Refurbishing AC connections on vintage receivers

[RickR]

I have some vintage 70s receivers I would like to bring up to today's standards.

The first thing that really annoys me are the two prong AC plugs with ancient wiring. But now we have the old problem of knowing where to stop. My take is: If I'm going to upgrade the AC plug and cut a hole in the back panel to fit in a new IEC receptacle, I might as well replace all the AC wiring and connectors up to the transformer.

Along this line of thought I have come up with some ideas that need to be worked out. While this looks straight forward, I'm not an electrician, so feel free to point out possible issues.

• Replace the two prong AC plug with an IEC receptacle. Adding a ground looks like a simple connection from the IEC ground to the metal chassis. Some vintage receivers have a mega-ohm resistor (or maybe a capacitor) across the hot and neutral wires. Can/should this be removed if I have grounded the chassis to the IEC ground wire?
  
• Get rid of old female 2-prong accessory connections with the newer Nema 1-15 receptacle. Nema 1-15 has one prong (neutral, I think) that is wider than the other prong. Unfortunately, I haven't found any suitable replacements on Mouser or Digikey. I'm thinking with careful use of a dremel I might be able to safely widen the original plug, or maybe not.
  
• While I'm at it, I might just as well replace the wiring on this side of the transformer. What is the correct gauge to use for AC connections? Any ideas on single or stranded wire?
  
• Once you go this far down the rabbit hole, other things can get involved. Things like the on/off switch could be replaced but this is starting to look like a waste of time and components. Do you see other issues down this rabbit hole?

Thanks for any ideas.

 

[WaynerN]

You don't need a ground wire. It would be nice to have a polarized plug...that has the wide blade on the neutral. I'm sure the other wiring is fine. The hot wire goes to the fuse and the switch (illegal to switch the neutral). You could simply not use the convenience outlets on the back.

Your receiver was UL listed as it was built back then. It would be OK to replace the non-polarized plug with a polarized plug. any other modifications will void the UL listing of the unit. Not that anyone cares.....

My rule has always been that people that don't know what they are doing, probably should not do what they don't know. But that was the old days.....

 

[Grenadeslio]

Why? I see no advantage in adding another connection into the chain unless you're into rolling power cords.

 

[WaynerN]

Why? I see no advantage in adding another connection into the chain unless you're into rolling power cords.

The non-polarized plug is a new day problem because the neutral can end up fused and switched. I've replaced all of my non-polarized.

 

[savatage1973]

IME, an "exercise in futility", and you have to be careful to determine how the original unit was grounded. Not all are the same, so employing a separate ground can cause "issues". If the unit was built with 2-prong, non-polarized cord, it must be what the manufacturer intended. You can certainly replace the cord if it has deteriorated/damaged insulation, but I wouldn't recommend hacking into it to "upgrade" to an IEC socket, unless you do your homework on your specific unit.

If it worked for all those decades with what it has, it must have been "enough". I have a couple "monster receivers" that have pretty "pathetic-looking" plugs, but they are fine. My Krells OTOH w/IEC power connections do "like" the 10 ga power cables on them (but they draw 8A each at idle).

 

[Aerobat]

I'd just sacrifice a new extension cord with the same wire gauge and use the existing strain relief. Make sure polarity is right and you're done.

 

[ducati_EL34]

Whatever is there is what goes back in. As for switching the neutral, while it may be against code, it is not “illegal”. The NEC is only a code for minimum level of safety. It is not a guarantee against fire or shock hazard.

 

[RickR]

First, thanks for the insights into AC circuits and grounding. I'll keep that in mind.

As to the question of polarizing or not polarizing the AC circuit, a bit of history of the two prong plug might be helpful. Back in the 60s and 70s, many (most?) homes did not have 3 prong grounded sockets, nor did they have the polarized sockets, either. Two prong non-polarized plugs reached the most people. Today we have grounded and polarized sockets; so why not use them?

The other idea I see in this thread is: If it ain't broke, don't fix it. This idea coupled with the limited sonic returns for a lot of labor are wise words indeed. But I'm new at this, so I might journey down the rabbit hole a little further than most.

 

[ducati_EL34]

First, thanks for the insights into AC circuits and grounding. I'll keep that in mind.

As to the question of polarizing or not polarizing the AC circuit, a bit of history of the two prong plug might be helpful. Back in the 60s and 70s, many (most?) homes did not have 3 prong grounded sockets, nor did they have the polarized sockets, either. Two prong non-polarized plugs reached the most people. Today we have grounded and polarized sockets; so why not use them?

The other idea I see in this thread is: If it ain't broke, don't fix it. This idea coupled with the limited sonic returns for a lot of labor are wise words indeed. But I'm new at this, so I might journey down the rabbit hole a little further than most.

I am also into ham radio. Many of those receivers had no input transformer and were directly wired from the AC nonpolarized plug. I am smart enough to check that the plug is inserted into the outlet properly, so that 90-110vac does not appear on the metal chassis. That is fine by me.

My view is, I do not need to be protected from myself.

If you do not feel comfortable with mid 20th century standards, then you should probably go with a polarized plug.

 

[RickeyM]

I wish larger than stock strain reliefs were readily available. Much easier than installing an IEC socket.

 

[restorer-john]

Whilst I quite like IEC sockets, they are no safer than a hardwired lead. There are plenty of situations where I've experienced arcing from IEC plug/sockets where the two are not mated well together or they get moved slightly when pulling cables behind gear.

As for butchering the rear panel or sockets to change the old two pin convenience sockets, I think that's a totally foolish idea. It will reduce the resale value and likely make those sockets brittle or unsafe. If you don't like them, don't use them. Put some of those plastic kid-safe dummy plugs in them. They are on the back, after all. Not exactly a spot where kids are poking paper clips in, are they?

If you are worried about polarity on an old two pin lead, confirm the active and neutral and put a red marker around the base of the active pin.

Sometimes (most times actually), modifications for modification's sake are counter productive.

 

[reydelaplaya]

You just said it yourself, dude:

...If it ain't broke, don't fix it...

And John gave you the best reasons why.

Whilst I quite like IEC sockets, they are no safer than a hardwired lead. There are plenty of situations where I've experienced arcing from IEC plug/sockets where the two are not mated well together or they get moved slightly when pulling cables behind gear.

As for butchering the rear panel or sockets to change the old two pin convenience sockets, I think that's a totally foolish idea. It will reduce the resale value and likely make those sockets brittle or unsafe. If you don't like them, don't use them. Put some of those plastic kid-safe dummy plugs in them. They are on the back, after all. Not exactly a spot where kids are poking paper clips in, are they?

If you are worried about polarity on an old two pin lead, confirm the active and neutral and put a red marker around the base of the active pin.

Sometimes (most times actually), modifications for modification's sake are counter productive.

 

[blhagstrom]

I have yet to find any vintage stereo that did not work properly with a good non-polarized AC cord.

I have found “hacked” vintage stereo with big ass 3 wire grounded cables that hummed.

Neutral and common go to the same joint in the fuse panel and for AC, the electrons will “wiggle” on all the lines. Adding electron “wiggle” to the chassis is stupid. Many of the chassis panels act as RF shields. Adding a connection to the AC line to that shield just cannot be good.

I’ve been down this hell hole before and gotten ripped up for arguing against grounding the chassis so I’m done.

Hack away.

 

[bimasta]

Cutting that hole isn't easy, and can easily go wrong. And the reward is negligible. As the others say, a new plug, maybe a new cord, and leave well enough alone.

 

[hamrules]

I replaced the three outlets on my Onkyo TX-8500MKII with polarized outlets, no ground, just a two prong polarized receptacles that replaced the originals. No cutting or nothing like that. Did the cord/plug with a two prong polarized. Works fine and looks like original unless you really look hard.

 

[Pyrrho]

I have some vintage 70s receivers I would like to bring up to today's standards.

The first thing that really annoys me are the two prong AC plugs with ancient wiring. But now we have the old problem of knowing where to stop. My take is: If I'm going to upgrade the AC plug and cut a hole in the back panel to fit in a new IEC receptacle, I might as well replace all the AC wiring and connectors up to the transformer.
...

If wiring from the 1970's bothers you, you will either need to replace all of the wiring inside your 1970's equipment (it all is old 1970's wiring unless it has been replaced; the wiring in the transformer is old, too; why don't you try to replace that as well?), or you should (and this is a smarter option) just forget about 1970's equipment and go with something new enough to make you happy. (A third option, of course, is to stop worrying over something about which there is no need to worry [and leave well enough alone], but people don't tend to react well to that sort of suggestion.)

If you want "today's standards", then you need to replace everything that is old. It is easier to buy new than to replace every part of an old unit with new parts, particularly as one would also need to update the design to new standards (which means, you need the knowledge of an electrical engineer to do this). (Not to mention that "new" parts for old equipment, of course, could easily be old stock made decades ago, so you would need to avoid those if you really wanted things made to current standards.)

Frankly, I think you are wasting time and effort and resources, that, at best, will make little to no difference, and, at worst, wreck gear that is perfectly usable as is (and possibly burn down your home and kill and maim people there). Since you say that you are not trained in this sort of thing, you would be well-advised to leave it alone and not modify anything. It works with the old style plug, and has not burned down your home, right?

Of course, you can do as you please. But if you imagine replacing a power cord brings everything up to current standards, you are fooling yourself.

 

[fred soop]

Replacing with a 3 wire grounded cord now connects the chassis to earth and can cause all kinds of ground loop problems. There are ways to deal with this but it gets complex and there will probably be insufficient space in a vintage unit. Refer to https://hifisonix.com/ground-loops/

If you are keeping the 2 wire cord, the standard leakage test is to connect a 1.5 KΩ resistor and 0.15 µF capacitor in parallel. This is then connected between the chassis and a known good ground. The unit under test is to be disconnected from all other equipment except the AC line. Connect an AC voltmeter across the RC circuit. Voltage is to be read under 4 conditions - power on, power off, both orientations of the plug. Voltage is not to exceed 0.35 V RMS AC for any of these conditions or there is excessive leakage. The vintage equipment should be able to pass this test. Even if the leakage test passes, be aware that there is still no protection for a major internal fault.

I also measure voltage without the RC network and mark the plug for proper orientation.

Additionally, I remove all the line capacitors and replace with a modern XY capacitor. Digi-Key BC2379-ND, Vishay VY1472M63Y5UQ63V0 is good for both X and Y applications. Connect line to line. If the unit is a receiver that may have an outside antenna, a high value resistor, 820 KΩ, should be connected from line to chassis to dissipate any static charge from the antenna.

 

[FOH Engineer]

I'd just sacrifice a new extension cord with the same wire gauge and use the existing strain relief. Make sure polarity is right and you're done.

Common sense rears its ugly head with a direct , concise, and financially responsible answer that does the needed upgrade.

I get it, you can buy 500 dollar a foot cable and impress but in the meantime your electric utility spans perhaps hundreds of miles with the cheapest bid on cable available to arrive at your house...

 

[SoCal Sam]

I can think of only one instance where a manufacturer down-spec'd a power cord. Marantz spec'd a shockingly thin power cord in the 2600 (300 wpc) which may have contributed to the 2600's infamous transformer early death. The previous year's 2500 receiver (250 wpc) had a thick power cord similar to a small appliance. The 2500 was quite reliable and didn't suffer transformer failures like the 2600.

 

[Grenadeslio]

I can think of only one instance where a manufacturer down-spec'd a power cord. Marantz spec'd a shockingly thin power cord in the 2600 (300 wpc) which may have contributed to the 2600's infamous transformer early death. The previous year's 2500 receiver (250 wpc) had a thick power cord similar to a small appliance. The 2500 was quite reliable and didn't suffer transformer failures like the 2600.

That cord would have to be very small AWG to be the culprit lol.