Nikko NT 950 Tuner, no stereo, low level mono output.

[svetolik]

Hello to everyone after sometime!
This tuner had above average sound in terms of low distortion and level of detail represented, so I think it is worth to spent some time to repair it and have all day listener.
I have checked reference voltage on the Sumida Discriminator Coil for Sanyo LA1235, between point marked x and TP1. it is 0.045 mV. There is no deference with trimming pot for muting threshold and nothing happens when I touch front end ceramic filters both with level of mono output or getting stereo signal. At one moment I have measured 19Khz pilot signal on the LA 3401 FM MPX demodulator IC.
Discriminator pots are blocked, there is a cracking sound til switching mute and narrow/wide band and sometimes output level gradually goes low.
After switching wide/narrow band output level comes back on on the same level which is also low.
What to measure next?
I will buy some electrolytic capacitors.

[orbanp1]

You should check the sensitivity of the FM section of the receiver.
You would need an FM signal generator for that, preferably with a calibrated output.
An FM receiver should produce "normal" output with an input at the antenna in the uV range, and when measuring from the IF-amplifier input (10.7MHz), it should work with an input in the 10uV range.
These values could vary.
The NT-950 service manual specifies 1uV at the antenna input for the FM-IF alignment.

If the sensitivity is off, chances are that the ceramic IF-filters (CF101, CF102, CF103, CF104) are not working properly.
The IF-circuit in that receiver is such that the ceramic IF-filters do have DC voltage on them, and that is known to damage the ceramic filters eventually.
Luckily such ceramic filters are available and cheap.

Regards, Peter

[svetolik]

Thanks for the answer, CF103-5 13.5VDC ,on the legs of that ceramic capacitors connected to the outputs of three BA401 in serial connection.
I do not have FM signal generator but I have to check IF-amplifier, how to do that?
I will change ceramic caps anyway, have you or anybody any suggestion what to choose?
Is it necessary to have extra cap leg for the ground?
Also the output level is normal during searching between station manually and then goes down when signal is found.
There is no exact point mentioned in the manual how to measure volts on the TP1, where is the second point to measure from?
I have just measured 0.065mV between TP1 and pin labeled x on that three pins next to Sumida discriminator coil.

[John_BS]

I'd suggest as a start you download the data-sheet for the LA1235 and then check all the DC voltages on the various pins - Vcc should be around 12V.

Check that voltage out of pin 13 rises significantly (towards 3V or so) when you go from no FM input connected to a strong local signal applied from a decent aerial .

The service manual is most unhelpful in giving no DC voltages, so I think you've have to work out the various HT's by downloading the datasheet for the regulator IC and substitute the values on the tuner schematic.





John

[svetolik]

Thanks, confirmed, voltage rises to 3.2V on the pin 13, when aerial is connected.
I have found only some modification of LA 1235 and Sumida discriminator coil circuits but without data needed.
I will download la 1235 data sheet, but I have found similar problem with some Onkyo receiver on the net, same chip but different coil. Maybe that manual has voltages labeled.

[svetolik]

Originally Posted by svetolik

There is no exact point mentioned in the manual how to measure volts on the TP1, where is the second point to measure from?
I have just measured 0.065mV between TP1 and pin labeled x on that three pins next to Sumida discriminator coil.

Are the pins 8,9 and 10, actualy three pins from Nikko where I have measured referent 0V between TP1 and point x, in order to confirm aligned disriminator coil?
Those measured points are actually before and after r121 6.8kOhm on the Nikko or resistor 4.8Kohm from the schematic from LA1235 data sheet attached?
It is important to know in order to eliminate misalignment od discriminator coil.

[John_BS]

R211 value changes according to the coil-set used: the Sanyo data gives three options, including another Sumida (SNY-074-1919A).
If Pin 13 rises as you say, the IF and front-end may well be OK.
Pins 8,9 & 10 should be at the same potential and around +6V above 0V. When the tuner is on-tune, the voltage between pin 7 & pin 10 should be close to zero.
I don't know where 'x' is so can't comment.

John

[svetolik]

"Pins 8,9 & 10 should be at the same potential and around +6V above 0V. When the tuner is on-tune, the voltage between pin 7 & pin 10 should be close to zero"
Confirmed. Voltage 7-10 pins opsite, 0.7V without signal, 5.7V with strong signal.
Pin 11 11.5V and there is no change when switching mute off/on or FM/AM.
No voltage on pin 5 under same switching too.

[svetolik]

"I don't know where 'x' is so can't comment."

This first circuit is LA1235 implementation from Luxman RX101 receiver.
Those test points corresponding to Nikkos, I would say.

[John_BS]

I think 7-10 voltage should be close to zero when the receiver is on tune?

J

[svetolik]

Now 7-10 pins V is 1.4V both with strong an weak signal.

We should solve this problem. On brodcast Radio Beograd is story about Mihajlo pupin now.

[svetolik]

Voltage on the pins of LA1235 muting off, mono output signal is present with attenuation:

1-3. 3V,
4-5. 0V,
6. 1.9V,
7. 11.9V,
8-10 6.3V
11. 13.5V
12. 6.25V,
13. 3.1V,
14. 0V
15. 6.8V,
16. 3.45V

There is a change on the base of tr101 and 104 from 0V-1.8V when switching wide/narrow while switching mono mute off/on gives no change.
Also switching W/N causes crackling output and gives signal back after occasionally fading.
Sometimes there is a crackling sound from the board like relay, when touching with probe TP1 point.
I will check switches.

[svetolik]

Sorry, my mistake,checked everything again.
TP voltage has to be measured on to r121, for sure and it is 5.4V away from zero volts.
Also, voltage on the pin 6 should be 6.5V instead of 1.8V.
Does someone knows values of caps insde the
Sumida Discriminator Coil for Sanyo LA1235 FM IF Circuit, have this part or idea how to wind it manualy?
It is so much far from reference, so I will put some caps between pins 8 and 10 in parallel to cap inside discriminator.

[orbanp1]

Originally Posted by svetolik

...
Does someone knows values of caps insde the
Sumida Discriminator Coil for Sanyo LA1235 FM IF Circuit, have this part or idea how to wind it manualy?
It is so much far from reference, so I will put some caps between pins 8 and 10 in parallel to cap inside discriminator.

The circuitry inside the LA1235 is probably very similar to the circuitry inside the CA3089.
The CA3089 datasheet does have data on the discriminator coil, look it up.

Regards, Peter

[John_BS]

I've a suspicion that the problem may be in the tuning - the output from pin 7 is not used to keep the front end on channel.
If this is the case, it's more likely that an error in the DC output on pin 7 is caused by mis-tuning, rather than a fault in the Sumida. Can you disable any afc and manually tune the receiver such that the voltage on pin 7 = voltage on pin 10? If so, at that point the audio output on pin 6 should be healthy?

Do you have a frequency counter and or scope?

[svetolik]

I don't have AFC control, it is right on the pin 7 and stations tuned manualy, auto tune can't detect anything, is right on their frequency.
If I touch with probe pin 9, output signal has normal, higher level.
I will pull board and check more, there is some mismatch in values of some el caps around LA1235 and I am measuring infinitive capacitance on the caps inside Sumida coil with probes on the r120.
Voltage on the pin 7 remain 12V, without,or strong signal.
I have only simple DIMM with frequency counter and questionable memory.

[svetolik]

Originally Posted by John_BS

R211 value changes according to the coil-set used: the Sanyo data gives three options, including another Sumida (SNY-074-1919A).
If Pin 13 rises as you say, the IF and front-end may well be OK.
Pins 8,9 & 10 should be at the same potential and around +6V above 0V. When the tuner is on-tune, the voltage between pin 7 & pin 10 should be close to zero.
I don't know where 'x' is so can't comment.

John

Originally Posted by orbanp1

The circuitry inside the LA1235 is probably very similar to the circuitry inside the CA3089.
The CA3089 datasheet does have data on the discriminator coil, look it up.

Regards, Peter

The value of parallel cap inside IF coil for CA3089 is 100pF.
I am measuring infinite capacitance here, so this might be the cause of excessive voltage over 0V on the TP1. Does someone has opened such coils?
I will open coil and cut connection of that cap, there is no point to examine further before fixing this fault.
In one moment til measuring voltage changes around reed relay that switches output signal, one probe only once was strongly attracted by it. Not sure is it working properly, that crackling during switching wide-narow should not be present as occasionally attenuation of output signal.
It is ferrite dust type core. I can see small shinning particles.
Electrolytic caps seems OK.

[svetolik]

Here is the picture from the bottom of similar IF coil from the net.
I would say that this two same elements are caps in IF coil. There is only one small extra coil in addition to the two ordinary coils.
Schematic diagram is for similar IF coil but wit two smaller coils.

[svetolik]

Changed cap 114, pin 8, no more fading out, but voltage remains out of specs 1.8V and have reading double capacitance of new cap soldered on the board.
I was suspicious about oscillating.
No stereo signal yet.

[svetolik]

I have made a mistake, it is cap 114 on the pin 6.
It is nice mess with caps leakage regarding temperature change and opportunity to explain implementation of LA 1235 and finally have reference voltages here.
I am measuring 0.9V between pins 9 and 10 on the r120 where is cap inside IF coil connected in parallel.
Touching TP2 and points that are connected to the IF coils, causing backing output signal to the full level, so maybe there is oscillation too.
After changing cap 114, sensitivity has go lower.
I will be grateful for ideas how to improve original circuit too.
Measured voltages on the LA1235:
1-3. 3V,
4-5. 0V,
6. 1.9V,
7. 11.9V,
8-10 6.3V
11. 13.5V
12. 6.25V,
13. 3.1V,
14. 0V
15. 6.8V,
16. 3.45V

Part of reference voltages from LA1235 data sheet is on the picture.