Vintage Hallicrafters SX-100 Mark 1 Shortwave Receiver Restore: Alignment and other tests

In this episode the Mark 1 is set up for IF alignment using a Spectrum Analyzer. The dial is calibrated using the unit's digital display. A drift test and audio tests are performed.

[@mikemurray1517]

Really interesting since I'm getting an SX-100, but your camera is terrible. Audio is mostly good - you could use a remote mic for better quality.

[@tomstafford1605]

I have an sx-100 and am having trouble getting the notch to function. I believe i have it as well aligned as I can without a spectrum analyzer. The coil has continuity (about 10 ohms), both pots are working, all resistors In the area seem within spec, I replaced all the paper and electrolytic caps, swapped out the 6c4 tube with no luck. The off switch works but doesn't affect the function. I get attenuation when I adjust the two pots and a very slight rise and lowering of signal on every rotation of the frequency coil. If you have any hints from your experience I would love to hear them. It's driving me nuts!

[@i82996]

Before the next step have you checked the resistors?

[@tomstafford1605]

@@i82996 The resistors seemed reasonably close but maybe I will go through again and verify. I have been assuming the mica style domino capacitors don't go bad but then I noticed that the two large ones that surround the depth adjustment are 7800pf! They (c103 & c104) are also not listed on the parts list 😬. I read that they are likely paper caps in a mica like package if they are this large which I wasn't aware of. I guess maybe it's time for me to build a capacitor tester. It seems bizarre that the two pots always attenuate the signal regardless of the switch shorting out the notch coil.

[@i82996]

@@tomstafford1605 the micas are micas and mine tested right on w/ no dc leakage. Will look over schem and reply 2 nite

[@i82996]

A couple more things to try before rebuilding the 2nd IF mixer box. The Mica between the mixer box shielded lead and the filter box, C31, 470 pf, was leaky in my latest set and had to be replaced. A mica high voltage cap can be replaced just to make sure, and there should be no drift effect. The R75 15k pot and the R76 5k pot with the switch should be serviced. Desolder and remove them and clean them with De-oxit. Test with a meter for a smooth increase and decrease, and for continuity/no continuity on the switch. When reinstalling, be careful not to over-torque R-75 with the attachment nut or the knob will not turn. Test as much as you can after reinstallation. I don't expect a problem with mica cap C-102 in the notch circuit. If you can measure inductance the notch coil should be between 2.5 and 3 mH. 10 ohms seems consistent with that. There should be a difference between the switch on and off. When re-installing the switch verify all wiring to the schematic. I don't know of any differences between the different Mark designs in this area, so any schematic should do. You don't know (unless you bought this one new) who has done what inside these things. If still there is no difference between on and off and no real peaks and notch valley, I would go back to the start of the alignment procedure in the book and start again with a signal generator and analog meter on test point B. I found that the directions have to be carefully done (contrary to my nature in these things) and a good analog alignment can be done of the 50.5 kHz second IF without the bells and whistles of a spectrum analyzer. Then follow the notch adjustment procedure in the book exactly. I am thinking the problem is most likely in the pots and switch as a first choice since you say the switch is not turning the notch off. When off, there is no effect at all adjusting the notch frequency. When on, the notch effect is prominent when R75 is good and adjusted properly. The notch filter is a part of the entire 50 kHz 2nd IF which begins with T3 and T4 inside the mixer box and associated caps and ends at T5 and T6 and associated caps including C37, R29, and R30. V13 is the first 50.5 kHz IF amp, and V6 is the second 50.5 kHz IF amp. The switch between USB and LSB comes before the 2nd IF and does not affect the notch which is applied in the 2nd IF signal. If nothing above works you are probably facing a rebuild of the mixer box since the IF coils for V13 are inside the box. Once you are inside that, you might as well rebuild everything inside since this is the second most difficult thing to do to restore an SX-100.

[@tomstafford1605]

@@i82996 Thanks so much for the in depth response! I realigned again and followed the notch adjustment instructions to a T with not luck. I think the core might not be moving in and out of the coil! Might be internal damage! The black core that is visible in the end of the tube isn't moving. I thought it must have been some type of cap but when when I lightly touch it it moved. I am wondering now if the shaft broke off from the core.

[@cfcreative1]

Man you so need a better camera.

[@dennyoconnor8680]

Un-watchable.