AC volts on Viking Valiant chassis

[n2awa]

Full story (with photos) here:
http://www.mbzponton.org/n2awa/valiant.html#audio

Overview for this forum:
Received several reports of distorted audio on my signal peaks. I began checking the basics, and discovered some AC line voltage on the chassis ground stud! Depending on how the power plug (original two-conductor style) was positioned in the wall outlet, I measured either 31 v AC, or 61 v AC on the chassis. This explained those friendly little zaps I was getting when I touched the cabinet.

Note:
My QTH does not have 3 prong outlets so I have not changed the power plug on the Valiant as of this writing.

To (hopefully) fix the problem, I replaced the six "low"-voltage-coil bypass capacitors (C65, C66, C67, C69, C70, C71) where the AC line comes into the transmitter. They were all tied to ground (see schematic detail in link, above)If one (or more) of those ceramic capacitors was leaky, it could explain why there was some AC line voltage on the chassis.

After replacing the capacitors, I still found 20 vAC on the chassis.

My question to this group is: where should I be looking to find the remaining volts on the chassis?

Thank you

[WQ9E]

Hi Jeff,

The short answer is you need to provide a good ground for the chassis.  Even with good capacitors there will be AC appearing on an ungrounded chassis, you have 6 .005 caps in parallel which do have high reactance at 60 hertz but if your chassis is floating then you will still measure significant AC voltage with a high input impedance DMM or VTVM.  If you have a scope handy you can touch the tip of the input probe and you will find that your body has significant AC on it according to the scope.

You could have leakage from a transformer to the chassis (a good reason to always ground your chassis properly) but I imagine that your capacitors are the reason for the AC on the chassis.

If you have the chassis grounded and are still measuring significant voltage between your chassis and your reference ground then either your chassis ground or ground reference point has a grounding issue.

A basic safety rule for vintage gear is ground should be the first wire connected in installation and the last wire disconnected during removal.  This is especially important for gear like the Valiant which fuses both sides of the AC plug in its original configuration.  If you decide to keep this plug then you need to provide a very good ground for the Valiant and keep it connected, identify clearly the side of your outlet which is the "hot" or load side and which side of the Valiant plug connects to SW-7.  In this side of the plug (connected to SW-7) place the proper 8 amp fuse and in the other side place a 20 amp fuse (both standard "blow") and make sure that the side containing the 8 amp fuse connects to the hot side of the outlet.  This will offer pretty good protection against the neutral fuse blowing which creates a significant safety hazard, particularly if you Valiant is not grounded properly.  Under some failure conditions if the neutral fuse blows and the chassis is not grounded the case will rise to 120 volts AC and you could become the "return" conductor between the Valiant and a properly grounded piece of gear.

Rodger WQ9E

[WQ9E]

Jeff,

I could not tell for sure from your photos so check the following:  take a look at the wire leading from the LV transformer to the 866A cathode leads.  In earlier Valiants regular hook-up wire is used and this wire has the DC HV on it also which will cause the wire to breakdown and short to the chassis.  The result is generally a dead LV transformer.  If your wiring is original and you are using the 866A tubes then either replace this wire with proper HV wire or sleeve it with insulating tubing.  It you are not using 866A or the 3B series equivalent tubes (i.e. you have changed to SS rectifiers) then disconnect the 5 Volt leads to the LV transformer.  This is not related to your AC on chassis issue but is important for the continued health of you LV transformer!

Rodger WQ9E

[n2awa]

Thanks for your two replies Roger.

[WQ9E]

You are welcome Jeff!

It is a great transmitter and a sentimental favorite of mine since I used one as a novice. 

Rodger WQ9E