Kenwood KA-701 issues. Please help

Prime Mover

Active Member
Hi there. This is my first post but I've been having a good read through the AK forums for some info regarding some issues I've been having with a recently acquired KA-701 amp.

I just moved to the UK from Australia (leaving my vintage gear behind :sigh:) and was looking for something to drive some B&W bookshelf speakers I tracked down (had some and liked them so I bought another pair). After doing some research I eventually decided on a KA-701 which I found on eBay in "perfect working order" of course. It arrived shortly after and fortunately was well packed and looked to be in excellent condition.

Before hooking anything up I did a quick DC offset check and everything looked good (under 20mV for both channels). The protection relay also clicked on as it should a few seconds after power on. So I decided it was at least safe too hook up my speakers and a source.

After powering up and playing some music everything was great (very happy especially as I could not audition the unit before purchase). The musical enjoyment was however rather short lived. After about a minute or two the left channel made a "pop" and became quite badly distorted for a few seconds. This was followed by another pop and a return to normal, again for a few seconds. The process would repeat over an over. The amp was fine when completely cold but when even slightly warm it would exhibit this behaviour.

Fortunately despite leaving most of my gear back in Oz I did manage to get my scope and multimeter over so I set about tracking down the issue (I'm willing to give the seller the benefit of the doubt and I really don't want to have to track down another amp). Despite being more competent (and confident) with digital than analogue circuitry eventually I traced the issue to the pre-driver section (specifically Q9, a 2SA899). I confirmed that this was the cause of the popping by swapping that transistor from the left side to the right side of the amp. The popping followed the transistor so a new one (along with a few spares) has been ordered.

After getting the left side working I set about checking the bias. Before stealing the 2SA899 the right side was exactly as specified in the service manual (26mV) but after repair, the left side was quite high at around 70mV so I used VR1 to adjust it down. However even at the lowest setting it would only go as low as 45mV. I checked the voltage across D1 (an STV-4H) and it appears normal (2.25V or so) so I'm a bit stumped as to why I cant get the bias down and I'm wondering of anyone here might have some insight as to what could cause this? I haven't replaced any of the caps yet but I have ordered them but it doesn't seem likely to be the cause. I have a pdf of the service manual with schematic but it's a bit too big to attach (5.3MB).

Even with the bias at 45mV things still seem to stay reasonably cool. Perhaps I am worrying too much?
 
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