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Old 24th July 2014, 07:41 AM   #1
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Default Kenwood KA-3500 and my blooper

whilst attempting to adjust the bias on this amp, my probe slipped and i shorted the Qe23 side of Re47 to the Qe19 side of Re43. this introduced an immediate hum, and amp now draws excessive current. i immediately shut down the amp. thinking that i may have stressed the smoothing caps Ce29 and Ce30, i replaced them (not cheap-GAK!), but still have the same hum and current draw.
any suggestions you guys might offer would be greatly appreciated.
best,
aidan
(red faced and embarrassed)
http://flipperpie.com/KenwoodKA-3500Service.pdf
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Old 24th July 2014, 08:04 AM   #2
Mooly is offline Mooly  United Kingdom
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Not the caps... you will have blown one or more output transistors and possibly the drivers as well. Check all the low value resistors around the output stage for damage too.

(Its a dead easy repair if done methodically but you will need suitable (alternate) replacements and please please please use a bulb tester to prevent damage)
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Old 24th July 2014, 08:08 AM   #3
Mooly is offline Mooly  United Kingdom
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These will give you an idea of what is involved. All similar amps follow exactly the same faultfinding procedure.

Marantz PM-66SE KI - bin or repair?

Kenwood Ka-1500 sounds weird(youtube link included)
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Old 24th July 2014, 09:41 AM   #4
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Mooly, thanks very much for your quick response. i have a variac, and can run the amp at a reduced voltage. at 30V the amp hums, and draws .7 amps. before my "repair" (read: blunder), the draw was a comfortable .2 amps.
Question: would blown output or driver transistors cause such excessive current? i am off to work soon, but will check back later today for any tidbits of assistance you can offer. the resistors/transistors mentioned in my post are on the 12th page of the 13 in the pdf service manual, bottom right corner of that page in my link.
again, thanks so much to you any any others for any and all assistance offered. the amp belongs to my son, and i was hoping to bring it to him in toronto when i visit next week. i am so peeved with myself!
aidan
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Old 24th July 2014, 09:45 AM   #5
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A variac is OK (in many ways ideal) but it can still deleiver lots of current even at low voltage output, something the bulb doesn't. Yes its fine... just wind it up slowly.

The symptoms are classic. Measure those transistors in circuit using a diode range on your meter and I'm sure you'll find one or more reading really low across the collector emitter junction.
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Old 24th July 2014, 10:07 AM   #6
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ok, will do. i'll rig up a dim bulb device tonight and report back then.
if not too much to ask, can i impose upon you to walk me through the checking of these transistors. i am of the vintage where i am very comfortable with hollow state gear, but have little-to-no experience with SS devices.
if you have a moment, could you perhaps take a quick look at the schematic and nudge me as to which components i should purchase today? due to the sentimental value of this amp, cost is of no concern (well okay, of some concern)
again,
aidan
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Old 24th July 2014, 10:20 AM   #7
Mooly is offline Mooly  United Kingdom
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All these need checking. Tbh I would automatically replace the outputs and drivers (the four on the right). The other transistors may/may not have suffered but they cost buttons.

You won't get the originals, you'll have to fit alternatives. The physical style of the outputs determines what will be suitable, the others its less important.

But you really need to do some basic checks first just to confirm what has happened.
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Old 24th July 2014, 10:24 AM   #8
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Shorted transistors in many possible areas such is the Vbe multiplier , driver or output area is the cause of excessive current

In no case i think that an open resistor will cause excessive current draw

I am sorry to say that if you have blown the outputs you are doomed since these are totally unobtainable

In this case choosing replacements will cause mounting issues and the choice of replacements is a very tricky story since this amp is already marginal and poor choice of even good transistors will end up to catastrophic oscillation .

If outputs and drivers are to be replaced both bias scheme and compensation needs to be replaced with other values

You got your self a real trouble ...

Nobody please rushes in to suggest 1943-5200 or MJE 15030 for drivers and outputs been tested a million times and not working properly there ...

Been there a million times
Kind regards
Sakis
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Old 24th July 2014, 10:27 AM   #9
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I can see the output transistors being a problem because they are physically an old obsolete type looking at the picture in the manual. That means you need to be sure you can physically mount a replacement on the heatsink successfully using a single nut/bolt together with the correct insulating kit for the replacements. Its all easy stuff, just something to be aware of.
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Old 24th July 2014, 10:30 AM   #10
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holy schmoly. molly, tyvm, and east electronics, as well, thank you so much. i'm off to earn my bread and butter, and will report back tonight.
cheers for now,
aidan
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