SX-850 tuner issue

Cajunchuck

Cajunchuck

Active Member
I've just acquired a new to me SX-850 and so far I love it. I have a question though about radio reception/antennas. I'm using a piece of speaker wire to one of the 300 ohm terminals and am pulling in a good many stations. Even with a good signal though I'm hearing a lot of distortion. Is this coming from an issue with the reciever or is it caused from the makeshift antenna I'm using? Guy I bought it from had it recently recapped and tested but he didn't seem so sure if they went through the tuner section. Thoughts? I'm new to all this vintage stuff. It sounds great on the other sources though.
 
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Get a proper antenna first then evaluate. As a minimum use a folded dipole as they shipped with the unit when it was new.
download the owners manual from hifiengine and read up on operation.
 
rcs16 said:
Get a proper antenna first then evaluate. As a minimum use a folded dipole as they shipped with the unit when it was new.
download the owners manual from hifiengine and read up on operation.
Click to expand...


Thanks. Headed to worst buy later to see what they have.
 
Go to their web site and see if they even sell a FM dipole antenna, or you maybe wasting your time and gas.
 
OK I've noticed something else. It seems the longer stays on the worse it gets. Starts out barely listenable and by about twenty minutes or so it's a garbled mess. Could this mean a heat-related issue? I would also like to at least do some voltage checking with a meter to see if it is in spec. Any recommendations for a meter would be appreciated. Guy I bought it from said a few caps/transistors had been changed (nothing in the tuner though).
 
It looks like the power supply has been worked on. It still retains some original caps but it also looks like the tech has replaced the old paste on the mounted transistors. Some care has been taken.
If the set works using the other inputs, Aux, Tape Mon, then the power supply should be considered operational.
Now if all the issues are isolated to the tuner then a check of the supply voltages to the tuner board should be considered.
Does AM work satisfactory? It has a built in antenna, the one mounted to the backplate.
Use a known good FM antenna for checking the FM reception.
 
I have an sx780 on my bench right now. No cover over the VC. I wonder why one old cap was left in the power supply? The corroded part is just a trimmer with carbon build up. Marantz used these alot on their amp boards for bias and dc offset. Ive replaced many of them. They tend to not want to remain stable.
 
What happened with your FM antenna search? It is nice to follow up on recommendations, esp something like a cheap FM dipole.
Cajunchuck said:
It seems the longer stays on the worse it gets. Starts out barely listenable and by about twenty minutes or so it's a garbled mess. Could this mean a heat-related issue?
Click to expand...
Yes something is faulty. Check the power supply to the tuner first. it should be 13.5V. It could also be dirty switch contacts. Has this units switches been thoroughly clean with de-oxit D5. Usually the first line of defense. Read up on how others manage this task on sx-x50 units, they share many common problems and solutions.
Many of these old components show tarnish, it does not necessarily mean that they are faulty. testing tells you if they are faulty, not always looks.
My sx-950 has a cover over the RF front end. I have seen others that do not have one. It does not mean that it is necessary for the tuner to work properly.

When you are looking for a DMM, for audio work, look for accuracy, AC performance, like what is the AC frequency response, that way it can be used to measure audio freq signals through the range up to at least 20KHz. I use old test equipment such as a HP 3478A. Problem with old equipment is that it can fail at any time and if you are not up to fixing them then they can be useless.
You also need a sine wave source in order to provide a stimulus to be able to trace a signal thru the ckts.
Investing in test equipment can be a expensive and if you only use it once it can be a waste of money.
 
Well I've pulled the tone amp boards and can tell they have been cleaned recently. I was told it has been completely deoxited and "a few parts changed" in the power amp/power supply sections. He showed me the bag of old parts that came out of it. It really sounds beautiful except for the tuner. I can pull in some AM. No luck yet on the antenna but I'm headed to Radio shack today for some deoxit and hope they have one. Will report back tonight.
 
Well I was able to procure from RS an FM dipole antenna. Hooked it up and was able to pull in stations a little better but the distortion remains. In fact it's sometimes worse with the stronger stations and gets worse as it warms up.
 
Good, so the tuner needs troubleshooting. Where to start.
take the composite(FM det O/P/4ch) out of the back RCA and feed it into a tape montitor1,2 play or aux in jack on another receiver/int amp to determine if it is before or after the stereo decoder.
 
good move - but my bet's on alignment. "good signal but distortion" Lower slug of the T6 IF detector to get it on 10.7 MHz

need an undercut plastic tool and a very very light touch.

check signal strength and sound versus tuning meter center indication, also see if less distortion can be found by altering tuning.

to adjust (NOT TAKEN LIGHTLY - you can BRICK your tuner!!!):
detune off of any stations then adjust lower core IF the tuning meter is off center.
if necessary, disconnect antenna and put a short across the fm antenna pin(s) to ground.
ANY residual signal leaking in will leave you worse off than when you started.
 
Cajunchuck said:
View attachment 855854
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Is the cracked insulation on the cap a potential issue? If so I'm fairly confident I could change it out. The tuner will have to wait a while. I'm new at this stuff but when I get addicted to a new hobby I can't cram enough info into my head fast enough LOL. I see whoever has worked on it before has substituted nuts/bolts for the little plastic clips that hold the boards in the chassis. Does Mouser sell these? There are several more that are totally missing or broken and I'd like to make it right. While I'm at it is there anything else I could order for it? The big black caps in the PS have tops that look bulged a little. Is there anything that can go wrong with these old units that will prove unfixable? If so I'd like to change out the parts to avoid disaster in the future. Sorry for all the questions but I'm in love with this thing and want it to live another forty years.
 
Hard to say, it only the sleeve.
Mouser probably sells them, I do not have a # for you, look around
Bulging caps are usually indicate don the bottom, not the top.
Look around for a parts/BOM list, ecaps,esp those blue Sanyo tanatalum,2sa725,726 are well know ones.
Do my tuner test and follow mtf advise first.
 
rcs16 said:
Good, so the tuner needs troubleshooting. Where to start.
take the composite(FM det O/P/4ch) out of the back RCA and feed it into a tape montitor1,2 play or aux in jack on another receiver/int amp to determine if it is before or after the stereo decoder.
Click to expand...

I will as soon as I can scrape up another reciever. Appreciate the advice!
 
Check the FM in MONO and STEREO. Leave it in MONO for at least the time you left it in until it started to distort. Switch to STEREO If it's NOT distorting inMONO, but is distorting in STEREO, then the problem is most likely in the Multiplexer. To get the most from the MPX the tuner has to be aligned 1st. Then align the MPX.. It sounds mostly like a Heat stressed transistor is breaking down as it warms up in the MPX. As for which one, you'l have to do some detective work.

The cracked insulation in that cap is a non issue. If it was part of a Voltage Doubler and sitting on a phenolic wafer, then it would have voltage on the cap case. An uninsulated cap in this case could be a big problem. With this one there should not be any voltage on the case (you can check it with a voltmeter if you are worried about it) and it can wait until you do that particular board.

Larry
 
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