after 79 trouble free hours, Cornet2 develops hum.

ScottRT · « **on:** 5 Jan 2009, 08:49 am »

I built a C2 for a fellow VA inmate, burned it in for 75 hours... it sounded great. Shipped him the unit with the same tubes it was burned in with. After 4 hours in his system his speakers made a 'pop' noise and started to hum.

I had him try all the usual stuff, but he was unable to swap in a diff rectifier tube as he had no spare for that. (he is running a Mazda 5Y3GB) He has swapped back in his old phono stage ( a PH-3) and it is working just fine.. no system hum.

After so many hours working fine during burn-in, and working fine in his system for 4 hours, I guess some component must be failing??

He is shipping it back to me, and the first thing I will swap out is the rectifier tube to see if thats all it is. If not, my next thought was to replace the electrolytics and the 2 transistors... Does that sound logical.

Any other advice on what components to trouble shoot first?

TIA
Scott T.

tubesforever · « **Reply #1 on:** 5 Jan 2009, 06:37 pm »

Scott I got pops like this from my old Prec Fidelity C7 when the wipers were loose on the tube pins. Why not have your customer use some 600 or 1000 grit sand paper, clean off the nickle tube pins, and then see if that cures the issues.

Also have them unplug and replug everything at the back of the C2, their line level amp and amplifier.

Sometimes the silly stupid stuff is what causes the most grief.

One last thought, If the ground wire from the C2 became broken anywhere along the line it could also create a minimal popping noise during the disconnect, and then you would have the hum from the ground loop. Have the customer also check that ground line from the C2 back to the turntable's ground and arm ground. Sometimes people stretch these wires so tight they disconnect at the turntable end.

Because the unit runs silent with the PH-3 I figure the tube socket or even a disfunctional tube is likely to be the culpret.

Hope this helps.

hagtech · « **Reply #2 on:** 6 Jan 2009, 03:53 am »

Pop noise would be a loose connection. Broken solder joint or loose tube socket? Something like that. Maybe loose return on the interconnects?

jh

ScottRT · « **Reply #3 on:** 10 Jan 2009, 08:43 am »

Found it... C207 has leaked electrolyte, and now leaks about 220 ma of AC current, 20 ma of which is still measureable at the output jack of the left channel.

I thought Nichicon had decent QC. I hope the other 4 of this same value don;t suffer the same fate.

Scott T.

denverartist · « **Reply #4 on:** 11 Jan 2009, 04:10 pm »

Thanks for this info. The hum on my Cornet 2 is mild (both channels), wondering if it is similar problem. How did you measure? A meter set to mA, w/ probe in the output jack?

Thanks,
Pete

ScottRT · « **Reply #5 on:** 11 Jan 2009, 10:49 pm »

My bad, Pete.... I meant to write mv, not ma. And yes, between the output jack and the ground lug.

Make sure your tranny has a good ground. If you don't run a dedicated wire, you need to grind away the anodizing to allow a ground path where things bolt together on some brands of enclosures.

Scott T.

tubesforever · « **Reply #6 on:** 12 Jan 2009, 08:42 am »

I understand the Panasonic ED 47uf 450v caps might be unobtainium....another victim of European electronic requirements. I understand they replaced them with something EU friendly of similar specs.

What I like about the ED caps are they are the quietest caps regarding distortion next to BlackGates and those are not available in this UF value and voltage.

If you are worried about the Nichicons try the Panasonics. Mine are so smooth and effortless that I can almost forget that I am listening to a system. If they are still available I should buy another 12 or so....These are awesome caps. They made a noticeable improvement over the Nichicons in my system.

Scott, the cap might have frozen during transport and cracked when heated. If so, the failure was not your fault. Electrolytics rarely fail this new. However winter has been hard and cold!

Scott which resistors did you use on the C2?

Cheers!

ScottRT · « **Reply #7 on:** 12 Jan 2009, 11:00 am »

Hey Tubes....
We talked about Kiwames', but he just purchased the standard ones off the list.

He also told me that as soon as he got the package (and he lives on the Michigan/Indiana border) the unboxed it and immediately fired it up in his system. Perhaps ther is somethinng to that freeze theory! I will tell him to give it a few hours to aclimate before putting the juice to it.

If I can find some Panny caps I will get those.

Scott T.

denverartist · « **Reply #8 on:** 12 Jan 2009, 05:49 pm »

Scott, or Mr Hagerman, what "should" I be measuring at the output RCA jacks in mV? Not zero. With inputs and output cables disconnected, the Cornet 2 idling, I get fluctuating low mV readings measured at both outputs. I would think some is normal (?) Or not.

Tranny is well grounded.

Thanks,
Pete

ScottRT · « **Reply #9 on:** 13 Jan 2009, 12:06 am »

Hey Pete:
On my good one I see 2 to 4 mv in AC at the outputs with nothing connected to the inputs.

Scott

denverartist · « **Reply #10 on:** 13 Jan 2009, 12:44 am »

Thanks for info. I am getting + and - 20 or 30mV, then it will settle down and fluctuate around +/- 10mV. But would that be a cause of mild hum? That is an electronic question where my knowledge is limited. I could just change the 47uf/450V PS cap and see if that has an effect.

Is there a way of checking leakage to filaments? That can cause hum - I know from experience. Volts are w/in spec at 6.15v

Pete

Mr Ed · « **Reply #11 on:** 13 Jan 2009, 02:51 am »

I have a low level hum also, when I measure from the ground lug to the output jack I did not get a reading.
If I measure using the output jack ground I get a very similar reading to what
Pete has described.
I am some what in the dark here also

Mr Ed

GRD · « **Reply #12 on:** 13 Jan 2009, 04:17 am »

The output will move around with changes in the line voltage before it settles back to zero. So if there's a momentary change in line voltage (say the fridge turns on), it will show up as several mv on the output and then trend back to zero as the spike drains out out of the output cap (through the grounding resistor if unconnected). Not a problem if the line voltage is the source.

hagtech · « **Reply #13 on:** 13 Jan 2009, 05:27 am »

I think it should measure about 0.1mV, but you have to use a wideband audio voltmeter (like an HP 400) with a needle. Not a DVM. It is not an easy thing to measure properly, so it is best to do by ear. Without anything connected to input, the hum should be well below the "hiss" level.

jh

denverartist · « **Reply #14 on:** 13 Jan 2009, 06:21 am »

Since most Cornet 2 owners have no hum, then those who do may conclude that we have a leakage somewhere. I know my input and output RCAs are isolated, and grounds are in order, so not sure what the source is. It's not hard to replace some PS caps, but that sort of random replacement can be tedious. Oh well, it keeps us out of the bars =;^)

Pete

tubesforever · « **Reply #15 on:** 13 Jan 2009, 08:32 am »

Hum can be caused by many causes. Ground loop hums, improperly shielded wires, cold solder joints, and worn out tubes. These and 100 other reasons can all cause hum. Proximity of unshielded cables to a big bad Hammond transformer is one that some might easily overlook!

You need to try different IC's, different equipment orientation and other things to find out why your system is humming.

If you have isolated the hum down to your C or C2, then you might want to carefully inspect the solder joints and check out the electrolytics for leakage.

From my experience, I did not like the sound of the Nichicons after my build. They sounded spitty and grainy in my system. Worse of all I could hear a noise floor that really upset me. I don't want to hear noise. I like to hear silence in between the notes.

Perhaps Nichicon's QC is slipping or perhaps they are shifting to EU requirements without issuing a new part number.

For whatever reason, my C2 did not sound a silent and quiet as ScottRT's unit with earlier Nichicon caps I heard for an entire weekend....(thanks Scott...that was one fun week end!)

I chose the Pansonic ED caps based on the recommendation of several Audio Asylum members whose tastes and recommendations match my own. (While this can be dangerous....it does allow you a base line in order to compare recommendations).

The ED caps have surpassed my expectations. The noise level is so low that it defies description. With the addition of the Russian FT-3 teflon caps in a bypass position, the overall performance of the ED caps have eliminated my desire to try motor run caps for these B+ positions. (Good thing because I might need a second case because of the physical size of some of these oil caps!)

Should everyone go out and pull their Nichicons? No way! Some Nichicons will match up with anything on the planet. It was just my Nichicons that did not produce the magic I heard at ScottRT's place. There can be lot to lot variation in production runs especially if a manufacturer is redressing the product to sell to the EU contingent.

If you have hum and can find not way to cure it, then see if the Panasonic ED 47uf 450v caps are available. These are smooth and quiet caps. Add a teflon bypass to this cap and you are in for a treat.

Cheers!

Mr Ed · « **Reply #16 on:** 14 Jan 2009, 01:11 am »

I found my source of hum.

One of my intercnects from my SUT box, now very quiet!

Cheers
Ed