help with checking vintage amp, dc offset and bias.

[jindy30]

Ok, as a baby step i bought an old Kenwood Stereo receiver, KS-4000R to learn some basic electronics.

I want to check the dc offset and bias. When i opened it up i was surprised to see so many adjustment dials. Can someone tell me which one is the adjustment for the dc off set and which one is for bias. Also, where do i measure the bias voltage, across which resister?

Thanks in advance

Jonathan

 

[mhardy6647]

Well, no, but...

^^^ These are IF transformers in the radio section of the receiver.

The two coils near the multi-ganged AM and FM variable (tuning) capacitor in the bottom photo likewise -- all part of the radio's tuned circuits.
Don't touch 'em!

Do you have a service manual?
EDIT: https://www.hifiengine.com/ does not.

I did find a piccie of the receiver there.

Totally unfamiliar model to me. 1980, per HFE. Reminds me of the entry-level KR-2600 of a few years prior, but with more knobs 'n' buttons.

https://www.hifiengine.com/manual_library/kenwood/kr-2600.shtml

 

[MaxwellsEq]

Ok, as a baby step i bought an old Kenwood Stereo receiver, KS-4000R to learn some basic electronics.

I want to check the dc offset and bias. When i opened it up i was surprised to see so many adjustment dials. Can someone tell me which one is the adjustment for the dc off set and which one is for bias. Also, where do i measure the bias voltage, across which resister?

Thanks in advance

Jonathan

Most of those photos are of the tuner. Unless you have a service manual, lots of test gear and experience, I would leave those alone.

To measure DC offset, put a load resistor on the output and measure the speaker terminals with a DC voltmeter.

Any adjustments affecting bias or DC offset will probably be near the power amp section. But without a workshop manual or at least specifications for setting things up, you won't know what to adjust or what to adjust it to. With some test gear you may be able to measure distortion and adjust for best SNR.

 

[Tryphon]

Btw many service manuals can be found on Hifiengine website

 

[MaxwellsEq]

Btw many service manuals can be found on Hifiengine website

@mhardy6647 checked but couldn't find one, see post #2

 

[mhardy6647]

I didn't check audiokarma's 'digital docs' section, though.
EDIT: Just did. nada.

@Tryphon's comment above gave me pause.
More to the point, it caused me to reflect on the notion that HFE might also list "Kenwood" products under the TRIO brand name, as Kenwood products were sold in many markets, for many years, branded TRIO.

Nope. No TRIO listings.
Oh, well, I tried...


This receiver, at least to the eyes of a US hifi buff of a certain age, is an odd duck. The "KS" nomenclature is odd for Kenwood of that time, too. In those days, their receivers' model numbers carried the "KR" prefix.

But, then again, I don't get out much.

EDIT^2 Oh, I did see this, with a few interesting tidbits (including a possible explanation for that "KS" nomenclature):

Kenwood KS-4000R Receiver

Does anybody have information about an old, very basic model, the Kenwood KS-4000R receiver? It only has one pair of speaker outputs - with RCA plugs no less - and phono/tape monitor inputs with no AUX. I was wondering about the years made, output and if it was a decent unit or a throwaway...

audiokarma.org

This, too:

 

[jindy30]

Hey, Thanks for looking. the manual is on e bay/servicemanuals.net, i guess i should get it.

So the red, yellow and black dials in the pictures are just for the radio receiver?

what about the chucking blue dials, i turned one of them quiet alot.

Most of those photos are of the tuner. Unless you have a service manual, lots of test gear and experience, I would leave those alone.

To measure DC offset, put a load resistor on the output and measure the speaker terminals with a DC voltmeter.

Any adjustments affecting bias or DC offset will probably be near the power amp section. But without a workshop manual or at least specifications for setting things up, you won't know what to adjust or what to adjust it to. With some test gear you may be able to measure distortion and adjust for best SNR.

Thank you, yes no need to touch the tuner electronics.

I did order the service manual so hopefully tuesday i can open it up again and take a look, I am wondering where to test the bias. Dc off set i have seen is simply the DC current on the speaker outs, but the bias is across a resistor. so not suere about that.

How do you put a load resistor on the output btw?

FYI, KS is for "System" and the 4000-R is the Receiver part of the system. It certainly is an odd duck, and althougm y speaker are mus matched WNYC has never sounded better.

Thanks again Jonathan

 

[jaybird396]

If this model is from 1980 it might have those "power pack" modular output devices that had no adjustment. Many receivers from that era used them as opposed to discrete components with adjustments. But your best bet is to try and fin a service manual.

 

[mhardy6647]

If this model is from 1980 it might have those "power pack" modular output devices that had no adjustment. Many receivers from that era used them as opposed to discrete components with adjustments. But your best bet is to try and fin a service manual.

That's a very good point. STK modules, a/k/a (I think!) "Thick Film" integrated circuits.
Of course, if it does, @jindy30 has been nosing around under the hood quite a bit, so they would have noticed if it does.

source: https://www.usaudiomart.com/details/649613354-beautiful-kenwood-ks-4000r-stereo-receiver/

IF (!?!?!) the image on this page is indeed the innards of a KS-4000R, it appears to have discrete outputs, or at least not STK modules.

source: https://web.joongna.com/product/55298552

For comparison, here's a typical STK module in situ in a very entry level receiver (Technics SA-80) of broadly similar ilk to the titular Kenwood.


Sorry about the dust!
The STK module may be seen mounted to a fairly respectable heatsink towards the top left of the image.