"hiccups" in Allied 495 (Pioneer) FM tuner

gadget73

junk junkie
Subscriber
My Allied-branded Pioneer SX-990 has recently begun giving random problems with the FM tuner. Every once in a while the audio crackles in and out. It doesn't do this on any other function. I have messed with all the controls and switches, and they were recently cleaned. The signal meter does not drop out when this happens. The stereo light is burned out, so no idea what that would be doing.

Does this sound like a familiar symptom to anyone? I haven't brought it home from work yet to poke around at. Its all original inside, so probably overdue for some service by now anyway. I'm hoping for a cracked solder joint or something along those lines, but if anyone knows of specific potential problem areas I'd love to hear about it. When it comes home it will get a cleaning, new lamps, and a complete checkup at least. The volume control was replaced years ago with the wrong value, and the original knob is long gone so I'd also like to try and sort that out while its on the bench.

Thanks
 
The audio output from the tuner section on those receivers goes through the head amp (phono preamp) section. It could be a noisy or intermittent transistor on the phono preamp board unless you're sure the phono section is working properly. If it's happening on both channels but the tuning meter isn't reflecting it maybe look at the power supply for the phono preamp board.

EDIT - Sorry, I was mistaken - the above information pertains to the SX-1000, SX-525 and SX-625 series receivers, but not this model. The tuner section on this model is NOT routed through the head amp section.
 
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Sporadic crackling noises usually point to small signal transistors. Try to narrow the problem, check if AM does the same and so. That will isolate the problem.

Many Pioneer SX have problems at the function selector switch and broken solder at some points.

A freeze spray can be helpful to identify a noisy transistor. Tapping on the unit or on the PCB with a wood or plastic stick can reveal some intermittent contact (broken solder)
 
The phono stage has been a little hissy for years. I don't run a turntable with it though so I've paid no mind to it. Perhaps its time to do something about that. Thanks for that bit of info, you may have identified the problem. Some years ago I did pull that phono board out, possibly one of the wire re-attachments is failing.

AM doesn't seem to do it, but I have no AM reception in here. Its just a high pitch whine that I cannot stand to listen to for any amount of time.

I'll swap it out for the Yamaha HT reciever someone gave me and give the old Allied a gander at some point. I've had it for way too many years now to just dump it. Its been mine for probably 20 years at this point, its pretty much what started all this.
 
Bear in mind too that the phono section is loaded with 2sc458 transistors which don't age well so one of them could have an intermittent short which might be knocking out the B+ rail for the head amp. There's also a 50uf 25v smoothing cap on the B+ rail for the head amp which could be doing the same thing (if it's the head amp causing the problem that is).
 
I'll have a gander at all those things. I appreciate the pointer at the phono stage though. Should be easy enough to work out, voltmeter on the supply and a scope lead at the in and out to confirm whats going on. The problem is that its very intermittent so catching it in the act might be difficult. I want to make sure I catch it before something fails in a bad way and causes bigger problems. I envision this thing standing on end on my bench with cables hanging out of it for a while.
 
Bear in mind too that the phono section is loaded with 2sc458 transistors which don't age well so one of them could have an intermittent short which might be knocking out the B+ rail for the head amp. There's also a 50uf 25v smoothing cap on the B+ rail for the head amp which could be doing the same thing (if it's the head amp causing the problem that is).
Marantz used these as well. I keep a bag of replacements on hand every time I pickup another Marantz. They were used on some of the line stage boards as well. Did Pioneer use them their also? I ask because I have a Pioneer SX-780 under my bench waiting its turn.
 
Marantz used these as well. I keep a bag of replacements on hand every time I pickup another Marantz. They were used on some of the line stage boards as well. Did Pioneer use them their also? I ask because I have a Pioneer SX-780 under my bench waiting its turn.

Not by the time of the SX-780, Pioneer stopped using them very early on. I don't think the SX-626 even used them.
 
The 2SC458 also used on the PS board, Also 2SC871(noisy) is on the Top Ten Worst transistors list, used on the control board. KSC1845F as the replacement for the 2SC871.
 
Yep, I generally use Fairchild KSC1845 to replace the 2SC458 too. The letter suffix indicates the packaging style - BU indicates bulk, TA indicates tape ammo style (taped on a cardboard strip), F probably indicates free and so forth. I buy them by the hundred count (BU) because I work on a lot of gear and they are an adequate sub for so many small signal Q's.
 
awesome, thanks again. When I get a chance to get into this thing I'll update with what I find. Hopefully I can make it glitch on the bench. It does seem to carry on more when cold vs hot at least. At work it runs 8 hours a day 5 days a week, and it mostly acts up in the morning. Might have to get some freeze spray to force the issue.
 
Finally opened it up for a look. Not sure if this has anything to do with it, but I bet it is not helping. This appears to be a capacitor across a rectifier on the power supply board. I haven't plugged it in to see if it shoots sparks but the char marks and the blister on the ceramic make me suspect its dead. Two of those diodes do not appear original either, the solder work on the back side is not as neat as the others, besides the case style being different.

I also found a 10 amp fuse in place of the 3 amp. Shame on me, I probably did that many years ago.

100_0692.JPG
 
of course its working perfectly fine right now. Tapping the various boards and connections doesn't do anything. No fireball from the power supply area. Short list I think will be rebuilding that power supply board and replacing the lamps. From there I may just run it a while to see what it does. It definitely could stand a full rebuild but I don't want to dig too deeply into it before figuring out what the crackle noises are.
 
Looks like it might have taken a lightning hit at some point (which would explain the new rectifier diodes). Can't imagine why the snubber caps weren't changed if that all happened at the same time though. :dunno:

BTW have you seen this?
 
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Hm, I could actually use some parts. The volume control would be worth the price of admission.
 
Well, I'm wrong. It is not an SX-990. Its an SX-1500TD. Its very close to the same unit except the power amplifier is 2x the wattage. The power supply board number was the key. Amp board number matches the 1500TD as well. The other boards are the same in both units except the head amp. The Allied 495 uses the W15-049 head amp that the SX-990 uses, the 1500TD uses a W21-001. The SX-1500TD has a mic input, the SX-990 and the Allied 495 do not, so that might be the difference.
 
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