Kenwood KA-6100 & Relay

[Bskitter]

Hey Guys.

I have an old Kenwood ka-6100 Amplifier. I bought it a while ago off ebay in very good condition.

After a year I started hearing distortion on the left channel after inspection I had found the idling current on the left channel was unstable and fluctuated.

After this I decided to recap the entire Amp and also found a transistor gone bad.

I replaced it and then all was well until recently the distortion came back and now when I push the amplifier the protection relay cuts the connection and then reconnects after a few seconds.

Could anyone suggest what could be happening and where I should begin testing ?

[coromonadalix]

while it create distortion  you have dc component going out to the speakers i presume, thats why the protection kicks

Check if you have an biais transistor or thermistors on the heatsink, i had one gone defective on a sony home theater, dc going out to the speakers ...

Check qm-17 or qm-18 and the thermistors thm1 and thm2,  you should check on page 10 your biais current adjustments

wich transistor has gone bad in the past ??

with no sound, compare the 2 channels in voltages points ...

[Bskitter]

I measured the channel voltages with no sound.

Left: -0.008 volts
Right: 0.000 volts

The transistor that went funny was Qm17. (considering it was that channel) it could of been Qm18.

Bias currents are good

20mV on both sides.

[coromonadalix]

while its on with no sound after a while, lets say 30 mins to 1 hrs,   do you have dc componet at the speakers terminals

or

maybe with a 1khz tone  at low volume  or when increasing the volume, when the distortion is heard, take measurements ...

are the 43 volts and 25 volts supplys  ok ?

Are your speakers ok, no voice coils damaged or deformed,   if you change input or output channels, is the distortion still on the same channel ?

Are the 47uf caps and 1uf values on each side changed too ?



Its what we call a direct coupled  biais / predrivers / output  amplifier transistors,  sometime  to get rid of a problem a friend had to change them all,  the problem was getting into the first stage differential transistors because of the feedback line, rm77 rm78  in this case.

Ex: qm10 qm12  has a current regulator (qm14 with dm2), dm2 serve as a current sink.

[mzacharias]

Don't rule out Cm81 100uF bipolar capacitor. Also, the relay contacts in older amps should be (carefully) cleaned or the relay replaced as this is an important source of distortion and erratic channel operation.

[Bskitter]

do you have dc componet at the speakers terminals

Could you please elaborate on this question ?

The supply voltage is 30 Volts

The distortion is only one the left channel. The distortion will on be present when the volume is very low (Note I use the computer to control the volume and leave the physical volume knob in a fixed position)

If I raise the volume the distortion will stop.

The ceramic caps were not replaced.

@mzacharias

I replaced the 100uF Cap. But it was from China so it could be a cheap copy.
 

[coromonadalix]

do you have dc voltages on the speaker terminals, when distortion appears ...

I asked to change speakers sides, to null the possibility of external or physical problems,  to be sure It is the amplifier the problem,  when its possible  i isolate the preamp section, and i short the left and right input,  to be sure the audio source is the same on the 2 channels,  after that i compare the voltages at different points or sections,   measure  the supply voltages if there is an ac voltage onto the dc voltages IE: capacitors dying or need to be changed ...  check the transistors biais,  check if there is an dc voltage going into the speakers,  if i can, i isolate the dc or anti pop speaker protection circuit, and supply a very small volage to see if it kicks correctly,  if every thing seems ok, and the preamp is isolated from the amp section, im sure the problem IS the amplifier section.

As seen on previous post, check the relay(s),  forgot to mention that, bypass them with an short wire, or a jumper, but be carefull, if you have dc component out it may damage your speakers, physical load of 4 or 8 ohms is recommended, and a scope will help

As i said the biais and pre driver and output trasistors are direct coupled,  meaning if one of theses is out of specs, a leakage , bad thermal conduction (compud grease), or a bad thermistor in this case,  you have to remove them,   check the gain (hfe) use the diode test and resistance test between base, collector, emitters pins, B-C B-E C-E and in reverse polarity, check visually any parts who may have bad or cold soldering ... check the TO3 outputs transistors screws for tightness, i dont know if they are socketed, check the contacts too. 

Google transistors checking  you'll have some tips and tricks, bad or cold soldering too,   since i dont have your amplifier in front of me  loll  it may be simple as a bad contact ??

A good trick would be to pen mark the good side parts and compare them with the defective channel ... removed from the pcb,  this amplifier shouldn't cost a fortune to repair    9 transistors per channel  ??


An other friend just realized he bought "cheap" chineess transistors for a psu and had lots of stability problems, before he said  maybe the new parts are wrong/bad ... check your parts sourcing/distributor.


EDIT :   generate a 1khz tone, 1 v pk to pk on line in, make the amp distortionate, if you have acces to a scope follow the signal path on each side to see where it gets ugly on the bad one ??