Got a Cornet2 with hum in both channels, then sudden low volume

0 Members and 1 Guest are viewing this topic. Read 1644 times.

MaskdBagel

  • Jr. Member
  • Posts: 31
Hoping someone can help me begin troubleshooting here. I have a Cornet2 between a MM cartridge (2.5mv, I think) and a flea watt SET amp. I noticed a week or two ago that there was a 60 Hz hum in both channels. Only noticeable if I turned way up, so I didn't mind too much. Today, though, I suddenly get very, very low volume playing records. Maybe 20-30% volume? The amp is fine - tested with my DAC - and I'm assuming for now that the turntable doesn't have anything funky going on. The sound is perfectly clear, and uniform in both channels, so I don't think I have a bad cable.
I don't necessarily expect anyone to say "Oh, yes! This specific thing!" but with the collective genius here, I'm hoping for suggestions on where to start investigating. I have a soldering iron and DVM at the ready! No tube tester, though... :)

Help?

Thanks!

galyons

  • Full Member
  • Posts: 442
Check the voltages at the test points as indicated on the board, then report back.  List which test point, the expected voltage and tested voltage.  R219L - 155V = (tested voltage).

Both channels is most likely power supply.  Can you test your rectifier tube?

Cheers,
Geary

MaskdBagel

  • Jr. Member
  • Posts: 31
I'll check. Is there any way to test the tube with a DVM? I don't know too much about tube testing.

galyons

  • Full Member
  • Posts: 442
We will get a good idea from the test points.
Cheers,
Geary

underdawg

  • Industry Participant
  • Posts: 358
sounds like a cap gave out

hagtech

Yeah, check your DC power supply voltages.  Maybe you lost a cap in the B+ circuit.

jh

MaskdBagel

  • Jr. Member
  • Posts: 31
Alright, just finished my chores for the day and opened it up. Here's what I got. Some high numbers, but nothing insane.

R107: 387 expected 375
R204L: 158.3 expected 150
R204R: 158.0 expected 150
R211L top: 354 expected 330
R211L bottom: 323 expected 300
R211R top: 353 expected 330
R221R bottom: 321 expected 300
R212L: 157.1 expected 150
R212R: 145.2 expected 150
R219L: 165.3 expected 155
R219R: 153.8 expected 155

A little asymmetrical at 212 and 219, but it's never been audibly off, so I guess it doesn't matter terribly.

Any ideas? Thanks so much for the help!

Kent
« Last Edit: 17 Feb 2013, 04:09 am by MaskdBagel »

galyons

  • Full Member
  • Posts: 442
Those voltages are fine, higher most likely due to 120V+ line voltage at your place.  Pretty much all the caps & resistors after the power supply caps are  are "dual mono", so not likely a culprit there for both channels down. 

Is the led lit? Do you have approx. 6vdc between R223 and R104?  If so...

Could be one of the 12a*7's went.  Do you have known good spares to swap out?

Could also be your cartridge. Check all the connections at the cartridge.  If a removable stylus, make sure that it is firmly seated into the cartridge body.

Cheers,
Geary

hagtech

The numbers look ok.  Must be a bad connection somewhere.

jh

MaskdBagel

  • Jr. Member
  • Posts: 31
Well, at least it was a chance to get the meter out. It had been a while. Looks like a pair of ICs was terminated badly. I think the amp got moved recently, and they must have gotten bent badly or something. Anyway, they were both wonky, which is kind of crazy, but I switched them out and all is well.  :roll:

Thanks for the input anyway.