TU-9900 stereo indicator

[KeithD]

Just picked up a TU-9900. Stereo indicator light does not come on when FM stations are tuned in. The light came on when I flipped the tone switch, so I know the light works and is not burned out. I can't say for sure if it's really tuning in stereo and the light is just not coming on, or if I am not getting stereo reception. What should I be looking at/testing for this problem?

 

[kevzep]

Just picked up a TU-9900. Stereo indicator light does not come on when FM stations are tuned in. The light came on when I flipped the tone switch, so I know the light works and is not burned out. I can't say for sure if it's really tuning in stereo and the light is just not coming on, or if I am not getting stereo reception. What should I be looking at/testing for this problem?

Headphones is what you need, and then switch between FM mono and FM Stereo. You will certainly be able to hear the difference if the stereo is working.
There is a possibility the VCO is off slightly, that can affect it, as can the discriminator, and a miss-aligned IF section...
But assess if its really getting Stereo first.

 

[KeithD]

OK, yes, will check that. When I switched from stereo to L+R on the amp I did notice a difference, but will give a listen through headphones and report back.

 

[kevzep]

OK, yes, will check that. When I switched from stereo to L+R on the amp I did notice a difference, but will give a listen through headphones and report back.

You need to do it on the tuner.
The function switch, you need to switch it to FM AUTO, then compare it to FM MONO....this way you will be operating the MPX circuit in the tuner rather than summing whatever it is going through the amplifier.

And yeah much easier in Headphones.

 

[KeithD]

Yes, understood, but there is no headphone jack on the tuner, so I will need to take the RCA L/R out jacks from the tuner into headphones. I will need to find that kind of adapter in my stash.

 

[KeithD]

OK, listening to tuner output in headphones, I hear no difference switching between FM Auto and FM Mono. So appears I don't have stereo output.

 

[kevzep]

OK, listening to tuner output in headphones, I hear no difference switching between FM Auto and FM Mono. So appears I don't have stereo output.

Yeah sorry, I meant you can listen in the amplifier, but just switch between FM AUTO and FM MONO, I might not have made that clear, but looks like you figured out what I was on about.

Okay so no difference between FM AUTO and FM MONO, now, how strong is the signal on the S meter? You do need quite a strong signal, probably over 3.5, ideally over 4 to get good FM STEREO.
You do have an antenna connected right?

 

[KeithD]

Yeah sorry, I meant you can listen in the amplifier, but just switch between FM AUTO and FM MONO, I might not have made that clear, but looks like you figured out what I was on about.

Okay so no difference between FM AUTO and FM MONO, now, how strong is the signal on the S meter? You do need quite a strong signal, probably over 3.5, ideally over 4 to get good FM STEREO.
You do have an antenna connected right?

Yes, antenna connected and getting strong signals on many stations - around 4.5 on the signal meter. Very impressed with the reception I am getting, definitely my best tuner, just no stereo.

 

[kevzep]

Yes, antenna connected and getting strong signals on many stations - around 4.5 on the signal meter. Very impressed with the reception I am getting, definitely my best tuner, just no stereo.

What I do with a tuner which has this problem, is I actually give it a good alignment as there are many parts of an alignment that could be out which cause loss of Stereo signal.
9 times out of 10, it brings the stereo back.

At a pinch and a shot in the dark, you could get your tweaker, and carefully adjust the VCO one way then the other, noting where you started, it might just be the 76khz Oscillator is off slightly.

If you get the service manual you'll see a diagram in there which has the locations of the adjustments. You're looking for the VCO.
Tune into a known FM STEREO station, and give it a tweak as outlined above.

I think from memory, the 9900 has a selectable IF bandwidth, Narrow and Wide? If it does, make sure its on wide to start with.

 

[KeithD]

What I do with a tuner which has this problem, is I actually give it a good alignment as there are many parts of an alignment that could be out which cause loss of Stereo signal.
9 times out of 10, it brings the stereo back.

At a pinch and a shot in the dark, you could get your tweaker, and carefully adjust the VCO one way then the other, noting where you started, it might just be the 76khz Oscillator is off slightly.

If you get the service manual you'll see a diagram in there which has the locations of the adjustments. You're looking for the VCO.
Tune into a known FM STEREO station, and give it a tweak as outlined above.

I think from memory, the 9900 has a selectable IF bandwidth, Narrow and Wide? If it does, make sure its on wide to start with.

Thanks Kev, I don't have the plastic/ceramic adjusting tools. I should be able to borrow some from the local tech on Monday and give that a try. As of yet, I don't have the test instruments to do any part of the alignment procedure. I did just pick up an Eico 232 VTVM from the tech, but that's not going to help much in this situation. I'll try adjusting the VCO as you stated. If that doesn't do it, this will be going back to the seller. Paid too much to have it not working...

 

[KeithD]

Took a look at the SM, and actually, this issue is mentioned with remedies. One of which is adjusting VR603 (76 kHz) you mentioned on the MPX board. I see those trimmers don't need special tools. Will give that a try. As you can see other causes are bad transistors TR608 and 609 on MPX board. I've taken a shot of the SM listing for those transistors. I could pull and test these, but what would be the replacements if they are bad?

 

[PE9ZZ]

VR603 is your starting point. Tune to a strong signal and slowly rotate it with a small screwdriver. Note the original setting. At some point the STEREO light should come on. If not something else is wrong.

Tjerk, 9ZZ

 

[KeithD]

VR603 was in about center position. Very slowly moved it all the way in one direction, then the other. Stereo light never came on.

Assume this means there is a faulty transistor or IC according to SM?

 

[kevzep]

VR603 was in about center position. Very slowly moved it all the way in one direction, then the other. Stereo light never came on.

Assume this means there is a faulty transistor or IC according to SM?

Not necessarily, I would be surprised if there is a bad transistor or the PLL chip is bad, I have seen that only on very rare occasions.
I am afraid whats most likely happened here is someone has fiddled with it and thrown the discriminator and/or the IF alignment out, this is more likely the problem rather than blown transistors.

The transistors around that circuit are audio transistors though, nothing special, so easy to get replacements.

 

[KeithD]

Not necessarily, I would be surprised if there is a bad transistor or the PLL chip is bad, I have seen that only on very rare occasions.
I am afraid whats most likely happened here is someone has fiddled with it and thrown the discriminator and/or the IF alignment out, this is more likely the problem rather than blown transistors.

The transistors around that circuit are audio transistors though, nothing special, so easy to get replacements.

Out of curiosity, Kev, what would you charge if this came to your shop with this problem and owner wanted it fixed?

 

[kevzep]

Out of curiosity, Kev, what would you charge if this came to your shop with this problem and owner wanted it fixed?

Well its hard to say, sometimes these faults can be tracked fairly easily, other times not.
There is a couple of hours to do a full alignment, then if there is an additional amount of time to track the NO stereo fault, that could run from 30mins to 2hrs...
I like to keep these things fluid depending on the customers budget....
I would never say its going to cost "X", otherwise I'll end up chasing something for a few hours which I don't get paid for...
Hope that helps...

I am pretty sure with your unit, the discriminator or IF is out.....

 

[KeithD]

Thanks for that.

Here is another observation, when I push in the "Calibration Level" button, I get the test signal and radio sound is muted. However, the stereo indicator light works with this button pushed in, as do the signal meters. If I tune to a strong station the stereo light comes on, and in between stations the light goes out. Any idea what this is indicating???

 

[rcs16]

However, the stereo indicator light works with this button pushed in, as do the signal meters. If I tune to a strong station the stereo light comes on, and in between stations the light goes out.

is that not normal operation?

 

[KeithD]

is that not normal operation?

I don't know. Just seems odd that I get no stereo reception/light when tuning in stations, but when I depress the Calibration button, I get the stereo light when I tune in stations. I thought it might give some insight to what the problem is for Kev or other experts who might be looking at this thread...

I might check TR608 and 609, since they are first on the troubleshooting list. Can't hurt. Also wondering if a cap could be the problem. A lot of small ones on the MPX board, and they are most likely to drift. Might be a problem if one is in a critical circuit for stereo processing?

 

[kevzep]

is that not normal operation?

Hi Rick, yes that would be normal operation.
Keith is wondering why he only gets FM STEREO when the calibration function is in operation.

I haven't had a chance to study the circuit yet, I will get to it though....

I am still standing by my original assessment that it needs aligning, Discriminator and or IF issues I am thinking.

You really need a Multiplex gen and RF gen to properly assess what is going on...