[rtm00x]

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

Poty,

I tried again, I am only getting around 330k if I have one set of outputs connected. Should I be measuring imprudence one DMM when checking from 312 to output RCA's? I did disconnect the ground from chassis and measured from rca shells to case, it did not give me a reading indicating there was no connection between the two.

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

Connecting only one channel does not help reduce the hum, it is still present. I do have it connected on the same outlet as the power amp. Jim, any thoughts on the hum/buzz coming through C101? I ofcourse can't hear it, but can feel it if I put my finger on C101.

Today I went ahead reflowed all the solder joints on the bottom of the board (the side that is facing up towards top of the case). I also cleaned both sides of the board with 91% isopropyl, the hum is still very much there. One thing I noticed is that the transformer is not connected to both "S" terminals, it does not have any wires to connect to it. I did check the transformer connections against the info that is on the board, looked good. Also, there are points on both sides of any given part to solder, should I put some solder down on both sides or just the back of the board?

BTW. there is a lug on the parts list. I couldn't find the part using the part number. I can't find a lug of any sort on the chassis, maybe I am not looking for the right thing though.

Appreciate you guys sticking with me on this.

[galyons]

Here is a thought....the Hammond 370BX is electrostatically shielded.  There is a "barrier" shield that is between the primary and the secondary windings.  The electrostatic shield keeps primary noise out of the secondaries. The shield is grounded, via a gray wire, that is usually in one of the end bells.  You can bring this wire out to your star ground at the chassis grounding point. You may have to solder an extension piece of wire to do so.  If it is grounded to a transformer mounting screw, it maybe creating a ground loop or not grounding at all.


Cheers,
Geary

[poty]

...I am only getting around 330k if I have one set of outputs connected. Should I be measuring imprudence one DMM when checking from 312 to output RCA's?

Sorry, cannot understand "measuring imprudence one DMM".
Both output "sets" are connected together, so if something (in your case - power amplifier) is connected to one "set": the input resistance of the connected device (in your case - power amplifier) is applied in parallel to the output resistance (R312). The result: you measured the input resistance of the next device, not the R312 when something is connected to the either output. You should check why you don't see the R312s on the output when nothing is connected to both sets. One "leg" of the R312 should show zero Ohm to the ground, the other - to the center of the output RCA (channel-to-channel of course).

I did disconnect the ground from chassis and measured from rca shells to case, it did not give me a reading indicating there was no connection between the two.

OK, the question is lifted.

Also, there are points on both sides of any given part to solder, should I put some solder down on both sides or just the back of the board?

Usually if you put enough solder to one side you have visible amount of solder from the other side.
Comparing your first and last photos I have a feeling that the 2 big white capacitors at the left (on the first photo), between the 12AU7 sockets, is soldered to heating ground, not to the ground plane (I mean - there are holes for them in the PCB and it seems nothing is soldered into them). Could you check the assumption?

[rtm00x]

Sorry, cannot understand "measuring imprudence one DMM".
Both output "sets" are connected together, so if something (in your case - power amplifier) is connected to one "set": the input resistance of the connected device (in your case - power amplifier) is applied in parallel to the output resistance (R312). The result: you measured the input resistance of the next device, not the R312 when something is connected to the either output. You should check why you don't see the R312s on the output when nothing is connected to both sets. One "leg" of the R312 should show zero Ohm to the ground, the other - to the center of the output RCA (channel-to-channel of course).OK, the question is lifted.Usually if you put enough solder to one side you have visible amount of solder from the other side.
Comparing your first and last photos I have a feeling that the 2 big white capacitors at the left (on the first photo), between the 12AU7 sockets, is soldered to heating ground, not to the ground plane (I mean - there are holes for them in the PCB and it seems nothing is soldered into them). Could you check the assumption?

Poty,

So I checked both R312's, got 0ohms on one end against shell of RCA, and around 330k with other end against the center of RCA.

The big capacitors are correctly connected, I checked all four. their leads are bent around their big bodies and connect to correct points on the pcb.

Here is a thought....the Hammond 370BX is electrostatically shielded.  There is a "barrier" shield that is between the primary and the secondary windings.  The electrostatic shield keeps primary noise out of the secondaries. The shield is grounded, via a gray wire, that is usually in one of the end bells.  You can bring this wire out to your star ground at the chassis grounding point. You may have to solder an extension piece of wire to do so.  If it is grounded to a transformer mounting screw, it maybe creating a ground loop or not grounding at all.


Cheers,
Geary

Geary,

I considered taking the transformer apart but I don't have  the means to do it at this time. Do you think this is an essential step?


Another thing I noticed. I kind of killed the right channel, there was some solder on C301R that looked a bit dull, this was on the side of the board that had parts on it. So I thought to reflow it, well long story short the right channel stopped working. I did eventually get the right channel working again. But while it was down I found that the balance knob doesn't do much.

So the right channel was down, I turned the balance knob all the way to the right, the left channel was slightly lower but was still playing at a fairly decent volume. I would imagine turning the balance knob all the way one way should kill the channel on the other side, is this correct? Any ideas on this? I did look over the solder joints on the balance knob, it looked good.

[galyons]

Geary,

I considered taking the transformer apart but I don't have  the means to do it at this time. Do you think this is an essential step?

The 370BX is a premium Hammond PTX.  The electrostatic shielding is part of the premium and very beneficial in eliminating noise/hum from the line.  There have been several occurrences, on the circle, of taking the shield's ground wire to the star ground eliminating hum.

The end bells come off pretty easily. Just carefully remove the long machine screws.  Be sure to get all of the washers back in the correct place.

Cheers,
Geary

[hagtech]

I'm not seeing anything obvious.  Yes, the grey wire inside the transformer connects to one of the mounting screws.  This can be made to connect to chassis.

You earlier said you did some "finger" tests.  You feel vibration on one of the capacitors?  Is the vibration also on the power transformer?  Does the hum/buss level change depending on where you touch the chassis?  This is one way to track it down.

jh

[bregez]

Here is a long shot, but check those 12AU7 tube sockets.  The super thin gold plating in those sockets has been known to delaminate from the substrate causing all sorts of noise issues that are difficult to trouble-shoot.

[rtm00x]

The 370BX is a premium Hammond PTX.  The electrostatic shielding is part of the premium and very beneficial in eliminating noise/hum from the line.  There have been several occurrences, on the circle, of taking the shield's ground wire to the star ground eliminating hum.

The end bells come off pretty easily. Just carefully remove the long machine screws.  Be sure to get all of the washers back in the correct place.

Cheers,
Geary

Geary,

Went ahead and did as you recommended, took off the bells, found the gray wire, connected it to another wire using a butt connector and attached it to chassis ground (where other ground wires are connected to). This did not help with the hum.

I'm not seeing anything obvious.  Yes, the grey wire inside the transformer connects to one of the mounting screws.  This can be made to connect to chassis.

You earlier said you did some "finger" tests.  You feel vibration on one of the capacitors?  Is the vibration also on the power transformer?  Does the hum/buss level change depending on where you touch the chassis?  This is one way to track it down.

jh

Jim,

Yeah C101 is buzzing. I can also hear it, it is really high pitched buzz. I don't hear the high pitch when I put my finger on it, it just buzzes like crazy. Honestly I think if I can figure out the source of the buzz that is affecting C101, that would be it. I did try to part other parts or parts of chassis as I was touching C101, made no difference on the hum.

Here is a long shot, but check those 12AU7 tube sockets.  The super thin gold plating in those sockets has been known to delaminate from the substrate causing all sorts of noise issues that are difficult to trouble-shoot.

Bregez,

I checked the tube sockets, they look good. I have also reflowed all solder joints on the tube sockets. No difference.

[galyons]

Geary,

Went ahead and did as you recommended, took off the bells, found the gray wire, connected it to another wire using a butt connector and attached it to chassis ground (where other ground wires are connected to). This did not help with the hum.

Jim,

Yeah C101 is buzzing. I can also hear it, it is really high pitched buzz. I don't hear the high pitch when I put my finger on it, it just buzzes like crazy. Honestly I think if I can figure out the source of the buzz that is affecting C101, that would be it. I did try to part other parts or parts of chassis as I was touching C101, made no difference on the hum.

Bregez,

I checked the tube sockets, they look good. I have also reflowed all solder joints on the tube sockets. No difference.

C101 is before the rectifier.  Ceramics sometimes do buzz at high frequencies. It is generally the laminations vibrating.  Doesn't generally happen in better quality ceramics, which the spec'd capacitor is, but....   I would switch it with C100. If it buzzes in C100, replace it.  I don't think that this is THE hum issue.  You have already determined that the hum is 60Hz.

Somewhere in the circuit a ground is not right and letting the noise into the signal path.  Are all of your test point voltages correct? 

If they voltages are all correct, then I would test all of the DC points (B+ & H+ for AC (millivolts) & 60Hz (frequency)).

Geary

[rtm00x]

C101 is before the rectifier.  Ceramics sometimes do buzz at high frequencies. It is generally the laminations vibrating.  Doesn't generally happen in better quality ceramics, which the spec'd capacitor is, but....   I would switch it with C100. If it buzzes in C100, replace it.  I don't think that this is THE hum issue.  You have already determined that the hum is 60Hz.

Somewhere in the circuit a ground is not right and letting the noise into the signal path.  Are all of your test point voltages correct? 

If they voltages are all correct, then I would test all of the DC points (B+ & H+ for AC (millivolts) & 60Hz (frequency)).

Geary

Geary, what would be the points on the pcb for measuring AC and frequency for B+ and H+?

[poty]

So I checked both R312's, got 0ohms on one end against shell of RCA, and around 330k with other end against the center of RCA.

This is wrong. The R312s is not connected by one end to the center of RCA. It makes a big loop for charging the output capacitors (which are at big voltage by the way).

[rtm00x]

I think this is a bit over my troubleshooting ability. I'm going to take this to a local stereo repair tech tomorrow, will keep you guys posted.

Appreciate all your help!!

[Speedskater]

About where is your local stereo repair shop?

[rtm00x]

About where is your local stereo repair shop?

littleton, colorado.

[Speedskater]

That's to far for me to come over. 1300 miles. Won't be in Littleton till April.

[rtm00x]

That's to far for me to come over. 1300 miles. Won't be in Littleton till April.

Appreciate the offer speedskater. I'll let you guys know how it turns out, if I get it back still with the hum I would be open to sending this out to someone who wants to take a crack at it.

[hagtech]

High pitched buzz tells me something isn't grounded that should be.

jh

[rtm00x]

I took it to the tech. He hooked it up, he said there was nothing wrong with it. I could hear the hum on his system as well, but this was if he turned the volume on the receiver he had it plugged into all the way up. He told me to to get a three prong to two prong adapter for this preamp, and anything else that might be on the same outlet. I might try this on my system. On my system I do have it plugged into a power amp with no volume control.

I really wouldn't imagine the hum to be within the operating specs of the clarinet. If the preamp his humming or buzzing, you may not notice it when music is playing but it is affecting the output in a negative way. I know enough to know that in a high fidelity system the quieter the system the better. I know that tube equipment does have more noise than SS, but its tube hiss, this is not tube hiss. So I still think there is something off here, the hum/buzz I'm hearing can't be within specs. Just not sure if it's outside the preamp or not, so going to try the two prong adapter approach tonight.

Tech said to take it back to him if I still hear the hum.

Jim, I wanted to ask you if the balance on this preamp completely attenuates one channel or if it just lowers one and boosts the other? On mine I noticed that when I have the balance say all the way to the left, the right channel is just attenuated a bit and the left is turned up, and the opposite if I turn the balance all the way to the right.

Thanks and happy holidays!!

[galyons]

My Clarinet and Cornet2 are dead quiet with 102dB/1W/1M horn speakers.  Audible hum is not normal, but if at normal listening volumes  the hum is audible, then there is an issue.  If the hum is only audible at abnormally high volumes and/or "ears to the speakers", then it may be just fine.

Cheers,
Geary

[hagtech]

The balance control is an attenuator.  Middle position has both channels attenuated 3dB.  All the way to one side is 0dB and 6dB (or something like that, I forget the exact numbers I used).  So it does sound like one channel does get louder. 

jh