Ka4002 right speaker only works with loudness button but no bass.

[DChips2]

Hello all
I joined the forum a little while ago and have been reading through posts with great interest.

As per the title, I picked up a 4002 a couple of weeks ago that has a problem with the right side. With the loudness off, I can barely hear the right speaker at all, with the loudness on it comes up almost to volume, slightly lower than the left, but with no bass. There doesn't seem to be any crackling or distortion, I know this may seem strange, but the sound is still sort of OK.
It looks all original inside to my untrained eye. I have cleaned all the switches with contact cleaner which made no difference.
My own thoughts are the output transistors, probably naively, I just don't have the knowledge or experience, and I don't want to make things worse.
I do have some experience with electronics, and I can wield a soldering iron in as much as I would have no problems changing caps or resistors etc.
Any advice is gladly taken and I appreciate the time that anyone takes to look over this post.

Many thanks
David.

 

[eizner23]

Did you clean the separate/normal switch on the back?

 

[avionic]

As per the title,

Clean that control. Then clean the rest of the switches and controls while your in there.

 

[avionic]

Welcome to AK BTW.

 

[DChips2]

Hi guys

Thanks for the suggestions and the welcome.
I never cleaned the seperate/normal switch at the back, I'll give it a go, and give everything else a going over too. I will report back

 

[DChips2]

Hi guys

I did as suggested and cleaned all the switches and also the seperate/normal switch at the back with de-oxidising spray and contact cleaner, but the problem still persists, no difference. Can anyone suggest where to go from here.

Many thanks

 

[quaddriver]

question - the the loudness on this a tap on the volume pot? if so, test the pot for resistance...rule out 'mechanical' issues

 

[DChips2]

Hi QuadDriver
Thanks fo the suggestion.
I believe the loudness is tapped on the volume pot. The pot is rated at 50k. With the loundness off resistance is measured from 0<31k, with the loudness on, 0<48.5k.

 

[DChips2]

I have also checked the output at the speaker terminals, A = 15mv/15mv, B = 15mv/14mv, is this in range for the 4002 ?
Can anyone suggest where to go from here, or what to check.
I appreciate all help and advice.

Thanks
David

 

[avionic]

is this in range for the 4002

Nothing to worry about.

 

[eizner23]

Quick test...Try running your phone line out to main-in, switch unit to separate mode. If that works then issue is likely in preamp/tone circuit.

Set phone volume at 25% initially.

 

[spark1]

If you connect the headphone jack of your phone to the "main in/power amp", be sure the volume setting of the phone is at zero before powering up the Kenwood. Then very gradually turn up the volume on the phone.

Probably obvious, but you will need a cable with asmall stereo jack at one end (to fit the phone's headphone jack) and a pair of male RCA connectors at the other end.

 

[DChips2]

Hi guys
I did as suggested and used the main in and gradually increased the volume and both speakers play fine .
So I need to test components on the pre-amp / tone circuits. Is there anything I should look for, or is it just a case of testing values until I find something out of whack ?
Thanks again for the advice and help.
David.

 

[eizner23]

You can grab a copy of the service manual from hifiengine.com and start checking the test point voltages with a multimeter on both channels of the EQ Amp (preamp) and Tone Amp boards.

 

[DChips2]

Hello everyone
Firstly I know its been a long time since I started this thread, but I had a lot going on and what with the pandemic and such, there were things I had to prioritise. Anyway things seem to be as close to normal as they can be, and I have some time for myself once again, so naturally I began thinking of my little amp. So over the past few weeks I have been reading through AK trying to expand my knowledge so that I may understand the advice I am given.

I have a copy of the service manual, but as far as understanding the schematic I'm not really there, I can see the seperate parts of the circuit but I am not sure about the test points. Can I assume that for instance the pins with odd numbers are the left channel and the pins with even numbers are the right channel ? and if thats the case can I compare them. So for example if I check pin 31 to gnd then pin 32 I should get roughly the same output ? Would this be correct or am I way off ?

Many thanks for all the previous replies and I really appreciate any advice given.

David

 

[DChips2]

Hi all

Just an update, the right channel is now working, although frustratingly it started working after I unclipped the wires that run along the left hand side as you look from the front. I was unclipping the wires so I had a little more room to use my probe and as soon as I did, the full wave appeared on my scope (for clarification its only a dso 150 so almost a toy, but it worked) but I can't replicate the fault.

So after a couple of days I recapped and changed the 458 transistors as there was a terrible hiss and replaced a couple of resistors that had been previously changed but for values that were slightly more or less than stated in the schematic. I used 2240's that I already had to replace the 458's. Now the little amp is extremely quiet and sings beautifully.

I wouldn't have been able to do any of this had it not been for Audiokarma and the invaluable information in which they bestow upon it. So thanks to everyone for the information, and special thanks to @dlucy @eizner23 @hopjohn @Hyperion as through their various posts I came to understand a lot more than I anticipated.
I have another (my brothers) 4002a to recap and I will change the 458's as it exhibits the same loud hiss.

So thanks to AK as you have made 2 old codgers very happy