Help needed with Harman Kardon a220 “the lute”

I have to ask as I saw your other thread, but are you using the tube shields?? They prevent stray RF and it comes through the speakers as hum so make sure they are installed.

Let's see some pics of your finished unit.

EDIT: Damn! Thinking of all kinds of stuff here! :D Are your speakers phased properly? If they are not, it will cancel out the bass and sound dull and lifeless. :thumbsup:
 
Boy, you really are a novice. :D The heaters in those two tubes (can't make out V1 or what) are tied together in a series connection and are part of the phono preamp circuit. It is normal.

It sounds like you just need to pay attention to your hum issue. Tubes are fine! :thumbsup:

I would disconnect that ground wire. It isn't doing any good.

If it is quiet with the volume all the way down, a capacitor on the mains side isn't going to help as it sounds to me like you have a microphonic tube. Pull the 12AX7 between the 7408's and have a listen. Turn the volume up and down and check it out. You can also swap those 12AX7's around.
What about the 12au7? Could that be a problem? If I pull the 12ax7 or the 12au7 I get no output. Could I try putting a 12ax7 in place of the 12au7 and see what happens?

With the volume all the way down, hum is gone, but there is still a crackly noise coming through speakers with varying intensity

I also noticed the hum is always in the right channel, regardless of how I turn balance knob
 
What about the 12au7? Could that be a problem? If I pull the 12ax7 or the 12au7 I get no output. Could I try putting a 12ax7 in place of the 12au7 and see what happens?

With the volume all the way down, hum is gone, but there is still a crackly noise coming through speakers with varying intensity

I also noticed the hum is always in the right channel, regardless of how I turn balance knob
Ok, I discovered that the tung sol tubes increase the hum dramatically.
 
Ok, I discovered that the tung sol tubes increase the hum dramatically.
The unit now refuses to play anything through it... nothing can come easy can it

Anything through it other than hum, that is. And I’m using the tuner input, not the phono, so it’s not because I removed those tubes
 
Last edited:
Don't put a 12AX7 in the place of the 12AU7. Although they both have the same pinout and are preamp tubes, the gain on them is different. The 12AU7 is a medium MU twin triode tube with a gain factor of only 20. The 12AX7 is a high MU twin triode tube with an amplification factor of 100.

Only swap between the 12AX7/ECC83's.

Do not swap them when it is powered up. Most of us assume that most people know this. :rolleyes:

You didn't destroy your amp, but its possible you took out one of the resistors connected to the 12AU7 tube socket. Easily checked with a meter.
 
Don't put a 12AX7 in the place of the 12AU7. Although they both have the same pinout and are preamp tubes, the gain on them is different. The 12AU7 is a medium MU twin triode tube with a gain factor of only 20. The 12AX7 is a high MU twin triode tube with an amplification factor of 100.

Only swap between the 12AX7/ECC83's.

Do not swap them when it is powered up. Most of us assume that most people know this. :rolleyes:

You didn't destroy your amp, but its possible you took out one of the resistors connected to the 12AU7 tube socket. Easily checked with a meter.
The thing is okay, but it is stuck on the tuner stereo input for some reason.
 
Don't put a 12AX7 in the place of the 12AU7. Although they both have the same pinout and are preamp tubes, the gain on them is different. The 12AU7 is a medium MU twin triode tube with a gain factor of only 20. The 12AX7 is a high MU twin triode tube with an amplification factor of 100.

Only swap between the 12AX7/ECC83's.

Do not swap them when it is powered up. Most of us assume that most people know this. :rolleyes:

You didn't destroy your amp, but its possible you took out one of the resistors connected to the 12AU7 tube socket. Easily checked with a meter.
For some reason the audio works if I plug it into the tape out, but not any other inputs
 
It's backfeeding from the tape output. It will do that. Did you say the selector switch is stuck on the tuner input? Is the knob slipping or is the selector switch jammed? If it's jammed, you will need to check out why. If the knob is slipping, I hope it hasn't destroyed the knurling on the plastic knob.
 
It's backfeeding from the tape output. It will do that. Did you say the selector switch is stuck on the tuner input? Is the knob slipping or is the selector switch jammed? If it's jammed, you will need to check out why. If the knob is slipping, I hope it hasn't destroyed the knurling on the plastic knob.
Well it’s not stuck actually I thought it was but I realized I accidentally put the audio into the tape out and not the tuner, so I’m still getting no audio. I probably blew a resistor like century tek said
 
You will have to use a meter to be sure, but I did that once with a tuner preamp which used the 6EU7 (same as a 12AX7) and fried the 1M plate resistor. It is something you should check especially if it was powered up at the time
.
 
You will have to use a meter to be sure, but I did that once with a tuner preamp which used the 6EU7 (same as a 12AX7) and fried the 1M plate resistor. It is something you should check especially if it was powered up at the time
.
Would a blown resistor be open or something else?
 
Alright most resistors seems fine if not all slightly out of spec, but not out enough to seem like they’re blown. There is one blue gray orange silver that’s supposed to be 68k, but it is reading 238 ohms. Could this be the culprit?

There was another resistor of the same color measuring something similar. Am I reading the bands wrong somehow?

The resistor in question pictured below

Is there any other places I should look for failure other than resistors?
 

Attachments

  • 76CC22E0-DFD3-4238-933D-5212D028C754.jpeg
    76CC22E0-DFD3-4238-933D-5212D028C754.jpeg
    71.9 KB · Views: 26
Last edited:
Out of spec resistors will usually measure high. You would be looking for an open resistor.

Blue, gray, orange is 68K.
 
Out of spec resistors will usually measure high. You would be looking for an open resistor.

Blue, gray, orange is 68K.
Well then I should probably replace those 68ks since they’re measuring at 230 ohms. I couldn’t find an open resistor but I’ll double check. What is the wattage of the resistors?
 
Out of spec resistors will usually measure high. You would be looking for an open resistor.

Blue, gray, orange is 68K.
Is it really necesarry to disconnect the legs? Most resistors were measuring close to what they were supposed to be. C4 was shorted to c3b for a second a while ago when the unit still worked. Could that have blown something? The unit still worked After so I just replaced c4 and didn’t think about it
 
Only if you don't how to read the results! :cool:
Well, I figured it out. When the phono preamp tubes are installed, the unit is extremely noisy, but plays. When I remove the phono preamp tubes and try to play something through the unit, it won’t play. The two phono preamp tubes were tung sols, so would replacing those lower the noise? And what brand should I replace them with for lowest noise?
 
Back
Top Bottom