Strange Grounding Problem - My best guess: ghosts.

[spice house]

Longtime reader - I've looked around quite a bit but i cant seem to find this problem.

My grounded grid pre has a ghost infestation all of a sudden. It stopped working for no apparent reason (no part of the system was changed), and i discovered this: For some reason, if i ground the PCB, it stops passing audio.

I had a star grounding centered on the PCB so that the mains earth, - from each rca, and the PCB GND were terminated at one point and it worked great for a year+.

Now, if either the mains earth or the - from the RCAs touch any part of the GND on the pcb, the thing stops passing audio. It becomes silent, though the tubes are still lit.

Since there is no GND to the PCB there is a hum, though the circuit still passes audio. The hum starts when the unit is plugged in and gets worse when it is turned on.

The wierd bit is this is only sometimes! it mysteriously stops, then its ok, then it works for a week, then today, if it's grounded it won't pass signal. Normally if the ground becomes connected it will increase slightly in volume and the hum will disappear.

I have tried: rewiring the whole unit, changing the tubes, eliminating all extraneous components so there is only the PSU and in and out from the pcb connected directly to RCA, plus the grounding changes above.

Is there something I'm missing?

 

[M Gregg]

Spice,

Can you post a schematic?

Regards
M. Gregg

 

[atmasphere]

My grounded grid pre has a ghost infestation all of a sudden. It stopped working for no apparent reason (no part of the system was changed), and i discovered this: For some reason, if i ground the PCB, it stops passing audio.

I had a star grounding centered on the PCB so that the mains earth, - from each rca, and the PCB GND were terminated at one point and it worked great for a year+.

Now, if either the mains earth or the - from the RCAs touch any part of the GND on the pcb, the thing stops passing audio. It becomes silent, though the tubes are still lit.

Since there is no GND to the PCB there is a hum, though the circuit still passes audio. The hum starts when the unit is plugged in and gets worse when it is turned on.

The wierd bit is this is only sometimes! it mysteriously stops, then its ok, then it works for a week, then today, if it's grounded it won't pass signal. Normally if the ground becomes connected it will increase slightly in volume and the hum will disappear.

I have tried: rewiring the whole unit, changing the tubes, eliminating all extraneous components so there is only the PSU and in and out from the pcb connected directly to RCA, plus the grounding changes above.

Is there something I'm missing?

This sounds like an RF problem, and it can be related to the grounding scheme.

Here's how you set up the ground:

Set the chassis ground to the middle pin of the IEC power connector- if you have a 3-wire power cord, this is the green wire.

The circuit ground cannot be at chassis ground! Keep it separate and star it to the power supply, which is *not* chassis ground, BTW.

At the signal output, install a resistor about 10 - 100 ohms between the circuit ground and the chassis ground. You could also install this resistor between the power supply ground and the chassis ground.

This will take care of ground loop problems, getting shielding from your chassis and problems with RF (as long as you have your layout ducks in a row). Your house AC wiring, as long as it is to code, will take care of everything else just fine.

There are other ways to execute grounding. They all have their strengths and weaknesses. This is the one that I have found to work the best. You may find that the circuit tends to be a little quieter, blacker backgrounds, that sort of thing.

 

[spice house]

This sounds like an RF problem, and it can be related to the grounding scheme.

Here's how you set up the ground:

Set the chassis ground to the middle pin of the IEC power connector- if you have a 3-wire power cord, this is the green wire.

The circuit ground cannot be at chassis ground! Keep it separate and star it to the power supply, which is *not* chassis ground, BTW.

At the signal output, install a resistor about 10 - 100 ohms between the circuit ground and the chassis ground. You could also install this resistor between the power supply ground and the chassis ground.

This will take care of ground loop problems, getting shielding from your chassis and problems with RF (as long as you have your layout ducks in a row). Your house AC wiring, as long as it is to code, will take care of everything else just fine.

There are other ways to execute grounding. They all have their strengths and weaknesses. This is the one that I have found to work the best. You may find that the circuit tends to be a little quieter, blacker backgrounds, that sort of thing.

Huzzah! a 33 ohm resistor did it. Its dead silent now. My mistake, the actual mains earth is from the 3 prong to chassis, doh. This works because the resistor stops a ground loop... somewhere? oh well either that or magic. Thank you

 

[spice house]

Sadly, the same grounding scheme that worked yesterday does not work today.

Again, now it only works un-grounded, hum and all

 

[spice house]

any suggestions to find/fix this strange problem? I want to move this unit into my studio, but i cannot until the ghostbusters make their move.

 

[pedroskova]

I take it that you've measured voltages throughout your circuit when it's working properly and when it is on the fritz...and they're the same?

If your psu is on the pcb and grounded to star ground, you might try grounding from the first psu caps to chassis and not from the star ground connection. Keep your safety (earth) ground as is.

 

[Soonerorlater]

Intermittent faults are very hard to solve. They are usually caused by changes in temperature. Something might be shorting, something might be going open. Changing grounding schemes might be solving the symptons not the problem. IMHO a thorough quality inspection is in order

 

[spice house]

Oh great! Thanks for all the comments, I'll probe around to the best of my abilities, hopefully I'll discover something that will translate into "oh replace input cap" or something of that nature

Also, I just noticed the amp is acting quite strange along with the pre... When the the preamp is properly grounded, the way it was originally, and 'not passing audio' like i said, i checked the other inputs of my amp and ALL of them, except where the actual RCAs its plugged in to, have very low music playing, slightly distorted, at about half volume. whaaaat? Lifting the ground on the preamp pcb, this phenomenon goes away.

I hope that made sense, but it must be passing audio if i can hear the music on other (wrong) inputs? very strange! i feel foolish i didn't notice this earlier. I didn't think to use a multimeter on the output of the preamp, doh!

Hopefully this rings a bell for someone, and thank you for your time

 

[dmills]

Methinks the thing is oscillating up in the RF somewhere.

Getting quiet distorted audio in strange places can be diagnostic of this as the following stage demodulates the RF (And grounded grid glass fets can make wonderful RF amplifiers).

One thought, that feedback taken from the output divider will be susceptible to phase shifts caused by cable capacitance and if the thing is only marginally stable that might be sufficient to case it to take off.

Try sticking a modest value resistor in series with the output connector to help isolate the feedback network from the cable, also possibly a few pf of cap placed carefully may help matters.

Regards, Dan.

 

[bill_a]

Hmmm, are we looking at an off-line power supply (no transformer)? Your schematic is not clear about where the AC is coming from. Off-line PS's are creepy, terrible and dangerous.
Bill

 

[atmasphere]

The power supply also looks like it is a bi-polar design. So there is a very good chance you are exceeding the cathode/filament arc-over voltage when the preamp is first turned on. After warmup there is probably no problem. However, if a tube develops a short due to this issue, quite often it can be intermittant!

 

[pedroskova]

I don't think it's anything anything related to the signal path -- I'd have to believe in both channels being effected equally at once. On the other hand, both channels are fed by a single psu and both channels share a common grounding scheme.

 

[M Gregg]

spice,

Quote:I hope that made sense, but it must be passing audio if i can hear the music on other (wrong) inputs? very strange! i feel foolish i didn't notice this earlier. I didn't think to use a multimeter on the output of the preamp.

I assume you have tried the pre amp disconnected from the power section? Also that the fault is no different with or without the power amp connected?

Have you tried a temporary power supply from another amp for the preamp section to see if it is the supply section? (with the Pre amp PSU disconnected).

This should give you a starting point ( Is it the PSU or the preamp)

Regards
M. Gregg

 

[M Gregg]

spice,


I guess you already know this, however I do not know if this is part of a larger installation that contains a power Tx.

Please note that if you do not have a Tx on the supply any fault current is only limited by the supply. Also the chassis Gnd will carry any leakage current through the caps.

Regards
M. Gregg

 

[spice house]

Oh my lots of great responses! thanks guys I'll try to answer them all.

First, today is a good day since it has decided to work properly! It may have had something to do with leaving it on for 2 days? But sometimes after i disconnect everything and take it apart and put it back together it works right away after that.

"Hmmm, are we looking at an off-line power supply (no transformer)? Your schematic is not clear about where the AC is coming from. Off-line PS's are creepy, terrible and dangerous.
Bill"

I have a power transformer that supplies 200ac and 12.6v ac to the power supply.

" The power supply also looks like it is a bi-polar design. So there is a very good chance you are exceeding the cathode/filament arc-over voltage when the preamp is first turned on. After warmup there is probably no problem. However, if a tube develops a short due to this issue, quite often it can be intermittant!"

I have replaced the tubes. The unit sometimes works immediately, if it has been disconnected, and is working now after being on for 2 days or so.

" I don't think it's anything anything related to the signal path -- I'd have to believe in both channels being effected equally at once. On the other hand, both channels are fed by a single psu and both channels share a common grounding scheme. "

This is true!

"I take it that you've measured voltages throughout your circuit when it's working properly and when it is on the fritz...and they're the same?"

I will try to do this and post my results. its working right for once so i can get this data.

"I assume you have tried the pre amp disconnected from the power section? Also that the fault is no different with or without the power amp connected?"

I don't have a another power supply to try it with. I ordered another amp to just to be sure, so ill try that as well. I'm going to take the multimeter and measure the output now that it's working.

"Methinks the thing is oscillating up in the RF somewhere. "

If it were oscillating this wouldn't be intermittent?

Thanks for all of your help! I will return with measurements + data. I wish the thing was more consistent with working/not working, hopefully i can trace a cause.

 

[spice house]

It is working:

120vac from mains

I made a mistake the transformer actually supplies 220ac

The outs from the PSU are 222 (-) and 226 (+) also 12.08 for the tubes.

Each tube is getting 12.1 v

the outputs register about 3v

I turned it off, waited til i hear no audio. power back up - still works. tried unplugging it too, still works. tried taking out the tubes and putting them back, still works. tried replacing the tubes with others, still works.

I imagine the only way i could get it 'not work' is to leave it off and cold for an extended period of time... before i test that, is there anything else i should measure while it works?

 

[atmasphere]

I would check the DC resistance of the grounds of the circuit to whatever is ground in your power supply. Similarly I would check the DC resistance of the B+ and B- points to the outputs of the supply. In all cases this should be nearly zero ohms.

It sounds to me like you have a bad connection somewhere. I am thinking its in the power supply somewhere- maybe the AC to the transformer is getting interrupted?

 

[spice house]

I would check the DC resistance of the grounds of the circuit to whatever is ground in your power supply. Similarly I would check the DC resistance of the B+ and B- points to the outputs of the supply. In all cases this should be nearly zero ohms.

It sounds to me like you have a bad connection somewhere. I am thinking its in the power supply somewhere- maybe the AC to the transformer is getting interrupted?

resistance between psu ground and circuit ground is 0

resistance between chassis gnd and and psu gnd is 33, though idk if that matters.

I have double checked ac - tx connectivity and rewired it, so there shouldn't be any trouble there.

I can't find any info on where the b+ and b- points are

220vac psu input is 0.05 ohms on one side and the other is 80ohms to gnd

the 12.1vac psu output is is 8ohms to gnd

shooting in the dark here, though i suppose these should be all ok since its working? hopefully when its not one of these measurements will be out of wack.

 

[M Gregg]

spice,

I take it that the heaters are on when it fails? They do not get dim or go off when it fails?

Regards
M. Gregg