qx949 fm drifts during warm up

john stumpf

Super Member
everything works marvelously on this set save for this issue.i read all i could in general on this issue,the info was varied and ran the gamut from 'live with it" to specific maladies such as a change of value of something in the front end, everything from capacitors.resistors or the local oscillator. i was tempted to leave it alone, but my curiosity got the better of me this morning. i turned it on and tuned it in after i removed the tuner cover and replaced the cabinet to speed the warmup. sure enough within an hour, with the fm muting on it had drifted enough to silence the station 100.3. using a q tip moistened with alcohol i carefully swabbed the local oscilator 2sc717 and lo and behold the station tuned in immediatly. this seems to suggest that the 717 needs to be replaced. they dont seem to be widely available but as time goes by there will come a time when they are totally UN available so i feel that if i can find a genuine part now is the time to deal with it. i also plan to deal with its neighbor a 2sc461. UNLESS of course im missing something and thats why im here. i fully realize i will have to realign the front end but that isnt an issue as i have a fm signal generator. by the way, does anyone know a SINGLE source for the 717 and the 461? shipping is a real killer to here/
 
Looking at the circuit, you might actually get by with a KSC945Y in place of the 717. It specs the same or better than the 717 except for frequency, but the fT is still 300 MHz, which should be plenty for that circuit.

The 2N6428 might be a nearly perfect replacement for the 2SC717, specs equal or better in all categories.

PN4209 or PN4355 might be suitable in place of the 461 on the same basis.
 
thanks watthour. i even have a ksc945 on hand but alas, nothing for the 461. ill try your number's on flea bay and see if i can get some with reasonable shipping rates. mouser had it all between the pn4209 and the 4355 do you have a preference?
 
I do not think that a ksc945 nor a 2n628 are the right parts for a >100MHz VHF oscillator.
You want a bjt stated for use as a VHF amp,osc,mixer. Look at the minimum ft spec, not the typical.
I believe that you want a bjt with an min ft of 500MHz for a stable VHF oscillator.
A sub for the 2sc717 can be a 2sc1906, 2sc2995, 2n5770, SS9018, MPSH10, 2n(PN918). always compare pin outs. There are more but this should be sufficient.
Try changing the 2sc717 first before changing the buffer 2sc461 which can have a bit of a more relaxed ft spec.

Good luck
 
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:lurk:

I had an SX-949 that would drift, years ago, flipped it.

Drift problems show up once in a while. I’m still practicing tuner repair, too busy fixing other things and dealing with life to really get some serious learning done.

FWIW, when I recap a unit, I recap the tuner and that brings things back a lot. Small e-caps always seem to be the biggest trouble makers since they are on the signal path for the most part. I am not rigged for alignment but other than aligning the dial pointer, I’ve never had a problem with a tuner after a recap due to new caps. As said, they work a lot better with new caps.
 
i hear you and do the same. i use stacked film wimas whenever possible. this 949 's tuner has been recapped as well. in the dozen pioneers ive done this is the first time i/ve encountered this.hopefully its the last defect that will turn up.God only knows how hard it was to rebuild . to rcs, if u have a good sub for the 461 i'm all ears as nothing on this set is easy to get at and i sure want to be DONE!!!! are watthours subs adequate in your opinion? i dont want to slight anyone in this discussion .
 
Watthour knows his stuff.
I don't have any recent memory on those parts and am too swamped to research it.
Doing you're own study on the subs is a good idea if you work on electronics.
 
i dont have the required technical training. i'm also very leery of using cross reference lists unless i get them here.caps arent a problem transistors are
 
You're giving me a lot of credit without cause. RCS is correct about the fT of the replacements. I mentioned that you MIGHT get by with the KSC948Y in that position, but the 2N6428 could be a near perfect replacement with its minimum fT of 100MHz. Since a lot of us seem to have a collection of 945s in the parts bins, it would be worth a try if a better part isn't on hand, just to prove the bad original. The 2N6428 is rated at 100-700 MHz. Like RCS specified. The original is 650 MHz and having something with a minimum well above the operating frequency is safest. The numbers he specified would probably also work (better in terms of speed), but watch the voltage ratings since some of them get pretty low.

Now that there is more time to dive into this circuit, it seems the peak BC voltage should be below 7V, and EB below 1V. Allowing for safety factor, a 20V rating should be safe in this installation. Considering a 108 MHz input signal and LO at just under 11 MHz, operating at 120MHz reliably will be critical. Given that the gain of the original is at least 40, suitable replacement in the Mouser inventory would be:

2N5570
2N2369
 
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If you suspect the 717 is bad, you could temporarily install a 945 if you had one to prove it out, then order the correct replacement if the 945 works, even at the lower end of the FM spectrum.

As for the 2SC461, it appears that a 2N2369A would suffice.
 
again,thank you all. ill order up a 2n6428 and a 2n2369a from mouser.i did find a seller with the oem types but he hasnt responded with a solution for the ebay global shipping (screwing) program. i'd rather pay mouser. as i said the damned tuner on that set (like everything else) isnt easy to access the bottom of and so doesnt easily lend itself to experimentation. but its my fault too as i should have known something was up when my alignment was drifting while in process. BUT another lesson learned and something to watch for BEFORE a rebuild gets under way! CHEERS!
 
Do yourself a favour and get a higher ft bjt, they are so cheap, some 2N5770 & SS9018 are perfect choices.
 
Excellent point. For the few pennies in components, and the cost of shipping, it only makes sense to have some options.
 
That would have been for the 2SC717. I believe the 2N6428 should work there for this circuit without problems. The 461 may need to carry more current but wouldn't necessarily need the fastest switching.soif the original has failed the 2N2369 should handle that duty fairly well.
 
The U.S. cavalry forces had encountered problems caused by the fact that their horses became sick and often died after eating Philippine "sawgrass."
 
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