SX-42 abnormal bias 7F8 local oscillator/mixer

[WB4AQL]

Using a poor schematic, I`m trying to get this thing working. With band switch in 540 to 1620 kc position I`m getting -2.5 VDC which is correct, even though sensitivity is reduced. Band switch in 1.62 to 5 mc position I get -5.0 VDC. Any band switch above that I get O VDC. Receiver is non-functional on those bands.

Does anyone know how bias voltage is supplied. I know the SX-42 is notorious for bandswitch problems, but this one seems to be in very good condx. The receiver was restored by a reliable previous owner who says it was working FB before he sold it.

Any ideas would be appreciated!

[w3jn]

It means the oscillator isn't running, or you're killing the oscillator by attaching your probe.

A better way to tell if the osc is running is with a scope.  Just get the probe in close proximity to the plate circuit.

[WB4AQL]

Thanks Johnny. I`ll try to get the scope on it this weekend. I`m feeling a little more confident about probably having isolated the stage. I had another "new" tube to try, but no joy.  Other pin voltages on the 7F8 seem to be OK, so maybe something in the surrounding resonant circuitry.

73, Michael

[The Slab Bacon]

42s and 62s are a bit of a pain in the ass to troubleshoot with the schematic. (I have owned several and repaired many) there are a bunch of subtle circuit and schematic changes. they had a handful of circuit changes and never publicized them very well. I have had 42s that looked more like 62s (as far as the circuitry goes) and vise versa.

Be sure to recap it, and check out ALL of the resistors, you will definately find quite a few bad ones. Also check the RF choke that isolates the front end filament circuit from the rest of the radio, I have seen a few of them that were open.

DO NOT SPRAY ANYTHING OILY ON THE BANDSWITCH! ! ! ! YOU'LL BE SORRY

[WB4AQL]

Thanks Slab. Yep. I`m finding out a bunch of variations. I`m actually working with 2 schematics trying to make sure I`m on the right track. I will definitely look at the RF choke because even the BC signals I`m hearing are not near as loud as they should be. Radio in great shape overall and has nice potential for good AM audio, plus it looks cool.

Thanks for the headsup on the bandswitch. Learned that the hard way long time ago!

Michael  WB4AQL

[The Slab Bacon]

Mike,
       I sent a mint cosmetic condition one literally "up in flames" about 20+ years ago by spraying the band stitch with greasy TV "tuner cleaner"    


The current one that I have now doesn't match up to any of the schematics I have or have seen. I use them as as a basic map and go the rest of the way by "flying by the seat of the pants"   

[The Slab Bacon]

If all of your resistors in the circuit check out ok but the biass is low and the plate voltage is high, You could have a tooob with low emission.

[WB4AQL]

Yeah, I don`t know if the 7F8 is as notorious as the 6C4 for ones that won`t work in certain oscillator circuits, but having been working before I got it, a bad tube would make a lot of sense. OTOH, over the years it seems tubes are often the most reliable components in our vintage stuff...

Michael

[The Slab Bacon]

OTOH, over the years it seems tubes are often the most reliable components in our vintage stuff...
Michael

I have found over the years that bad tubes are a lot less common than other component failures. I always tell newbies that the tube tester should be the LAST piece of test equipment you should reach for. If you have a bad tube, when you do your voltage tests, everything will be way off. Keeping in mind that circuit component values were designed to work with properly performing toobz.

[KM1H]

The 7F8 can have short life when driven hard by leaky caps but is not a PITA like the 6C4.

Having worked on dozens of 42/62's over the decades I'll agree they can be a PITA.

With the 2nd RF amp switched out for the lower 2 bands that could be a place to look for trouble in the switch. Replacing a wafer is a huge PITA and usually winds up with the customer not wanting to spend the money and I often wind up parting the sets out.

Carl