hallicrafters sx-25 - Page 1
Forum section: electronics / repair

author: FarmerJon1066
author_rank: Newbie
timestamp: June 07, 2015, 12:05:33 am
content: Howdy All,I'm trying to repair a hallicrafters sx-25 radio receiver, I've seemed to isolate the problem to a resistor coming off of the 80 Vacuum tube going to the 6F6-G tube, the closest of the two to the 80 Tube, The issue i'm having is a buzzing/humming noise from the headphone and that resistor smoking and getting up around 150-175F. Any idea's would be much appreciated!I found these schematics onlinehttps://images.campyent.com/sites/default/files/schematics/hallicrafters_inc_sx_25_pg11-2.pngOperators pdf i found online alsohttps://docs.google.com/file/d/1Zu3bqQambug23GfR3snZGFkGda_bN5_bG_gK3eVhSGpsBO4bRMsa3dvOwSra/editThe color code for the resistor is orange, orange, brown silverThanks!I outlined the resistor in question in red

author: PaulAm
author_rank: Frequent Contributor
timestamp: June 07, 2015, 01:16:36 am
content: You may want to try the antique radio forum for this one.http://www.antiqueradios.com

author: monz
author_rank: Contributor
timestamp: June 07, 2015, 03:11:25 am
content: As another has said, the antique radio forum is the place to go for this.Orange orange brown is a 330 ohm resistor, since it's larger it's rated at a couple watts. I can't quite see where it's connected, between ground and something. But R36 on the schematic is a 350 ohm 2 watt resistor, this must be it - value may have changed during manufacture. It'll like get warm during operation. It provides bias by making the cathode more positive than the first grid. The cathode is pin 8 of the 6F6, you count clockwise starting at pin 1 just to the left of the key notch in the socket. You can find pin outs online or in an RCA tube manual.If it's getting too hot to touch, smoking, or discoloring, it's getting too hot. Verify it's still around 330 ohm (the silver means it's 10% tolerance), it may need to be replaced if it was damaged from the over heating.At minimum the electrolytic capacitors must be replaced. Likely the source of the hum. Do not be tempted to place new capacitors across the old, the old must be disconnected from the circuit. Since this capacitor is mounted to the chassis, leave it in place for aesthetics with new caps below (some disassemble and hide the new ones inside). Note it's a multi section type with the capacitors sharing a common negative connection. A shorted capacitor could take out an irreplaceable part such as the power transformer.Next the paper capacitors need to be replaced - these have a high failure rate and leak DC. Many look like paper tubes covered in wax, the one on the volume control needs to be replaced too, it's a bakelite or other plastic package. Use same or exceeding voltage rating. You can find more info on this over at antique radio forum including where to buy replacements. Note you may not find values like .05?F, use modern value .047?F. Back then they called this mfd or mf, same as micro (?) today - pico was mmf then.I can't quite read the number, but the coupling capacitors between the 6SQ7 plates and the 6F6 grids, if leaky, would put plate voltage on the grid causing excessive current through that resistor.Check those by disconnecting one end and measuring the resistance. You should see a quick dip in resistance as the capacitor charges then go to infinity, a good capacitor does this almost instantly, second or two. If it doesn't go to infinity or does so slowly, it's leaky. If it checks ok, replace it anyway, it will go bad or is malfunctioning when high voltage is across it. No need to check the actual capacitance, they don't lose capacity, they become leaky.The output is high impedance, if you connect a (8 ohm) speaker to it, the output will be very low. You'll need a matching transformer. The original matching speaker had this mounted inside.The manual/schematic doesn't show voltages. You should find a high voltage from ground to the 80's cathode (200 to 350 or so volts), the cathode will be positive - the cathode is the heater on this tube, pin 1 or 4, the thicker pins. High voltage AC on the other two. You'll find high voltages at the plates and screen grids. On the cathodes you'll find zero to a few volts and on the grids zero or a negative value.This radio does need an antenna to operate - connect several feet of wire to the antenna connection, if working, you'll get local AM stations and some shortwave. Performance will be very good once recapped and re-aligned.Be careful due to the high voltages.

author: N2IXK
author_rank: Frequent Contributor
timestamp: June 08, 2015, 02:09:33 pm
content: Yes, a unit of this vintage is going to need ALL the electrolytics and paper coupling caps replaced. Really no point in troubleshooting (or even powering it up) before this is done. The "postage stamp" mica caps fare much better with age, and should still be OK. There 2 larger black "domino" shaped caps in the second picture (to the right of the 100 uF/10V electrolytic) which look to be "micamold" molded paper caps. They should be replaced, as well.If you want to retain the original look above the chassis, the new electrolytics can be put inside the original aluminum twistlock can. Google "restuffing electrolytic caps" for info.You REALLY shouldn't apply power to this thing anymore until the caps are replaced. Leaking or shorted caps can take out parts that are really hard to find these days, like IF and power transformers.