Kenwood KW-44 hum - diyAudio
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Old 23rd April 2009, 08:30 PM   #1
DaveWM is offline DaveWM  United States
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Join Date: Apr 2009
Default Kenwood KW-44 hum

Hi folks new guy here.

I have a kenwood KW-44, recapped all the old JCP brand coupling caps (every single one tested bad). Plays great but for a slight hum.

I checked all the filter caps, they test fine (heathkit cap tester for value and leakage under rated voltage). Hum is low and not related to volume. I looked at the power supply B+ using a scope. there was quite a bit on the supply to the audio transformers. the rest of the supply was very clean.

When I got it some one had tacked on a couple extra 100uf caps on the factory doubler caps. It helps some but still there. It 1st I thought the orig were bad and had just been left in the circuit.

Anyway I was able to get a schematic for a KW-30 which is very close. I checked the voltages, the B+ was a little low not much but low by maybe 10-15v. Next I checked the AC current of the HV winding, per the kw-30 it should have been about 245ma, well it was 580ma. hmmm. I find it hard to believe it should be more than double, but with out the correct schematic...

The bias is about 14v and it gets it thru a cathode resistor that feeds all 4 of the 6BM8 tubes. the bypass cap is ok (40uf) Resistor is a 1k power WW and it checks out. the phono preamp tubes get there filament supply from this as well. they seem fine, no shorted filaments. The Grids get a bias from a tap out of one of the filaments. the schematic shows a cathod voltage of 20v (so each of the 12AX7 tubes gets 10v. one of the 12AX7 pin 9 (center tap filament) is used to get 5v that is fed to the grids thru 470k resistors.

The actual cathode voltage is about 18.5v and the tap pin 9 is a little over 4v.

As a test I pulled on of the phono preamp tubes out (not the tapped one, that would get rid of the grid bias) and the cathode bias shot up to about 24v with 0 at the grids (connected thru the 470k, and part of a filament to ground). Sound seemed fine on the FM tuner, secondary HV current dropped down to around 260ma (what the schematic called for on a KW-30). the ripple with the reduced load was about cut in half.

I am hoping someone out there may have comments on the current draw/and maybe know where I can get a schmatic

Thanks

Dave
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Old 25th April 2009, 08:04 PM   #2
DaveWM is offline DaveWM  United States
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Still fighting it, not sure if my orig take on what the current should be, I think I was reading wrong based on the scheamatic, 92ma DC was what I get when I read between the cathode and the + fo C302. I replaced the two diodes in the power supply, and added back all the extra filter caps (2*100uf for the doubler section and a 100uf at the 236v to ground that feeds the OPT's.

Voltage is better 231v with all the added filter caps, but I think I am masking the real problem (it still hum more than it should).

I am going to try looking at the scope again, compare the B+ ripple to a non-humming set (again). argh...

I have attached a schamatic of a KW-30, very close the only diff seems to be the KW-44 has an extra IF stage and a MPX noise filter (the noise filter in the schematic is very simple, the one on the KW-44 is much more complicated has its own small pcb with a bunch of discreet parts). It really threw me for a while as the KW-44 has 16 tubes and the schematic list tube V1-V16 but there is no V5. So I guess the KW-30 came after the KW-44 and they just did not bother to change the tube numbers.

Oh well I tried to attach the schematic, too big 500k I will see if I can figure out how to make it smaller (pdf) and add later.
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Old 26th April 2009, 10:19 PM   #3
DaveWM is offline DaveWM  United States
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Join Date: Apr 2009
well think I got it, seems the gain of the triode fo the 6BM8 is picking up hum from the wiring between the grid and the output of the preamp. I have another receiver which has no hum, I look them over closely and noted that the VC on the quiet one (allied 333) is after the 12AX7 preamp, so when VC is set low the grid of the 6BM8 is effectively grounded, however on the KW-44 the VC controls the the grid of the preamp 12AX7 so any noise after it gets to the 6BM8! ugh. I bet all the Kenwoods that have this design suffer from low level hum, Not a problem as soon as the signal come up some but with the VC all the way down you will hear it. I was sure it was in the power supply, but I confirmed it was not by just grounding the grid of the 6BM8 triode and dead silence (as it would be if they only put the VC there instead way upstream of all the tone controls).

Anyway I hope this helps someone that has hum problems. Ground that grid and see if it goes away before worrying so much about filter caps (that is if they seem ok as this one did).
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