KA-5002 right channel is out

eizner23

Super Member
Not completely out, i get a little sound out of it. Occurred after I heard a crackle and pop in the right channel. I hooked up a Dynaco preamp and the issue doesnt persist, so perhaps the receivers preamp is at fault.
Preamp board as rebuilt prior to the loss of sound, worked fine for over a year, only original parts in that board are the mylar caps and diodes.

I suspected bad transistor in preamp. Pulled them all out (KSC1845) and all tested fine. Replaced them anyways. Service manual doesnt really have many test point measurements for preamp circuit, so my diagnosing stopped there.

Gave the balance/vol pots a good deoxit faderlube cleaning, along with the Separate/Normal switch with D5 but the issue still persists. Rechecked the soldering, looks fine.

My question is what other circuits are in-line with the signal path that could be suspect? The tone circuit looks as if its bypassed when Bass/Treble knobs are switched to "Off" position, and I wonder if the low/high filter boards are bypassed when not engaged? I hope someone can confirm this for me or give a little help.
 
I would assume the tone circuit remains in the signal path so long as there is no dedicated tone defeat switch to route it elsewhere. You mention cleaning the volume and balance pots but not the tone controls so you might give that a try. I always suspect the normal/ separate switch in these. Because they often remain in one position for long periods of time they can be difficult to clean the first go round. Clean that switch diligently. Attack it from both sides.
 
thank you hopjohn. I have cleaned the normal/ separate switch at least 5 times during the past 6 months and I get both sides pretty good. The symptoms of this switch being dirty or corroded on this model is a channel completely drops in and out intermittently, or for a long period of time.

There is no dedicated tone defeat switch, only an Off position on the tone knobs. The tone circuit has two transistors that I am going to test next unless someone has a better idea.
 
Low filter has a pair of 2SC871. Known trouble makers.
High filter has a pair of 2SC458. Known trouble makers.

I've been getting and working on those old Kenwoods lately and the 2S458 and 2SC871 are 95% of what I find bad in them.
The other 5% is the old caps and dirty switches/controls.

Now, I just shotgun all the caps and those transistors without blinking an eye.
 
I think you're on the right path. You've seemed to have eliminated the amp section being the issue routing a second unit to the main ins with success. So I would preclude, as you do, that the problem exists in the preamp. There are two different schematics based on which serial # you have with a cut off point from one to the other at 430000. That'd be helpful to know.

Also looking at the schematic tells me that the tone controls are in play at all times. The off position on the bass / treble controls here simply means flat or zero. The low/high filter circuits are bypassed in the off position.

What source input are you using? You never mentioned.
 
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Thank you very much for reviewing the schematic and confirming if the tone controls are always in play. I have been testing this unit using AUX and Tuner inputs.
Based on your input I am going to check the transistors in the tone circuit and I will post back. I didnt realize there were two revisions of the preamp board, but i suspected so as the part number on the PCB didnt match the one listed in my original service manual. My service manual does contain a service bulletin for 3 different revisions to the amp board.

I do not use the low/high filters and they have not been touched, all original, since I didnt think they were in play when in the off position. Thank you for confirmation on this.
 
Just to be clear. Understand that the signal does pass through the switch itself even if the circuit they control is bypassed by their position. So those switches need cleaned whether you intend to use them or not.
 
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