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Amcron DC300a overhaul.

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Category: General
Forum Name: Amp Forum
Forum Description: The 'Stopping Jake Fielder moaning constantly' forum description...
URL: https://forum.speakerplans.com/forum_posts.asp?TID=105157
Printed Date: 04 October 2022 at 10:23am
Software Version: Web Wiz Forums 12.05 - http://www.webwizforums.com


Topic: Amcron DC300a overhaul.
Posted By: jbl_man
Subject: Amcron DC300a overhaul.
Date Posted: 24 April 2020 at 4:47pm
Good evening guys,hope everyone is keeping well,and safe. Smile

Whilst sitting here stuck at home, took the opportunity to overhaul this lovely old amp....

Crown DC300a from 1973/74.  Bear in mind this is 47 years old.

I'm getting 0.01 and 0.02v DC  both channels at output, so all ok there, but sound is VERY weak and muffled (max output is about half a watt,on both channels.) Output PCB rails read fine, both channels giving + - 51v. OK. All output transistors tested,all fine. All preamp board diodes and transistors test fine.....i'm getting the correct + - 9.5v power supply at the ic pins....must be something on that main centre preamp PCB, as is affecting both channels.....any ideas? As i'm a bit stuck now.

Ps,i have to say,this is beautifully made,lots of nice shiny copper on the PCB's, really nicely built back then.









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Be seeing you.



Replies:
Posted By: jbl_man
Date Posted: 24 April 2020 at 4:48pm


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Be seeing you.


Posted By: Pinyorouk
Date Posted: 24 April 2020 at 10:34pm
Maybe you need to use an oscilloscope and signal generator to check signals at different points.


Posted By: kedwardsleisure
Date Posted: 24 April 2020 at 11:08pm
interesting it's both channels. Dodgy IC socket?
Old silver mica capacitors as used by crown suffer from what the yanks call 'silver mica disease'. They can go leaky and eventually short. Might be worth checking. Not something generally used in european amps.

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Kevin

North Staffordshire



Posted By: jbl_man
Date Posted: 25 April 2020 at 11:39am
Cleaned those four presets, now have 0.00v DC at output both channels, but output very weak and distorted. Fitted new ua739 ic, still the same.  Geek

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Be seeing you.


Posted By: Earplug
Date Posted: 25 April 2020 at 12:31pm
Originally posted by Pinyorouk Pinyorouk wrote:

Maybe you need to use an oscilloscope and signal generator to check signals at different points.


+1

The only way to really pin down the problem.




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Earplugs Are For Wimps!


Posted By: APW
Date Posted: 25 April 2020 at 2:15pm

Check C4 and C5 (the two caps closest to the Red Switch) as they are part of a diode pump (in conjunction with D1,D2, R1,R2 and D3) that generates about 70volts that feeds both channels driver circuit. Check you have about 100volts with respect to ground on one end of the 2k2 resistor R2…. R2 should have about 30Volts across it.




Posted By: jbl_man
Date Posted: 25 April 2020 at 2:33pm
Thanks APW,will have a look.....i did find a couple of resistors way-out of value, (r101 and r201) on the input offset on both channels, should be 2 meg, one was reading 780k,and other 540k,that cant be helping, have ordered some new ones. Will check what you suggest when i have fitted these. 


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Be seeing you.


Posted By: madboffin
Date Posted: 25 April 2020 at 5:30pm
On anything like this, I would change all the electrolytic capacitors on the circuit board. 50 years is well over their expected service life and they will all have gone leaky or acquired a high ESR (equivalent series resistance). Their service life depends (inversely) on temperature, so it's best to replace them with 105 degree rated types instead of the standard 80 degree version.

I'm surprised that you have found resistors gone low in value - carbon types normally drift high as they age.

I must get around to servicing my vintage Turners...


Posted By: jbl_man
Date Posted: 25 April 2020 at 5:35pm
Yup...have just changed the 10uf 150v, that Alan suggested above as was physically bulging and leaking. Will do,have ordered a batch of of other ones too. These old things are so nice to work on, lots of room around the PCB tracks, simple to solder, not everything crammed in like the modern stuff,with a hairs-width between the tracks. One slip with the iron,and it's scrap. Yuk.


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Be seeing you.


Posted By: GAZ.
Date Posted: 25 April 2020 at 11:29pm
And I’m sure with a few new components this amp will live on for another 50 years.
I can’t see any of these new amps surviving that long.
Any evidence of previous repairs or does it all seem original?

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100% Earth Moving Bass


Posted By: jbl_man
Date Posted: 26 April 2020 at 1:30pm
All completely original Gaz, original Crown (RCA) output devices, only repairs i can see is new front volume pots, not suprising after nearly 50 years. There are 3 service stickers on the back,dating from the 80's,90's and 05's so i guess someone has cared for it regually.

 Looking at the mechanical construction and attention to detail, i can see why these were twice or three times the price of anything else back then.

Yes likewise, love to see a Powersoft or FFA or MC2 after fifty years. Wont be much that can be repaired i bet! LOL


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Be seeing you.


Posted By: Robbo
Date Posted: 26 April 2020 at 2:04pm
Could that one possibly going into use on your Hi-Fi Ian


Posted By: Robbo
Date Posted: 26 April 2020 at 6:19pm
Certainly takes me back a bit seeing one in pieces again Ian---I had 28 of the buggers when Slade packed in touring and sold off all the PA gear and they were all in double racks with 4 way fan cooling.
We had to repair a few and I think most of them had seen a soldering iron at least once in their lives---Charlie, Slades FOH engineer used to buy them cheaply in the States and smuggle them back to the UK bolted inside the bands backline cabinets in order to avoid paying duty--Nod was very careful with money back in the day


Posted By: djeddie
Date Posted: 27 April 2020 at 9:01am
C'mon John, when's the book coming out?

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Chas n Dave : it's like Drum and Bass but with beards.             E=mc² ±3dB


Posted By: concept-10
Date Posted: 27 April 2020 at 10:11am
Originally posted by djeddie djeddie wrote:

C'mon John, when's the book coming out?

Agreed, John you are a gold mine of cool stories, a book is essential :)


Posted By: Robbo
Date Posted: 27 April 2020 at 10:17am
Its quite funny you mentioning that Eddie as I have started a few topics and got the format sorted while we have been on lockdown---even got the book title sorted now as well(don't ask).
Its going to have to go on the back burner for a while now though as I have just started work on my Mini Cooper S getting it up to 225bhp as I am hoping to be able to use it back on the road later in the summer---Cheers--John.


Posted By: jbl_man
Date Posted: 27 April 2020 at 1:14pm
Dont worry about the the Mini Cooper, it's the book we need, you can spill the beans on all the showbiz prima donnas John!  Edited to 'Mr.X' in case they sue of course! 

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Be seeing you.


Posted By: djeddie
Date Posted: 27 April 2020 at 4:40pm
Originally posted by Robbo Robbo wrote:

Its quite funny you mentioning that Eddie as I have started a few topics and got the format sorted while we have been on lockdown---even got the book title sorted now as well(don't ask).
Its going to have to go on the back burner for a while now though as I have just started work on my Mini Cooper S getting it up to 225bhp as I am hoping to be able to use it back on the road later in the summer---Cheers--John.


Is that a proper Mini or a BMW John? If it's the former then that's OK, if it's a Beemer then you know the book is the way to go!!

Sorry for hijacking your thread Ian! But, I do refer to these older amps as Helen Mirren's and Joanna Lumley's... maybe old but I'd still have a go and you know they're still gonna work!!!


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Chas n Dave : it's like Drum and Bass but with beards.             E=mc² ±3dB


Posted By: Robbo
Date Posted: 27 April 2020 at 7:31pm
I would prefer Helen Mirren as the only blow job that you will get from a Crown DC300A is when it goes DC


Posted By: djeddie
Date Posted: 27 April 2020 at 9:08pm
Ah, a Beemer then!!!LOL

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Chas n Dave : it's like Drum and Bass but with beards.             E=mc² ±3dB


Posted By: Robbo
Date Posted: 27 April 2020 at 10:04pm
Yeah--As far as I know you cannot get a classic Mini up to 225bhp without putting a Honda engine in it.
Don't knock the Beemers until you have tried one Eddie as they are surprisingly good--Mine is one of the very first ones with the supercharger and drives amazing when it was a standard car but with the mods I am doing, it will be even better.
BMW certainly did a good job on these early Cooper S models--they even managed to get them leaking oil and water just like the originals used to


Posted By: jbl_man
Date Posted: 30 April 2020 at 1:41pm
Originally posted by APW APW wrote:

Check C4 and C5 (the two caps closest to the Red Switch) as they are part of a diode pump (in conjunction with D1,D2, R1,R2 and D3) that generates about 70volts that feeds both channels driver circuit. Check you have about 100volts with respect to ground on one end of the 2k2 resistor R2…. R2 should have about 30Volts across it.




Ok...just to give you an update....changed the 2 capcitors around the diode pump, C4 and C5,and also changed 2 out-of-value resistors on the input offset pots. Result is exactly as before,very low,distorted output. 

Now measuring each side of the resistor (R2) to ground on the diode pump you mentioned,i get 61v one side,81v the other.

Have also changed the ua739 ic, incase that was faulty.same result.

But output DC is perfect, 0.00v left. 0.01v right.

Any ideas from here?




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Be seeing you.


Posted By: Robbo
Date Posted: 30 April 2020 at 3:50pm
We had a bad batch of DC300As once with very weak and tarnished jack input sockets---I presume you have checked them Ian as it could be a real daft problem.


Posted By: APW
Date Posted: 30 April 2020 at 3:54pm
I think its time to get the scope out!!


Posted By: Tonskulus
Date 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. 









Posted By: jbl_man
Date 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.


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Be seeing you.


Posted By: Tonskulus
Date 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.




Posted By: jbl_man
Date 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.






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Be seeing you.


Posted By: Earplug
Date 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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Earplugs Are For Wimps!


Posted By: Tonskulus
Date 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  :)



Posted By: jbl_man
Date 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.







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Be seeing you.


Posted By: jbl_man
Date Posted: 13 July 2020 at 2:29pm
And the parts tally...




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Be seeing you.


Posted By: madboffin
Date 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.




Posted By: Earplug
Date 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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Earplugs Are For Wimps!


Posted By: 4D
Date Posted: 13 July 2020 at 3:35pm
Nice one Ian persistence pays off... 

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DMZ. "The bass was intense. Girls were literally running up to stand next to the subs"


Posted By: djeddie
Date Posted: 13 July 2020 at 5:00pm
 Bravo Sir!

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Chas n Dave : it's like Drum and Bass but with beards.             E=mc² ±3dB


Posted By: APW
Date Posted: 13 July 2020 at 5:09pm
Originally posted by madboffin madboffin wrote:

.....
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.
......


As Kevin pointed out on the first page of this thread old silver mica caps can suffer from "silver mica disease". I doubt it was the issue in this case but in something this old I'd change them as a precaution.



Posted By: kedwardsleisure
Date Posted: 13 July 2020 at 10:55pm
great result. Always a relief when you fix that elusive fault and hopefully a satisfying sense of acheivement

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Kevin

North Staffordshire



Posted By: odc04r
Date Posted: 14 July 2020 at 8:41am
More than likely! Or the schematic that made it into the net archives was just one of many in the original library given out at one point.


Posted By: jbl_man
Date Posted: 14 July 2020 at 8:51am
Thank you gents. I will not be beaten by a 47 year old amplifier! Of course,it doesn't help that the official schematic is wrong....the voltages they give,and some of the components dont match at all,including at least half a dozen resistors,transistors and capacitors that simple dont exist on my board. I guess they modified these on a yearly basis,and no-one bothered to update the schematics.
Confused

I even emailed Crown direct,and asked if they had the correct schematic for my amp, the reply, "Thank you for your enquiry, we do not release schematics,these are only available to our official service centres" suggesting i take the amp there!

As if anyone is going to do that with something this old.

They even had the cheek to send me one of those customer satisfaction surveys afterwards.....so that will be Zero, zero,zero,zero,zero and zero then.

LOL

In the slim chance of anyone from Crown ever reading this,dont be so bloody stupid,it's not as if anyone is going to steal the design of this thing after all this time.


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Be seeing you.


Posted By: jbl_man
Date Posted: 14 July 2020 at 8:53am
Originally posted by odc04r odc04r wrote:

More than likely! Or the schematic that made it into the net archives was just one of many in the original library given out at one point.


Yes,this exactly. There appears to be three version out there,unfortuantly none relate precisely to my amp here,so was a lot of guesswork involved filling in the blanks.


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Be seeing you.


Posted By: kedwardsleisure
Date Posted: 14 July 2020 at 9:23am
Possibly the result of a long production run. Components become obsolete or expensive, better parts become available, feedback from the customer, new design staff, all conspires to change the design over the years. It's the same with the macrotechs. There are many circuit revisions and one bears little resemblance to another.



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Kevin

North Staffordshire



Posted By: boominbob
Date Posted: 26 April 2022 at 9:38pm
Hi guys. 

There are 7 different main pcbs on dc300a / dc300a mk2

With overlap between mk1 and mk2

5 different versions on mk1

3 different versions on mk2

Early mk2 had mk1 pcb. Essentially early mk2 was a mk1 turned upside down with different mains switch and relocated level pots. 

Apparently mainboard was redesigned due to the main 739 ic going out of production. 

Mk2 had drain resistor across one smoothing cap. Drain resistors across pre drivers and a solid state switch across main ic supply. All these mods were to stop farting and whistling at switch off. 

Always follow number on underside of pcb

Spent a lot of my life working on them.

Good luck. 



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