[Ferrstein]

Hello!  I have a problem with my Cornet2 that I'm hoping someone can help me with.  I first had a hum on the output when I was using a Linn Arkiv (150uV output) and an Altec 4722 step-up.  I figured I didn't need all that cartridge complexity and the Arkiv was shot anyhow, So I went with a Linn Adikt MM cart (6.5mV output).  Anyway, my Cornet2 still hums. 

So, today I took the unit apart, disassembled the transformer and ran a lead from the gray ground wire to the chasis ground lug, resoldered every connection, and twisted all the AC wires together (I twisted the two red wires on the secondary, along with the two green wires).  The unit is in a Lansing enclosure and except for some high-end capacitors it is a stock build.  I put the unit back together and the hum is still there.  I threw the mono switch, and the hum got quieter by quite a bit.  In stereo mode, the hum only comes from one channel.  I can switch or remove the inputs from the cartridge and the hum always stays in the same channel.  When I switch the outputs the sound will finally changes to the other side. 

Any ideas?  I already tried a cheater plug, unhooked the ground lead from the turntable, etc... but because the unit only hums in one channel I figured it is probably more likely a component failure than a problem with the power supply or installation.  Anywhere I should look in particular?  I have Vcaps in a few positions, and Auricaps in a couple.  Since this sounds like perhaps a component failure, should I start replacing parts?  Is there a good way to zero in on the problem?  I do have an oscilloscope, but I'm still new to electronics so any pointers in that area would be greatly appreciated.

Thanks in advance for any advice!

[thorman]

HI : I am by no means an expert but I would closely inspect my headshell wires etc and any connections from your amp to your cartridge connection..You could have a bad headshell wire or connection etc..It just seems strange that this all happened changing cartridges ..Hopefully its something simple you overlooked....

[Ferrstein]

Actually the hum was present with both cartridges.  I did try plugging this turntable into my other preamp (a Counterpoint SA5000) to see if it would hum in that situation, and all was well.  Plus switching the input wires from the turntable into the Cornet2 doesn't change what I'm hearing... the hum stays on the same side.  The only way to get the hum to change sides is to switch the output wires from the Cornet 2.

That's why it appears that the hum is actually being picked up or generated somewhere inside the Cornet2.

Thanks for your input!

[hagtech]

Ok, so the hum is on the output side?  I wonder if there is a broken shield in a cable.  Or perhaps the shield of one of the CORNET2 RCA jacks is not soldered?  Or maybe the shield contacts the chassis?

jh

[Ferrstein]

Thanks, Jim... I'll take a look.

Probably not the cables because I am switching them at the output of the Cornet2.  When I do that, the problem switches sides.  Could be something wrong with the RCA jacks, though.  It is the standard duplex unit you list for your kits.  Perhaps one of the signal straps is cracked or something.  I'll give it a once-over this evening.

[Ferrstein]

Okay, so the RCA jacks checked out...

I did find a problem, though... but I don't know how to go about fixing it.  When I checked the voltages on the board, one channel is reading only 60 volts where it is supposed to be reading 150 volts (at R204 and R212).  Both channels are reading fine at the 330v and 300v test points.  Looking at the schematic it seems like there could be a million reasons why the 150v reading might be low.  Are there any good places to start?  I am getting a lot of voltage drop across R209 on that channel (200+ volts), where R209 on the other channel only reads 120 or so volts across it.  I thought that was the problem, but when I turn off the unit and measure the resistance, both R209L and R209R show the same value. 

One odd thing... the channel with the hum is not the channel with the low voltage readings.  Strange, yes?

Thanks in advance to anyone who can point me in the right direction. 

[hagtech]

Probably a wrong value resistor or 1/2 of the tube is bad.

jh

[Ferrstein]

Thanks, Jim.  Can most of the resistor values be checked in the circuit? 

I'll try a fresh set of tubes first...

[hagtech]

Just look at the stripes (color codes).

jh

[Ferrstein]

Okay, so it was a bad tube.

Now I'm back to hum in both channels. 

I never did get an MM cart for this, which would probably solve the problem.  Right now I'm thinking my cart or tranny is causing hum.  If I unplug the inputs the hum goes away.

[tubesforever]

I suspect that the rca cable's ground is somehow touching the chassis of your Cornet2.  I had a friend use shrink wrap over the outside cover of his rca jack and that cured the problem.  Only the inside ground and center signal post need to be exposed. 

Dang RCA's.  I wish there were better connectors out there for audio that were standardized to our hobby.