Kenwood KA-3500

[audiobliss]

I just hooked up the Kenwood to my 'porch system', using my portable CDP as the source. She turned right on, but there's a good bit of static when the volume is turned down low, but turn it up and it sounds good. Well, the right speaker does. I walked over to the left speaker, and it was hardly doing anything. Put out about as much static as it did music. I switched through all the inputs, and the results were all basically the same.

Why do you think the left channel is so statiky?

[audiobliss]

What surprised me the most is when I turned it off. It kept playing just as strong as if it were on for about six seconds, and then slowly faded for another few seconds.

I'm going to see if I can pop the top and take a few pics.

[audiobliss]

Here are a few pics under the top. It's collected a good bit of dust over the years. The fuse seems to be stuck, too.

[mhardy6647]

Decent little amplifier. Could be as simple as dirty pots or switches. I'd recommend checking for DC on the outputs, though, to be on the safe side (I think/assume it's a direct coupled amp, so a shorted output transistor could put DC at the output).

EDIT: I usually use a small, flat-blade screwdriver under one of the fuse's "ends" to persuade it to come out. Just use a bit of finesse.

EDIT^2

[audiobliss]

Here are some pics of the faceplate and of the front of the unit with the faceplate removed.

What can I use to clean the faceplate that won't remove the lettering?

[audiobliss]

Decent little amplifier. Could be as simple as dirty pots or switches. I'd recommend checking for DC on the outputs, though, to be on the safe side (I think/assume it's a direct coupled amp, so a shorted output transistor could put DC at the output).

May I be so ignorant as to ask how I would check for DC at the output?

[mhardy6647]

With a digital multimeter (DMM) or a volt-ohm meter (VOM), or a vacuum tube voltmeter (VTVM), set to measure DC volts. The audio signal is AC. Any significant DC voltage at the speaker terminals can cook a voicecoil in a speaker (I learned this the hard way). The KA-3500 may have "protection circuitry" (probably does), but better safe than sorry.

I usually use Windex for cleaning, but once in a while the lettering isn't Windex proof. You can use a Q-tip and clean around the lettering!

[audiobliss]

Well, I don't have any idea if I did it right or not, but I just used Dad's "Digital probe" to measure the voltage at the outputs in DC mode. The reading I got wasn't constat, but fluctuated between -21 and -24.

What's that mean?

[mhardy6647]

If you hooked the probe's ground (black) to the speaker's black terminal, and the probe's "+" (red) to the speaker's red terminal (or vice versa) and saw + or - 21 VDC... that's not good.

The DC balance (offset) of modern SS amplifiers is somewhat adjustable (see http://audiokarma.org/forums/showthread.php?t=5634 ), but this is a very large offset. Were both channels the same?

[audiobliss]

Well, I got my KA-3500 back out yesterday to finally clean it out, clean the pots/switches, get the settings right, and see if she'll actually work.

What prompted this? Reading back through my Affordable$$Audio 'zines, I ran across the DIY article about how to use a voltmeter and then the one about setting up vintage equipment (in the January and February issues, respectively). Thanks for the info in the articles, Markmarc; without those articles I would have been lost!


So, anyways, after cleaning it out and spraying down the pots and switches good, I connected my voltmeter to the outputs to read DC and got a reading of about -26mV or so for both the A output, and then with B selected, the same with the B outputs. I got out my screwdriver and started turning the DC-offset pots...slowly at first. But I saw NO correlation between how far I turned the pot or in which direction I turned the pot, and the reading on the voltemeter, so I'm going from one extreme to the other, scratching my head wondering what in the world is going on.

DOH! I obviously don't have DC-offset pots, and I've been playing around with the bias pots! *beats head against the wall*

So now, to the point of this post. Both the A output and B output read similarly (horrible). So, then when I selected 'A+B' and checked the A outputs...same horrible reading (just over -30mV, i.e. about -28mV). HOWEVER, when checking the B outputs with 'A+B' selected, I got a steady reading of only -0.1mV on both the left and right channel. What does this mean? When (if) I use this, should I have it on 'A+B' all the time and use the B outputs?

Well, now I'm going to try and figure out how to (re)set the bias pots since I (stupidly) messed them up.

Thanks!!

[audiobliss]

Haha...I just realized I don't know what the bias pots should be set to. Dadburnit.

[mhardy6647]

If my post count were higher. If I knew you better. And IF this weren't cyber-space...

I'd whack you upside the head with a rolled up service manual!

To paraphrase my favorite line (the only funny line?) in the movie version of "Hitch-hiker's Guide": Pots are not toys!

[audiobliss]

Hahahahahaha. I think I'd deserve that one, too. After fiddling around with it a bit, I've also realized I have no idea where to take a reading from to set the bias pot to where ever it's supposed to be. I think I'm just going to hook up some speakers and pray.

[mhardy6647]

Keep them driver and output transistors cool, baby :-)

EDIT: I'd start low and bias "up" 'til they start to sound decent. I understand tube bias much better than transistor bias, but I assume the concept's the same.

[audiobliss]

Well, I took it out to the porch and hooked it up to my old speakers and tried it out.

First off, the bias pots make absolutely *no* noticeable difference to the sound that I can discern.

Second, the left channel, like last time I hooked it up, is all crackly and at a lower db level than the (clean, great sounding) right channel. BOTH left channels are that way, ouput A and B. So, I just hooked the speakers up to the right channels only and selected A+B. Don't get stereo, but I get good sound that way.

So, any thoughts as to what's wrong?

[heiney9]

Probably very dirty contacts on the volume and/or balance control. You won't necessarily hear much difference when the bias (DC offset) is incorrect. But you'll cook the voice coils in the speakers and/or fry the output tranny's (oscillation). I know your just getting your learn on, but if you want any hope of saving that Yammy or the speaks, figure out how to correct what you've messed with. ;)

H9

[audiobliss]

I cleaned *everything*, and *everything* moves very smoothly. Could they still be dirty?

[heiney9]

I cleaned *everything*, and *everything* moves very smoothly. Could they still be dirty?

Yes, possibly. Define clean. Deoxit? If so then some of the output tranny's are on the way out already. Does it crackle only when moving the controls or does it kind of break up with the beat of the music? If it goes away or become much less at higher volumes then it's most likely control/switch related. It's possible even if you cleaned well with Deoxit or similar that the controls are just old and are on the way out. Most likely the output tranny(s) are on the way out.

H9

[audiobliss]

When I say clean, I mean sprayed liberally with a can of contact cleaner that I picked up from Radio Shack. Spray, turn, turn, turn, spray and turn, turn, spray, yada yada. I've never done that before, but I'm confident I was pretty thorough with that.

Is there another way to clean switches/pots/buttons?

The crackling in the left channels does not seem to be related to any of the settings, but rather is independent (and constant). I can't say that it really crackles with the music, though I'm not sure. From memory I'd say it just crackles period. But when I get home today I'll play around with it a bit more.

Is there anyway I can find a schematic for this piece/a service manual short of buying one off ebay?

[faster100]

you should hose down the pots and switches, id spray alittle, turn and let dry.. spray alittle more, turn and let dry.. like that.. not spray spray spray turn spray.. and fire up. also let it sit for awhile before you plug it in, longer then the 15 minutes they reccomend.. it is still liquid and needs to evaporate and dry before electric starts flowing through it again... Id get the deoxit kid from rat shack.. its way better then the lube stuff.. it cleans and then you have the pro gold that comes in the kit.. downside. the cans are very small and go alot faster then the big can of rat shack brand

[danger boy]

where did you get the Kenwood? who lost owned it? why did they get rid of it? sounds like that channel is going out.. and may need more then just a cleaning.. repairs may need to be done.

this is just me.. but if it was a piece i wanted to keep.. i'd take it in for an estimate on repairs. you may f*ck it up more than it already is.

[audiobliss]

For a little clarification on how I cleaned it, I was pretty thorough with it. I doused with one or two long sprays, turned the knob, flipped the switch, or punched the button a LOT, then did the others. Then went back and did the same. And then it set to evaporate for about an hour.

db - I bought it from Goodwill for $10. If it ends up working and being useable, awesome. If not, then I've at least learned a little about poking around under the lid.

[heiney9]

For a little clarification on how I cleaned it, I was pretty thorough with it. I doused with one or two long sprays, turned the knob, flipped the switch, or punched the button a LOT, then did the others. Then went back and did the same. And then it set to evaporate for about an hour.

db - I bought it from Goodwill for $10. If it ends up working and being useable, awesome. If not, then I've at least learned a little about poking around under the lid.

You got it at the right price lol. From your photos it looks really dusty. Sounds like you did a decent job of cleaning (Deoxit is the best, IMO) and it's possible the reason it was at the GW was because it had issues. You could probably get it fixed if it's a minor problem for about $100, may or may not be worth it to you. At this point since you know nothing about repair there's not much you can do even with a service manual (I'm pretty much right there with you). I understand how electrical circuits work, but couldn't fix one if my life depended on it.

If you are going to keep it it's not a bad idea to run down a service manual. I have one for my Nak receiver just because if I ever have to take it in, the tech might not have the service manual and, I'd rather have him work on it with a manual than without.

Good luck

H9

[audiobliss]

Thanks for the advice. It's definitely not worth $100 to me! I wouldn't even spend about $50 on it to get it to work.

I might play aroudn with it a bit. Or maybe I'll just play it like it is, just running everything off the right channels.

[heiney9]

Play around, but be careful ELECTRICITY KILLS

Stay away from the main power supply and the bypass caps at all costs even if it's powered down. Those caps aren't that big but they will store electricity even after the power is off.

H9

[audiobliss]

Thanks for the word of caution. I'll try to be keep my hands to myself. :)

[Phasearray]

Interesting, over a year later I have the same problem with the same amp. Dead left channel. Cleaned it like crazy and it has the same problem.