Trio KT-900 - trying to fix problem tuning stereo

[audiogymp]

Hi all
I'm fishing for a bit of help trying to fix an issue with this tuner. Hopefully someone out there has the time/inclination. It's a nice sounding tuner and I'd like to get it working, if I can.
The issue is a bit unusual. It will tune into stereo (with some effort) and sounds good. Once tuned, it will stay tuned for days. But in order to get it to tune in, I need to turn the dial quickly and then stop at exactly the point where it will tune to stereo. Generally a number of goes, sometimes many are required. Approaching the station frequency more gradually, as you would normally do rarely works. It's been like this since I got it several years ago but it's gradually getting harder to get it to lock onto stereo.

The service manual is at the following address:

Kenwood KT-900 AM/FM Stereo Tuner Manual | HiFi Engine

www.hifiengine.com

The issue suggested that it might be the contacts on the tuning cap, so I cleaned those and the control switches. This made no difference.
SoCal Sam has suggested cleaning the FM Oscillator (I understand this to be a variable cap very close to the tuning cap) but I'm not sure the symptoms match this fix. Anyway, I'd prefer to put off messing with settings until I've exhausted other options.

I've read that drift in the power supply can cause issues, so I checked this. There are discrepancies but I'd appreciate some input on whether this is the problem and trouble shooting.
The power supply (called VOLTAGE REGULATOR) is in the schematic at G3-4. It has three marked voltages (top down): 0.6V; -18V; and 14V.
I measure these at: 1.2V; -23.35; and 13.26V, respectively. The first two deserve most attention.

The AC output of the transformer is shown in the schematic as 18VAC on either side of the centre tap. I measured these as 16.7 and 16.5VAC.

16.7VAC goes through D26 and is filtered by C93, so with a low current draw (multiply by 1.4) gives close to the measured voltage of -23.35 (-18V on the schematic). A possible explanation is that there is reduced current draw from this voltage source, so it sits at the rectified-filtered voltage.

The measured voltage of 1.2V (0.6V on the schematic) is way out. Interestingly this voltage source converges with the negative voltage source (-18/23.35V) on Q13 (C5 on the schematic), which is the only drain on the negative source, save for a couple of 10M resistors in that vicinity of the schematic. Q13 is a PNP and the 0.6/1.2V source feeds the base, so elevated voltage would tend to reduce E/C current and the draw on the negative voltage source (-18/23.35V). But R115, 820K (also at C5) is in series with the Q13 collector, so query how much difference this might make to voltage at the power supply?

Suggestions much appreciated.

Cheers

PS - I've included a picture but I'm not sure it adds much.

 

[Punker X]

Would have tried to align it before chasing electrical faults. Miss aligned IF and or discriminator will give you the symptoms you are describing.

 

[audiogymp]

Thanks, Mike
I have a CRO and simple signal generator - will need to check frequency limits.
Can I do it with this or does it take all the special kit?

 

[audiogymp]

My CRO goes up to 100MHz but my signal generator only goes to 2MHz.

I made some measurements at Q13 - voltage here is related to tuning status.

The base of Q13

• It is fed a relatively by constant voltage of 1.2V (that should be 0.6V) from the power supply by R112 (R112 only varies on the supply side by about 10mV with tuning status).
• The Q13 side of R112 is never close to the supply voltage.
• Between stations, the voltage on Q13 base is 10.6V-11V, generally one or the other.
• When the tuning is close enough to a station for a strong signal, the voltage drops to 5.322 or 5.2321V
• When the tuner locks into stereo the voltage drops by a further 3-8mV

Q13 base and emitter are both fed from the power supply of 1.2V (that should be 0.6V) via resistors and in the case of the emitter a voltage divider. The collector is fed by the -23.35V source via a 820K resistor (R115). So the positive voltage at Q13 base must come via D13.

The collector of Q13, which drives the gates of Q6 and Q7, responds to tuning status as follows.

• Between stations, the voltage on Q13 collector is -21.5 to -21.6V (ie very little EC current).
• When the tuning is close enough to a station for a strong signal, the voltage goes up to about 5.85V.
• There wasn't any clear difference when the tuner locks into stereo.

It appears that Q6 and Q7 at the positive voltage of Q13c turn on the two channels of output that come from terminals 4 and 7 of IC5 and allow the output to reach the output terminals of the tuner. So it appears that this part of the circuit is working consistent with the way it is meant to function notwithstanding the deviations in supply voltage. This suggests that the supply is not necessarily the problem.

Any thoughts from someone knowledgeable about tuners would be much appreciated. If it is possible to do some kind of rough alignment without all the necessary equipment, that might be enough.

Cheers

 

[audiogymp]

Hi Mike
I'm not getting much uptake on this thread but could I impose on you for a further comment? I've read on about alignment and I note your attitude to amateur tweaks. However, you mentioned misaligned IF as a possible cause. The manual lists an adjustment called "IFT" (item 7 in the list of adjustments). Is this the adjustment of the IF you referred to (adjustment is at T1, which appears to be the oscillator circuit at C1 of the schematic)? Interestingly, L5 on the PCB is labelled "FM IF" but there is no adjustment of it included among the alignment adjustments in the manual.

The "IFT" adjustment is the square can in the tuning cap compartment in the attached photo (to the right). It doesn't look too fiddley, so I should be able to return it to position if I need to. I was thinking I might give it a small adjustment to see if it affects locking onto stereo.

I've accidentally had some modest success with the issue, so I'll explain it in case it's useful to someone with the same tuner.

There are three screws on the outside of the tuning cap compartment (photo to the left). I mistook these for grounding contacts for fins of the tuning cap and set about tightening two of them. This affected tuning. I surmise that the head of the screw forms one side of a capacitor and the capacitance varies as the plastic washers are compressed.

I found that turning the screws increased/decreased the signal strength. It was easy enough to dial in the strongest signal but the tuner would not lock into stereo at all with this adjustment. However, when I adjusted away from the strongest signal strength I found an adjustment where stereo function worked better. It's still not working like it should, so I'll consider tweaking the IFT adjustment. But if you're doing what's described in the following thread, these screws appear to be the adjustable caps for the FM gangs (not the oscillator).

Tuner Alignment for Dummies

Lately, I have been experimenting with doing my own tuner alignments with pretty good results. This is in no means better than getting it professionally done, but will return excellent results for older tuners out of alignment and let you enjoy it that much more. I've looked all over the web for...

audiokarma.org

Cheers

 

[Punker X]

Sorry I really don't have time respond as you would like. Correct those are trim caps. Those should be adjusted around 106Mhz. The LO adjustment would be in the upper part there, usually a trim cap that is better quality. The alignment procedure is in the manual is junk and doesn't cover these. I would peak these for 106 and the IF also. Do step 1 and 2 for the tuning meter. Then check the VCO at 76khz. This should get you stereo. Something flaking in the muting circuit could cause problems, but you need to make use it is getting the correct input from the tuner.

 

[audiogymp]

Epilogue

This is where I ended up.

I don't have an RF signal source, so I wasn't able to do the tuning meter or VCO adjustments.

The IFT adjustment in the manual involves adjusting T1, which appears to adjust a variable inductor. This made little difference to signal strength or locking into stereo. I returned it to where it was. I also tried the trimmer on the PCB marked 'FM IF' (also an inductor). This also made little difference to signal strength or locking into stereo and I returned it to where it was.

As Punker X said, there was another trim cap in the tuning cap assembly, which was in the compartment furthest from the wheel. This is the tuning cap for the oscillator (I'm not sure what 'LO' refers to but I assume its another way to refer to it). For the most part, this trimmer adjusted the reception frequency for a given place on the dial. I adjusted it so that reception of a station at 106.7MHz was at the correct frequency on the dial. I also adjusted the trimmers for the three FM gangs for max signal strength at this station (ie towards the top of the dial).

All in all, I basically followed the instructions in the thread, Tuner Alignment for Dummies (including Punker X's comments) but tried a couple of other things and reset them to where they were.

The tuner is now useable again. It still takes a deft roll of the wrist or a few to get stereo on the stations I listen to but this is a major improvement on where it was.

I expect that there is still some maladjustment that affects locking into stereo. However, I have given it enough of a nudge to make it useable again.

I marked the position of the trimmers before adjusting them and found that I could restore settings without problems.

Cheers

 

[amptramp]

As an explanation, "LO" means local oscillator.