Realistic STA-47 Tuner

[bwinterstine]

I have (as a hobby) a pair of STA-47 Receivers from back in the late 1970s. One is my personal receiver that I bought as a teen; the other; a test subject.

On the spare set, I am trying to get the tuner to work correctly. FM Mono is strong but it will not bring in stereo. If I mess (a little) with the primary tuning coil for the detector circuit, I can get the Stereo indicator to light and I get stereo sound. BUT, the audio is VERY DISTORTED.

I replaced the multiplex Sanyo LA3301 chip thinking it was the culprit. I also know that the chip is a good chip as I removed it from yet another in which it was working correctly. No difference after changing out the chip.

Any clues as to what would cause very good MONO but very distorted stereo?
bwinterstine, Today at 10:36 AM#1

[Cruisin Marine]

I would suspect decoupling caps being bad in or around IF or demod or MPX chips (or some other instability), as that would affect the higher MPX switching frequencies and maybe not the normal Mono receiving. It might be quicker to pad some appropriate 1nF, 10nF and 100 nF caps across the appropriate supply lines. A scope on the supply lines may be a good indicator, but probably faster to just add extra decoupling caps to tame it down if it is being naughty.
Worth a try anyway.

[frsimen]

It may be that the alignment of the two tuned circuits associated with the LA3301 has been adjusted by someone in the past.

IF that is a possibility, you could try making a small adjustment to T110 to see if the stereo light can be encouraged to light more readily. For the distortion, a small adjustment to T111 may help. This type of decoder isn't the easiest to adjust without test gear, so only make small adjustments! Mark the start position so that you can return the cores to their original settings if there is no improvement.

[Cruisin Marine]

Frsimen is right, check alignment first of course- I made my my usual mistake of assuming things like that have already been done!

[bwinterstine]

Thanks for your suggestions. Here is what I know/did:

1. If I turn the Primary Coil for the detection stage; T104, the 19K pilot is forwarded to the LA3301 chip and it then shows 19K and 38K at pins 1 and 10. But, the audio is very bad/distorted.

If I then feed, from TP 21 (just after the detector/input to LA3301) from the bad receiver to the good receiver, the bad/distorted audio is clean but I do not get the 19k pilot.

2. If I turn the Primary Coil the opposite way, the 19K pilot disappears and the mono audio is good and clean on the receiver.

I have checked all the voltages and resistances comparing the good and bad receiver. All looks good. I replaced the two capacitors for the 19 and 38K signals at the LA3301.


There are a number of ceramic and mylar capacitors on the board ... do they really fail?

I have examined the good and bad circuits from the beginning of the IF Chain through the detector and compared them; on an analogue scope. Visually, they appear similar in shape, frequency and amplitude; but I know this is not a great test. I do not have a sweep generator so I may need to invest in one. BUT, this is a hobby for me and I do not want to spend a lot of money. The coils have definitely been mucked with.

[frsimen]

T104 is the ratio detector primary tuned circuit. It should be adjusted for the best mono reception, monitoring at TP21. Its setting along with the secondary tuning will have a big impact on the distortion on both stereo and mono signals.

Once set for good reception it can be left alone. If the other IF transformers have been adjusted, that will affect both mono and stereo reception, but don’t adjust them unless you are sure that they have been tampered with.

If you have a signal generator, switch off any modulation and inject a signal at 10.7MHz (or the frequency of the ceramic filters if it’s not 10.7MHz) to TP3

Connect a DC voltmeter meter across C143

Adjust T104 primary, T103, T102 and T101 in that order for the maximum voltage on the meter. Reduce the level of the signal generator output as you go along, to avoid the circuit limiting and masking the adjustments. Tou can use a local broadcast station instead if you don't have a signal generator but it's not the ideal method.

Adjust the secondary of T104 for zero volts measured at TP21 to the chassis. If using a broadcast station, adjust for the lowest distortion instead.

To fix the stereo problem, you need to focus your attention around the LA3301, which is a filter type stereo decoder. If the alignment of that isn’t correct you will get good mono results but distortion on stereo.

The signal fed from the working set via TP21 will have a lower level of 19kHz due to the combined effect of C144 in both tuners.

In the absence of a stereo generator, you will need to use a broadcast station with a good stereo signal to align it as best you can.

Connect your oscilloscope to TP17. Adjust T110 for the maximum 19kHz signal.

Adjust T111 for the maximum output on the left and right channels with minimum distortion.

Adjust R171 for best separation, although that may not be easy to judge.

The above decoder alignment instructions have been adapted from those for the JVC 4VR-5446X tuner, which uses the same decoder chip, as I can’t find the instructions for this Realistic model.

You will notice that T111 affects the distortion, so your problem could easily be incorrect alignment of the decoder.

Mylar and ceramic capacitors are generally reliable, that’s not saying that they never fail. If adjusting the alignment doesn’t sort out the fault, it would be worthwhile checking the electrolytic capacitors which decouple the supply to the LA3301 (C202, C213 and C215).

[bwinterstine]

Thank you so much for the detailed response.

I want to try all of your suggestions; and I will in time. But, I am certain the tuning of this receiver was changed. I have it receiving channels by "ear" but I do not really know how accurately I have it tuned.

I have tried unsuccessfully multiple times to pass the 19k to the LA3301 when tuning for undistorted mono. If the audio is clear, the LA3301 will not detect stereo.

I do not have quality signal generator or sweep generator; that may be my next step.

I did make a 88Mhz FM transmitter and sent a cheap generator sine wave through it. Used it to tune the radio by ear comparing it to the k ow good receiver. With that, I was able to receive all the stations. The cheap 9v signal generator was a eBay kit. Not anything useful for tuner alignment.

[bwinterstine]

I was able to use a simple FM transmitter and 19k sine wave to get the receiver to tune into a quiet station and turn on the stereo indicator. As soon as I add the 19k pilot to the transmitter the stereo indicator comes on and the noise/distortion starts. Without the pilot the tuner is dead quiet when tuned to the transmitter.

[frsimen]

At this stage you need to carry out the alignment on the decoder, as described above in post 6. That should sort the problem out if it is just an alignment issue.

[bwinterstine]

I wanted to close this out. I purchased a vintage Lafayette signal generator and a vintage Knight sweep generator. After getting both of these working correctly I was able to align the FM IF and was able to adjust the Dectector S curve (it was way off). The tuner works great now. But, much of the distortion was being added at the first stage of the preamp which turned out to be bad electrolytics. Thanks for all of the suggestions.