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9090DB FM advice

PF Sound

PF Sound

AK Subscriber
Subscriber
I'm just finishing a restoration & have come across this issue. When in FM stereo mode, I have to turn the volume all the way up to hear it at all, & I'm getting no "FM stereo" lamp (grounding pin 9 lights it up). If I switch to "mono" it plays loud & clear. The muting switch has no effect. The tuning needle & SS are both working & the unit actually tunes in stations nicely. I've checked all the voltages on the mpx chip & they all jive but there's no change on pin 9 when tuning on/off stations, just a constant 8.5-9v..........bad HA1196? I've tweaked VR3 & VR4 to see if I get any change but nope.
I ordered a replacement IC from the bay (semtrnous), but I was curious about something I found. This unit, although otherwise showing no signs of work, had R107 snipped & a 4.7k in series with R72 to give around 26.7k @ VR4. C57 measured 379pF......just wondering if I should correct this to spec or not sweat it.
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Do you have 76Khz at TP02? Pretty uncommon to have these chips fail. Much more common to have alignment issues. Without a good discriminator zero and signal strength. Tune and signal volume pots and the muting/indicator adjustment all have to adjusted correctly.
 
I neglected to check that. I already removed the old 1196 & installed a socket, but it'll be another day or two til the new one arrives so I'll put the original back in & check.
 
Following the instructions for the VCO using a freq. counter, I have nothing at TP02.....just garbled numbers. I don't have stereo mpx input ability so I'm just shorting TP03 to chassis (tried with no short too) as instructed & checking at TP02 while tuned on/off stations. Tweaking VR04 had no effect at all.
 
Interesting....... I have nothing but garble at TP02, but when I clip onto the R72/R107 "junction" (pin 16) I get a spot-on 76khz when tuned to a station, & it drops a bit when off station (hard to find an off-station spot with this unit).
Odd though......TP02, as marked on the board, is before pin 2 on the HA1196, tied to C28 & L02. I can't find it on the schematic although there is a "circle" near VR04 which seems to indicate a test point.....& when looking at page 9 in the SM, they have TP02 & TP03 transposed. I'm guessing the "circle" near VR04 on the schematic is the real TP02 & the 76khz is present where it should be.
 
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I guess pin 12 should go to near 0v when stereo is detected....& it isn't.....stuck on about .720v no matter what. Going backwards, I've got 0v on the base of TR05 because I've got 0v coming out of D08. Somewhere between TR04 & TR05 I've got a problem.
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I've now got output on cathode of D08.....peaked it to 2.15v. It was sitting around 1.8v & adjusting T04 moved it up down. I can adjust VR03 now but it doesn't seem to matter how far I run it to change the base of TR05, I still get no change of the .72v on pin 12 of the HA1196. I'm stumped now .....don't know if I'm even on the right track.:dunno:
 
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