KR-4140 DC Offset?

RandomAVdood

New Member
Hi all, I have recently acquired a KR-4140 from ebay for pretty cheap, I have cleaned it up, installed new LED fuse lamps for the dial and tuner meter, and cleaned all of the controls with deoxit to prevent static. It works quite well and pushes my older Allegro speakers effortlessly however the sound seems a bit off. I replaced all four the transistors with NOS NEC 2SD155's which are what are supposed to be in this unit, the previous owner had mismatched transistors two of which that were General Electric branded for some reason, the other two original NECs. It sounds better now and it no longer sounds different between the right and left channels however I am getting around 180 mV on both the left and right speaker outputs. I tested with all tone controls zeroed, balance in the center, volume control at the minimum position. Is this normal? It doesn't seem to have the same low end oomph and warmth that my lower end KR-2120 did. The tuner also seems a little weak but I will address that later. I really would like to get this working to its full potential because I absolutely love how it looks. Thanks!
 
Does the DC drop when speakers hooked up? Schematic shows a 12K feedback resistor going to a low (1.8 volts) junction and this should drop way down when there is a load. Will slowly creep up when load removed. As to other issues - no clue at this time.
 
I'm going to put this on hold for now, until I can get a good DMM. Mine was just spitting out random values it turns out, I kept trying and eventually, when it gave the little "beep" of it registering something I was able to get about 13-15mV on both channels, I think the sound quality issues may lie in the tuner which has been acting finnicky. Thanks for the help anyway, FYI for anyone with a 4140 I do not believe the DC offset is adjustable. Thanks again
 
The entire KR-2120, 4140, 5150, 6160 line has cap-coupled output and those caps are an important part of bass response (they form a filter, I believe). You may want to replace the two big electrolytic caps, one each at the tail end of each channel in the main amp circuit, and see if that restores your bass response. These should be high on the list of things to replace.
 
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Finally tore into this thing as my town is pretty much locked down, so I've been working on some projects. I replaced the three large capacitors mounted up top on the chassis, the two audio coupling (C171, C271) and I believe a filter capacitor (C304) for the power supply. I replaced them with Nichicon Golds of the same value and slightly higher voltage, and wow! The size difference between these 70s era and modern capacitors is crazy. The quality of the bass and sound quality in general seems to be vastly improved, I'm no audiophile but to my ears it definitely sounds better. When I had just replaced one of the coupling capacitors I hooked the receiver up to a pair of pretty crappy test speakers and there definitely was a sound difference between the right and left channels. With the unit hooked up back on the audio cabinet with my pair of rebuilt Realistic MC-1000s it sounds excellent! I'll sure to make a post soon when I get to testing the DC-offset. Thanks for the help everyone, the old Kenwood is sounding healthy now and will sure to be getting more use! On another note, are there any other capacitors I should be getting replaced or should I leave the unit alone for now?
 
With the unit hooked up back on the audio cabinet with my pair of rebuilt Realistic MC-1000s it sounds excellent! I'll sure to make a post soon when I get to testing the DC-offset.

Great work!

On another note, are there any other capacitors I should be getting replaced or should I leave the unit alone for now?

The sound difference you're hearing is the result of the caps you replaced degrading as they age toward failure. All the electrolytics will fail... at some point. The questions are "when they fail, will they take out anything important or unobtainable?" and "will I notice the sound difference as they slowly degrade"?

There isn't anything unobtainable or replaceable, as far as I know, in your KR-4140. So, many people would wait. I would not. At a minimum I would replace all the ecaps in the power supply, the most-likely place for damage.

You are the only person who'd know how likely you are to notice a slow degradation in audio performance. Again, it'll likely continue to play, just less and less accurately.

And others will chime in soon here with their viewpoints. Whichever course you choose, you've always got AK'ers willing to help out.
 
So I put my good multimeter on the speaker outputs at 200mV range and observed the following results:
Without speakers connected: about 180mV on both channels consistently.
With speakers connected: about 1.2mV on both channels consistently.
What should I be looking into next? I do have the service manual and there seems to be no mention of adjusting the offset, as others have mentioned.
 
So I put my good multimeter on the speaker outputs at 200mV range and observed the following results:
Without speakers connected: about 180mV on both channels consistently.
With speakers connected: about 1.2mV on both channels consistently.
What should I be looking into next? I do have the service manual and there seems to be no mention of adjusting the offset, as others have mentioned.
For cap coupled amps, there is no need to adjust dc offset. It should be close to 0 unless your coupling caps are bad. If you really want to measure the dc offset, you need to connect loads to the speaker terminals, which you did already.
 
For cap coupled amps, there is no need to adjust dc offset. It should be close to 0 unless your coupling caps are bad. If you really want to measure the dc offset, you need to connect loads to the speaker terminals, which you did already.
Ah, that makes more sense. So with 8 ohm loads connected (my speakers), it sounds like 1.2mV is a pretty good reading for this amp. Thank you! Thought my faithful 4140 was screwed up for sure.
 
This is a good ref for adjusting Kenwood units and it includes KR-4140 http://akdatabase.com/AKview/albums/userpics/10007/Kenwood Audio Adjustment.pdf

This is a good ref for adjusting Kenwood units and it includes KR-4140 http://akdatabase.com/AKview/albums/userpics/10007/Kenwood Audio Adjustment.pdf
Found link provided for supplement document regarding the KR-4140. Thanks for posting! Can you anyone advise on comparisons in my photo’s. I picked this unit up from a guy that said he only used for an amplifier. Still the radio did appear to work, but he said it didn’t have an antenna connected. So when purchased, did not know the history of any previous work.

As I‘m reviewing the capacitors status, I noted today in the manual a deviation. Color Pics are of the KR-4140 am working on. Image 4185 indicates a solder joint to tuner which was found broken off. I have soldered back Based on location. Unit is working well on AM but very poor FM reception.

Now today noticed in manual (img_0213) appears to may have originally been a 300 Ohm lead? Not sure where is shown on schematic. Wondering if was removed in an update or just plain missing? Not even sure where is shown on schematic. Can you advise? I see terminal numbers 1&2 and 5&6 but not quite clear where numbers are located on tuner.
I know someone was inside the tuner as cover screws were missing.

I have the schematic enlarged as a block from original manual schematic. Helps with the eye chart once printed. The two areas on attachment I am guessing is circled in orange. Basically FM in/out of tuner.
(
In meantime I am awaiting arrival of an oscilloscope.) Will be learning on the. O-scope too.
 

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