Help needed with Harman Kardon a220 “the lute”

[century tek]

Nope. It looks like (according to the schematic) they are also being used as the bias voltage to the 7408 tubes so they need to stay. A lot of manufacturers did it this way. It was to reduce hum in the magnetic phono satge.

Looks like you are going to have to check every resistor (there aren't too many in this amp) plus make sure your capacitors are all oriented and connected properly to ground (chassis).

 

[Odizzieus]

Nope. It looks like (according to the schematic) they are also being used as the bias voltage to the 7408 tubes so they need to stay. A lot of manufacturers did it this way. It was to reduce hum in the magnetic phono satge.

Looks like you are going to have to check every resistor (there aren't too many in this amp) plus make sure your capacitors are all oriented and connected properly to ground (chassis).

Every resistor is slightly out of spec, but not extremely. The only other capacitors are the three in the phono preamp, the jelly bean shaped ones. I’m not sure what to replace those with. I’m definitely going to redo some of my soldering, because it wasn’t the greatest, and it doesn’t looks very pretty, so I’d like to clean it up. I’ll check back once tubes are replaced and I redo some of my soldering

 

[century tek]

Its an on going learning experience.

 

[Odizzieus]

Its an on going learning experience.

I guess I could also try placing a capacitor between the mains. Should I try a 120v 50uF or something?

 

[s-petersen]

Not unless you want an explosion!

 

[Odizzieus]

Not unless you want an explosion!

What Value would you recommend?

 

[cademan]

None. That's not your problem. It seems that you have input issues. If it's quiet with the volume all the way down, concentrate on the 12AX7 and 12AU7 tubes between the 7408 tubes.

 

[s-petersen]

What Value would you recommend?

Later, you may want to add a safety cap, but it is not really nesessary

 

[century tek]

I think that I would also be looking in that area between the power tubes as well but make sure that you are using the tube shields, prolly check the tubes themselves and all of your ground wiring to the new capacitors. A hard-to-find hum is the one thing that I really hate chasing down in amplifiers. I can usually remedy the situation with my own stuff but its hard to diagnose over the internet.

 

[Odizzieus]

What about the random crackling noise in the speakers that is present with volume all the way down

 

[Odizzieus]

I think that I would also be looking in that area between the power tubes as well but make sure that you are using the tube shields, prolly check the tubes themselves and all of your ground wiring to the new capacitors. A hard-to-find hum is the one thing that I really hate chasing down in amplifiers. I can usually remedy the situation with my own stuff but its hard to diagnose over the internet.

The phono preamp only works with the old tubes, not the new sovtek 12ax7lps. Any idea for why this is happening?

It is in the speakers, just extremely extremely faint. The tubes glow and work in the other spots on the unit, just not the phono preamp for some reason. The old tubes work fine, but are noisier.

If I do one old tube, one new tube, the music is quiet, and sounds like it’s motorboating

 

[Odizzieus]

Honestly I think I’m just gonna sell the dumb thing. It’s starting to cost too much to repair and idk what the problem is anymore.

 

[s-petersen]

Maybe a member lives near you, and can help you with it, approximately where are you located?
You are done spending any real money on it, You should start checking some voltages, most important relating to your problem would be the 19v at C3d

 

[Odizzieus]

Alright I’m reviving this thread. So far I’ve replaced the filter capacitors twice, the jelly bean capacitors, the voltage doubler capacitors, any other electrolytics, and the diodes in the rectifier. Anyone other things that can introduce a hum into the circuit that is not related to grounding?