WHY would Altec Lansing place a 1.5M resistor from 2-wire Mains cord to Chassis GND?

[nevetslab]

While I have this Altec Lansing 316A AM/FM Tuner up and running, and have ordered replacement 68uF/200V Radial Electrolytic caps to replace the 200V/60-60-40uF Cap Can, I've been looking at the wiring from the AC Mains cord entrance into the chassis, the 2-wire Accessory Outlet and this terminal strip that mounts the rectifier diode to feed HT Secondary AC thru it to the cap can. And at the tie point where AC Mains ties to a wire that I think goes to the shielded power switch, it also has this 1.5M 1/2W Carbon Comp resistor to Chassis GND. The AC Plug isn't polarized, so that line could either be AC Line or AC Neutral.



Meanwhile, still puzzling over WHY there is no AC Mains Protection Fuse in line with the Primary of the power xfmr. I see a Silk Screen label on the rear left corner of the chassis above the AC Mains Outlet UL Approved 1157. With what I see inside, HOW could this product design get UL Approval with no Mains Protection fuse and one side of the AC Line connected to the chassis with a 1.5M resistor?? Is there something I'm missing here? I forgot to order in-line fuse holders last night when I ordered replacement filter caps that I might be able to install inside the 60 yr old Mallory 200V 60/60/40uF Cap Can, or mount them separately, with a plate over the capcan hole to mount insulated turret terminals for the new caps.

[Helmholtz]

If there once was a death cap, the resistor might have been used to discharge it and thus avoid shock when touching the disconnected mains plug.

[Tom Phillips]

...puzzling over WHY there is no AC Mains Protection Fuse in line with the Primary of the power xfmr..

It's interesting that you bring this up because just last night I discovered the same issue with two Heathkit Model AA-1 Audio Analyzer units. They are date coded 1959 and 1962. The schematics do not include a mains fuse and the physical units are also un-fused.
Very surprising!

[mozz]

Well, you have to look at the year these were made and UL code at the time.

[nevetslab]

While there isnt' a death cap present, I don't know the history of this unit, as it was given to me years ago, and sat tucked away until the last couple weeks. From the shabby soldering job I see on the CapCan (definitely NOT Altec's work), some maintenance work has happened on it.

I have Altec Service Doc's (dated 1962) on their 314A and 315A FM Tuner, and looking at the primary side of the wiring there, I DO see they used a 10nF/500V Disc ceramic cap across the Power Switch, but not taking it to ground. On both of these models, there's a 1M 1/2W resistor from Line to Chassis. So, that appears similar to this model I have.

I haven't removed the shield that's placed over the power switch. I suspect there may be that ceramic cap across the switch, as I see on the schematics for the 314A and 315A FM tuners. No Death Cap shown on either schematic here....but have that 1M resistor from Line to Ground. Never seen that done before.

I looked thru the rest of the Tube Amp mixers and Tube Power Amps of that era in my Altec Binder, and NONE of those have this mysterious 1M resistor between AC power Cord Line to Chassis Ground.

[Jon Snell]

The reason is quite obvious once you know.
These are radio sets and as such are connected, in the old days, to an outside aerial. the outside aerial can become charged with static electricity and the resistor discharges any static voltage to the local mains reference coltage through a resistor with a high enough resistance not to become a danger to the user.
A capacitor, like the old fashioned death cap, will not discharge static, it may store it and in time become a danger to the user.

[g1]

'Widowmaker' amps have even lower value resistors from line (depending on 2 prong plug polarity) to chassis. 68K in some of the silvertones. With cap in parallel to add to the shock potential.

[nevetslab]

The reason is quite obvious once you know.
These are radio sets and as such are connected, in the old days, to an outside aerial. the outside aerial can become charged with static electricity and the resistor discharges any static voltage to the local mains reference coltage through a resistor with a high enough resistance not to become a danger to the user.
A capacitor, like the old fashioned death cap, will not discharge static, it may store it and in time become a danger to the user.

That explanation makes sense.

Now, as I would only have this connected to a dipole antenna, made from 300 ohm antenna wire, joined to the two arms from the middle, and looks like this store-bought Dipole I have tacked to the wall above the bench, and perhaps later if I continue to use this after I've replaced the 60 yr old capcan and axial caps in the chassis, I'd be inclined to replace the 2-wire AC Mains cord with a 3-wire cord, now grounding the chassis via that cord. I'd then remove this 1.5M resistor. Does this seem like a good idea?

No space on the rear panel to install a fuse holder, so probably will place one in an In-Line Fuse Holder. Nothing like having a bag full of them in my storage locker...old inventory from BGW Systems, and NO CLUE where that bag IS. Sigh.........

As it is.....having a good deal of hum in the audio.....there's sure a lot of channels I can't tune in, so it sure needs tuner alignment. Never done that, nor do I have the 10.7MHz generator with the marker facility to do that sort of thing, as is called out in the Altec 314A and 315A service doc's. I do have Synthesizers that will tune to that with good precision, but...all that is uncharted territory for me. I'd of course do far better with some cheap used tuner of recent technology, and retire this old dog.

[nevetslab]

Yesterday the replacement electrolytic caps I ordered came in. I looked at the cap cans that CE Distribution had, and price for the 3 and 4 section 350V 50uF cans was over $40 + shipping. More than I wanted to spend on this aging rusty Altec tuner. I found some suitable 68uF/200V Radial Electrolytic Nichicon caps at Digi-Key, bought five and had them shipped via USPS to me for less than $12 total. I had some 22uF/160V axial caps to replace those 60 yr old 10uF/160V caps, so the next step was ugly.

I unsoldered the 3-section Mallory 200V-60uF/60uF/40uF cap can, and found this was not the original part. Some hack years ago had installed this, keeping the former terminals on the original cap can on the wires and somehow solder-mashed that glob onto the terminals of this replacment cap-can. Ugly mess. After I got the three wires removed, along with the GRN wire from the power xfmr that was soldered to one of the twist-lug ground terminals of the cap-can, it dropped out of the chassis, leaving me with a square hole with rounded corners, and four slots where the twist-tabs had been.

I had pondered the thought of removing the guts of this cap-can, and perhaps installing the selected 12mm dia caps I bought as replacements inside this can. Something I've never attempted before, and had serious doubt about it anyway. I pried the outer lip of the aluminum can off of the solder terminal ring, got it free, after having to break off the ground tab from the cap-can. After I managed to somehow get the phenolic insulator out without it breaking apart, I found the three wide terminals of the three caps of the capcan sitting on the surface of a potted can



So that left me with a square hole in the chassis. I looked at the twist-tab ring I had removed, inserted if from the top side to have a look at it, and decided to make use of it, if I could find any of my 16AWG copper scraps I had from BGW's machine shop (used for ground straps on our computer grade buss caps on many of our products). Found some material wide enough to make a square cover plate, so took my calipers, got the dimensions I needed to cut a copper square plate. Then, located three holes of three of the corners thru which I'd punch mounting holes for insulated solder turrets to attach the (+) lead of the radial caps I bought, with their ground leads attached to the twist tabs. Mounted the turrets, then installed that copper plate, twisted the tabs of the ring, and spent time to solder the copper plate to both the turrets as well as to the plated steel chassis (which nicely accepts solder from a very hot iron).

This morning, I brought in an in-line fuse holder that I bought from O-Riley Auto Parts, having forgotten to buy one for line voltage when I ordered the caps. Wish I had, as this automotive part has 12AWG wire crimped onto the 3AG-size fuse terminals inside the nylon housing. Removed the 2-wire power cord from the chassis, after unsoldering it from the terminal strip and accessory outlet where the power cord was attached. Elongated the strain relief hole to handle the 3-wire power cord, and installed that.



Everything powers up fine, initial DCV at turn on just about hits 200VDC then drops down to 160VDC at the first filter stage, second stage sits at 144DC, 3rd stage is at130VDC, then the last two stages at the axial 22uF/160V caps sit at 128VDC and 118VDC.

For the time being, I left that 1.5M 1/2W resistor attached to the BLU Neutral Mains wire, connected the BRN Line Mains wire to the in-line fuse and it's now connected to one side of the Pwr Xfmr Primary, the other side being the switched side side from the Neutral.

Hum level dropped, though the tuner still needs to be adjusted. That I think is beyond my skill level. Never went thru that learning curve. I suppose I could find some basic text on the internet on how to do it. I do at least have a Wavetek Synthesizer that could be dialed in to 10.70000MHz, as well as one from Analogic.

So far, this has only cost me about $25 for the 6CL8A tube, the in-line fuse holder and some new electrolytic caps. And time to cobble it together.