[rtm00x]

Simple shorting plugs....take a cheap RCA interconnect, (the ones that get shipped with most consumer products), cut the connectors off leaving about 1/2" of wire. Strip the wire and twist the center signal wire and ground wire together. That shorts the input to ground when you plug the sorting plug into the preamp's input RCA's.  You only need to short the pair of input RCA's that are switched into the circuit. 

Once the inputs are shorted and the preamp powered,  set your DMM to frequency.  Put the red lead into the output RCA to make contact as center pin and the black lead to the outside of the RCA to ground.  Make sure that the red lead does not contact and short against the ground.  Check both R & L outputs.  The DMM will display the frequency of the hum at the output.

Cheers,
Geary

Geary, thanks for the tip on shorting the inputs, learned something today!! I made the shorting connectors. I set my DMM to hz, set red lead inside the output RCA and the black lead on the shield of the same RCA connector. I did this for all four outputs. For all four outputs I got a reading of 60hz.

Thanks for the help Geary!

[galyons]

Geary, thanks for the tip on shorting the inputs, learned something today!! I made the shorting connectors. I set my DMM to hz, set red lead inside the output RCA and the black lead on the shield of the same RCA connector. I did this for all four outputs. For all four outputs I got a reading of 60hz.

Thanks for the help Geary!

So now you pretty much know that you have a grounding issue.  Try the cheater plug to lift the ground at the IEC plug.  Remeasure.  You can measure back along the signal path to try to find where it is being introduced. Start at the inputs and work toward the outputs.  You could also have a bad solder joint in the grounding circuit.  I would re-flow the solder joints.  It's quite a bit of work, but sounds like it arrived to you with the issue.

Grounding issue can be tough to track down.  Be deliberate and careful.

Cheers,
Geary

[rtm00x]

So now you pretty much know that you have a grounding issue.  Try the cheater plug to lift the ground at the IEC plug.  Remeasure.  You can measure back along the signal path to try to find where it is being introduced. Start at the inputs and work toward the outputs.  You could also have a bad solder joint in the grounding circuit.  I would re-flow the solder joints.  It's quite a bit of work, but sounds like it arrived to you with the issue.

Grounding issue can be tough to track down.  Be deliberate and careful.

Cheers,
Geary

Geary,

I disconnected the internal ground connector that was connected to power cord, turned the amp on and tested just the same as I had before. This time it started a at 60hz again but once I touched the black lead to the ground of the rca connector it went down to 0hz within a couple of seconds. Is the chassis ground the problem here?

[galyons]

Geary,

I disconnected the internal ground connector that was connected to power cord, turned the amp on and tested just the same as I had before. This time it started a at 60hz again but once I touched the black lead to the ground of the rca connector it went down to 0hz within a couple of seconds. Is the chassis ground the problem here?

Where was the red lead?  The only chassis grounds are on the back to the grounding post. The rest are circuit grounds.

[galyons]

Are all of the standoffs plastic?  Hard to tell in the picture.

[rtm00x]

Where was the red lead?  The only chassis grounds are on the back to the grounding post. The rest are circuit grounds.

Geary,

The read lead was in the output rca connector I was testing at the time, and black lead was on the outer ground of that same connector.

The standoffs are all plastic, although two of them arrived broken.

[galyons]

Geary,

I disconnected the internal ground connector that was connected to power cord, turned the amp on and tested just the same as I had before. This time it started a at 60hz again but once I touched the black lead to the ground of the rca connector it went down to 0hz within a couple of seconds. Is the chassis ground the problem here?

If I am understanding correctly...no hum if the AC ground is lifted?

[poty]

It seems something changes during the experiments...

[rtm00x]

If I am understanding correctly...no hum if the AC ground is lifted?

Geary,

I tested this last night, connected the clarinet to power amp with the ground connector left unconnected, the hum still came through. I wanted to ask you, how would I go about testing where the hum is originating on the board? Would I put black lead to chassis ground and red lead on component leads?

It seems something changes during the experiments...

Poty,

I can see how it would seem this way, I am kind of questioning my own results. I remember when I first got the amp, I powered it up, got the hum, but then the hum persisted after I disconnected everything. At this point the hum gradually fades away and is gone within about 10 seconds or so.

[poty]

It's been unplugged for 5+ minutes and my speakers still have the hum.

...the buzz/hum cuts off as soon as I pull the power cord out of the clarinet...

Poty, in both instances the hum fades away gradually.

I powered it up, got the hum, but then the hum persisted after I disconnected everything. At this point the hum gradually fades away and is gone within about 10 seconds or so.

Only the latest 2 descriptions are the same.
Never mind.
Having such case I'd think of several things, but I do not have the Clarinet and do not know for sure its design so can't describe the precise steps to check.
1. Check the wire from the PCB to the ground point at the back side of the enclosure. I'd also try to unplug it from the PCB and listen what will happen.
2. Check if the shells of the RCAs are isolated from the case.
3. To eliminate the possibility to induce the noise from the following gear - unplug the following gear from the mains (with Clarinet on) and listen what will happen.

Your case reminds me the bad or dry electrolytical capacitors, but then there should be no noise after powering off or unplugging from mains...

[rtm00x]

1. Check the wire from the PCB to the ground point at the back side of the enclosure. I'd also try to unplug it from the PCB and listen what will happen.
I checked the wire going from pcb earth to ground on chassis, it looked fine. Also tried unplugging the ground connector on the pcb side, the hum became louder. I also tried sanding off the black paint from the chassis where the ground is connector to expose bare metal, this did not help.
2. Check if the shells of the RCAs are isolated from the case.
The RCA's seem to have a isolation material built in, they look like they are from BOM.
3. To eliminate the possibility to induce the noise from the following gear - unplug the following gear from the mains (with Clarinet on) and listen what will happen.
The only thing I had plugged in were the shorted rca connector I had previously made, also tried with nothing plugged in.

Your case reminds me the bad or dry electrolytical capacitors, but then there should be no noise after powering off or unplugging from mains...
Any way I can test the capacitors?

Thanks for the help Poty!

[hagtech]

I should have stayed on top of this.  My guess is that you have portions of chassis that are not connected to EARTH.  The black anodize is an insulator.  I got around this by using a dremel to grind off the anodize where the "nuts" go, thereby giving a metal to metal contact when the pieces are screwed together.  Also, take a countersink to all the screw head areas and drill them out.  Doing this will make sure all the pieces connect to the back panel, which has the EARTH lug and connection.

jh

[rtm00x]

I should have stayed on top of this.  My guess is that you have portions of chassis that are not connected to EARTH.  The black anodize is an insulator.  I got around this by using a dremel to grind off the anodize where the "nuts" go, thereby giving a metal to metal contact when the pieces are screwed together.  Also, take a countersink to all the screw head areas and drill them out.  Doing this will make sure all the pieces connect to the back panel, which has the EARTH lug and connection.

jh

Jim,

The only nuts I see are the ones underneath the top cover, holding in place the transformer. Also the one that is used to hold the ground against the chassis. I sanded down the point on the chassis where the ground is connected to, the hum persisted. Should I also sand down chassis points that are making contact with the nuts used for mounting the transformer (the four of them)?

The locations where screws are used already are countersinked. I have attached some pics with some examples.

One thing I found, the pcb says the fuse should be a slow blow fuse. The fuse installed is fast acting. Does this matter?

Thanks for the help Jim!

[poty]

1. Check the wire from the PCB to the ground point at the back side of the enclosure. I'd also try to unplug it from the PCB and listen what will happen.
I checked the wire going from pcb earth to ground on chassis, it looked fine. Also tried unplugging the ground connector on the pcb side, the hum became louder. I also tried sanding off the black paint from the chassis where the ground is connector to expose bare metal, this did not help.

Well... At least you are sure the connection is working.

3. To eliminate the possibility to induce the noise from the following gear - unplug the following gear from the mains (with Clarinet on) and listen what will happen.
The only thing I had plugged in were the shorted rca connector I had previously made, also tried with nothing plugged in.

Saying "the following gear" I was thinking about power amplifier!

... the bad or dry electrolytical capacitors, but then there should be no noise after powering off or unplugging from mains...
Any way I can test the capacitors?

Do not be in haste. There are not enough evidence for the diagnosis. I'd take serious attention to the Jim's words about shielding. It was the next thing I was going to suggest after checking the power amplifier influence. At this point of experimenting there are only 3 possible source of noise left:
1. Some leakage from the power amplifier (but you tried the device with other power amplifiers with the same effect).
2. Weak tubes (SRPP schematic used in the Clarinet is more demanding to the top tube in terms of cathode-heater voltage, so some good NOS or old tubes may behave not very good at this point). (but turning the device off should eliminate low-frequency hum almost instantly)
3. External EMI stray pick up.
IMHO, the third point is much closer in explaining the device behavior than others.

P.S. By the way - is the hum depends on volume control/balance control position?

[rtm00x]

Well... At least you are sure the connection is working.Saying "the following gear" I was thinking about power amplifier! :)Do not be in haste. There are not enough evidence for the diagnosis. I'd take serious attention to the Jim's words about shielding. It was the next thing I was going to suggest after checking the power amplifier influence. At this point of experimenting there are only 3 possible source of noise left:
1. Some leakage from the power amplifier (but you tried the device with other power amplifiers with the same effect).
2. Weak tubes (SRPP schematic used in the Clarinet is more demanding to the top tube in terms of cathode-heater voltage, so some good NOS or old tubes may behave not very good at this point). (but turning the device off should eliminate low-frequency hum almost instantly)
3. External EMI stray pick up.
IMHO, the third point is much closer in explaining the device behavior than others.

P.S. By the way - is the hum depends on volume control/balance control position?

poty,

Yeah I tried it with another amplifier, I also have three different sets of tubes (rectifier & 2 12AU7's), the hum persisted with all of them. I don't think it EMI either, the owner before me had the same issue with the hum, and again I tried it in a different room with same results.

The volume/balance/select have no effect on the hum.

If you look at the last pic I posted for the side panels of the chassis, there are metal clips that used to connect the top and bottom plates to the chassis, the anodized paint underneath those clips is not yet sanded. Tonight I will sand these contact points down to expose bare metal. I am also going to pick up a 250V 1A slo blo fuse on my way home to replace the fast acting fuse that was in there.

The other thing I am going to try is reflowing the soldering joints for the rectifier tube and the 2 12AU7's, there is some residue on there, I don't know if you guys can see it in the pics I posted.

[poty]

Just in case (if you have time after you sandwork):
- With powered off device - measure the resistance between the central pins of output RCAs and their shells.
- Try to power on the device without V301 (the 12AU7 closer to the rectifier tube).
But I agree with Jim that most probable thing is lack of shielding.

[hagtech]

Are the RCA shields isolated from chassis?

jh

[rtm00x]

So sanded down contact points on case, 20 points on the case (8 on each side panel, four on the bottom panel). Made little to no difference on the hum.

Just in case (if you have time after you sandwork):
- With powered off device - measure the resistance between the central pins of output RCAs and their shells.
- Try to power on the device without V301 (the 12AU7 closer to the rectifier tube).
But I agree with Jim that most probable thing is lack of shielding.

The DMM got no reading between the central pin and the shell. If I connected one set of RCA's on the output and left the other not connected, the one left unconnected measured at about 325kohms's.

Tried powering it up without the 12AU7 closest to the rectifier, hum persisted.

Are the RCA shields isolated from chassis?

jh

The RCA's are shielded, they have a plastic shield around each RCA connector.

[poty]

So sanded down contact points on case, 20 points on the case (8 on each side panel, four on the bottom panel). Made little to no difference on the hum.

Tried powering it up without the 12AU7 closest to the rectifier, hum persisted.

Very strange... The tube should disconnect all relating to power supply from the output. The only thing that is left - EMI.

The DMM got no reading between the central pin and the shell. If I connected one set of RCA's on the output and left the other not connected, the one left unconnected measured at about 325kohms's.

If all was done to the original schematic the DMM should read around 330k regardless any connection to the output. Check R312 and all connections from the output RCAs to the board. There maybe a tiny crack in a trace or bad solder joint... Check all traces by the DMM (RCA_centers ->C303, R312;RCA_shells -> ground; R312 -> ground).

The RCA's are shielded, they have a plastic shield around each RCA connector.

As far as I can see the only connection of the PCB to the ground point is the wire at the back of the device. It is detachable, so it is easy to check if the RCA shells are isolated. Disconnect the wire (you already did so early in the thread) and measure resistance between the RCA shells and the case (where the disconnected wire go). It should be open.

[hagtech]

Wow - you seem to have us stumped.  What happens if you listen to only one channel (disconnect the other)?  Is the CLARINET plugged into same outlet box as the power amp?  Maybe a photo of the inside of CLARINET can help us look.

jh