HQ-129X AC Ground Issue

[WE1X]

About 2 years ago I updated a nice HQ-129X that has worked brilliantly. I'm amazed how good the AM audio sounds.

A week or two ago I moved the receiver to a different shelf and began noticing what appears to be AC hum. I placed it on the bench and began poking around. The electrolytics are a few years old and I doubted that was the problem. Upon turning the power off via the bench's variac I received one mother of a shock touching the metal toggle switch. Well that seemed to be a good symptom. I measured 118 volts from the toggle switch to the chassis. This happens with the receiver on or off. Note my update included a 3-wire AC cord. Also note that when the receiver is on the rack it has ground braid from chassis to a common buss. I've not noticed any problem when touching the chassis when the rig was on. 

I tested the continuity of the AC cord's ground plug to chassis. On occasion I get continuity hence I believe there is a short in the cord. Continuity with the hot and neutral is fine. What stumps me are the following (a) why didn't the 2 amp fuse blow (there is good continuity with the fuse and fuse socket) while on the bench and (b) there are no obvious wires or components touching the chassis that shouldn't.

Thoughts?

Harry

[KA2DZT]

Not sure how the 129X is wired but,  if the receiver has by-pass caps on the AC line, remove them.  Next,  the new 3-wire cord should have been wired with the hot (black) wire going to the fuse first then the on-off switch.  The ground to the chassis and the neutral should always remain connected.

So, check the wiring, very often in old equipment the switch was wired in the neutral line.  Also check the ground connection from the chassis to the ground pin on the plug.

Seems you may have a two-fold problem.  One, you have AC leaking onto the chassis, most likely caused by leaky caps on the AC line.  Two, no ground connection from the chassis to the AC ground buss, or a ground connection that only makes contact sometimes.  Last,  make sure there is no AC leaking onto the chassis from the antenna line.

If there is AC on the metal chassis and you were touching the receiver when you touched the toggle switch on the Variac you would get a shock.  The leaky AC is probably not enough of a short to blow the fuse or the fuse is wired in the neutral line.

Fred

[PA0NVD]

Harry did measure 118VAC between the toggle switch and the chassis, so it should be a faulty (leaking) switch. There was NO voltage between the chassis and the ground.

[WE1X]

Fred and Nico,

Thanks for the replies.

Just to sum up:

(a) the is basically no continuity from ground prong on plug to chassis
(b) there is 118V on the chassis
(c) when I touch the chassis and variac toggle switch I get a good shock (with the receiver ON or OFF)
(d) there is good continuity between the chassis and all ground points, SO239, etc.

The schematic shows one lead of the traditional 2 prong AC cord going to fuse the other directly to switch.

Attached is a pic of the current wiring. Black goes to fuse. White (neutral) to wire directly to switch. Ground to chassis.

[WE1X]

Forgot to attach pic

[PA0NVD]

ok, the toggle switch at the variac, I understood at the receiver. Sory for that misunderstanding So it seem to boil down to a double fault, no continuity of the ground in the cable and a short from the live wire to the ground. Looks like the green wire in the plug is loose and touching the live prong. Isn't it better to start with a new cable and plug?

[WE1X]

Nico,

Yes. However, I'm a bit worried that somehow AC is leaking onto the chassis and where is it coming from?

If it's the Power/AF Gain switch then I assume the simple answer is a replacement.

Thanks,

Harry WE1x

[PA0NVD]

In the power cord plug, the connections are quite close and not separated by a good isolation. So if the ground wire came loose, it may touch the live wire.
Does the receiver has noise caps from the AC to the chassis like Fred suggested? I should change them, saw many leaking. In general they didn't pull the fuse, but ran hot!! (when there is a good ground to the chassis)

[KA2DZT]

Those white components that are right there in the pic are caps.  One looks like it is connected to the fuse holder, other end to ground.  If so, those are the AC line by-pass caps I mentioned in my first post.  Remove those caps.  They are most likely leaky.  Additionally you don't need AC by-pass caps

Fred

[WE1X]

Fred,

I actually removed those when troubleshooting and the voltage went up to 140.

In any event, you guys got me thinking about the switch. Upon close inspection I noticed the insulation on the wire from the neutral to the Power/AF Gain switch was cracked quite a bit. It appears to be original. I replaced it along with a new AC cord. All seems well. Chassis voltage down in the millivolts and hum seems to be much lower.


Thanks,

Harry

[W6TOM]

  You mention moving the receiver to another shelf, would that be another AC outlet it is plugged into?? If so you might also check that the output doesn't have the neutral and hot wired backwards. The fact that in the new configuration you hear an AC hum when the receiver is off indicates that this might be a possibility.

   One of the worst shocks I ever got (My Fault) was when I was wiring up and old windmill water pump that had been converted to run on AC. The motor was from the 1920's and I had shut the power off at the breaker, I grabbed the Neutral, White Wire. It was HOT, they were switched.

   After I got over the very unpleasant shock I got to thinking what had I done wrong. First I should have checked that the wiring was correct but also when the breaker was open the pump motor had an ever so slight AC hum, leakage, which I should have noticed and thought to check the wiring.

[WE1X]

Tom,

Thanks and something to consider. Yes, when I moved it from one shelf to another I plugged it into a different outlet strip.  Will check it out.

Harry