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Kenwood 7020
Kenwood 7020
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Old 25th August 2019, 05:57 PM   #1
Mission770F is offline Mission770F  United Kingdom
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Default Kenwood 7020

Evening all

I acquired one of these monster 90's amps recently for restoration. Top of the Kenwood line in 1990/91. Dual transformers, massive 15,000uF Elna capacitors, all good stuff. This one was caked in dust and while it sort of functioned it sounded like a worn out amp. I've restored it by cleaning it thoroughly and re-capping it throughout (many of them were failing badly), but i'm now having trouble resuscitating it!

Tracing the audio signal through the circuit(s) (with my equally ancient Kenwood oscilloscope!) it gets as far as the front panel PCB, enters a pair of op amps connected with the loudness circuit but doesn't emerge. Bypassing this circuit with 'source direct' gets it to the little volume control daughter board, but again, it goes into the associated volume control op amps but doesn't come out. All the circuits/op amps seem to be correctly powered, the main power amp DC offset and bias are all ok, and it's possible to get some very nice sound out of it (albeit very quiet) by bypassing the volume control with the audio signal that's made it this far - so the power amp seems ok.

Weirdly, when you really crank up the input signal the sound 'breaks through' (even with the volume right down), albeit very rough?!

I'm at a slight loss so I thought i'd canvass some opinions :-)

Hope you can help!
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Old 25th August 2019, 08:58 PM   #2
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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You have + - 20volts on the chip?
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Old 25th August 2019, 09:35 PM   #3
Mark Tillotson is offline Mark Tillotson
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Could the opamps have cooked in the thick layer of dust? Are they socketed?
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Old 25th August 2019, 09:55 PM   #4
latala is offline latala  United Kingdom
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Kenwood 7020
d
Does the unit have pre-amp to power links that are missing?
Trev
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Old 25th August 2019, 11:52 PM   #5
MAAC0 is offline MAAC0  Portugal
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Following Schematics helps a lot...

KENWOOD KA-7020 SM Service Manual download, schematics, eeprom, repair info for electronics experts
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Old 26th August 2019, 12:34 PM   #6
Mission770F is offline Mission770F  United Kingdom
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Thanks for the thoughts so far. I measured the voltage on the op amps across pins 4 and 8 (is that correct?) and got 28.2v...which seems a little high (according to the schematic it should be 20v)... Incidentally the op amps are NJM4565D's and NJM5532's.

...i'll go away and check all the voltages across the board to see where we are.

Sadly the amp doesn't have defined pre-power amp connectors as such. I bypassed pretty much everything and fed the audio signal straight into the main board and got some nice sound out of it (albeit very quiet).

Strange one!
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Old 27th August 2019, 04:24 AM   #7
PRR is offline PRR  United States
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> and got 28.2v...which seems a little high

I wonder if the two 20V regulators have failed (WHY??) and are dumping nearly-raw 31V to the opamp chips. (They won't live long.)
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Old 27th August 2019, 08:38 AM   #8
sgrossklass is offline sgrossklass  Germany
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That's actually a little low.

OP, please measure both pin 4 and pin 8 voltage relative to chassis ground, separately. Also verify continuity between chassis ground and ground on the volume control PCB (pick from pin 3 = GND on CN7, pin 5 + 6 on the input connector, R121-124 etc.).

Did any board-to-board connections have to be undone during the recapping? The screwup may be hiding there.

BTW, what sort of bellend would think it a good idea to supply a +/-18 V rated opamp like the NJM4565 with +/-20 V on a regular basis? Kenwood amp designers, apparently. Maybe the original plan was to use 5532s throughout or something.
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Old 27th August 2019, 06:03 PM   #9
Mission770F is offline Mission770F  United Kingdom
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Thanks for the input which has been very helpful. Thought i'd post that the problem is cured (in case anyone else in future might be searching for fixes).

Turns out there's a moderately complex power supply on the main board which supplies the front panel PCB and the volume control PCB. This was behaving very strangely with +37 volts on one rail and 0.9 on the other(?!). I checked all the capacitors in this circuit and replaced C111 (33uf)...but only because I dropped it on the floor and couldn't find it. Also did a small repair on the PCB (the pad on C107 was broken), and it worked. Voltages swung back to +20v and -20v and all is well.

Amplifier sounds really good.

May well consider upgrading the opamps to something that can run on 20v :-)
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