problem with older kenwood amp

[shagone]

Hello, this is my first post here. i have been into electronics for about 12 years now. first in the Marines and now as an electronics engineering tech. i have also worked part time as an installer in small shops and on the side in my garage.

i have repaired a few amps in the past but nothing major(cold solder joints, burnt traces) and i have recently begun to get more in-depth replacing fets and caps after reading Perry's tutorial and getting some more appropriate test equipment. i plan on fixing more amps for fun and maybe even to make a few bucks on the side once i get confident enough. so far i have fixed 7 amps in the past week.

here is the broblem, i have an older made in japan kenwood 2 channel amp with built in eq on the top. forgot the model # kac-???eq i think. the amp powers on but when i feed it a signal from my home pre-amp that is connected to a sony home cd deck i get massive feedback and my system shuts down(due to overload i guess). i checked the rca inputs on the amp and thay show 1.5 voltc DC with nothing plugged into them. i looked for shorts but could not find any so far and there are no shorted power supply fets or output transitors from what i checked while in the circuit. i also re-flowed the solder on the RCA jacks thinking they may have been bad but no luck.

any ideas? this looks like a neat old school amp and it is in good shape except for the feedback issue.

[anatech]

Hi shagone,
Look for an open circuit common to ground resistor (or blown trace).

Those older amps were sometimes much better than the cr*p they sell these days. I think the entire market lost the concept of sound quality completely these days.

-Chris

[shagone]

Thanks! i'll try that and post my results.

yeah the older stuff was so solid. my mid 90's kenwood system sounded so clean and strong in my 92 mitsubishi eclipse with infinity kappa greens up front and a sealed 12'' pioneer impp sub in the back. i was on a budget but it still sounded better than a lot of the stuff i hear today.

my friend just got some new infinity kappa's and i wasn't very impressed. i told him to go with CDT but oh well, he liked the flashy looks of the kappas i guess???

[ppia600]

Also be sure to connect the rcas outer "ground" connectors to the ground terminal of the kenwood amp if you are testing it with a home unit. There will be no ground reference for the signal in the amp if you don't, and it will either do what you are talking about or not make sound at all. Try that first

-get a length of wire long enough to reach from the amp's rcas to its ground terminal
-strip back several inches on one end and wrap it around both rcas while they are plugged into the amp (around the metal exposed section on the outside of the rca)
-connect the other end of the wire to the kenwood amp's ground terminal

[anatech]

Hi ppia600,
I thought it was only those newer amps that ran without a reference. A 10 ~ 100 R resistor could be added between signal and power commons to achieve this without causing a ground loop.

-Chris

[ppia600]

I was thinking that it was the older ones that needed the ground reference and the later amps that began to be capable of running with floating ground signal sources (better at noise rejection) Didn't the older amps use the amp's main ground terminal through a resistor for the rca ground? And the newer (mid 90's and up) use the common/center rail of the power supply through a resistor for the rca signal ground? Of course when they started using op amps, everything changed. Correct me if I'm wrong, I'm not absolutely sure.

[shagone]

i just checked and there is a good 100 ohm resitor between signal and ground on my amp. there is a cap there too but it does not seem to be shorted out. any other ideas as to what i should check?

good to know that some newer amps run without reference on the signal. any idea wich amps do this?

[anatech]

Hi ppia600,

Didn't the older amps use the amp's main ground terminal through a resistor for the rca ground?

Only a few badly designed ones. As far back as the seventies, they were smart enough to resistively couple the common points.

I was thinking that it was the older ones that needed the ground reference and the later amps that began to be capable of running with floating ground signal sources (better at noise rejection)

No, see above.
The newer amps that had no connection between common points were designed that way to reduce warranty claims. Smoked resistors were common with guys putting their own stuff in and poor install shops. Some of those guys were just plain stupid beyond belief. I don't think a ground reference should ever completely depend on the head unit or electronic crossover. Some of these amps actually run some current through the RCA common. Huge no-no!

Of course when they started using op amps, everything changed.

No, when they started using op amps the designers got much lazier. Not all of them, but enough did. Discrete input stages require the same treatment. It's more a case of keeping the invertor noise out of the input signal. Engine noise also, but inverter noise could be deadly on tweeters. The earlier amps used lower switching frequencies on average.

-Chris

[anatech]

Hi shagone,

i just checked and there is a good 100 ohm resitor between signal and ground on my amp.

Darn! Could have been an easy fix.

Check all the connections between the EQ board and teh rest "of the world". Try to bypass the EQ. You may find open traces and controls on this EQ board. Water can get in there.

-Chris

[Perry Babin]

Can you post the model number of the amp?

Someone here may have a service manual for it.