SX-626 problem w/ tuner

[nickdinatale]

Been running this unit for a few weeks with no issues. Suddenly upon startup, it fails to get FM signal in the middle of the tuner, but still will at the far ends. After about 20 minutes, the problem manages to right itself and tuning isn't a problem. Any thoughts as to what this may be are greatly appreciated. Thanks!

 

[dlucy]

I'd start with checking the voltages around the FM board and comparing those to the schematic. Have you picked up the SX-626 schematic from hifiengine.com yet? That's a good starting point.

 

[I LIKE MUSIC]

It is likely that you should focus on the FM front end, local oscillator and the FM RF mixer.

Since the tuner still works at the lower end and upper ends of the FM band, that indicates that everything downstream from the FM mixer is likely okay, although there can be exceptions.

The RF front end of this tuner is fairly simple, just 3 transistors. 1 FET RF amplifier, 1 BJT for the local oscillator and 1 BJT for the mixer. A failure in any of these three areas could cause your symptom.

The problem is likely in this area.

v



A good place to start is cleaning the main tuning capacitor.

How to clean a tuner (analog)

Then I would look at the local oscillator, circled in blue.

 

[nickdinatale]

It is likely that you should focus on the FM front end, local oscillator and the FM RF mixer.

Since the tuner still works at the lower end and upper ends of the FM band, that indicates that everything downstream from the FM mixer is likely okay, although there can be exceptions.

The RF front end of this tuner is fairly simple, just 3 transistors. 1 FET RF amplifier, 1 BJT for the local oscillator and 1 BJT for the mixer. A failure in any of these three areas could cause your symptom.

The problem is likely in this area.

vView attachment 1000891



A good place to start is cleaning the main tuning capacitor.

How to clean a tuner (analog)

Then I would look at the local oscillator, circled in blue.

Wow, thanks for this. I really appreciate it.

 

[I LIKE MUSIC]

You are very welcome.

 

[dlucy]

If you haven't worked on the tuner portion of a unit before then you need to know that adjusting any of the variable inductors (coils) or the variable resistors (trim pots) can result in the need for some expensive test gear (to realign the tuning).

No need to be afraid of diagnosing the problem or working on the circuit, though. The only thing to avoid is adjusting (turning one way or the other) any of the coils or pots on the FM boards.

 

[Binkman]

Just note don't use deoxt or lubes on the tuner cap. brush out dust bunnies. perhaps whole tuner board is dusty? same brush out bunnies.

I LIKE MUSIC is pretty much dead on. Try mounting your dipole antenna vertically. Sometimes local stations change and I found vertical mounts to be much more apt to lock in better with fm mute off.