Kenwood km-207 common ground?

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blownaway
blownaway Posts: 15
edited May 2020 in Vintage Speakers
So I just picked up a kenwood km-207 amplifier among some other old stereo components from a family friend for free. I can't seem to find schematics to look at, I probably just suck at searching and can't find a free service manual. I also tried looking into older posts on how to check with a multimeter, but they were gone. When checking with a multi meter I have gotten between 3 and 8 ohms on the a terminals, but no continuity between the b terminals. Is this amp a common ground amp?
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  • F1nut
    F1nut Posts: 49,794
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    Test between the negative outputs of the A binding posts with the amp unplugged.
    Political Correctness'.........defined

    "A doctrine fostered by a delusional, illogical minority and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a t-u-r-d by the clean end."

    President of Club Polk

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  • mhardy6647
    mhardy6647 Posts: 33,029
    edited May 2020
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    I didn't find an SM, either, but I did notice something interesting in the OM (from hifiengine -- the OP presumably found that. ;) ).

    a6w4217agbns.png


    The NOTE (see red arrow) [edit] seems to imply that the A and B speaker outputs are wired in series rather than in parallel. Not necessarily a clue vis-a-vis whether the amp is common ground or not... but an unusual configuration. This suggests that the amp isn't very tolerant of low impedance loads. Just thought that was worth pointin' out.

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  • blownaway
    blownaway Posts: 15
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    F1nut wrote: »
    Test between the negative outputs of the A binding posts with the amp unplugged.

    Between the negative a posts I had varying ohm loads, between 3 and 8 ohms each time I tested. Unfortunately I couldn't look back on the old how to test threads since they have been deleted. Is that good if I use a jumper?
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  • blownaway
    blownaway Posts: 15
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    mhardy6647 wrote: »
    I didn't find an SM, either, but I did notice something interesting in the OM (from hifiengine -- the OP presumably found that. ;) ).

    a6w4217agbns.png


    The NOTE (see red arrow) [edit] seems to imply that the A and B speaker outputs are wired in series rather than in parallel. Not necessarily a clue vis-a-vis whether the amp is common ground or not... but an unusual configuration. This suggests that the amp isn't very tolerant of low impedance loads. Just thought that was worth pointin' out.

    Thank you for pointing that out... I had seen that, but wasn't sure what to make of it. I don't plan on driving the speakers hard off of this amp, as I will be getting one more suited to do so, but I would definitely like to be able to enjoy them until I get another if it is a common ground.
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  • F1nut
    F1nut Posts: 49,794
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    blownaway wrote: »
    F1nut wrote: »
    Test between the negative outputs of the A binding posts with the amp unplugged.

    Between the negative a posts I had varying ohm loads, between 3 and 8 ohms each time I tested. Unfortunately I couldn't look back on the old how to test threads since they have been deleted. Is that good if I use a jumper?

    I just told you how to test. You should not get varying readings.
    Political Correctness'.........defined

    "A doctrine fostered by a delusional, illogical minority and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a t-u-r-d by the clean end."

    President of Club Polk

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  • motorstereo
    motorstereo Posts: 2,049
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    FWIW I drove my 2.3 Polk sda's for a short time with a KM209 which is from the same family of cheap 80's Kenwood rack mount gear. Supposedly they put out 200 wpc but no doubt low current. They will make music but the Polks are deserving of much much better.
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  • blownaway
    blownaway Posts: 15
    edited May 2020
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    F1nut wrote: »
    blownaway wrote: »
    F1nut wrote: »
    Test between the negative outputs of the A binding posts with the amp unplugged.

    Between the negative a posts I had varying ohm loads, between 3 and 8 ohms each time I tested. Unfortunately I couldn't look back on the old how to test threads since they have been deleted. Is that good if I use a jumper?

    I just told you how to test. You should not get varying readings.

    Evidently my multimeter isn't worth a crap or batteries are about dead... because I can't get a consistent reading between each time I test. I was almost certain that it should be the same. If I try it a few times in a row it shows roughly the same(+ or - .1), or if I wait a few minutes after turning the meter off and on and try again it gives a completely different number. It is unplugged. And I misspoke when I said how to test, I should have said how to interpret the results.
    Post edited by blownaway on
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  • RTally
    RTally Posts: 14
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    blownaway wrote: »
    Evidently my multimeter isn't worth a crap or batteries are about dead... because I can't get a consistent reading between each time I test. I was almost certain that it should be the same. If I try it a few times in a row it shows roughly the same(+ or - .1), or if I wait a few minutes after turning the meter off and on and try again it gives a completely different number. It is unplugged. And I misspoke when I said how to test, I should have said how to interpret the results.

    An easy check is to touch the meter test leads together. If you read anything other than a dead short (zero ohms), your meter is crap. Either fix it or get another one.
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  • mhardy6647
    mhardy6647 Posts: 33,029
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    Weak batteries will give erroneous results for resistance. Usually when I pop the battery case open on a DMM here -- there's been a battery leak! :(

    Now -- if it is a 9 dollar Horrible Harbor Freight DMM -- yeah, buy a better one. Pronto. ;)

    Other possible explanations might be poor continuity due to Sturm und Drang stress and strain in one or the other test probe leads (e.g., a broken or 'failing' wire in one, or both), or the connection(s) at the meter (e.g., inside the banana plug housing) -- or the probe tips. As mentioned above, shorting the probes together and verifying a zero ohm reading is (always) a "best practice" with a DMM/VOM/VTVM before using for resistance measurements. B)
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  • blownaway
    blownaway Posts: 15
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    So I went and got a new higher quality meter after trying new batteries. The new one reads 0.2 ohms with the probe leads together, is that acceptable? My readings with the new meter are 0.5-0.6 between negatives with the new meter. The 9v battery seemed kinda weak that I had, so I will go pick up a new one later... can retest and repost if needed.
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  • JayMX
    JayMX Posts: 435
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    This thread caught my curiosity, so on my lunchbreak I peeked into the service manual for the KM-208 (couldn't find KM-207, but I suspect they're similar.)

    Strange A/B switching circuit in play. Definitely common ground before it enters the speaker selector section:
    xiam8yv74f32.png
    Full manual is attached. Good luck!
    Current Collection: Monitor 4a (Peerless), Monitor 5B (Peerless), Modified Monitor 7b (Peerless), RTA15TL (SL3000), SDA CRS+ (194’s), SDA SRS 2.3TL, R100's, R200’s, R300 🤩
    Pairs that have passed through: Monitor 4b (Peerless), Monitor 5a (Peerless), Monitor 5b (SL1000), Monitor 5b (SL2000) (3x pair), Monitor 7b (Peerless), Modified Monitor 7c’s (194’s), Monitor 10a (Peerless), Monitor 10b (5x pair), RTA8, RTA8TL, RTA 11T, RTA12c (194's), SDA CRS, SDA 2 (2x pair), SDA 2a, SDA 2b, SDA 1b, SDA 1c, SDA SRS 2 (2x pair), SDA SRS 3.1TL (198’s) (2x pair)...and more to come, it’s a sickness.
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