Kenwood Ka-7100 project

[mikepick]

I got a Kenwood Ka-7100 from the Auction Site for a reasonable price, took a bit of a chance on a cheaper unit that was of unknown functional condition but looked cosmetically ok. Been interested in a higher-power unit since my KA-3500 recap, I like this era of Kenwood.

It was delivered this week, and it seems like its functionally in good shape. I tested it on a dim bulb, and once that passed, I tried headphones, tested DC offset, and finally speakers with a source in AUX.

It's got dirty pots, switches, and connectors, so it took some fiddling to get it going, but I finally got it sounding pretty good through a pair of Smaller Advents.

Yesterday I had a chance to take it apart and inspect it further. I had been reading some of the many threads on 7100 repair here and was prepared to look for a few things. Sure enough, it had suspect looking joints under the middle foam piece around Q34 / C41 / R84 / R85. I cleaned those up and resoldered them, and am trying placing the foam support farther forward so it doesn't sit below those joints but on the edge of the board.

I found two issues in the inspection:

+ one of the plastic corner supports on the back of the case is broken. Not a big deal, I think I will remove these anyway when I eventually make a wooden case for it.

+ more seriously, the volume pot shaft is broken. There was a fair amount of slop in the volume know initially, which I chalked up to loose set screws, but I don't think I am going to get a tight fit on the know without figuring out a repair or replacement of the volume pot.

The plan, I think, is to recap it, put new micas and grease on the outputs, and try some of the heat mitigation ideas others have tried, such as heatsinks on Q31 / Q32.

 

[mikepick]

Finally got back to this one, had another look and listen.

On second listen, it seems like one channel is a bit low in volume. Switching between Mono-Stereo-Reverse cause the low channel to move left-right, and it seems to be in effect on both the AUX and Tuner inputs, so my first guess is to look at some preamp voltages (although I need to test another input cable as well to be sure.)

I did try a fix on the volume shaft with some Loctite Concrete / Metal epoxy, curious to see how this holds up.

 

[dstariha]

Depending on how the Loctite works you may want to find a piece of brass tubing sold at hobby shops to slide over the shaft. Then carefully bore out the knob so that the set screws bite on the brass not the Loctite.
Of course, this is dependent on how much metal you have to work with on the knob.

Good luck and have fun.

 

[mikepick]

Good tip re: brass tubing, worth a try if the epoxy doesn't work.

One difficulty is that I can't find a good picture of a whole shaft to compare. There is a big divot visible in the photo above, in the inset groove, that doesn't seem right either, so I think I may fill that in as well.

 

[dstariha]

The channel around the shaft may be there to locate the knob. I probably would not fill it in. The divot appears to be a later mod, maybe also to locate the set screw. It may be worth looking around to see if a new or near new volume control can be found.

 

[mikepick]

Update: I've refurbed the insides of this amp, next up will be trying to fix up the exterior.

I didn't take any pics of the preamp and controller board in progress, but they went reasonably well.

One thing I hadn't anticipated was that the Elna Silmic caps are basically the same dimensions as the original caps, unlike the Nichicons which tend to be much smaller. Going up in voltage can mean awkward fitting in tight spaces like this preamp board.



Filthy power board! In the upper right there are a few new caps where I had pulled a few components to see if I could track down an issue.



I wanted to pull the outputs and put in new micas and compound, so that was the next step. It turns out that you don't need to remove the heat sink to do this, as the outputs simply drop in to the little boards under the heat sink, but it did make it easier to clean everything.



Reassembled and cleaned power board. I replaced the electrolytic caps, cleaned and reassembled the heat sink and outputs, added small heat sinks to Q31/32, replaced resistors R75-76 with 1W metal films, cleaned up the fuses and holders, and touched up the solder points around Q34 where I think the foam padding caused it to overheat.

I made a mistake while composing my mouser order and did not get a bipolar cap for C39, so that will have to wait until I put together another order.

 

[mikepick]

Finally, a few assembled pics. After assembling and a dim bulb test, I set the DC offset and bias and then changed out Ve1/2 for Bournes pots. The old offset pots where VERY touchy, a hairline move would swing it 20-40mV. The new pots are much easier to dial in. I didn't bother with the bias pots since they were actually very easy to dial in precisely.



I used little stacks of window foam insulation tape to provide support for the power board. For the center front I shifted the foam forward so it supports the lip of the board but doesn't cover the solder points around Q34.

 

[dstariha]

Looks very nice. I am guessing your fix for the volume control shaft worked well. Congrastulations.

 

[mikepick]

Looks very nice. I am guessing your fix for the volume control shaft worked well. Congrastulations.

Well, that's a little funny. I ended up filling the divot in the shaft with epoxy and I think the set screws have a better grip now.

But I'm starting to think there is something else going on with the pot. When I had the control board out, I flooded the pot with Fader Lube to clean it up, and it worked well enough to feel and hear the detents again, where before it was mostly a smooth rotation. Now there is a point where I can feel the detents, and then I turn the pot all the way down, and then it seems to slip an eighth further and the detents cannot be felt until it is turned up to half or so again.

Needs more testing, I haven't had a good chance to run it at volume yet.

The other question I have is, how does a wooden case attach to the chassis? In most units I have seen, the bottom cover has tapped holes for the foot screws, but mine has feet attached by clips. Do I need to tap new holes, or find a way to attach screws where the current feet are?

 

[mikepick]

I finally had some time to revisit the "slipping" volume pot, I figured that there must be some stop which was not engaging and causing the pot to over-run its normal axis.

So I desoldered it from the front board, pulled it out and disassembled it.



Basically it's about peeling up these little pins at the different levels that clamp it together.



Back end off, the wipers on the rotary dial and the contact pads are pretty dirty.



Fully disassembled. There is a fair amount of grease in the top which I guess is meant to give it some tension when turning, and prevent it from seizing.



Close up off the top end.Basically the bearing sits in that groove and rides over plastic ridges in an insert, and that's how you get the detent feedback as you spin it. in the circle is a tab which is bent down from the top cover. It functions as the stop at either end as you turn the knob.

It's a bit hard to make out, but the pin is bent such that it was pushed back out again. So when you turned the volume all the way down, it functioned as a bit of a ramp instead of a hard stop for the brass plate. The plate lifted a bit and the bearing goes under it, from the cutaway where it sits, and so it was stuck and didn't rotate until you go all the way in the other direction and reset it into its groove. So no detent.

I managed to get the pin straightened again, and cleaned and reassembled the pot. Popped it back in and its working, so far so good, although I am still treating it a bit warily and the low end of the rotation... I suppose I will use it normally once I get more comfortable.

 

[dstariha]

Great job on a part not meant to be disassembled. I love repairing things like that as it provides a great sense of self-satisfaction. Bask for a while.

Dave

 

[Brad Lehmann]

I love to take things like that apart. Seldom manage to get them back together correctly, though. I picked up a 7100 a couple of weeks ago along with a KT7500 so I'm now enjoying some good tunes in the house. I did have to do some cleaning on the pots and slider switches. It kept losing the right channel when good and warmed up. I think that it is fixed now. Now I want to buy another project amp to tinker with.

 

[Wilfried]

I read your thread with much interest, working on a KA-7100 right now myself.

I was wondering which capacitors you used for the main filters. The general recommendation here seems to be one of the Nichicon's with 12.5mm lead spacing (typically KW), but from your pictures it looks like you used something different.

Also great to see how you fixed that volume pot, and how it looks on the inside. I had a slight mishap with mine: I like to desolder switches and pots for cleaning, because my contact cleaner from Kontakt Chemie leaves a mess on the PCB. While carefully straightening one of the pins, it broke off where it was bent.. Still fixable but really frustrating, never happened to me before. I looked like it was over-bent at the factory, fatiguing the metal pin.