Another KA-3500 with some issues

soldeerbout

New Member
Another Kenwood KA-3500 with some issues

Hi All!

First post here at AK.

I’m doing a restoration on a Kenwood KA-3500 (European model) and could use some technical advice.

The problems:
1. There was a lot of distortion at both channels when volume is turned up. (solved)
2. When you turn on the amp you hear a loud scratchy sound, volume knob does not effect the amplitude, its only on the right channel, and lasts about 4/5 seconds

What i’ve done so far:
- Cleaned the knobs and switches
- Replaced the powercaps (problem 1. solved) and a few small capacitors around the rectifier. C27 t/m C31

Now I’m trying to locate problem 2. It must be in the poweramp section (or powersupply??) because I disconnected the right channel signal wire between preamp and poweramp (blue one), problem was still there. Further more, there are no sources connected. When you switch the power on and off fast, the problem does not occur. Also when you switch-off the amp and then turn up de volume, the sound is there for a few seconds.

This amplifier does not have a delay relais for the speakers to get signal. Somebody told me that there is circuit on the poweramp board that has to have this function, anybody know more about this on the KA-3500?

Really like to get this puppy going again, first try and solve the problems and then do a total recap.

Hope you Guys have some helpful thoughts on this one.

Thank you in advance.
 
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There is no relay on this model. The delayed switch off is normal.

The noise on one channel in the poweramp could be several things but being as it is predictable at 4/5 seconds, it suggests a cap related issue. (try C2,8,14,16)
 
Hi John,

I replaced the caps with following values:

C2 50V 0,47uF
C8 16V 47uF (Original 10V)
C14 16V 220uF (Original 10V)
C16 50V 47uF

Unfortunately the start-up problem on the right channel is still there.

Do you have any other suggestions, or should I just replace all the capacitors of the power amp?
 
I have replaced C22 and C24 as well, still same issue.

when I was replacing C22 I saw the legs of transitor Q20 are very much corroded. could this part cause the problem?
 
Soldeerbout:

Welcome to the forum!

The KA-3500 is a well respected amplifier - once restored it sounds very good - much better than almost any of the modern amplifiers available on "Sale" at the big box stores right now.

That said, you need to be very very careful - the D587/B617 output transistors are impossible to find/replace. The same is with the C1735/A850 drivers.

On the couple of KA-3500's I've worked on, the Q19/Q20s (A850) were always found to have black oxidized legs. I don't believe that is the reason for the scratchy sound in your unit.

Have you measured the DC Offset? The input differential pairs are an easy fix if the offset is high.

Strongly recommend a complete re-cap. You will not regret it.

Rgds
Mayank
 
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