Kenwood KW-70 Receiver

[ALBQ]

Found an old Kenwood KW-70 tube receiver from the 1960's in pretty good shape - would like to restore. Looking for a schematic or service manual. Any help is appreciated.
Thank you -ALBQ

[ALBQ]

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[bournville]

These people have a copy of the schematic only which they can supply for $8.00...

http://www.agtannenbaum.com/k_cat.html

[Gary P]

Sams will sell you a photo copy of the KW40 service manual for $15.00. It is 10 pages. Had everything except the FM alignment procedure, the part I wanted most. Fortunatally, the FM section worked fine after cleaning the sockets.

Sams website

Sound quality wise, the stock unit is just so-so. Way too many "features" in the way.

I did some extensive mods to my unit and now it is a very good sounding unit. Basically the mods were converting the output stage to triode connection and removing the line stage section along with all the tone controlls and the blend circuitry. It also benefits from new power supply , coupling, and bypass caps. Now the output of the selector switch connects directly to the volume pot and that drives the driver stage directly.

One limiting factor to good bass response is the output transformers. The transformers over load on low frequencies at nowhere near full output.

One of the output transoformers shorted from B+ to ground about a year ago. Replaced the outputs with a pair of 1950's vintage Rockola 6BQ5 juke box transformers. This was a good upgrade. Modern transformers to look at that will fit in the chassis are the Hammond 1608 or 1609.

The KW 40 is in daily use now in my ex wifes studio. She loves it! Sounds vastly superior to the NAD solid state reciver that it replaced.

Gary

[ALBQ]

Gary,
Thank you for the reply. Question - what B+ does the unit run and which 6BQ5/EL84/7189's are you using. I did a quick measure without ouput tubes and measured ~450V. Guessing will be less with tubes installed. Based on some reading, I ordered a quad of EI EL84 Gold pins. Hoping will be ok to run hot.
Thank you -ALBQ

[Gary P]

You should be fine with those tubes. I'm running a quad of JAN Sylvaina 6BQ5's in mine.

The original tubes were 6BQ5's so I don't think you will have problems. The B+ will drop quite a bit when you get the outputs installed..

Gary

[ALBQ]

Gary
Thanks for your the help. The sockets are labeled 7189 but, no way willing to spend the money considering what the unit is and value. Thank you -ALBQ

[ALBQ]

Gary,
The Ei's don't appear to work. They start to glow red within a minute or two. One channel appears worse than the other but, neither looks good. Please advise.
Thank you -ALBQ

[ALBQ]

All,
My saga with this old Kenwood continues. I bought a quad of Russian EL84M (6P14P-EB - 7189 sub) and after a couple minutes, the plates start to glow red. The receiver plays music but, I can't run it in this condition. Does anyone have a schematic of the amplifier section they can send me? I really do not want to buy one - or put any more money into it - until I can figure out whats going on with it. I have heard that Kenwood used the same output configuration/transformers in several models so, maybe more options in turning up a schematic - and really only care about the amplifier section as everything else works fine. Both channels behave the same, voltages seem to match and there is no sign of anything burnt.
Any help is appreciated
-ALBQ

[SHiFTY]

Sounds like you have a capacitor or bias problem. First step will require replacement of the coupling capacitors to the output tubes, generally the ~0.1uF 600V jobs connected to the grids of the output tubes (EL84 pin 2). Do this first, old capacitors don't last.

It should be quite easy to determine whether your amplifier is cathode or fixed bias. Look at the underside, if the output tubes have a large power resisotr from pin 3 (cathode) to ground, with an electrolytic ( eg100uF 100V) capactior in parallel, it is cathode bias. There may be one resistor per pair of tubes or just one for all four. You should replace the capacitors if this is the layout.

If it is fixed bias there will probably be bias adjustment pots nearby, and it will be a bit harder... You will need to check that you have negative bias voltage at the output tube grids. Replace old bias diodes and capacitiors.

If you can't get it working (red plate glow is BAD!) I would suggest a (reversible) rebuild of the power stage. Convert it to cathode bias for more reliability, it will take all of 4 components. It is worth it, because "playing music" suggests the transformers are ok! After all your work so far, might as well!

Let us know how you get on!