Kenwood KA-7100 clicking and very low volume

bharry

AK Subscriber
Subscriber
I picked up a just about mint condition KA-7100 yesterday knowing that the right channel was a bit quiet and a bit scratchy. I got it home, opened it up and cleaned all the pots and switches with Deoxit and applied some Faberlube. Later, when I turned it on the relay clicked and everything worked perfectly, for about 30 seconds. After that the volume dropped to the point I could barely hear it.

This is what it is doing exactly. I power it on and in about 3 or 4 seconds it clicks on but the volume is almost nonexistent. The volume dial does two things. When it is turned all the way down and I turn it up a tiny bit it sounds like the relay clicks again, if I go back and forth at the lowest point on the volume dial it clicks repeatedly. It does the same thing when the volume is turned all the way up, it clicks when the volume is almost at maximum.

Does any one have any idea where I should start? I have a DMM, soldering iron, and most of the basic tools needed. I also have an oscilloscope but I don't really know how to use it, it was given to me.

Thank you for reading.
 
I should add that this happens even if I have nothing hooked up to the inputs.

Also, when I initially set everything up I hooked up two pairs of speakers. One set was 8ohm, one was 6ohm. Somehow I mixed up the connections and hooked up a 6ohm speaker and an 8ohm speaker to both the A & B channels. When I turned the amp on I realized later that it was set to the A+B speaker setting. Could this have anything to do with causing the problem?
 
I took the cover off of the relay and it looks brand new inside. Turn the input and tape knobs back and forth goes the relay to go crazy. Not sure what's going on here.
 
Also - with the volume set to minimum - hook the red DMM probe to + speaker terminal and the black probe to speaker -, set the DMM to DC mV, and measure what is being output there - ensure speaker selector has that speaker output turned on
 
Thanks for the advice guys. While I was going through all the recommended steps I noticed there was heat being generated near the volume knob and a faint burning smell. Unplugged it and left it alone for awhile. Took the front faceplate off and detached the board from it. Two caps next to the volume knob don't look so good. I'm including a picture. Does it look like this is my problem? Do I just replace these two caps for now?

Rz4v0b3.jpg
 
my guess this is glue to hold down the caps. If you decide to replace C27,C28, remove the glue. I use lacquer thinner. Others use acetate. I'd suggest searching for ideas. both are pretty toxic solvents and there might be better options.

I still think you have dirty contacts - rjsalvi's post #4 and acmmty's post #7 is probably the issue and fix.
 
That actually looks like assembly glue, but that doesn't necessarily mean that the glue has not become corrosive of conductive. Measurement should give you an indication.
 
I think I see a small ring on D2 cathode but I'm not sure.
Check your soldered connections.
Anything that looks iffy reflow to little fresh solder.
Please measure the voltages (reference to ground) at both ends of resistors R65 and R66.
Also please check Q31 and Q32 solders on the power supply
 
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I think I see a small ring on D2 cathode but I'm not sure.
Check your soldered connections.
Anything that looks iffy reflow to little fresh solder.
Please measure the voltages (reference to ground) at both ends of resistors R65 and R66.
Also please check Q31 and Q32 solders on the power supply

I will do what you suggested this evening.

Between getting my wisdom teeth pulled (at 47!) and then a root canal, plus a few vintage audio purchases I got sidetracked, but this Kenwood deserves better than that so I'm jumping back into trying to fix it again.
 
I measured both caps and got 396 for one and 378 for the other. That's good right?
 
Both are high; Typical of an end of life capacitor but for now they're fine.

Sorry, TBH I am not an expert.
Here are several kenwood guru that could help you better than me.

While this happens you can make some measurements.

Just remember:
Do not remove the amplifier from the dim bulb tester http://www.geek-tips.com/2015/11/22/dim-bulb-tester/
Be careful not to make shorts with the multimeter tips.
After turning off the amplifier and before touch anything always discharge the large capacitors from the power supply .

Could you measure this?
Also - with the volume set to minimum - hook the red DMM probe to + speaker terminal and the black probe to speaker -, set the DMM to DC mV, and measure what is being output there - ensure speaker selector has that speaker output turned on

The preamp has capacitors Ci23 Ci24 at the outputs (cap coupled) so there should be no DC voltage at the output of the pre amp to the power amp. If the 2 channels have the same problems with no signal (and probably with the offset) I would think about problems with the power source.

Please measure the voltages reference to ground (Black probe to chasis) at both ends of resistors Ri65 and Ri66. (+/- is important)
Like this:
Ri65 -26V -20V
Ri66 26V 20V
 
I would check the power supply first. Shame the 7100 doesn't have the separation pins. If you haven't yet, clean the tape monitor switches again. Is the speaker relay acting intermittently?
 
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