"Opto lock" drift in a OPTONICA ST-7405 tuner.

markn2wae

Mark T N2WAE
My ST-7405 tuner has a strange problem.

Depending if the tuner is hot or cold, the "opto lock" control high "good" or low when indicating it is tuned.

For example, depending on the temperature of the set, when I tune in "104.30" on the dial and let go of the knob, it will go to 104.29, 104.30, or 104.31.

I have found a coil inside the set to "fix" the problem, but next time there is a temperature change, it's "off" again.

I have no way of knowing if the "mis-tuning" is just a display error or the actual signal is being mis-tuned also.

There is no switch to defeat this action other than your fingers touching the tuning dial, I do like this feature if it can only be made to work properly, other wise the set plays fine.

So far, I haven't found a schematic or service manual for it yet. :sigh:

Mark T. :music:
 
I've got a similar problem on a big AIWA 9700 dual quartz FM tuner. Early Q PLL analogue/digitals are a nightmare to work on.
 
A quick check of the local oscillator frequency will help determine if the tuner is actually changing frequency.

There may be a part in the AFC circuit that is changing value with temperature.

Is tuning accomplished using an air variable capacitor?

Can you follow the circuit back from the display to see what drives it?
 
I just bought an ST-7405 a couple of days ago. I was searching for a service manual when I found your post. I was looking for a manual because I want to re-cap and align it. I did not find a service manual for this model yet but I did find one for an Optonica ST-7100h at hifiengine.com. I still don't know what the differences are yet, they look the same. The big problem is that the manual is in the German language so I'm still searching for a manual. The schematics in this manual might help you.
I checked mine to see if it had the same problem as yours. Mine doesn't drift but when it locks onto a station the digital readout is high by 0.01mhz. I have a cheap transmitter that connects to a phone or mp3 player that I tuned my tuner to, and it also read high by 0.01 mhz. This little transmitter transmits RF within +-2khz.
After looking at the ST-7100 manual I found that in the alignment section it says shorting TP601 and TP602 disables the opto-lock, at least if you can trust Google translate. There is a TP601 and TP602 labeled on my ST-7405 board in the same location but the TP601 pin is missing and there is only a hole filled with solder
 
I just bought an ST-7405 a couple of days ago. I was searching for a service manual when I found your post. I was looking for a manual because I want to re-cap and align it. I did not find a service manual for this model yet but I did find one for an Optonica ST-7100h at hifiengine.com. I still don't know what the differences are yet, they look the same. The big problem is that the manual is in the German language so I'm still searching for a manual. The schematics in this manual might help you.
I checked mine to see if it had the same problem as yours. Mine doesn't drift but when it locks onto a station the digital readout is high by 0.01mhz. I have a cheap transmitter that connects to a phone or mp3 player that I tuned my tuner to, and it also read high by 0.01 mhz. This little transmitter transmits RF within +-2khz.
After looking at the ST-7100 manual I found that in the alignment section it says shorting TP601 and TP602 disables the opto-lock, at least if you can trust Google translate. There is a TP601 and TP602 labeled on my ST-7405 board in the same location but the TP601 pin is missing and there is only a hole filled with solder

Maybe the next time I open my tuner up again, I can locate the coil that I tried adjusting and that information might "fix" your problem (as you stated, you have NO drift). :scratch2:

Mark T. :music:
 
Well I looked at the schematic and it shows the chip used to drive the display is a frequency counter. Its measuring the local oscillator frequency, but the local oscillator frequency is 10.7mhz higher than a tuned station so it must subtract it. The local oscillator must be off or maybe the ceramic filters are off center and the local oscillator was adjusted to the match them. It would only take +-5khz off from the local oscillator to change the lower digit.
 
A schematic would certainly help. You need to find out whether the LO is drifting or the discriminator (the DC voltage could be varying and causing the tuning voltage to move around.) or the PLL circuit. If the LO is drifting, the tuning voltage should change to compensate, so I think it's more likely the fault is in the discriminator or the PLL circuit.
 
Tuner back together, working good.

Well, the tuner is back together, it needed a bit of cleaning and slight touch up of the front end (the IF adjustments where never touched).

The variable resister talked about above is just an adjustment for the red and green "center tuned" lights (when the set is "locked" this adjustment insures that the green light is full on and none of the side red lights are on).

I put contact cleaner on the tuning capacitors ground points at the rotor shaft, being careful not to get any on the movable plates or the adjustment screws then all the trimmer pots on the PCB (did not rotate any of them except for the one mentioned above).

I did a SLIGHT adjustment to the oscillator trimmer (high end) and coil (low end) to get the dial to read correctly (along with the digital display) and then touched up the antenna, RF, mixer and other "high end" adjustments (didn't touch the Low end") and now ALL stations come in with a higher readings on the signal strength meter. :yes:

Time will tell in the winter months when the room temp. is lower if the "drift" is still there.

Thank you all for the help! :tresbon:

Mark T. :music:
 
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