KA-7100 recap

_Attila_

Asking & asking & aski...
I’m about to start recapping my beaten-up KA-7100 (see here), and as this is the first time, I’ seeking for some help with the choice of the components (caps). I was thinking about Elna Silmic II for the audio path and Panasonic FM for the others (following EchoWars recommendations). Of course, I’m open for any suggestion. The main problem for me is to identify which caps belong in the audio path and which not. Some are obvious, but not all. I have the schematic of the amp, and can partially read it (I’m a biologist...)…
From what I read, the electrolytic caps between 0.1uf and 1uF will be replaced with stacked film caps: Any recommendation of what brand and series to choose?
Should I stick with the original values for the caps, or go one level up for the voltage?
Anyone has maybe already a caps list?

I was also thinking about replacing the transistors (not the output transistors): does this make sense?
What about the diodes?
Any other suggestion?

So far I’ve got through the parts list, and started checking with what’s on the board: Already find some differences.

Any thoughts?
 
Hi Attila.I think if the transistors don't show symptoms you can leave them alone.They should have a half life of 50 million years or so.
Good question on the audio path.
Also I think staying with the original values is best.
As for the caps list I have used the amp itself and just write down the cap designation such as C409 or whatever and the values.Then be sure you got them all with a count of whats on the board against what the schematic tells you is in there.Its a small problem to have to order one capacitor at a time because you missed one or two.Then pay postage on one item and have to stall the project.
Take it slow.
Pictures are much appreciated.
Good luck.
 
At least I will change Qe34, the relay driver. It is know for dying very quickly, so I'll just order a couple with the caps.
For caps values: I though going one level up for voltage by sticking to the capacity would be better, as it reduces the stress on the caps and in this way extend their lifespan.

Another new problem I discovered yesterday evening: one thermistor (STP-41L) isn't "glued" anymore on its bracket. Any possibility to glue / fix it back? I will try to get a picture this evening.

If there is some interest, I could document the whole overhaul process.
 
Hi Attila,

I fix that with normal epoxy glue, shouldn't be a problem up to ~ 150°C.
http://en.wikipedia.org/wiki/Epoxy
When recapping I usually buy 105° condensers instead of usual 80°. They are better built and last more. I also go higher with voltages, newer condensers are smaller than old ones and going up with voltage means that you get physically similar condenser to old one and it works with higher margin to its limits. Just find the ones with equal raster of the old ones. I never buy audio grade condensers, I prefer finding ones that are not called audio superhyperdontknowwhat, have a normal cost and sound good. And I'm trying to avoid water filled ones, I don't want to recap every two years.

LIX
 
Hi lixaudio,

which brand do you use for recapping? Or at least, which ones sound good?
For the thermistor I'll try with thermal glue... I think I can get a small quantity for free...
As for the 105°C grade, I'm going Panasonic FM series (for the caps not in the audio path), so it's fine.
 
Could someone check in his 7100 the value of R1 and R2 on the power amp board? From the schematic I read 1.5K. In my amp: nothing, they are missing, it looks as if they've been cut out of the amp...
Would really appreciate some help here!

And as I have to replace them: carbon or metal film? Does something speak against the metal film resistor?
 
Hi Attila,

yes R1 and R2, the two on the input of the µPA63 are 1k5 on the schematics. I would put them in. I would put metal film.
Right now I'm using Jamicon 105°, I can get them for a fair price. I would prefer Rubycon ZX, but can't get them locally. Panasonic FM are suitable for audio path, anyway - what is not in the audio path, even your wall plug is :)) Aren't FMs water based - FM lifetime is 7000 hours at 105*C . Ok, you're not going to push the volume so high to reach 105° in the amp.
 
One more (important) question:
How to identify caps in the signal path? From what I read, caps in the signal path are the coupling caps, meaning the caps which are in series in the audio path. Right?
Meaning that caps that go to the ground are decoupling caps and are not in the signal path. Right?

To check if I got it right, here an example (please see attachment, part of the 7100's schematic), red indicate the signal path:
- Cd 17 (marked green) is in the signal path
- Cd 22 (yellow, left side) is a decoupling cap
- Cd 13 (brown, on the left on Cd 17) is a bypass cap. It's ceramic, but IF it were electrolytic, would it be considered in the signal path or not? I would just like to understand a little bit more the theory...
- Cd 7, 9 and 15 (blue): for me: bypass caps, right or not? Cd 15 is an electrolytic, and as I said, for me it's not in the signal path...

Or am I totally wrong here? I would really like to understand a little bit better the schematic and would be happy about any help!
Thanks a lot!
 

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The originals of many of the Kenwood manuals show the signal flow path for the Left channel using a bold line (others do the same thing with a red halo around the dark lines). The only signal path cap in your pic is Cd17.
 
EchoWars,
thank you very much for your input! It really helps!
And yes, you're right, the signal path is shown as a bold line for the left channel. I just added the red color to make it more clear here.

Here is my next question, regarding the replacement of Qe34 (relay driver transistor). I understand that the 2N3904 proposed by some people could get problematic as the voltage supply is 42V (Vmax=40 for the 2N3904). I also understand that the choice for the replacement isn't critical as Qe34 is used as switch. If I'm right, I just need to care about the Vmax, the current handling capabilities and the power dissipation. My choice would then be 2SC1318 from Panasonic: here. Does that sound right for you?

Again, thanks to everyone for the help!
 
Well, next batch of questions... :/

1. For a couple of caps, I need 3.3 µF 25V.
The caps are Cd17,18 , Ci19,20 , Ci23,24 (see attachments).
Cd17,18 are in the audio path (phono preamp) and I would say Cd17 and 19 get in the signal path when using the tone control (see question 2.)
This is what I can get for substitution:

3.3 µF 25V Panasonic KK
4.7 µF 25V Elna Silmic II
3.3 µF 50V Panasonic FC
3.3 µF 100V Panasonic FC
3.3 µF 100V Elna Silmic II

What would you choose?
For me, 100V is far too much, I read somewhere that electrolytics should be used in a range between 75 and 95% of their voltage ratings (not sure if this is true!)...

2. To see if I get the point (looking at the second attachment (Ci.jpg): when tone control is in use, the selector Si5 will change it's position, and Ci23 will be in the signal path, and then I'm lost, it gets too complicated for me :/ Which other caps would then be in the audio path? I would go for Ci17, 19 and 21...
In the same way, I would say that Ci25 is in the signal path if Si6 (subsonic filter) is open...

This is getting way more complicated as I thought as the beginning! But I willing to learn and understand! :yes:
 

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The schematic is NOT going to show you every possible signal flow path, and I can't either (C17, 19, and 23 are all in the signal path, depending on switch position). There's too much going on. You have the right idea though.

Use the 4.7µf 25V Elna.
 
This was a quick answer! Thank you so much for the help!
I know the schematic doesn't show every possible signal path, that's why I try to understand it!
 
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