The AM Forum

My S40B has developed a problem that has me stumped.  This is the one with the digital freq display, Soft Rock IF and a gas regulator in the oscillator.  I undid all these mods just to make sure I hadn't created the problem.

It works fine on 80 and 40 but as I go above 10 mcs, all sigs take on a raspy tone that gets so bad on 20 meters that it is nearly useless.  I hooked up with W3GL  on 20 this afternoon but conditions were so bad that he tried CW and SSB and I cold barely understand either.

I bought a complete set of new tubes for it last week but they didn't help at all.

Whatever is happening is before the 2nd IF because even the SDR IF exhibits the same behavior with out using the internal BFO.  The oscillator output looks clean over the entire range.  When I hook a sig gen to the antenna, I get a nice clean signal at the RF amp grid but after that it looks pretty cruddy but I can't tell where the crud is coming from.

Any ideas?  My experience with receivers is pretty limited.

js

Hum modulation in the local oscillator.  I'll bet if you tune in the LO on another receiver whose BFO is on, you'll hear a nasty 60 hz FM-modulated carrier.

There's a thread in the Receivers section of the Handbook forum here on some suggestions on curing hum modulation.  May or may not work in your case.

http://amfone.net/Amforum/index.php?topic=4983.0

Not sure whether to respond here or to JN's previous article but...

As stated, the LO output looks clean over the entire range.

I don't understand your description of the filament wiring.  All filaments have one pin grounded at the tube socket and the other pins are tied to the other side of the filament winding, not to ground.

And to repeat, the raspiness only appears above 10 mcs.

js

Cancel my last, or at least part of it.  Seeing is deceiving.

Upon closer inspection, the LO is not a clean sine wave at the higher freqs and the pitch in another receiver does indeed sound like the sigs in the S40.  So the problem is in the LO but I still do not understand the filament wiring suggestion.

js

Corrosion in the socket mounting hardware can cause this if the filament ground is through the mount. The greater than zero resistance to ground feeds a 60hZ signal into the circuit.

I am well aware of where it's happening.  I read your post.  That's usually where the trouble shows up; it seldom is noticible on the lower bands.

Each socket has two terminals for filaments.  One goes to a wire, as you stated.  The other one is grounded.

How is it grounded?  Is there only ONE connection at that pin, and it goes DIRECTLY to ground, not going thru any other pins of the tube socket, and NO other components are tied to that pin?  If not, that's your goal.

Many times the cathode, or suppressor grid, is connected to that pin, and it's grounded on the other side.  So there's 60 HZ AC flowing thru that wire as well as RF, which is a recipe for FMing.

Also, try connecting a bypass cap (.01 disc) across the fil terminals, with leads as short as possible.

If all else fails, you might want to look at the dress of the filament wire, and re-route as far from any components in the grid circuit as possible.

It can look like a nice sinewave on a scope, and still have enough 60-Hz fm'ing to drive you nuts on narrow modes. Conversely, the local oscillator sinewave can contain a significant amount of distortion and still sound clean.

If you can listen to the local oscillator on a good receiver it will tell you a lot more than the scope.

Pete K1ZJH

   The same treatment I use for 1st audio stages in speech amps.

I use balanced filament wiring to the first speech amp tube usually a AX7 or AU6.
 This makes a night and day difference in High impedance mike hum pickup. Do the standard stuff. Twist two equal small insulated conductors together to run from New floating (at tube) filament pins (neither grounded to chassis) to the filament transformer winding where one side is grounded to chassis (fine). No current is required to light one tube so use tiny wire. Make sure RF, filament bypass caps at first speech amp filaments are .01 from each side to ground. Now, we cannot have a ground loop bringing 60 Cycle hum directly up through our grid leak resistor to our control grid ! This great technique came from Peter W1VZR many years ago.

down at bottom  http://hamelectronics.com/k1deu/pages/ham/transmitters/am/pages/universal_speech_amplifier.htm    regards  John             

A ground lug riveted under the socket flange or to the chassis can also be a problem. Drill out the rivet in the socket and use a real serated lug or solder right to the chassis. On the chassis mounted lug, solder over it.

Next step is to clean the both variable cap wiper contacts and solder the wiper mounting rivet to the cap frame.

Without good grounds you wind up chasing your tail.

Dont assume its not the tube either especially if its a used one, filament to cathode leakage is not unheard of.

Carl
KM1H

What Carl said. Think of it this way. The radio worked as designed when new. Now what changes over time? The capacitors leak, the resistors change in value and corrosion takes its toll. There is usually no need to redesign the radio.

Sometimes you have to re-do design and/or construction shortcuts.   I had to do this on a HRO-60 and Super Pro (re-route the fil ground wire).  Once I did that, the radios were FB OM.

This is by no means an uncommon problem, and plagued some radios even when new.  Assuming that it worked without hum modulation when new is not necessarily correct.

I have tried many of the ideas suggested and a few of my own.  So far, no joy.

A few more data points:

1. When disconnecting the leads to the filament of the 6SA7, the raspy tone continues as the tube cools down.  The pitch changes of course and then dies but for as long as I can hear it, it is raspy.  I would expect that if it was coming from filament AC, it would go away when the AC was removed.

2.  I ungrounded pin 7 and removed the filament wire from pin 8 and connected both an external 6.3V fil trans and a 6 v DC source and nothing changed.

3.  The NOS tube sounds just like the one that was in the receiver.

In the unlikely event that both tubes are bad, I will buy another but it seems like #1 above would exonerate the tubes.

js
 

BFO tube could also be at fault (with the attendant fil lead issues).

Another thing to check for is DC continuity between the grid of the tubes to ground.  Should be on the order of a megohm or more, but not much more than 5 megohms (not having the schematic in front of me, I can't tell). 

If it's happening only on the higher bands I'd suspect a DC continuity issue in the grid of the RF tube or mixer on those higher bands.