Hallicrafters S-38 - AC on Switch Bracket - Any ideas?

[W4LTM]

Hi guys,

I am just finishing up a complete overhaul on a early model S-38 and I have AC on the "Stand By" switch bracket when powered on.  This bracket is completely isolated from the "hot" chassis (I even removed the rivet and replaced the grommet).  The radio plays well (other than a little hum) and seemed to align rather easily.  I have checked all of the resistors and possibly shorted leads and have found nothing.  I have looked to the point where my brain hurts.  Any S-38 owners have any ideas where I may consider looking?  It is my hope that by fixing this problem I will fix my hum as well.  This receiver did not appear to be modified.

The late model S-38 I just finished up does not have this issue, and even a side by side "what the heck did I miss" comparison did not prove helpful.

Thanks for any assistance.

Matt

[KA8WTK]

Not that I am sure it matters, but which model is the S-38? I have every one but the "B" and there were some slight differences. Maybe I can open one up and get an idea for you.

[W4LTM]

Bill,

This is the original S-38 with no letter.  This is the early version that has the old style chassis with the cardboard bottom cover.  The one I finished the other day is also a no letter version but has the later style chassis with a metal bottom cover.

Thanks,
Matt

[KA8WTK]

  Ok...My no letter S-38 has the metal bottom. The bracket for the standby switch is mounted with a grommet to isolate it. However, there is a cap from the chassis to the bracket.
  The standby switch on this one has one side to pin 5 on the V2 and the other side is through a resistor to ground. The schematic I have shows one side of the switch to ground and the other side through the resistor to pin 5 of V2. I don't think they ever built two the same!
  My best guess, if you have not replaced all the old caps, is that the one from the chassis to the case is leaking. In this one it is the cap from the chassis to the standby switch bracket. If all the grommets in good shape, the only "path" is through the chassis to case cap.
  I realize that you may have already replaced all the caps. In that case, I need to do some more thinking and we may need to talk in "real time". You can send me a PM with your phone number and I will give you a call this weekend.

[Bacon, WA3WDR]

How much AC is on the switch bracket?  As I recall, the S-38 was an ac-dc type of receiver, basically an "All-American Five" design with the older type octal tubes and bandswitched coils and a bfo switch to put the IF into oscillation for CW reception.  I loved mine when I had one.  Do you see about 60VAC to water-pipe ground with a high-Z meter on the offending bracket?  Is it more like 120V?  If the AC plug is reversed, does that help?  If you see 60VAC, the problem could be capacitive voltage division if there are caps to both sides of the AC line.  If you see 120V to ground, and/or reversing the AC plug drops it to about zero, then it could be straight capacitive coupling from one side of the line.  You could have a shorted or leaky cap, but because it's AC, some current can flow even if the cap is good. 

[W3NP]

Here is what I would do. Make sure the set is plugged in so that the hot side of the AC line is NOT on the chassis...this is where a newer polarizied plug would be good.

Now...if you are adventurous, touch a grounded object with one hand and touch the bracket with the other  ....it you just get a tingley feeling or no feeling at all....it is just capacitive leakage...if you get zapped  , it is somehow hot with AC (I doubt if it is).

If you are not that into living on the edge, you can "flash" the bracket with a grounded clip lead, OR , better yet...and this is what I have done in these strange nagging situations, take a small value fuse, clip lead it to ground and then touch the other end to the bracket. If it blows, the bracket is at dangerous potential, if it doesn't blow, it's just leakage. Then you can replace any caps that could be causing it, or call it normal for that particular radio.

I always use an isolated and metered bench AC supply (fancy variac) when working on such devices.

[W4LTM]

Hi gang,

Thank you for the great replies.  Well, it does appear to be capacitive leakage (I do not know why I did not think and check this to begin with - prior to wasting everyone's time.  Maybe I was just thrown off because the bracket voltage echoed the main "hot chassis" so I assumed it was hot with AC.  The first thing I did before tearing in to this radio was to add a polarized plug.   All the caps are new except for a couple of domino caps (don't see too many go bad), and many of the resistors are new, etc.  The particular cap in question going from the bracket to the chassis is a .22 @ 250.

"or call it normal for that particular radio." Yup, I will say that is true and leave it at that.

Bill - This early version has one side of the switch going to ground and the other going to a resistor and then to pin 5 on V2.  Just like the schematic shows.  My later version is wired just like yours with one side going to Pin 5 and the other going to a resistor and then to ground.

Thanks again,
Matt