Ampex MM-1000 Story...

OK now. I looked at the schematics - not real hard - and went back and read some of that which you wrote. I think that you have a deeper problem....

First you need to verify that you have proper house wiring with a protective ground. Get an indicator tool or put a GFI where you plug this beast in. (GFI is a great idea)

Typically, our houses (USA) are supplied with 3 wire power. 2 phases and a neutral. Connect the phases and you have 240 volts. Any phase to neutral and you have 120 volts. Having 2 phases is (one of the reasons) why we want to power our audio gear from a single outlet. If we power form 2 outlets that are on different phases we can end up with 240 volts between them. (rare but accidents do happen)

The neutarl line in the interesting thing. In theory it goes all the way back to the generator and forms a return for the current. In practice it goes to the neutral of the nearest power transformer. It should be grounded there and also at your breaker box where it enters your house. The neutral and the protective (green) ground are tied together there and should have a real earth ground (stake, cold water pipe etc) .

The three wires to your ampex should be Hot (black), neutral (white) and ground (green). The hot is at 120 volts and carries current to the Ampex. THe neutral (in theory) should be at ground and carries the current back to the generating plant. And the protective ground should be at the potential of the ground under your feet.

How far away from your ampex is the breaker box? If it is far then the ground under your feet (and the puddle of water you are standing in) could be at a different potential than the neutral wire (death awaits) or protective ground.

Put in a GFI and test your outlet. Be sure that the neutral and ground are tied together and have 0 voltage between them at the outlet.

Sorry for rambling on here.

Neutral is not ground. Take for example the case of a broken neutral wire. Power comes in throuught the hot wire, goes through the fuse/breaker and switch and to a motor. The other wire of the motor is connected to neutral and travels through you ampex to the neutral bus and is broken at one of the connectors (bad connector for example) The neutral wire from the motor to the break is at hot potential. You touch it and you die.

No part of the transport plate should be at any voltage above the protective ground. (not the neutral) This is to keep you from completing a circuit from the puddle on the floor to the transport plate. The plate and other metal parts should be grounded. This is an important test.

It may be that the plate is at protective ground and you have an elevated neutral.

OK, I've blabbed too much....

OH, one more thing. If you have a voltage you want to test you can wire a 120 volt lamp (lightbulb) in series with your meter set on AC current. a 60 watt bulb will pass a max of 0.5 amp at 120 volts so set your meter to the 1 amp scale if it has it. (120 volts / 60 watts = 0.5 amps). This will tell you if the voltages you measure have any power behind them without blowing anything up. Of course if the bulb glows then you know. Even a few mA is too much for me.

--Ethan
 
evm1024 said:
The neutral line is the interesting thing. In theory it goes all the way back to the generator and forms a return for the current. In practice it goes to the neutral of the nearest power transformer.
Click to expand...

I never thought of this, but you make a good point. If you think about what a transformer is, it's got two magnets, (one for hot and one for neutral) and each has the high-voltage contacts coiled around one side, and the house-side contacts coiled around the other side of each respective magnet. (There's fewer coils on the house-side of the magnet, and this in turn steps-down the voltage, if my understanding is correct.) These coils never actually touch the magnet, or one another, so you're right - the neutral can't actually go back to the generator, because the electrons physically don't get there - they stop at the transformer.
 
Muck, that's a good point and I will need to keep that in mind, BUT...let me see if I can convey this succinctly because I'm sure its hard to conceptualize what I'm dealing with from a few short paragraphs rather than having the machine in front of you. Might help to have page 8-5/6 of the schematics open in front of you...

  1. Mains power comes to the MM-1000 via the mains cable plugged into the wall receptacle, and on the other end that cable connects to the Power In connector on the chassis of the MM-1000.
  2. From the Power In connector there is an internal three-conductor cable that goes to the "breaker panel", a 2U panel on the front of the machine with a circuit breaker CB1 for a power switch. The mains power doesn't go anywhere else first, or at least its not supposed to.
  3. At the breaker panel the hot line is switched by the breaker CB1, and from there (via a screw terminal block TB1) the hot and neutral lines go into the bowels.
  4. The bowels: from the breaker panel the mains power goes to the 24VDC regulated supply via J1.
  5. The schematic shows that the mains power also goes to the 39VDC regulated supplies from TB1 to J706. Well, I'm gonna have to track down just how the 39VDC supplies get their power because there are only two wires coming off of TB1 (except for two smaller wires that power the cooling fan) and they go straight to 24VDC supply input J1.
  6. From the 24VDC regulated supply mains power goes to the control relay box via J2 (24VDC supply out) to J1 (control box in).
  7. From the control relay box the mains power branches to multiple relays and on up to the transport.

Here's where it gets aggravatingly fascinating because at present:

  1. The input connector to and output connector from the 24VDC regulated supply are disconnected
  2. The inputs AND outputs of the 39VDC regulated supplies are...disconnected
  3. The inputs AND THE outputs of the control relay box are disconnected
  4. The control panel is...yup...disconnected.

So basically all the multiple connections that are responsible for feeding any AC voltage to the transport are disconnected and yet there is still AC voltage present on the harness connector J2 at the transport. In fact, I went to the trouble of measuring what the voltages are and that is listed below.

I wondered about the internal cable that goes from the Power In on the chassis up and around inside the chassis to TB1 in the breaker panel so I made a temporary cable out of romex and connected it directly to TB1 to bypass the Power In connector and the internal cable...same problem...the transport plate is charged when power is "ON" and I measure for AC voltage between the mains ground and the plate...its not that cable.

And what is *really* baking my noodle here is that if there is voltage present at the plate, then if I disconnect the hot and neutral out from TB1 and measure for continuity between the hot or neutral outputs and the transport plate there *should* be some continuity there right??? Nothing. Zip, zilch, nada.

I haven't analyzed the following data yet, but here is what I measure on the respective pins of the harness connector J2 at the transport with all the above supplies and systems disconnected from their harnessess, and just to clarify this is with J2 disconnected from the transport as well:

pin1 = 21VAC
pin2 = 21VAC
pin3 = 21VAC
pin4 = 12VAC
pin5 = 2.4VAC (I'm not concerned about this...this is likely "noise" right? This big metal hunk and all the wiring is probably a noise magnet...)
pin6 = 2.4VAC
pin7 = 13VAC
pin8 = 13VAC
pin9 = 21VAC
pin10 = 2.4VAC
pin11 = 13VAC
pin12 = 22VAC
pin13 = 2.4VAC
pin14 = 2.4VAC
pin15 = 2.4VAC
pin16 = 2.4VAC
pin17 = 2.4VAC
pin18 = 2.4VAC
pin19 = 2.4VAC
pin20 = 19VAC
pin21 = 2.4VAC
pin22 = 2.4VAC
pin23 = 2.4VAC
pin24 = 13VAC
pin25 = 12VAC
pin26 = 12VAC
pin27 = 2.4VAC
pin28 = 13VAC
pin29 = 2.4VAC
pin30 = 2.4VAC
pin31 = 2.4VAC
pin32 = 2.4VAC
pin33 = 2.4VAC
pin34 = 2.4VAC
pin35 = 2.4VAC
pin36 = 2.4VAC
pin37 = 2.4VAC

SO...bottom line:

  • Stray voltage is getting to the transport via the main harness connector J2 (when J2 is disconnected from the transport the stray voltage is gone)
  • AC current is present on multiple pins of J2 when (AFAIK) it shouldn't be
  • The machine still operates normally (when everything is connected of course)
  • There is no continuity between the hot and neutral lines coming out of the breaker panel and the transport plate when J2 is connected.

So much for being succinct. :o

This is a side-note here...chassis grounding...look at that system schematic on 8-5/6 again, and particularly in the upper left of the schematic at TB1, J1 and the Power In connector. Again, J1 is the input of the 24VDC regulated supply. TB1 is that screw-terminal block mounted to the breaker panel. Notice that the ground from the Power In connector goes to the chassis, to TB1 and branches to J1. Guess what? On my MM-1000 the ground comes from the Power In connector straight to TB1, and then from TB1 to a stud on the metal breaker panel, and then...that's it. :eek: This is not good right? The breaker panel is just a 2U rackmount panel so even if IT is connected to ground, there is no gaurantee that it will conduct ground to the rest of the chassis. It is painted metal, and a fairly heavy coat at that, and that's supposed to ground the rest of the chassis?? And, you got it right, there is no ground to the 24VDC supply. I pulled J1 apart and there is just hot and neutral lines in there even though there is a lug for a ground. So I'm thinking regardless of anything else I need to run a ground line from TB1 to the 24VDC input J1, and also star-ground the rest of the chassis including the transport plate??
 
Ethan,

  • The Ampex is plugged into a 30' heavy-duty extension cord which is plugged into a Monster PowrePro2500 conditioner.
  • The 2500 is plugged into a circuit I wired in the garage about a year and a half ago specifically for audio. At that time I ran new 12AWG romex from a dedicated breaker in the service panel, and I also tightened all breaker connects to wiring and to their respective buss bars. I also confirmed the physical ground stake and the ground and neutral busses tied together in the panel.
  • There is probably about 35' of that romex between the panel and the outlet to which the 2500 is connected. All new materials were used in wiring that circuit except for the existing breaker and ground buss connections in the panel.
  • The 2500 has an indicator for "ground ok" as well as an indicator for "wiring fault" which simply addresses if the hot and neutral lines are reversed. The "ground ok" led is lighted (good).
  • I will check again to see what the voltage potential is between the neutral and ground in the receptacle, but I seem to recall it being really really small...like 0.
  • look at my last post particularly the not at the bottom of it about chassis grounding in my MM-1000.
 
Okay...just a point of interest...There were 14 pins on the transport connector that showed any voltage other than noise, and those 14 correlate to the only pins that actually carry any function (looking again at Transport J2 on page 8-5/6 of the schematics), so that's good in a way because the problem is isolated to something in the known system and not something weird that's not supposed to be there.
 
sweetbeats said:
So I'm thinking regardless of anything else I need to run a ground line from TB1 to the 24VDC input J1, and also star-ground the rest of the chassis including the transport plate??
Click to expand...


Hey Corey,

So on your machine, there is a lug for ground on J1, but nothing connected to it from the Power cable or from TB1?

So theoretically if you connected the "ground" lug of J1 to TB1, is there then a ground connection on the other side of J1 going to the 24 v power supply? In other words, if you did connect that ground from TB1 to the J1 lug, would it go anywhere or do any good?

What are pins 3 and 4 of the other side of J1 on the control box? Dummy pins?
 
Muck,

The ground pin on J1 on the 24VDC schematic ties to the supply chassis ground, but you are correct that, at this point, there is nothing connected to it from the mains ground and hence neither from TB1.

The good it would do to connect that would be for safety, and who knows...potentially noise in the audio path since the record relays are powered by the 24VDC supply.

I was talking to my father-in-law tonight who is an EE and he believes the problem is the lack of effective grounding to all the chassis components and that the neutral line is floating. I don't QUITE understand but based on my description of what's happening he highly urged me to star ground the main chassis and the transport plate and he said he's 95 percentcertain that will also take care of my stray voltage problem.

Pins 3 & 4 on J1 of the control box arenot connected to anything. Ampex just used a 6 pin Jones connector for 4 conductors...It *does* provide some separation between the AC condoctors and the 24VDC and 24V common conductors...it may have been used for something else in another application because, remember, the MM-1000 is a modified 2 inch quad-video transport.
 
The voltage differential between the ground and neutral leg of the receptacle to which the MM-1000 is connected is 169mV.
 
I don't know... :confused:

Um...Ethan?

Anybody?

Any comments on grounding being the simple cause of this strange issue?
 
sweetbeats said:
The voltage differential between the ground and neutral leg of the receptacle to which the MM-1000 is connected is 169mV.
Click to expand...

That is not bad. It represents (more or less) the resistance in the neutral wiring to the point where the neutral and ground are joined.

At the point where they are joined the difference between neutral and ground is zero. As you mover further away from the common point the resistance of the neutral wire increases and thus a voltage divider is formed.

--Ethan

PS consult your local code and licensed electrician before even thinking about any comment I might make about household wiring.
 
Right...okay. I have a friend who's a respected electrician...I'll ask. He's a stickler for safety and proper procedure. He came over once to check some things in our service panel and it was amazing to watch how careful and methodical he was.

So...What do you think about the grounding issue?

It makes sense if that is the issue; that the neutral is floating. I mean, it would explain why there is no continuity between the neutral and hot outputs from TB1 and the transport plate and yet there is voltage.

But...wouldn't...there *still* need to be some way that the neutral leg is connected to the transport plate for it to float...man this is a brain-bender for me...ground and neutral tied together...measuring between ground and the plate, but the plate isn't connected to...*anything*...so how could there be voltage there?

If I run a dedicated ground wire from TB1 to the transport plate, is that the solution or is that justmasking the problem? *How* do I determine if there is really stray voltage or if I have something floating?
 
I'm sorry I didn't get the time to dig into this more, but I think that if you are to solve the plate voltage issue, you'll have to take bold action: either put a light bulb across it and see if there's enough current to make it glow; or "burn out the short" with a high current power supply. Or, you could just completely disassemble the thing... :)
 
Burn out the short??

No progress on this...too many questions about what kind of chassis grounding I *should* have compared to what's there which is basically nothing, and whether or not that is a causative factor.

So a low wattage 120V bulb between the plate and the household ground yes?
 
Yeah, that's how I interpreted the posts above.

Didn't you mention a friend or relative who was an electrician? Have they given a knack at it?
 
sweetbeats said:
Burn out the short??

No progress on this...too many questions about what kind of chassis grounding I *should* have compared to what's there which is basically nothing, and whether or not that is a causative factor.

So a low wattage 120V bulb between the plate and the household ground yes?
Click to expand...

Yes, I related in a previous post how we used to burn out shorts in the lab - but it's a last resort, and sometimes destructive.

Try the low wattage bulb.
 
Okay...to make a long story short (no pun intended), I couldn't oven get anything out of a 25W bulb...so for all intents and purposes there is no current there. This is supported by the fact that I measure no *voltage* when I set my cheap-cheap-cheap DMM to VAC and probe between the transport plate and househld ground...its a cheap meter and the impedance is too high I bet, but the voltage is measurable with my Fluke 85.

So I temporarily ran a ground wire from the ground buss on the breaker panel to the transport plate and guess what? The voltage drops to 0.2mV *with everything hooked up and running*!!

SO...no real current to speak of and voltage drops to basically nothing when I tie the ground to transport plate.

Safe to assume this is just a matter of a floating neutral line and the proper solution is proper grounding?

I'm going to run the ground wire to the 24VDC supply is shown in the schematics, and star-ground from the breaker panel lug to the transport plate, the console chassis, and maybe even the overbridge.

What do you think fellas?
 
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