s-20r band 4 problems

[kc2etm]

hi i got an old s-20r recapped it new powercord fuse etc aligned the unit all works well except
the band 4 it has intermittent crashing static without moving any controls and it is very weak
compared to the other bands any suggestion of where to start looking
thanks
fred

[KC4VWU]

Man, I really wish I could get around to fixing up my S-20R's. I have 2 and have 0% accomplished except that they have been disassembeled and look really sad sitting on the shelf. Really neat looking rx's when fixed up and restored!

The static crash noises could be attributed to dirty bandswitches/controls, flakey tube(s). Clean all the switches meticulously and carefully with proper cleaner; don't just saturate them, that's risky business. Tube testers are great to have, but don't put 100% faith in them; they are just a small part of troubleshooting routine. You should have a scope to help see where the static problem begins in the rx chain.

Ditto for the low sensitivity, although many will argue that those as well as many of the older, lower end BA rx's, will have lower sensitivity on the upper bands.

There are probably a few of the OT's out there who may know of specific problem areas/ fixes for that rig.

Good Luck getting the old gal up and running 100%!
 
Phil 
 

[VA3AEX]

To add to Phil's suggestions I'd also suggest that you check voltages particularly the B+ on the RF tube where mine experienced problems (no voltage/bad solder joint here).

My S20R dates from '39 and I use it to monitor 80M activity when working on the many 'projects' I have on the go.  Selectivity/sensitivity is good with a simple dipole strung across the basement shack, but I haven't gotten around to a fix for the restricted non-HiFi audio.  Ideas?

Alex 

[WQ9E]

For band 4 crackling, C5 is also a possible issue but the band switch is most likely.  There could also be buildup on one of the trimmer caps causing leakage, adjusting the trimmer will usually impact the crackle and make it easy to identify.

A weak 6K8 will be most evident on the highest frequency band so substitute that if you can once you cure the crackling.  Some 6K8 tubes are very microphonic and can cause unusual noise.

Finally, all of the simpler general coverage receivers are not going to be at their best on the highest frequency range.  Sensitivity will be less and images will be much more noticeable.  Converters (both HB and commercial) that converted 10 meters down to a lower frequency were common as were preselectors which added some hopefully low noise gain  in addition to tuned circuits which reduce images.

I have an S-20R and it plays pretty well but I wouldn't want it to be my main receiver.  I have an S-14 awaiting attention and it is an attractive little unit but probably also weak on the performance scale.

[kc2etm]

Well i found if i pull one of the 6sk7 tubes
the one closet to the main and bandspread
adjustable cap the crashing goes away
and all bands still seem to work fine without
it put it back band 4 crashing static again
 
thanks
fred

[WQ9E]

Fred,

That is your RF amp tube and the receiver should work MUCH better with a good one in place and I am surprised you are receiving much of anything without it since there should be minimal coupling from the antenna terminal to the mixer without this tube.

If it isn't the tube itself, then check the components around the first RF amp for the problem.  I don't know if you recapped the receiver but if not the cathode bypass cap for this stage could be the problem.

[kc2etm]

I have recapped the unit the thing is
it only crackles on the band 4 setting
so it must be something before the tube
in the band 4 cicuit
thanks

[WQ9E]

Fred,

You are welcome and it should be easy for you to run down this problem. 

There isn't much specific to band 4 in RF stage.  If it is the band switch itself then moving it should change the crackle and I would suspect the switch contact on the primary of the mixer coil since plate current flows through this contact.  Otherwise, suspect mixer coil itself (probably the primary winding) for this range or if it is the RF coil it could be the trimmer cap across the coil or perhaps the coil itself. 

[KM1H]

The S-20R is prone to bandswitch arcing since B+ is on it at all times. Spray cleaners usually make it worse and DeOxit will destroy it if sprayed on.

If you still get reception with the RF amp removed that is a good sign of a conductive path somewhere. I just pulled that tube in my S20R and nothing is heard except weak BCB.

Carl
KM1H

[kc2etm]

ok so i fixed th problem it was a brokn wire on t14 the fine copper wire was intrmittntly conncting disconnecting
so all is well now i would like to install a three prong cord on this unit any tips the power transformr is not marked which tab should be netrual and hot or does it matter thanks
fred

[WQ9E]

Fred,

Proper wiring is for the black lead to go through the fuse and to the power switch and then to one of the transformer primary leads and the other primary lead goes directly to neutral.  For the transformer primary it does not matter which is connected to neutral or load; the key thing is that the neutral lead should never be fused or switched.

Congratulation on finding and fixing the problem.

[kc2etm]

Perfect thank you