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Amcron DC300a overhaul.

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Young Croc
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Post Options Post Options   Thanks (0) Thanks(0)   Quote APW Quote  Post ReplyReply Direct Link To This Post Posted: 30 April 2020 at 3:54pm
I think its time to get the scope out!!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tonskulus Quote  Post ReplyReply Direct Link To This Post Posted: 14 May 2020 at 1:35pm
Hmm. Have you checked the bias? Measure the voltage across driver stage 12ohm emitter resistors (location: output pcb's).  It should read about 0.35Volts.  I think you have service manual?

Few months ago I bought Amcron DC300A. One channel weak and distorted, bias was missing.  There was faulty transistor in bias voltage source, MPSA93.  After that, I also changed all electrolytics (except PSU filtering) and all zener diodes. Fine tuned the offset (output/input) to 0mV.   
Amp is now working like a dream! Its all original except those components. 

I really love this amplifier, so clean sounding and exceptionally tight bass response. Very good cone control and plays amazingly loud when needed! One great piece of audio engineering. 







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Post Options Post Options   Thanks (0) Thanks(0)   Quote jbl_man Quote  Post ReplyReply Direct Link To This Post Posted: 15 May 2020 at 8:37pm
Yes,im gradually working my way around it, changed all the electrolytics,and well over a dozen of the (expensive) silver mica caps too,as some were very discoloured where they have got too warm,and were reading all wrong. Replaced a few old carbon resistors that were off-spec for some nice new metal oxide ones.

Bias is within spec, 0.3v across the bias resistors on output stages. output mv is 0.01 and 0.00 per channel,but still distorted/weak output.  I have +90v at the diode pump,so all ok there. Will get there eventually.
Be seeing you.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tonskulus Quote  Post ReplyReply Direct Link To This Post Posted: 16 May 2020 at 11:28am
Good luck with it!  Just take some time, using scope and voltage meter. Compare both channels beetween the stages starting from the input stage.

Pretty staight forward amplifier it is afterall :)

I never changed those mica caps, even tho they seems to be darkened but it shouldnt be issue for mica types (I think they will last forever). Are you sure they were out of specs?  They are very low value caps so measuring technicue is critical. Not all capacitance meters are able to measure 1..100pF values reliably.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote jbl_man Quote  Post ReplyReply Direct Link To This Post Posted: 20 May 2020 at 6:50am
Ok,update time...have replaced all burnt capacitor.  Several old carbon resistors were out-of-value too,so replaced.Still same issue. IC replaced with new one.

Here are the voltages...clearly Q102, Q120, Q103 and Q220 cant be right...yet all test fine, as do all the diodes.

IC pin 1 should read +7.5v ,which feeds the base of Q1 (the silver 2N4515).....im actually getting -2.8, so something very wrong.






Edited by jbl_man - 20 May 2020 at 9:47am
Be seeing you.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Earplug Quote  Post ReplyReply Direct Link To This Post Posted: 20 May 2020 at 9:13am
Can you not get a scope? Checking output without load, then with load will  be a clue.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tonskulus Quote  Post ReplyReply Direct Link To This Post Posted: 20 May 2020 at 3:26pm
You sure all transistors and diodes are ok? You soldered them (and all other measured components) off for measuring ofcourse? 
Some transistors need to be matched for the rest of the circuit to work properly (as far as I know).  Thats why some of them has coloured dots painted on them. 
Just one thing to note, if you replace some failed transistors..

Good luck! :)

Here is mine under restoring.




And there it's doing her business  :)



Edited by Tonskulus - 20 May 2020 at 3:43pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jbl_man Quote  Post ReplyReply Direct Link To This Post Posted: 13 July 2020 at 2:24pm
Ok Gents, a long overdue update....

Well  i could not get this to run right, so removed every output transistor and driver,and measured each one,and then put them in matched sets of four,+ 1 driver, so all the transistors matched all the others exactly in the same rail. This times four.

Then had to reset the bias,as was far to high..(was 0.6v on all four rails,should be between 0.30 and 0.35)...this is done by replacing set resistors until the correct figure is obtained. We now have correct bias within-spec, and DC ,both input and output offset is now 0.00v per channel.

The remarkable thing is,there is no background noise at all,and i mean nothing. No hiss,no buzz,it is totally silent,even with your ear close to the speaker. The only way you know it's on is the power light.  We now have audio,and it is,very,very loud,and really punchy.

My advice to anyone planning on overhauling something of this age, replace EVERY electrolitic capacitor,and check every single resistor (as these old carbon ones dont age well,some were way-out of spec), and finally remove all the output devices,test each one,and arrange them in  matching rails.

Was it worth doing? To be honest,in terms of man hours,of course not,and the repair bill at any workshop would far exceed its value, but at least it is now saved and working.







Edited by jbl_man - 13 July 2020 at 2:26pm
Be seeing you.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jbl_man Quote  Post ReplyReply Direct Link To This Post Posted: 13 July 2020 at 2:29pm
And the parts tally...


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Post Options Post Options   Thanks (0) Thanks(0)   Quote madboffin Quote  Post ReplyReply Direct Link To This Post Posted: 13 July 2020 at 3:10pm
Well done, good to know it's working again. As for the economics, a restoration like this is really a form of entertainment, or a hobby, and we can expect it to cost us something!

For future information, I'll be very surprised if all those brown capacitors really needed replacing. They are silver-micas and are usually very reliable. And they often have a "burnt" look about them, it's just the brown encapsulation.

Carbon composition resistors are always suspect, especially high value types (100k upwards). They tend to drift high in resistance over the years (even in storage) and should all be checked and, if necessary, replaced with modern film types.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Earplug Quote  Post ReplyReply Direct Link To This Post Posted: 13 July 2020 at 3:20pm
Nice one. Glad to hear that you got it sorted.  Smile Thumbs Up Smile
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 4D Quote  Post ReplyReply Direct Link To This Post Posted: 13 July 2020 at 3:35pm
Nice one Ian persistence pays off... 
DMZ. "The bass was intense. Girls were literally running up to stand next to the subs"
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