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Help with Harman Kardon A401 Tone Control Please!

justinfranx

justinfranx

Member
I recently picked up a Harman Kardon A401 to use in a stereo console project I am working on. I have some background with soldering and repairing things if I have clear instruction but have not gone deep enough into stereo gear to know how to diagnose/ trace down trouble spots. I replaced the capacitors with a kit that I got on eBay and everything seems to work really well besides the balance control.

In the center position it sounds ok but kind of lack luster. Turned to either right or left and the volume increases and the music just seems more alive but it does not isolate the speaker on the side that is selected, it seems to just kind of play a little louder on that side.

I have tried different speakers, I have tried swapping speaker wire around and anything I could think of that may change it but no luck.

Do you think the pot itself may be not working properly? Something else in the amp needs attention?? Can it just be bypassed?

I’d really appreciate any help I can get!!
 
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  • The first thing to try with something that old is to spray contact cleaner (DeoxIT or CCI) into the pots. Since you've replaced the caps you're familiar with removing the cover. Viewed from the front the five pot controls--volume, bass, treble, balance, and function are on the right of the faceplate; you have to get at them from behind so you can get the spray can's little straw into any opening possible in the potentiometer case. (I know you're only concerned with the balance control here but you might as well treat all the pots while you're at it.) Once you spray a pot good, turn the knob back and forth several times. This is standard procedure to rejuvenate an old pot especially if the audio output sounds scratchy when you turn it.
  • If the contact cleaner treatment doesn't work then other possible causes are the pot itself is bad or one or more transistors may be weak. Replacing pots and transistors requires some disassembly/desoldering and installing new parts, so of course it's up to you how far you want to go before having a professional troubleshoot the problem.
Here's an interesting article on the subject: https://www.nutsvolts.com/magazine/article/July2015_Koebel

Good luck with your nice piece of vintage gear!
 
daybreak2350 said:
  • The first thing to try with something that old is to spray contact cleaner (DeoxIT or CCI) into the pots. Since you've replaced the caps you're familiar with removing the cover. Viewed from the front the five pot controls--volume, bass, treble, balance, and function are on the right of the faceplate; you have to get at them from behind so you can get the spray can's little straw into any opening possible in the potentiometer case. (I know you're only concerned with the balance control here but you might as well treat all the pots while you're at it.) Once you spray a pot good, turn the knob back and forth several times. This is standard procedure to rejuvenate an old pot especially if the audio output sounds scratchy when you turn it.
  • If the contact cleaner treatment doesn't work then other possible causes are the pot itself is bad or one or more transistors may be weak. Replacing pots and transistors requires some disassembly/desoldering and installing new parts, so of course it's up to you how far you want to go before having a professional troubleshoot the problem.
Here's an interesting article on the subject: https://www.nutsvolts.com/magazine/article/July2015_Koebel

Good luck with your nice piece of vintage gear!
Click to expand...
I did get a CRC brand contact cleaner from Lowe’s and tried it on the pots but honestly I don’t feel like it changed any of them (still have a little scratching when turning the bass knob). I may need to invest in the good stuff and give that a try.

I’m hoping it’s not the pot itself because the unit had a broken (still operable) shaft on the treble pot and I couldn’t find one that would be a drop in replacement. This could very well be due to my lack of knowledge of where to look, key words and terms to use in the search though??

There were some transistors that came with the kit I mentioned in the post but I have yet to install them since I was fearful of installing them wrong. I need to did a little deeper and learn how to read/ install those I guess. I’ll go read that article as well. Thank you for your help!!
 
justinfranx said:
I did get a CRC brand contact cleaner from Lowe’s and tried it on the pots but honestly I don’t feel like it changed any of them (still have a little scratching when turning the bass knob). I may need to invest in the good stuff and give that a try.
Click to expand...
FWIW, the CRC's website only mentions "cleaning", while you probably need something to deoxidize. It's possible that the CRC (I assume you got CRC QD) will deoxidize as well, but it seems like they would have mentioned it, if so. One thing you can try is turning the knobs back and forth rapidly several times to see if that makes any difference. If so, oxidation is a likely suspect.
 
Very good point, I’ll get some of the good stuff on order and try the other trick as well. Any particular deoxit product I should gravitate toward or just anything that says contact cleaner?
 
justinfranx said:
I have used a contact cleaner but not deoxit brand. I may need to try the real stuff!
Click to expand...
Well I guess it'd be worth a try if you want to invest in DeoxIT, after you decide on the particular type to use for old audio gear. Plenty of discussions about that.
The main thing with spray cleaning a pot is to get the fluid inside the case so that all the wiper contact areas get sufficiently wet. Just spraying the outside or the shaft won't work. This is a challenge if there are no openings to get into; one extreme solution is to drill a small hole carefully into the case, which requires skill so that you stop as soon as the drill bit breaks through the case wall and you do not let any metal "chips" get into the case. But that technique is a delicate project in itself, best used if replacement is not possible.
Finding a replacement shouldn't be too hard as pots are fairly standard. The things to match are resistance value (ohmage), case size, and shaft length. Chances are the shaft is 1/4" dia. to fit the knob, which may be push-on or set screw retained.
Yes, you want to be sure to install B-C-E transistor terminals same as original part.
 
daybreak2350 said:
Well I guess it'd be worth a try if you want to invest in DeoxIT, after you decide on the particular type to use for old audio gear. Plenty of discussions about that.
The main thing with spray cleaning a pot is to get the fluid inside the case so that all the wiper contact areas get sufficiently wet. Just spraying the outside or the shaft won't work. This is a challenge if there are no openings to get into; one extreme solution is to drill a small hole carefully into the case, which requires skill so that you stop as soon as the drill bit breaks through the case wall and you do not let any metal "chips" get into the case. But that technique is a delicate project in itself, best used if replacement is not possible.
Finding a replacement shouldn't be too hard as pots are fairly standard. The things to match are resistance value (ohmage), case size, and shaft length. Chances are the shaft is 1/4" dia. to fit the knob, which may be push-on or set screw retained.
Yes, you want to be sure to install B-C-E transistor terminals same as original part.
Click to expand...
Luckily these do have a nice opening so I should be able to give them a nice soak without too much trouble. And I should probably go ahead and replace the transistors for good measure.

The trouble I have had finding the same pot is that they are double stacked (not sure if that’s the correct term) and small in size. The one pictured is not a pcb mount but the one I am having trouble with is, so that complicates things a little.

IMG_3658.jpeg
 
justinfranx said:
Very good point, I’ll get some of the good stuff on order and try the other trick as well. Any particular deoxit product I should gravitate toward or just anything that says contact cleaner?
Click to expand...
There has been much discussion on this in the past. Deoxit D5 is the de facto standard here (here's a page describing some similar Deoxit products). I'm not going to recount or recommend any other brands, since that's what I use and it's worked for me, but a brief search here should find some alternatives.
 
swechsler said:
There has been much discussion on this in the past. Deoxit D5 is the de facto standard here (here's a page describing some similar Deoxit products). I'm not going to recount or recommend any other brands, since that's what I use and it's worked for me, but a brief search here should find some alternatives.
Click to expand...
Can’t argue with the folks who know far more about it than I do, I knew that was the preferred brand but was impatient when getting the crc since there weren’t any other options locally. Even if it doesn’t fix the issue, it will be good to have on hand. I really appreciate you guys!!
 
UPDATE: I finally had a little time to crack the A401 open again and try the Deoxit. While I do feel that it improved scratchiness in some of the other pots, it did not correct the issue I am having with the balance control. (Middle position seems dull, turned to either side the sound comes alive but does not isolate either speaker but doesn’t sound correct enough to just leave in this position.

Next step is to try my hand at replacing the transistors in this board. If that doesn’t work, I am open to just bypassing the balance control if anyone has knowledge, tips or instructions on doing that.
 
redk9258 said:
If turning the pot brings the sound to life, it's not transistors. It's a grounding or connection issue.
Click to expand...
This is great info! I will open it back up when I get a changed and double check my solder points and connections from when I replaced the caps.

I had to remove the some wires to remove the boards during that process. Thank you for you help!
 
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