Question re: grounding scheme and hum

[rude52]

Hello all,

I have had my Eros up in my system for over a year and love it.

On initial build, after one wiring mistake, all resistances and voltages were fine, and it was very quiet.  Over time, there has been a more pronounced hum that is volume dependent.  Also some RFI. And all seems like a lifted ground for various reasons.   

Resistances and voltages still spot on, so based on another post, I was checking continuity around the grounds.  Jacks are fine, ground buss is fine, but I got what I think is incorrect around the EF86s - on each, A5/A6 and B5/B6 went to ground.  I though only A2/A7 and B2/B7 were supposed to be?

Tracing it back, it seems to be from the heater voltage coming to C5 and C6.  Initially both were showing continuity with T23, which does not seem correct.  With some attempts to sort the wiring (no soldering), I now have C5 NOT to ground (or A5, B5) but still C6.

I don't read schematics, but something seems off.  Admittedly, my C5 and C6 are not the prettiest build, but before I bring out the iron, I wanted to see what is correct here.

Thanks,
Rich

[fullheadofnothing]

Does it hum without a turntable connected?

[rude52]

Hi Josh,

Yes, it hums without TT connected.  It goes down (but not away) when I attach a wire from the ground to Foreplay and/or when I touch the tube shields. 

Thanks,
Rich

[Doc B.]

Try cleaning the tube pins and RCA jacks.

[Paul Birkeland]

Yes, it hums without TT connected.  It goes down (but not away) when I attach a wire from the ground to Foreplay and/or when I touch the tube shields. 

Which Foreplay do you have? (1's and 2's and this issue might not be super surprising)

A good ground connection should be made through your RCA cables, and additionally each chassis plate is safety grounded through your power cable.  It would be a good idea to tighten up all the hardware on both pieces of equipment, just to be sure that those connections are well made.

-PB

[rude52]

Hi Paul,

It is an older lightly modded Foreplay 2.  I will check all the jacks as it would make sense that something came loose over time.  As I noted, there was no initial noise issue but it has increased over the past year.  Thanks for the advice.

I haven't had much time to work on it.  Last weekend, I tried to take Doc's advice to clean all the pins and jacks using deoxit, but then I realized he probably meant to touch up all the joints with the iron.   

Cheers,
Rich

[Doc B.]

I did indeed mean to clean the tube pins and the RCA jacks and plugs. It sounds like you have a contact that has gotten oxidized over time.

[jjvornov]

Rich

I'm running a new Eros into an original Foreplay. I can confirm a decent amount of hum. One thing I found helps was to run a jumper from the Eros ground to the Foreplay ground. I just used an empty input shield as that goes to the Foreplay ground bus.. The interconnect shields should be creating a common ground already, but it seems to help a good bit. Grounding to eliminate hum is mysterious thing.

James

[Paul Joppa]

The older Foreplays had a floating signal ground. Since then we have attached signal grounds to the chassis plate, along with safety grounds; I think about the same time we went to EIC power inlets. I seem to remember a long thread on the original Bottlehead forum about connecting the main signal ground T14 to the adjacent T13 which is a chassis ground. In most cases it solved a hum issue. So you might check whether that mod was done to your Foreplay, and if not, give it a try.

[jjvornov]

Paul-

Over the weekend I looked at the grounding of my original Foreplay to see if I could get the hum down as suggested. On mine there's a ground bus that links the chassis and all of the grounds of the RCA jacks, both input and output. I don't have the manual or schematic anymore. When you say T14, I assume you mean a terminal strip. There's not that many terminal strip lugs in my unit. There is a point where all of the grounds are connected, but that is a chassis ground. Is there another point that I should be looking at as signal ground?

James

[Paul Joppa]

I think that star ground is T13 - very close to the center of the chassis plate. So I expect yours was built using T13 instead of T14 as the star ground, and my previous note is not needed.