TX-9100 strobing/unstable stereo

[QSilver]

I finally found one of these after much searching. There's a little rust on the back grill but besides that, the case is in great condition.

FM and AM are great but the Stereo is not working so well. Basically what happens is that when you tune into a stereo broadcast, the stereo lamp starts to flicker slower and slower with an audible beating on the audio too. Over a few seconds it slows more and more until it settles on Stereo for about half a second before it loses it again and the lamp goes out. Then a few seconds later, it all starts again.

I'm not all that familiar with MPX circuits so I'd be grateful if someone could help point me in the right direction of what to measure or test.

 

[chazix]

Interesting presentation. It sounds to me like it might be a problem with the PLL "loop filter".

For starters, though, I think I'd suggest a check of the power supply to the MPX board (AWD-002-0, according to the elektrotanya schematic) - say, by measuring the voltage across C1 to see if it's somewhere in the 12-13V vicinity, and stable.

If the power supply looks good, you might just go ahead and replace C5 and C6, which are the likeliest suspects in the PLL loop filter. Afterwards, you'd want to adjust VR1 to the middle of the range where stereo locks in.

At the risk of being a mother hen, I'd be remiss if I didn't point out that a tuner the age of a TX-9100 would surely benefit from a proper full-on alignment. Cluck, cluck.

HTH,

chazix

 

[gslikker]

I was told by the Pioneer gurus "replace all blue Sanyo's" and although before the total recap it worked fine, ALL those little "solid aluminium" capacitors showed more leakage than normal electrolytic capacitors are supposed to leak.....
So now, also I am shouting "replace all blue small Sanyo caps"
Note, I do not have real tuner adjustment experience and I recapped it only.

 

[QSilver]

Thanks for the pointers, I forgot to mention that because it had quite poor reception, I went through and re-capped it already and replaced the 2SA725's and 2SC1312's with KSA992 and KSC1845. The blue sanyos got changed out for films.

I've just checked the capacitors around the MPX chip - 3.3uF is a new electrolytic (Panasonic). The other two, 470nF and 1uF are non-polar film capacitors.

Voltage to the MPX board on Pin 19 is a stable 13.27V

When I powered it up now, the stereo light flickers once some stations are tuned in the goes out and stays out. I adjusted VR1 but that only made it worse, and now I don't get any stereo indication. I have readjusted it so that Pin 2 shows me 19kHz on my DMM, however I'm not sure how accurate that is as I think the oscillator is damped or slowed when I connect the DMM.....

I have the equipment to align this and I have gone through the alignment procedure which has brought back quite a lot of the reception and provided I have an aerial attached most stations come in between 2.5 to 5 on the signal meter.

I've tried feeding in an MPX signal generated on my PC and fed to my FM modulator to give it a clean MPX signal and that didn't give me a stable stereo signal either.

 

[rcs16]

The 19KHz TP probably should be looked at with a high impedance, low capacitance probe, connected to a frequency counter or a scope input with a trigger or (X,Y mode) from an very accurate 19KHz signal. try this first.
I guess we can assume that a 19KHz pilot from your PC is accurate enough, so use that for your "Y" input to the scope. only 19KHz, nothing else.
Check the delay line, F1, measure the composite signal with scope on both sides to see how much it attenuates the signal, level difference.
The stereo decoder is an old IC, they say PA1310P, but I am assuming it is the old MC1310 part. It could be the IC or any of the surrounding passive parts as chazix suggests.
see where this gets you.
Good luck

 

[QSilver]

Ok, I'll try to test F1. What input signal to the tuner is needed? Anything specific or just detuned?

 

[rcs16]

Any signal, even noise, but a sine wave would be more useful, since you are comparing the level of the input to the output in this test to see how much it is attenuating the signal. It is just a hunch, nothing more.

 

[QSilver]

So, I tuned into a radio station and hooked each side of F1 up to my scope. Set both inputs to identical input sensitivities and overlapped the traces. They are identical, so I guess that means that there is no attenuation through the delay line?

I also probed Pin 2, the 19kHz test point and fed my scope with a 19kHz signal from an oscillator and set by a frequency counter on my DMM. I cannot get the oscillator to stop on 19kHz. It just drifts higher or lower. Also, the oscillator on Pin 2 is a square wave - just making sure that its supposed to be?

When tuned into a station, the beating from the stereo circuit, matches the amount that the local 19khz oscillator is running off by. Every time I tap the VR, it changes, so I might try replacing that to try and get some more stability.

Does any of this shed some light onto the problem?

I've been using the scope as opposed to a DMM for readings as I think the scope is less interactive with the circuits so less likely to skew sensitive frequency measurements.

 

[chazix]

Does any of this shed some light onto the problem?

Not for me (though that ain't saying much...) It sounds like the chip is just not able to lock on the incoming pilot at all, so if you're confident of the C5 and C6 replacements, I think it might be time to just go ahead and replace the chip. I believe a standard MC1310 will do the trick. Consolidated (http://www.ceitron.com/cart/index.php) is a trustworthy source, though shipping costs are irritatingly high.

Not trying to talk you out of replacing the VR, though. Couldn't hurt.