Kenwood KA-3500 and cone excursion on start-up

[minim]

Greetings and salutations,

I just had this amp repaired at a business owned by an old couple (and I do mean old, thirties vintage I reckon).
The amp works fine now but is behaving differently on start up. There is a noticeable deep 'thud' when switching
the amp on. I do not know if it's an issue. My concern is for speaker longevity. The initial symptoms prior to repair
were a loud tone (quite musical, like a 'B' note) emanating from both speakers, and full constant excursion of both
speakers, as if there was DC present. They apparently changed a resistor and in addition touch up some
'connections' (dry joints I assume). I am happy now with the performance of the amp, however it does also
seem to run warmer now. I guess noticeable warmth (not hot) rising through the top grille is normal?

I have attached a link to my youtube account, it is only a 7 sec vid.

I would greatly appreciate it if you would have a look/listen.

https://www.youtube.com/watch?v=KnWNoowrxhQ

Mitch

[g1]

Some heat is normal. Is this a unit you had working properly before? You said it is working differently at start up, so it sounds like you did not buy it in the broken state.
Thump at turn on is not uncommon, but if you are sure it was not there before, then there could be something that has changed. Is it possible you were turning off the speakers before powering on/off previously? That would get around the issue anyway.
If you are sure it is running hotter now than before, it's possible the bias has been set improperly.
Instructions for the bias adjustment are in the service manual available here:
https://elektrotanya.com/kenwood_ka-.../download.html

[The Dude]

......If you are sure it is running hotter now than before, it's possible the bias has been set improperly.
Instructions for the bias adjustment are in the service manual available here:
https://elektrotanya.com/kenwood_ka-.../download.html

^^^^^^^ Yes that. If the amp is biased too hot, it might also account for a more noticeable pop at start up. I have an old Yamaha amp on my bench for testing. It, too, has a pretty harsh pop on startup. I've just developed a turn on sequence to work around it. I shut off the speaker switch before powering down the unit. On power up, I turn the amp on, then the speakers. It saves the speakers from the short blast of DC at power up. Probably not necessary, but easy enough to do.

[minim]

Thanks guys.

Yes the exact trail of destruction was: I bought the amp. It worked great for a week (no thud on start-up even with speakers already on). The amp sh!ts itself with the aforementioned 'tone ' and full out constant speaker excursion. It was only idling at that point - the record had ended. I had it repaired. It now works and sounds great, albeit with the addition of the thud on start-up and I am 90% sure it's running warmer than before.

I look forward to checking the bias tomorrow. I am quite happy working on my tube amps and guitars etc, but hi-fi is not something I'm familiar with yet. And if I have to I'm sure I'll get used to flipping the speakers on after powering on.

Thanks for the link

[minim]

Well I think I've really screwed the pooch now...

I am so pissed off with myself I can barely type without using profanity every second word, but here goes:

Everything was going well enough, checking the bias. It was however extremely high to begin with...about 270mA. I got it down to 40mA. Then I let slip my meter probe, a tiny spark appears, and the 3A fuse blows.

I have shorted Q21. All three readings show dead-shorts with my ohmmeter. Really livid with myself. The problem is that these outputs seem rare as hen's teeth. I have scoured various pages and datasheets and the most conclusive thing I can come up with is a couple of threads on DIY Audio where an experienced Greek tech basically says "fat chance". He did sub some other transistors in one of his repairs, but went on to say he also had to change driver transistors and one or two additional caps - i.e. the subs weren't easy drop-ins.

The blown transistor is NEC D587A. The parts list in the manual reads: 2SD588. I have seen suggested replacements, e.g. http://alltransistors.com/transistor...ansistor=33645

The specs are similar. Any thoughts?

Has anyone had any luck tracking down these transistors from a reliable source? I found one or two on ebay in Asia for example, but have not had any dealings with them.

Mitch

[The Dude]

There are some on eBay. I'd have posted a link, but there are different case styles, used/new, etc. You can do an eBay search and see if anything suits you.

[minim]

Alright thanks for that. I've ordered some from a seller that has the least-bad feedback, so I'll see how that goes.