SX-838 FM radio troubleshooting.

[tutor_turtle]

AM is working fine. Even the lower power FM stations come in ok, but never light the stereo beacon.
KSA992's I have by the 100's. Sorted by gain (for use when differential pairs are needed)

I have a lot of hours into this restoration, 9 out of 10 condition, sonically it came out super nice.
I really would like this last issue resolved.
That you for all your help.

This is the semiconductor list for the 828.
All NPN except for the 2SK30-Y FET which I do have limited stock on.

 

[zebulon1]

I'm wondering if its coupling AM with the FM, when in FM?
I'm all over the place (I know!) and if It were on my bench I would be searching for clues without any complete understanding.
Make sure you have all the power supply feeds. The tuner uses a few of them. 12v and a 4.5v.
It looks like Q14 is the tie in for the AM and FM signals. When AM is selected the circuit biases the base of Q14 and supplies power to the AM tuner. If the transistor is shorted the tuner will be operating both AM and FM when FM is selected.
Something like that. Check my thoughts.
You could check and see if there's voltage on tuner board pin 17 when in FM.

 

[tutor_turtle]

It is possible it needs an alignment tweak. What kind of signal levels are you seeing on the lower power FM's. Do you need to turn the muting off to receive them?

Muting switch: yeah, it has no effect at all. It acts like the muting switch is always in the "off" mode.
Another clue.

 

[mrk229]

Full recap, checked polarity.
Voltage is 7.99vdc, off station.
With the lamp trying to light, voltage sags to 2.2V on the cathode & anode side of the diode and on the transformer side of the fuse. (blue wire)
Lamp is dimly lit with 2.2v across it.

Should only be AC on the transformer side (anode) of the diode. Pop a new one in.