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Pioneer SX-838 Tuner Issues

Childof60

Childof60

AK Subscriber
Subscriber
I picked up a nice SX-838 a few weeks ago in overall good condition. I have done the normal things including power supply, protection board, and power amp, and tone control recap and known transistor issue replacement - which has turned out well. So, moving on to the tuner it is a different story. While the AM section is fine the FM section has gradually lost the ability to pick up any station. My guess is either one of the RF stages has failed or the tuning capacitor has a corrosion issue. From the dirt I have found on the unit it was in a smoking environment - not terribly coated in yellow residue thankfully.

Does anyone have direct experience in dealing with similar tuner issues? I believe cleaning the tuning capacitor is the easiest step to try first using iso-alcohol, Pulling the board to get to the circuit board is a nightmare compared to the other Pioneer receivers.
 
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DO NOT USE ISOPROPYL ALCOHOL on the tuning cap! It will leave a residue which changes the capacitance of the tuner cap (finned structure). If you're gonna clean it, go to home depot and get some CRC Q-D Electronics Cleaner (contact cleaner, not the electric motor cleaner!) It's non-residual and won't change the tuning on the cap. SPRAY with the Fins fully extended, and retracted. At the point where the shaft goes thru the mounting base are some copper fingers (grounds between the shaft and base. Using a toothpick, spray some de-Oxit in a paper cup or a plastic cap for a spray can, dip the toothpick in the deoxit and put the drop/drip on each of the fingers. Exercise the cap back and forth full lock to lock.

You should be able to lift the tuner board, by following all the wiring bundles back thru the unit and making slack, by removing those leads from the bundle clips. It's gonna loosen the tuner string, so lock all pulleys and string with Masking tape. Wrap the main pulley at the tuning cap with tape. Then remove screws, and lift in a rotating manner (toward the front) while adjusting the wires for slack, CAREFULLY! The wiring is solid core and can be brittle in places.
 
Since the cleaner uses a propellant, and it has a very low boiling point, it may cause condensation on and around the VC, including within its trimmer capacitors. Allow plenty of time for the assembly to dry thoroughly, and don't get terribly disappointed if it doesn't magically fix everything when powered on. It may actually require some heating to dry out the assembly.

The 838 used the earlier Hitachi ICs in the RF section, and those have been fairly reliable. Diagnosis should be resonably easy with the right equipment.
 
i belong to several antique radio forums. some folks remove those tuning capacitors from the chassis,in your case the board itself and sometimes completely disassemble them (i dont recommend this),or even run them through the dish washer and rinse in distilled water. dry and re-lube is a must. i did this on a 1934 philco and it does work. i doubt that your 838 needs this drastic approach,but nothing is impossible. i suspect something else is wrong. to eliminate the cap as a possibility i'd use a NON- filming NO residue contact cleaner after applying deoxit d100 to the shaft wiper grounds. leaving it sit a couple of days cycling the hell out of it and then douche with the contact cleaner. condensate can easily be removed with a hair dryer. you then need to re-lube the front and rear bearings and put some fader lube on the shaft wipers,MAKING SURE you dont get any of the lube on the fins. but as i said,i doubt thats the problem.
 
john stumpf said:
i belong to several antique radio forums. some folks remove those tuning capacitors from the chassis,in your case the board itself and sometimes completely disassemble them (i dont recommend this),or even run them through the dish washer and rinse in distilled water. dry and re-lube is a must. i did this on a 1934 philco and it does work. i doubt that your 838 needs this drastic approach,but nothing is impossible. i suspect something else is wrong. to eliminate the cap as a possibility i'd use a NON- filming NO residue contact cleaner after applying deoxit d100 to the shaft wiper grounds. leaving it sit a couple of days cycling the hell out of it and then douche with the contact cleaner. condensate can easily be removed with a hair dryer. you then need to re-lube the front and rear bearings and put some fader lube on the shaft wipers,MAKING SURE you dont get any of the lube on the fins. but as i said,i doubt thats the problem.
Click to expand...

So, some progress. Using CRC Q-D did have some effect as it now at least picks up one station - the strongest in my area. This at least confirms there is some residual tar in the tuner and at least part of the problem. I think you are right s this is likely not the only problem. I will likely have to invest in better test equipment as this now looks like it will require a full alignment.
 
chazix; Isopropyl alcohol evaporates, but the water in it(however slight) leaves a residue that changes the capacitance of the tuning cap. Dust, smoke residues, environmental particles in the air can land on the tuner fins which can/will change capacitance on the tuner cap fins causing tuner to NOT receive stations. A non Film forming, non residual cleaner is best. (See my post #2 and John Stumpf's post #5).
 
ALSO remember, that NO ADJUSTMENTS be made before, or within 24 hours AFTER this zero-residue cleaner application.
I don't even TRY to listen to it afterward, I just put it on the side for 2 days, THEN come back to it.

You are dealing with infinitesimally small capacitance, anything and everything affects it.
 
remember the shaft wipers ground half the plates. just how dirty is this cap anyway? the trimmers being filthy (pictures would be nice) can also effect it. but again i doubt it, whatever you do dont TIGHTEN THE SCREWS!!!!
 
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