Kenwood KA-3500 recap

[Dingman]

Since swapping the 10K caps to 4800, I have not been able to reproduce the "static at turn on" issue (high DC on speaker terminals for a few secs)

I do have the proper value ordered and will install next week - 6800\50v.

Testing shows that a 100ohm pot in series with a 100ohm resistor for R15\R16 can bring DC offset down to about 5mv (both ch landed at about 25-28mv without adjustment), so I'll be doing that for final config.

 

[Dingman]

This should be last update. Installed 6800uf to match original specs. Those 10K's were just to large for this amp - I didn't check why, but I'd see high DC on outputs for a few seconds at turn on. Normal now, DC on startup doesn't exceed 1VDC.

I've still got just a little fluctuation on the DC offset - bias is stable. I've re-matched more 992's for the diff pair, no change. Installed some 970's - no change. Installed original diff pair - no change.
I shouldn't be worried about this - it's no more than 1mv change. So, I'm leaving it alone
I did install trimmers for the offset and I've got offset running about 4-5mv.

Re-assembled and "burning in" with some Minimus7's. SQ seems about right, but of course that's subjective and owner will have to decide if he likes it. He's running a Pioneer SA-8100 that I provided since I kept this kenwood way, way longer than I should have (sometimes, I just get involved in other projects\hobbies and audio just comes to a full stop).

 

[Dingman]

So, as I mentioned, I'm really just some guy with an AA in electronics, focused on motor control\automation. So I have a question.
I've been reading up on how to use a scope to check amp performance, and I think this isn't bad but not sure to what degree I would say "this is a problem" or "this is good".
So, here's a few pic of square waves out of the amp. Can I get some opinions? There's freq\voltage info in the screenshots.
These waveforms without a load:

The square wave isn't perfectly square?
EDIT UPDATE - I've been looking at other amps and reading on the web. This is a pretty good waveform.

 

[concertfiend]

Well I'm glad someone logged such a detailed thread on recapping this amp, I think I'm going to pass on using it as my first recap I'm excited to start my journey learning how to fix audio gear, but I think my Kenwood KR 5150 will be a better one for me to start on... There's just one big board smack dab in the middle, everything is easily accessible. I do like the 3500 but I want to move on to bigger and better things, so out it goes!

 

[cabby]

getting towards the end. No transistors (other than the two replaced earlier for repair) will be replaced. All electrolytics replaced. All ceramics left alone. Filter caps upgraded.
Not sure why I do this, but cleaned the fuse. And fuse holder.
View attachment 917761 View attachment 917762 View attachment 917763

If you look just adjacent to the fuse there is another slot on the board for another fuse. Why is that one empty? Mine is the same way, just one fuse. I looked at some nudies on Google and some appears to have two fuses? Any ideas what is going on?

 

[gort69]

Some that were sold outside the US have a voltage selector switch on the rear panel so they can be operated on 110/120V or 220/240V. Two primary windings in the transformer, each with its own fuse.

 

[cabby]

Some that were sold outside the US have a voltage selector switch on the rear panel so they can be operated on 110/120V or 220/240V. Each switch setting has its own fuse.

must be the euro ones. thx!!!

 

[mikepick]

Glad this thread resurfaced, the DC offset solution is of interest to me.

 

[Dingman]

Interesting to me is that I thought I was just wasting my time, and getting way too OCD on this thread.
I thought "no one in a million years is going to care about this".
But look! There is one (two?) person out there who read the whole long, drawn out thing. Heh, this work did span many months.

So, ya on the DC offset, I just stick a POT in there, dial it in to determine needed value.
Remove the POT, stick in proper wattage\value resistor.
As I recall tho, I never did get the DC offset to settle down completely. If this is the unit I'm thinking of, it drove me to distraction.

 

[RanDibble]

Thank You for YOur EFFORTS !

I received a KA-3500 as a gift, I intend to restore it and give it to my Grandchildren.

This thread has helped me out a lot.

They think good audio comes out EarBuds ??

 

[Dingman]

OMG, earbuds - noooooo!

I admit to being a total amateur on these repairs\refurbs - so if I can do it, so can you. Go slow and take tons of notes and pictures.
Personally, I thought this was a sweet sounding unit - ya, BOTL but I hooked it to some Klipsch for testing and I'm saying it sounded sweet at low volumes.
I haven't heard from the owner, I wonder if it's still working OK for him.