Building Foreplay 2.0 - issues at the no-power checkout stage

[htsmatt]

Hi everybody. I'm trying to finish up the Foreplay 2.0 I've had for years, but keep putting aside for other stuff....

Anyway, I'm at part 6 of the assembly process, the pre-power-up checkout.

I'm getting good resistance values everywhere, except:

1) I'm getting infinite resistance (no continuity) at terminals 3, 8 and 13. These terminals are continuous with the plate the amp is mounted and assembled on. The manual says there should be continuity (no resistance) between these terminals and the ground buss. I'm not sure how the ground buss makes contact with the plate (and therefore the terminals). Nothing connects to the terminals, except the green wire from the power cord that connects to terminal 13.

2) I'm getting 55KW (and slowly slowly climbing) at B9; the manual calls for 68KW.

Any help figuring this out would be much appreciated. I'm not too hot on the theory.

Cheers.

 

[JC]

OK, now I'm going completely from memory, here, so please take what I say with that in mind!

As far as your first question, I think the important thing is to make sure the chassis plate is attached to safety ground, which I seem to remember as tag 13.  So, if you have good continuity from the round ground prong of your AC plug to the chassis plate, you are safe.

Circuit ground will attach to the plate, either at the uninsulated RCA connectors, or by attaching circuit ground to safety ground at tag 13 if you have insulated RCAs.

As to your second question, as long as that reading is that high and rising, I think you're good to go.  As I recall, there is a capacitor across a 68k resistor at that point, and your meter is trying to cope with a capacitor charging up from the meter itself, so it may or may not take a very long time, indeed, for it to charge fully depending on your meter.  In the meantime, the meter "sees" the capacitor as another resistor in parallel with the 68k, and it is measuring the combination of the two current paths.

Anyone who still has a FP2 manual handy, please chip in here and correct me! 

 

[Grainger49]

The instructions for the FP 2 do not ground the ground buss to the chassis plate.  You have to put in a jumper from terminal 13 to (I believe) terminal 14 (should be part of the ground buss).

Then, after the jumper, you will read continuity to the top plate from all these terminals:

T3, T7, T8, T10, T11, T13, T14, T16 & T20

 

[htsmatt]

I think I understand: The FP2 instructions don't SAY to connect the ground buss to the chassis plate (and since I have insulated RCAs, the ground buss currently ISN'T connected to the chassis plate). The check-out procedures of the instructions, however, are written as if the two were connected (by a jumper or by uninsulated RCAs). So there's a little glitch in the FP2 instructions (if your FP2 has insulated RCAs). OK, I can live with a little glitch!

Since I have the insulated RCAs, I need to put in a jumper from terminal 13 to the ground buss (you're right, Grainger, it's 14, but there's already such a huge bunch of stuff joining at 14, I may run it to another point on the ground buss).

I don't have terminals 16 and 20! Or at least I don't know I have them. This kit has three strips of 5 terminals each, numbered 1-15 in the instructions.

Yes, Jim, I have good connection between the round prong of the AC plug and the chassis. I think I understand what you're saying about the second question too.

Thanks much to you both!

 

[Paul Joppa]

I'm going to try to recall the history here; the above advice is correct.

The very earliest Foreplays had uninsulated RCA jacks, which provided the signal ground to chassis plate connection. When we went to insulated RCAs, I left the chassis unconnected to the signal ground to interrupt any magneticly-induced ground loop. Somewhere in that time frame we went to 3-wire cords and safety-grounded the chassis plate. But this prevents the chassis plate from being an efficient ground plane for RF interference and electric field-generated noise. Eventually we figured out that the safety-ground loops were not a big deal as long as everything is powered from the same power circuit, so connecting the signal ground to chassis ground (T14 to T13) is almost always quieter.

 

[htsmatt]

Well, I made that 13-14 jump, everything checked out, and we're rockin' now! It sounds great. Thanks to all for the prompt and expert advice! Matt.

 

[JC]

See, you were basically done, and didn't know it!  Good for you!

The connection between circuit ground and chassis ground (14 to 13) that you made is basically optional.  As Paul Joppa notes, most of the time, it is quieter to have these two points connected.  But, by no means is this always the case.  It depends basically on what else is hooked up to the FP2, and how that other equipment does its grounding scheme.  In the archives of the original forum on Audio Asylum, I'll bet you can find at least a week's worth of reading on the subject!  I recall that in the system I used the FP2 in, a little 10 Ohm resistor between 14 and 13 was the quietest solution, a little trick I learned from 50's-era tube hifi gear.  The important thing was to make sure of the safety ground connection to the chassis, since that is, in fact, a safety issue with equipment with exposed metal parts and AC mains running around inside. 

Enjoy!

 

[Grainger49]

I agree with JC, except, this connection is to "safety ground."  That makes it mandatory if it doesn't increase noise.  It can save your life. 

In only a very few cases does it increase noise.  Try with an alligator jumper first.

If the noise increases try inverting the power plugs of your source devices before making a final decision.

 

[htsmatt]

Well, as I said, I already made the 13-14 jump. I soldered it. Not having any hum or noise problems but I'll keep those ideas in mind in case I get some noise. Thanks again everybody.

 

[JC]

You know, I'm probably a bigger proponent of safety than many, since I came up during a time when electrical devices weren't particularly safe, and I did a few years at commercial installation where safety grounds were a matter of code.  But, I'm afraid I'm going to have to take exception to this.  While I agree that tying safety ground and circuit ground together on the FP2 is a "nice to have", I'm not at all sure that it is any very likely safety concern to leave them separate.  The FP2 has a transformer to isolate line Voltage from the rest of the circuit.  The AC line is not attached in any way to anything other than the primary of this transformer.  The only exposed metal that is accessible during normal operation is the chassis plate and hardware, the transformer case, and the outer shells of the RCA connectors.  Of these, only the RCAs, if they are the insulated type, would not be already safety grounded.

Now, I'll admit that there may conceivably be some fault that could make the RCA shells "hot" from the AC line, but I think the possibility is so remote as to be negligible.  That would mean that circuit ground would have to come into contact with the "hot" side of the AC line without the fuse or other parts burning.  I have not been able to postulate such a fault that any other piece of gear with AC going in and RCA connectors on the outside wouldn't also be potentially subject to.

I really don't mean to be argumentative here, but we can only make a piece of gear with line Voltage running into it so "safe".  I believe the FP2 achieves that level of safety regardless of the 14 to 13 jumper.  Beyond that we would need to enter the realm of low Voltage, low current gear like the "Quickie" to get any safer.

Again, and most definitely in the context of putting safety above all else, and in the interest of fleshing out the knowledge base of the new forum, I implore those with more and better knowledge to show me the error of my ways.