I've seen multiple threads online about this issue but none of them have solved my issue and I'm wondering if my issue is stemming from somewhere else.
Working on a Sansui 5000x; everything is fine except the FM Stereo lamp will not turn off, in any mode (AUS, phono, etc). The thing that's confusing me is that FM works, and I can tune in a clear station, even with the muting off, but I cannot tell if that signal is truly stereo or not. I do know that the common transistors to replace on the MPX board did make a small change, but didn't resolve. I then recapped the board, checked the diodes, checked the VR adjustment pot... TR409 was replaced first, and prior to replacing it it would cause the lamp to flicker if I tapped on it. Pulled it, measured it a fews times on my Peak and it kept reading faulty so after it was replaced, the lamp now stays on with no change. I replaced the transistors 407 and 408, and then started shotgunning stuff out of frustration but nothing made a difference. I checked voltage at the adjustment pot and adjusted it while tuned to a good station and with static and it just gets a solid 7.5v with no variation, so with little tuner knowledge to begin with, is there any way for me to 100% know if the signal I'm getting in FM is actually stereo, outside of just listening and assuming it is? Because I'm concerned my issue is on a different board.
As far as I can tell, nothing has been modified, rotary switch is all original wiring, the FM lamp was attempted to be replaced at some point but it was wired to the correct pads on the selector board. I haven't gone any further diagnosing the power supply because I assumed if the board was getting the correct voltages, then the issue was something else. Any ideas on what to look at? I have no FM adjustment tools or knowledge so I don't particularly want to play with it...
Working on a Sansui 5000x; everything is fine except the FM Stereo lamp will not turn off, in any mode (AUS, phono, etc). The thing that's confusing me is that FM works, and I can tune in a clear station, even with the muting off, but I cannot tell if that signal is truly stereo or not. I do know that the common transistors to replace on the MPX board did make a small change, but didn't resolve. I then recapped the board, checked the diodes, checked the VR adjustment pot... TR409 was replaced first, and prior to replacing it it would cause the lamp to flicker if I tapped on it. Pulled it, measured it a fews times on my Peak and it kept reading faulty so after it was replaced, the lamp now stays on with no change. I replaced the transistors 407 and 408, and then started shotgunning stuff out of frustration but nothing made a difference. I checked voltage at the adjustment pot and adjusted it while tuned to a good station and with static and it just gets a solid 7.5v with no variation, so with little tuner knowledge to begin with, is there any way for me to 100% know if the signal I'm getting in FM is actually stereo, outside of just listening and assuming it is? Because I'm concerned my issue is on a different board.
As far as I can tell, nothing has been modified, rotary switch is all original wiring, the FM lamp was attempted to be replaced at some point but it was wired to the correct pads on the selector board. I haven't gone any further diagnosing the power supply because I assumed if the board was getting the correct voltages, then the issue was something else. Any ideas on what to look at? I have no FM adjustment tools or knowledge so I don't particularly want to play with it...
Seems like an alignment problem, especially if any parts in the RF circuitry have been replaced
without realignment afterward with the proper, required test equipment.
without realignment afterward with the proper, required test equipment.
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So the original fault was actually different and not the lamp being on all the time. One of the most common issues generally across any make and model is often just a dodgy 19/38kHz preset. Could that have been the issue all along and swapping parts has created another issue?
It is to late now 🙂 but that is the very worst approach really and particularly so with anything RF related. Even the physical positioning of parts and wires and so on can affect overall alignment due to stray capacitance. Always disturb as little as possible.
Without test equipment listening is the only way. It should be easy to tell with headhones and any music station. Mono will be a tight and central 'in head' image whilst stereo will be a much wider and more diffuse image.
Yeah I know, I realize I invite scrutiny for practices like that, but in this case I did test that transistor out of circuit and it was definitely faulty. So Just replacing that one first caused the light to go a bit brighter. When I was saying the lamp was flickering, it wasn't when moving the dial or anything like that. It was literally doing it when I tapped on the transistor. Once I saw it was stuck on with the new one installed, I tried taking a step back and replacing the ones before it and seeing no change.