Harman Kardon A-401 Treble Hissing Problem

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Hi Everyone,

In need of some help with a Harman Kardon A-401 control amplifier.

Im facing some loud hissing problem when i adjust the treble on the tone control board. The hissing sound stops when i turn the treble all the way down. Bass control is fine. Not too sure if its oscillation.

While poking around the amplifier as i can't think of anyway to fix this issue, i realise when i use my screwdriver to touch the top of some electrolytic capacitor, the hissing sound actually stops but some soft weird squeeling sound will appear instead.

Some measurements on the speaker output DC offset actually produce almost 200mv when treble turn up to middle and 10mv when treble is at minimum position.

Poweramp section is fine after a test with external preamplifier and voltages at transistor legs are quite close to the voltages specified in service manual.

Summary of things i did so far:

  • - Replace all resistor, electrolytic capacitor and wirings
  • - Replacement of all preamp transistor
  • - Swapping treble and bass potentiometer
  • - Replacing small caps(3300pf) around treble pot
  • - Replace trim pot and stabalization transistor
  • - Redid the grounding scheme

Attached is the service manual for this amplifier(only a few pages due to file size limit let me know if i need to upload more). Hope someone could point me in a new direction to make this amplifier a fully working one.

Regards,
Askae

***DISCLAIMER*** (Long story only read when you are bored :cool:)
Some background of my works so far to better understand the problems

Got this amp from the flea market for the size and look. And it powers up when testing(no speaker test). However when i brought it home and plug in to my speakers only 1 channel seem to work the other just has a loud hissing sound.

Being a naive newbie, i go ahead and order resistors and electrolytic caps base on the schematics and proceed with recapping and replacing all the resistors in the amp thinking it should solve the problem! (Big mistake :D)

After replacing all resistors, capacitors and almost all wirings i power it up this time with a dim light bulb tester. It powers up well but i face a big swinging DC offset at the speaker outputs (almost a volt at times) and i just could not get the bias to stable at 25mv as required from the service manual. The voltage is swinging too wildly and with the trimmer resistor all the way down, i could not get it below 60mv.

In order to determine if its the preamp section or poweramp section, i connected a external preamp to the poweramp section directly. To my surprise it works perfectly and sounding good too! which points my problem towards the preamp.

A check on the preamp transistor's leg voltages, the base of the transistor is putting out a astonishing 2volts! Convincing me to replace all the transistor in the preamp section from 2sc1344 to BC549(wanted BC550C but couldnt find them) After replacement, a check on all the voltage shows close values to the service manual tested recommended voltages. And i thought this is it! my first repair accomplished! But not so lucky! but at least it is semi working now!

Now the problem, after some inconsistency voltages reading DC offset off the speaker post and adjusting bias (At times 100+ mv swinging wildly at times 10mv) I begin searching again for things that need to be replace!(getting addicted :D)

While turning knobs here and there and testing voltages around PCB. I found out that the cause of the wildly swinging DC offset is actually the 50k treble pot in the tone section. Turning treble all the way down makes the DC offset stay at 10mv turning it midway increases the offset to almost 200mv. Same for the bias adjustment, 25mv when turn down almost 60mv when turn to middle.

After looking here and there and after so much time spent on repairing this amp and still cant find a solution to this, i got frustrated and go ahead and hook up a sacrificial speaker and want to see what is the sound i'm getting.
Turns out music actually plays with the treble all the way down. But its not sounding nice due to min treble. But everything else seem to work and the whole amplifier is very stable. The only issue i had now is that i cannot turn the treble up as it will cause a audible hissing sound(oscillation?) and when i turn the amplifier on with speakers connected, the start up will make the speaker produce a WONK(or is it call hum) sound. It sounded 2 times but no more after that.

With the treble section in mind, i go ahead and swap the potentiometer around (bass with treble) to eliminate the probability of potentiometer noise. But it didnt help. Next up, i replace the small capacitors around the treble pot(3300pf cap) didn't help either. The last thing i tried before asking for help is to change my grounding scheme. Didn't help either! i'm at my wits end, hope someone can help me with this issue. I think i've change almost 80% of the components in the board! And i have nothing else to replace :(

Learn alot while troubleshooting this amplifier and attempted repairs thanks to some excellent posts and advice from this forum. Now if i'm able to fix this amplifier completely, it will be a good ending! :D Now please point me to the things i do to get this amplifier working perfectly!
 

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