2018, 09:48:51 pm
content: Quote from: SilverSolder on September 05, 2018, 08:18:10 pmNo sound for FM. Dial lights, I can select an FM station but no sound.

author: Hextejas
author_rank: Frequent Contributor
timestamp: September 05, 2018, 10:00:26 pm
content: Quote from: Specmaster on September 05, 2018, 06:30:12 pmSpecmaster, would you check just the electrolytics, or all of them ? And do I need to unsolder them ? I was trying to put some logic to use to narrow it down. But I don't have enough knowledge to do it. It looks to me that both AM and FM use the same path. Into the 1266, then into and out of the 3401.Since the AM is ok, does that mean that these 2 chips are ok, or is it possible that a portion of the chip can be bad ?

author: Specmaster
author_rank: Super Contributor
timestamp: September 05, 2018, 11:41:28 pm
content: Quote from: Hextejas on September 05, 2018, 10:00:26 pmHighly unlikely that part of a chip like this could be faulty I would have thoughtI would only check the electrolytics and any tantalums, generally speaking other caps are highly unlikely to cause problems. Yes you would need to lift one leg of each in turn to check them and I would also identify which ones are in the FM section and only check those seeing as you have already checked the voltages and seem OK, likewise the AM section is working so no point in disturbing those is there?

author: SilverSolder
author_rank: Super Contributor
timestamp: September 09, 2018, 04:27:31 am
content: Quote from: Hextejas on September 05, 2018, 09:48:51 pmDoes the "Stereo" lamp light up when you select a station?If so, the RF tuner part might be working.

author: cncjerry
author_rank: Unknown
timestamp: September 09, 2018, 05:28:07 pm
content: when you turn the dial do you see the tuning meter change with a station you are tuning? Have you checked to make sure you don't have FM muting turned on?

author: Hextejas
author_rank: Frequent Contributor
timestamp: September 09, 2018, 06:38:56 pm
content: Yes the tuning indicator is turned on and I can see it change as I change the stations. And I forgot completely about FM muting.In fact I did not know there was such a critter.

author: Specmaster
author_rank: Super Contributor
timestamp: September 09, 2018, 08:06:38 pm
content: Yes FM muting is real but it should not prevent you from receiving any stations, it's supposed to to cut the hiss (static) between stations as you manually tune between them.

author: cncjerry
author_rank: Unknown
timestamp: September 11, 2018, 01:35:04 am
content: I have one receiver that will mute at a very high level. The other thing to check is to ensure you don't have the tape monitor punched in. I haven't operated a manual receiver is so long I need instructions...Jerry

author: mzacharias
author_rank: Frequent Contributor
timestamp: September 11, 2018, 11:19:11 am
content: Antenna connected?(had to ask - nobody had yet)

author: commongrounder
author_rank: Frequent Contributor
timestamp: September 11, 2018, 05:56:14 pm
content: Hi HextejasI just happen to come across this posting.The signal on the LA1266 pin 12 is the FM demodulated output. It is the raw signal that contains the audio, the 19khz pilot, 38khz stereo difference signal, and possibly other digital or "muzac" type signals. The input to the stereo demodulator chip, LA3401 pin 2, only wants to see the audio, 19khz pilot, and 38khz difference signal, as these are needed to decode the left and right channels. The other signals need to be filtered out, which is what the network that includes FL5 does (low pass filter). To test and see if there is a signal present at LA1266 pin 12, you need to be familiar with what an audio signal looks like on your scope. A slower sweep speed, with higher gain, will help. If you tune in a strong local FM station, and probe pin 12 you should see audio present (plus some lower level fixed frequency tones). If you see it there, move to LA3401 pin 2 and probe for a similar, but probably lower level signal, since the filter will have some loss. If it is present there, then move on to the output pins, 5 and 8. If you see a signal there, then it must be in the discrete circuitry following. There are a couple of muting transistors (Q402, Q409) that could be pulling down the signal if they are getting switched on inappropriately. See what you find and report back.

author: slbender
author_rank: Regular Contributor
timestamp: September 14, 2018, 02:17:51 pm
content: Not to steal any thunder, I ended up being "gifted" two high-end super tuners a few years back. Seems this fellow had a daughter who had a habit of plugging in any unplugged AC cords. Anyone want to guess what he used for his FM Antennas ? Yes he did. So no, not MY daughter, but I was gifted a silent Yamaha T-80 and a rare Sumo Aurora. Don't they look pretty?Steven

author: Specmaster
author_rank: Super Contributor
timestamp: September 14, 2018, 02:45:25 pm
content: Quote from: slbender on September 14, 2018, 02:17:51 pmPictures?

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: September 15, 2018, 04:37:08 pm
content: You didn't say whether you could see audio output from the oscilloscope in both AM and FM mode?Since you have output on AM, the post amp section must be OK. So the problem is at the LA3401 stage or earlier. Assuming you have an antenna connected, is something causing Mute to stay jammed on? If you turn mute off, do you get a hiss? It is also not unknown for a dry or loose joint to develop on the antenna connector.

author: slbender
author_rank: Regular Contributor
timestamp: September 19, 2018, 03:47:13 am
content: Mostly all black style... The Sumo I think is still in the attic can't get there right now.