tuner repair problem
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. Dave |
tuner repair problem
If you actually replaced 44 caps, you deserve to get this working!
Look for free-standing TO-220 voltage regulators. Freeze 'em or heat them up, see if any effect, or just replace them (heck, you've already replaced most of the PS). Might be worth a shot... Good luck. |
tuner repair problem
"Mr. Land" wrote in message oups.com... If you actually replaced 44 caps, you deserve to get this working! Look for free-standing TO-220 voltage regulators. Freeze 'em or heat them up, see if any effect, or just replace them (heck, you've already replaced most of the PS). Might be worth a shot... Good luck. I replaced the single +5VDC regulator last night. We'll see if the problem goes away. Dave |
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