Sansui G-8700DB Digital Display is off by 0.2MHz - Local oscillator vari-cap replaced.

[sbbrown]

The tuner in this receiver had issues but now functions.

The first issue I had was that the local oscillator variable cap. did nothing and the dial was way out of whack. The digital display was way off too. After replacing the variable cap. and tuning it to proper DIAL setting, it worked OK, with the multiplex light and center frequency display lighting up, as well as signal strength.

The next issue I've found and cannot fix is that the DIGITAL display is off by about 200KHz,(0.2MHz). When a known station is tuned, regardless of the dial calibration, the digital display is off 0.2MHz! Also the servo lock LED does not light up. The manual says nothing of the digital display, and there are no adjustments listed in the manual. There is a crystal and another variable cap. on the digital display board, but it does not seem to affect the frequency read out at all. My head is getting bloody from banging it against the wall : )

Any comments or ideas would be appreciated.

 

[PE9ZZ]

Did you do the AFC voltage adjustment and the dial calibration as per service manual steps 4 and 5 (page 3)?

 

[dewdude]

The digital display is largely its own unit...it looks like it's sampling the local oscillator frequency and generating a display. The LO signals for AM and FM run into a pre-scaler (F-3010) before going to the F-3088 board. The pre-scaler could be busted.

My first instinct would be to check and make sure the LO is actually where it's supposed to be. If you changed the gang cap it's possible you threw the LO off and just tuned the analog section around the new cap. This would cause the frequency to be off going into the board and cause it to display incorrectly as well as the servo lock not work. The trim cap on the board is part of the MSM5540's clock circuit; it's likely a microcontroller based on the fact it has it's own clock circuit.

There are some diode adjustments for changing the IF input to the board. But I think what you really should do is pick up a cheap frequency counter or if you have a scope that can tell you the frequency; you should look to see what the LO is actually putting out and make sure it's offset matches what the IF should be going into the board. The IF on the display board can be adjusted from 10.64 to 10.76MHz.

It's possible your replacement and alignment just knocked it all out of whack. Since it's an analog tuner with a digital read-out...this is easy enough to do.