I recently acquired a Sansui T-80 am/fm tuner. The sound was pretty good, but I had some flaky performance issues: The tuner would periodicaly lose its' quartz-lock and the "tuned" inducator would drift badly. Also, the entire digital display would periodically dim and lose some or all functionality.
I opened up the case and first off noted that the main filter caps on the power supply were leaky. Well, I figured, might as well bite the bullet and replace all of the electrolytics given that this tuner was built in 1981. So last night I replaced all 44 caps. I had trouble finding one of the spec caps, a 0.15uF 100V electrolytic, and so ordered and installed two film caps of this rating instead. All replacement caps matched the ratings of the originals EXCEPT those 0.15uF caps... I think a previous owner had run into the same problem with availability of this cap in an electrolytic and had substituted a 0.22uF 100V.
I cleaned up the boards, put the whole unit back together and powered it up. Well, first thing I noticed was the sound... low volume, tinny, no base at all. Whereas before I started 25% volume on my amp would shake the house, now I can easily go past 50% and still can't feel any thump in the floor. The tuner drift issue has gone away, I suspect a cap labelled "AFC bias" was the culprit there. However, I still have the dimming issue with the digital display; perhaps the +5VDC regulator that powers the display board is flaky? It's in a tough spot to blast with heat or cold without affecting a whole whack of other components.
I left the tuner on all night thinking maybe there is some sort of initial "burn-in" period and I THINK it sounded a bit better than at midnight last night, but who knows...
Can anyone tell me where I might start to diagnose the tinny sound problem? There are but a handful of coupling caps in the FM signal path, some in the IF amp section, some adjacent to the MPX decoder. I am inclined to toss back in the 0.22uF electrolytics that I replaced with film caps; they are output coupling caps just to see what happens.
Any assistance greatly appreciated.