Vintage tuner drift problem

[oldmarantz]

Hi all
I ve got a question about our vintages tuners and receivers.A lot have (or had) drift frequency problem.I meet this pb frequently.Have you found an easy solution to resolve the problem?

Dom.

 

[Lefty]

Hi Dom;

Well let me see if I can add value to you and the group on this subject.

First it is important that we get an operational definition of what you mean by drift. There are two different symptoms that sometimes get confused with each other.

First there is what I would call dial calibration error. This is where when you tune in a station by centering the tuning meter but you find that the dial pointer is not aligned to the actual frequency of the station. Frequently this can be one or several megaherz off. If the tuning meter stays centered over time then you don't have a drifiting problem but rather an dial offset problem. This is usually the results of dial cord streaching over time and can be corrected fairly easily if one wants but is not really nessesary. Just wanted to get this problem seperated from true drifting problems.

Second there is a true drift problem, where after you carefully tune in a station by centering the tuning meter, you find that over time the meter slowly moves off center. This is a result of drifting of the local ocsillator in the FM front end section. If there is drifting happening one should find that it is worst from initial turn on of the tuner or receiver then if you retune the station after 10-30mins later when the elecronics has had a chance to reach thermal equalization.

There has been three different methods that designers have used to prevent FM frequency drifting over the decades of tuner design.

1. The latest method applies to digital tuners where the local ocsillator is phase locked to a crystal reference oscillator. This method is robust and therefore drifting in digital tuners is VERY rare and if present represents serious problems in the front end circuits.

2. In less expensive and much older FM equipment an analog AFC (automatic frequency control) circuit was often used to continously readjust the front end local ocsillator by using feedback from the FM detector circuit. This circuit frequently used a front panel switch so you could unlock the AFC for easier manual tuning and then switch it back on once you have the station centered. I don't know why AFC circuitry is not common on the higher end vintage tuners and receivers of the 70s unless there is some tradeoff in detected audio quality when AFC is active. This would be a good questions to pose to the FM tuner group on yahoo if one wants to ask others opinion of why AFC fell out of favor in the 70s and up to the digital tuner era.


3. Most of the better quality vintage analog tuners I have run across relied on thermal compensation in the FM front end section to deal with drift. This is where hand selected capacitors are installed into the local oscillator circuit at final testing stage of manufacture. The service manual will sometimes show which of these certain capacitors may have slightly different values both in capacitance and or in temperature response. These capacitors can be selected to either increase of decrease in value with raising temperature. The bad news is if one is seeing drift in a tuner of this kind it is very difficult to correct for the average hobbyist. It requires a very accurate frequency counter to measure the drift and then cut and try different compensation capacitors to correct it. The good news is that I have personally never come across a noticeable drifting tuner or receiver in my 5 years of collecting this stuff. I have been amazed at how well that these better analog tuners and receivers perform in this area. I will tune in a station at first turn on and look again an hour later and detect no movement in the tuning meter. This is no easy task to design and build in and is a real indication of the talent and skill that was used in building this equipment.

So I hope this helped in understanding what causes drift and what methods were used to help prevent it. Sorry that I can't recommend specific repair recommendations. It is my opinion that FM repair and aligment is just out of reach of most of us. It just requires so much special test equipment and documentation to do it properly and problems can be made much worst by trying to just turn all those little adjustable caps, inductors and pots without all the right stuff. Also the fact that digital circuits have been around for over twenty years now the chance of finding a repair tech that really has knowlege and experiance in analog tuners can be hard.

Comments from others?


Lefty

 

[oldmarantz]

Lefty thanks for your comments I spoke about true drift problems.I had got this problem with 2 receivers (analogic) and didn t found something to stop this drift one had got a small drift not very disturbing but the other was ok at the switch on but sometimes 30 mn or 1 hour later it drifted very strongly... I lived with...

Dom.

 

[kkib]

Hi Lefty,

My Sherwood 3000V has a serious dial offset problem, but
I don't think it's due to dial cord streching. A station that
should appear at 92.5 appears at 103. I managed to bring
it back to 92.5 by adjusting one of the screw type caps but
then it developed a drifting problem (the station would drift
to another frequency in half an hour or so).

I thought about sending it for a checkup/alignment but,
$150-200 seemed too much for a unit that is worth much
less. I have the schematics for it and I checked various
voltages etc. and everything seems ok.

Any suggestions for possible causes for this. Thanks.

 

[Lefty]

Hi KKIB;

Yes, 10.5 mhz offset is a hugh offset and bigger then I've come across in my finds. Dial cord streaching does happen over time but not to that extent as the dial cord tension spring would never be able to keep the cord tight enough to function at all. I suspect that possibly that you had some kind of slippage on the main dial cord pully that attaches to the main tuning capacitor. This sometimes happens when someone continues to turn the front panel tuning knob even after the capacitor has reached it's mechanical limit.

The method I use to correct dial offset is to tune in a station of known frequency and then carefully loosen the set screw(s) on the main tuning pully so that I can turn the front panel tuning knob without causing the tuning capacitor to move. After I have the dial pointer at the correct frequency I then carefully retighen the set screw(s) on the main pully. This sometimes has to be repeated a few times as it is sometimes hard to loosen or tighen the main tuning pully without causing the capacitor from moving a little. This method does not cause any changes to the electrical adjustment in the front end circuits.

The problem with the method you chose is that while you indeed corrected the local oscillator frequency so that the dial pointer is now correct, you have upset the tracking of that tuner gang in relationship to the other tuning capacitor gangs. This means that the front end RF amplifer circuit(s) is not tracking to the same frequency as the local oscillator, therefore possibley causing a reduction of RF sensitivity and a reduction of image rejection.

As I stated in my first posting, proper electrical adjustments of a tuner front end requires special test equipment including sweep frequency generator and frequency counter, scope, etc. Many FM tuner and receiver service manuals I have seen don't even cover front end aligment and sometimes state that factory service is required.

Lefty

 

[backmd]

Hi KKIB;

Yes, 10.5 mhz offset is a hugh offset and bigger then I've come across in my finds. Dial cord streaching does happen over time but not to that extent as the dial cord tension spring would never be able to keep the cord tight enough to function at all. I suspect that possibly that you had some kind of slippage on the main dial cord pully that attaches to the main tuning capacitor. This sometimes happens when someone continues to turn the front panel tuning knob even after the capacitor has reached it's mechanical limit.

The method I use to correct dial offset is to tune in a station of known frequency and then carefully loosen the set screw(s) on the main tuning pully so that I can turn the front panel tuning knob without causing the tuning capacitor to move. After I have the dial pointer at the correct frequency I then carefully retighen the set screw(s) on the main pully. This sometimes has to be repeated a few times as it is sometimes hard to loosen or tighen the main tuning pully without causing the capacitor from moving a little. This method does not cause any changes to the electrical adjustment in the front end circuits.

The problem with the method you chose is that while you indeed corrected the local oscillator frequency so that the dial pointer is now correct, you have upset the tracking of that tuner gang in relationship to the other tuning capacitor gangs. This means that the front end RF amplifer circuit(s) is not tracking to the same frequency as the local oscillator, therefore possibley causing a reduction of RF sensitivity and a reduction of image rejection.

As I stated in my first posting, proper electrical adjustments of a tuner front end requires special test equipment including sweep frequency generator and frequency counter, scope, etc. Many FM tuner and receiver service manuals I have seen don't even cover front end aligment and sometimes state that factory service is required.

Lefty

Well the saga continues, setup the receiver with speakers and the signal continued to drift. Bought some component cooler and sprayed it on the transistors and IC Chips hoping to recreate the significant drifting.
The 2SC701
transistor ie TR3 was intermittent and caused significant drift with component cooler recreating the defect. Hit it a third time with component cooler and now I cannot get a single station. Have two 2SC701 transistors on order from Newark. Getting to this transistor is really a bear. I may gently cut some lines in the aluminum cage to gain access, trim the leads close to the transistor's body and solder the leads to the new transistor. Finally this intermittent drifting problem seems to make sense.

 

[AK 47]

I have a 70's tuner that has held rock slid on 90.3 for the 10 years I've owned it (St 8600) and anothe that hasn't budged from the same station for the two + years I've owned it (ST 9030), so they can behave quite well if there is not a problem. Chuck

 

[bhundu]

If I could pull this thread up again please.

Getting drift that resonds better when warm. Happens more towards the higher FM frequencies at start up.

Caps yeh???

 

[hsn_jones]

Have a Harman Kardon HK-670 from 1980. Was top of line receiver back then.

Having similar issue with FM drift. Power unit up, tune station in, then in about 30 minutes, have to retune the station. I probably have to move the pointer about .5 MHz or so.

Not sure if the second tuning matches closer to the dial calibration than the first.

Is this type of problem more common in older FM tuners? - as this one is 30+ years old.

 

[century tek]

FM AFC was supposed to help with FM tuner drift. Unless you have a newer tuner or receiver with quartz locked tuning, I'm afraid you are just out of luck unless you really want to dig into this and find the drift problem in general!

 

[Trio]

Actually any component in the oscillator or IF section can cause drift if it is excessively affected by heat. If the antenna is matched to the tuning gang on analog systems, drift is a little less of an issue. Sometimes it can be a transistor, but more often it can be a capacitor and less often a resistor. To some extent this is normal, but in extreme cases, it is indicatiing a failing part. Drift is more common with tube recievers just due tot he nature of tubes, but transistor circuits can suffer from it as well.

If the drift is only one way and stabilizes, it is likely a transistor. if it keeps varying in direction and the extent- a capacitor is suspect.