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Old 23rd Nov 2017, 8:51 pm   #1
benlucysam
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Default NAD 7130 Receiver fault

Hi everyone,
I have been gifted a faulty NAD 7130 receiver, and was looking for some guidance repairing it.
The issue is with the tuner section, the amplifier section works fine on all inputs.
So, the tuner display lights up, you can manually search up and down in both AM and FM modes, you can store a manually entered frequency. but it has no audio output in either AM or FM mode.
I can tune to a known frequency and I don't get the FM Stereo light, and the tuning indicators will not light.
Im assuming the fault must be common to both AM and FM, so chances are not the IF section.
I initially thought it would be the main Synthesizer IC Toshiba TC 9137,
so I have replaced this and the fault is exactly the same, so basically ruling this out, The only other common IC I can see is the Stereo decoder , LA 3390, and it would appear the AM signal does pass through this, I have ordered one of these, but don't really like the "shot gun " approach.
The only other point I have noted it the autoseek does not work, now I would have thought if the MPX decoder IC was faulty it would still seek and find stations ??
I know im stabbing in the dark but it should be an easy fix as it is a constant fault and affects both channels .
Any thoughts or pointes for other tests would be much appreciated.
Oliver.
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Old 23rd Nov 2017, 9:04 pm   #2
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Default Re: NAD 7130 Receiver fault

Have you checked the power supplies in the tuner area?

There probably isn't a lot in common between the A and FM radio sections.

What test equipment do you have?

David
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Old 23rd Nov 2017, 9:26 pm   #3
Biggles
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Default Re: NAD 7130 Receiver fault

If it's any help I once had a similar problem with a Technics tuner. It had me on the point of giving up as it was a very obscure fault. The supplies to the tuner were all low but not low enough to be obviously faulty. The synth wasn't locked. Anyway, after a lot of chasing about, the fault was traced to the chip which muted the audio output on AM and FM. It had gone faulty in such a way as to drag the supply down by a volt or so. Removing this chip brought the whole thing back to life, without the mute function. Replacing the chip with a good un cured the fault completely. But not before I had wasted time replacing the synth chip and fiddled about with the IF amps etc.
Alan.
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Old 23rd Nov 2017, 9:36 pm   #4
PJL
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Default Re: NAD 7130 Receiver fault

Both AM and FM pass through the LA3390 Stereo Decoder and it has a mute input. If the PSU looks OK then I would start by sticking a scope on the inputs/outputs of this device.
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Old 23rd Nov 2017, 10:20 pm   #5
paulsherwin
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Default Re: NAD 7130 Receiver fault

While a chip fault is possible, it's not likely. The first thing to do is check the switching arrangements, which are notoriously flaky in NAD gear as it ages, though this wouldn't normally affect the frequency display or LEDs. Do you have some Servisol switch cleaner or equivalent? (not WD40).

If that doesn't help, check the power rails with a meter.

Dry joints are a strong possibility.
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Old 23rd Nov 2017, 11:12 pm   #6
benlucysam
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Default Re: NAD 7130 Receiver fault

Hi,
I will be totally honest in that my knowledge and test equipment is limited, I use Electrolube ECSP switch cleaner, as I find it good and I can order it from RS through work !!
I have ordered an LA 3390 as it was about £3, and worth a shot, voltages appear OK ,my only thought is if the LA 3390 was at fault then I would get no sound, but would it not still tune?
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Old 23rd Nov 2017, 11:43 pm   #7
Biggles
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Default Re: NAD 7130 Receiver fault

The only reason a faulty audio chip was affecting everything on mine was because it was dragging the supply to the whole receiver down. Normally a fault in the audio stage would leave the rest of the tuner such as signal meter, stereo indicator etc working ok. I just thought I would mention my particular fault as it was a bit out of the ordinary and sometimes faults can produce unexpected effects. It may not have any bearing on your tuner problem.
Alan.
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Old 24th Nov 2017, 12:34 am   #8
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Default Re: NAD 7130 Receiver fault

As I understand it the Tuner section would process the signal after tuning into it. Then it would pass the signal to the stereo decoder. The fact that there is no stereo indicator light, means that the decoder is not finding the 19khz pilot tone that is broadcast by the station. So if the tuner is picking up a signal then the decoder is not working. However if the decoder is not getting any signal in the first place it will not see a 19khz tone to light the stereo light.
So you either have a faulty decoder chip or the tuner section is outputting no signal.
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Old 24th Nov 2017, 12:40 am   #9
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Default Re: NAD 7130 Receiver fault

I suspect that "the tuning indicators will not light" is a clue that there is no signal from the tuner to the decoder.

I don't have the schematics, so I guess that the tuning lights involve either the relative signal strength voltage from the IF chip, or else the AFC output or some combination.

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Old 24th Nov 2017, 5:18 pm   #10
benlucysam
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Default Re: NAD 7130 Receiver fault

Ok it looks like I've had success, it turned out that whilst the synthesizer and MPX ic are permanently powered, the AM and FM IF circuits have the power switched through the AM/FM selector switch, the PCB track to this had been damaged and no power was getting to the switch common.
I have repaired this and all seems good, well almost!!
The next minor issue is that I can manually tune it perfectly , but If I try to use the auto seek it skis straight past the strong stations but will occasionally lock on a very poor one, also when tuned perfectly the FM center tuning indicator left hand arrow is always lit , suggesting that it considers itself not to be tuned, the only time the green center indicators will light is if it has found a station on Autoseek, but this will be very poor, the right hand arrow will never light.
Im assuming this is just some form of internal adjustment?
When I first looked at the unit I applied switch cleaner to all the VRs on the tuner PCB and gave them a couple of turns back and forth but returned each to its original position, I didn't touch any of the VCs.
If anyone knows how to adjust this I would be grateful,
Many thanks to you all for your guidance so far, glad to get this going again.
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Old 25th Nov 2017, 1:44 am   #11
benlucysam
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Default Re: NAD 7130 Receiver fault

Can anyone explain how this circuit works, and is it likely to be adjustable or a component out of spec/failed?
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