Hallicrafters SX-140 reciever

[SethK]

I was given an old Hallicrafters SX-140 by a friend and I'm trying to fix it. Theres nothing obviously burnt out, I'm getting power and although the main audio output tube (6AW8A) looks blackish it is lighting up and when I plug in headphones I get some hum. It has an output rating of 3.5ohms and I don't have a 4ohm speaker to test it with but I get no audio with an 8ohm speaker.

I read that older electrolytic caps can dry out and go bad and when I tested them I found wild numbers, virtually all are way higher than the value printed on them and I'm thinking that maybe the dielectric inside has become dried out and lost a lot of insulating ability, does this sound right?

I'm ISCET certified as an associate level tech but I've never had a job in the field and the only thing I've done with RF gear is build it from schematics and I've never done anything with a piece of tube equipment before.

I'm thinking that I should replace the caps and see if it works, if not then put in a new tube. Am I on the right track?

Thanks,
Seth

[RichardH]

I, too, would suspect one or more faulty capacitors, especially if they're the old types with a wax coating to minimize moisture infiltration. Replace them with tubular plastic types - hopefully they're still available.

Look for a bad coupling capacitor between stages, it'll be connected to a tube's grid pin, from the prior stage's plate circuit, or maybe from a coupling transformer's secondary winding.

If you can get a schematic to learn all the tube pin connections, use a DC meter set to maybe a max voltage of 300VDC, to look for the presence of plate DC on the audio output tube's grid input. There shouldn't be any plate DC on a grid input.

Also, see if you can find one of those small pen-sized audio signal generators that put out a low-voltage 1Khz signal or similar and use it to input an audio signal to the audio output tube's grid. If the stage is working, you should here the tone in the earphones or via the speaker. Then work backwards into the earlier tube stages in a similar fashion to determine the point at which the weak introduced audio signal does NOT produce an amplified signal that's been passed through the previously-checked circuits. You've then located the faulty circuit; check and/or replace the components connected to the tube's pins.

You could also use any low-voltage audio source as test signal to introduce into the circuits being tested. Even a signal from a more modern-day AM-FM tuner's RCA audio output to an external amplifier can be fed into a circuit to be tested; just use an adequate length of shielded audio cable, with a suitable plug on the source-end, preparing the "probe" end by removing the outer jacket and braid for an inch or so, and maybe soldering a tinned test-tip end about 1/4" long on the copper conductor inside. Then you'll be ready to introduce audio wherever it's needed to evaluate circuit operation in the Hallicrafters...

[SethK]

Thanks Richard, I appreciate the help.

Seth