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Kenwood Model 600 and its fets

MeAN

MeAN

Kenwood addicted
Hello,
I've just received my "new" project: a Kenwood model 600 that I bought on Ebay.de
Among the many troubles that this poor unit is showing, at first glance I saw that some of the preampli section's fet were ripped off (don't know if because they were broken or just for "spare parts"), while other have been changed.
The fets are 2sk68A(N) 2sk68(L) 2sk68A(K) and the replacement used are 2sy18 and 2sy18/2.
Apart any consideration about the substitution, I'd like to place the original fets but I was only able to find plain vanilla 2sk68.
Is this fet suitable to replace also the 2sk68(*) and 2sk68A(*) or should I mess around and look for the very, absolutely exact match ?
Btw, the only working thing (after I changed the blown 500mA fuse) in this unit is the power lamp :tears:

Thank you for your consideration
Carlo
 
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Wow, that is really too bad. Sorry to hear about your Model 600. Was it advertised as working? How are the cosmetics, are they good at least? I can't help you with the FET question, sorry.
 
The 2SK68 and the 2SK68A are identical except the A version supposedly is low noise. It is probably not manufactured differently, just selected for low noise from the 2SK68. You can select them yourself, just build a small board with that section of the preamp circuit on it, and a socket for the transistor. Connect either a scope or a DMM to the output. Short the inputs and select for the lowest noise. As an alternative you can use a 2SK30 which is a very, very nice low noise fet with the same voltage rating and similar Idss. It would be a plug in replacement. They are dirt cheap, too, I think I paid $.25 each for them.
 
@dr*audio
Thank you so much Warren for your precious help and suggestion: you guys are great :thmbsp:
Now it's up to me choosing one solution or the other.
I'll have to decide if I'd prefer to keep the originality of the unit or make what it seems to be a small upgrade..

@Inspiribomb
Aaron, thank you for your solidarity.
Actually the unit was sold as defective - "no power on" - so that I cannot complain too much. I'd also wait before I blame myself beacause of the purchase: this unit could still be a bargain at 139 euro.
This weekend I'll go further with the investigations and I hope that I'll figure out better the situation.

Anyway, as I don't wish to hijack your thread on the vintage forum, I'll post here some pictures of the beast.
Btw, it's in fairly good cosmetical conditions: just a scratch on the lower part of the faceplate; at least I could still use it as a costly but nice paperweight :D

Bye for now,
Carlo
 

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A Kenwood 600 by any other name will still sound just as sweet! :yes:

I have faith that you'll get 'er up and singing! :thmbsp:

Scott (Really wanting to join the 600 club!)
 
It's done!
I've spent the whole weekend on the project, but I got rid of it :tresbon:

On the power supply board I had to perform a full recap, changed 4 transistor, 2 zener diodes and a couple of 1w resistors.
After this the final part of the ampli was working, but the pre was not.

I thereafter realized that the missing fets were from the phono section of the preamplifier: the other parts of the pre were intact - or at least all the components were there.
I started to check here and there about what was not right and I noticed that there was a heavy tension dispersion on the hull of the output transistor: I can really feel it just touching them. Further investigation showed that all the inputs RCA where showing a 16V tension on them. I suspected then a faulty condenser somewhere... must be in the pre board or tone control or filter board as the main amp section was working.
Then it came to my eye that one of the electrolytics on the tone board had a red mark on its top... I knew that the unit was serviced or at least it had been diagnosed and an estimation had been made but it was returned unrepaired (there was a sticker in german on the inside saying this: KV erstellt/unrep. zurueck).
I assumed that the previous tech identified the fault but he didn't repair it.
So I changed the cap and its twin for the other channel - just to be sure; the cap that was marked showed a large boulging in its bottom part, it was obviously to be replaced, but still when measured with a capacitor meter and tested with the MM showed correct values.. moral: don't rely on this instruments when checking for a bad cap :nono:

What to say.. the thing is now working :thmbsp:
Adjusted the bias - expected 25mV, was 12mv and 65mv(!!!) - then the DC-offsets that were -4.5mV and 25mv. and went for a listening session.

Now I have plan to completely recap the unit and to place new fets for the phono section, but at least I was able to hear the beauty's voice: really really really ashtonishing in its perfection!!!!!

Bye for now,
Carlo

PS.:some pics will soon follow.
 
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Excellent! :banana:

Gotta love saving another vintage piece... and such a nice one to boot! :thmbsp:

Scott :music:
 
Congrats Carlo! That is sweet, it is such a nice amp. I may be contacting you in the near future regarding both my 650 and KA-7100.
 
Congratulations, Carlo! Since the Fets are in the phono preamp, use the 2SK30s as replacement; you really want low noise Fets. The phono preamp in this amp is excellent, you will be very pleased. :tresbon:
 
Thank you everybody!

@dr*audio
I've just placed the order for the 2sk30s. Do you think that I have to select them in pairs basing on their IDSS or may I put them on the board without any coupling (I mean coupling for L-R channel, so to place equal fets in equal positions.. :scratch2: ) ?
Matching wouldn't be a problem as I ordered a stock of 22 just to reach the minimum order.. :)

cheers,
Carlo
 
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I have used them before without matching, but since you will have so many you may as well.
QD1 and QD3 should be a matched pair, QD9 and QD11 should be a matched pair. QD5 and QD7 do not have to be matched. The other channel is the same. Theoretically this would give you lower distortion, but since there is feedback in the circuit, I doubt if the difference is audible.
 
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Well, if *you* did, who the heck am I to object ? :D
I think that I will let the fate decide their matching..

Anyway, here're another couple of pics: that is the power board before and after cleaning/recapping (but I still didn't change the BJTs, the zener diodes or the metal film resistors).
Please note that the power board has a plug in connector (like a PC card) so that you can take it out, confortably repair and then plug it back... God, I *love* this ampli :thmbsp:

Cheers,
Carlo

PS.: For sake of couriosity: the caps that I used were mostly Panasonic FC (blu/gold), 4 philips very low ESR, a couple of Nitai bipolar (the yellow ones) and 1 Panasonic FA (just because I didn't find its FC counterpart).
 

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Carlo, are all the boards on the 600 like that, the "plug and play" type? I seem to remember seeing a bunch of wire wraps in mine, but I don't have it in front of me. If they are all like that, hell, I'd recap mine myself.
 
Unfortunately not. :no:
Especially the tone and filter boards are almost coupled together and it's a hell of a work to get there to change components (and.. did I say that you have to dismantle the whole faceplate ?).
But.. I didn't want to say this.. :)
You see, I just look at its positive facts.
Maybe it's because I'm pretty biased :D

Ciao,
Carlo
 
Nice job on that power supply board, Carlo! My amp came missing that board entirely, I had to make one from a pad per hole proto board! It is all hand wired, but does plug in as original; I found a proto board with an edge connector on it.
 
Wow! That's what I call extreme repair :thmbsp:
This encourages me: I'd have to do the same for the phono board on my Rotel RA1312. That is also completely missing and I had plans to rebuild it from scratch.

Cheers,
Carlo
 
Carlo,
Do you have a schematic for the Rotel? I have that amp also and have been unable to find a service manual for it. So far I've been able to fix it's problems without one, but I would love to have one.
 
@Inspiribomb

Here's how *it* would look with the missing lever... :thmbsp:

:) :) :) :)

Ciao,
c.
 

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dr*audio said:
Carlo,
Do you have a schematic for the Rotel? I have that amp also and have been unable to find a service manual for it. So far I've been able to fix it's problems without one, but I would love to have one.
Click to expand...

Of course I do. Unluckely it's one of the few that I only have on paper, otherwise I would have sent it to you immediately.
I'll try to scan it for you this weekend and send you afterwards.

Cheers,
Carlo
 
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